FEDERAL DEMOCRATIC REPUBLIC OF ETHIOPIA
MINISTRY OF WATER, IRRIGATION ANO ENERGY
DABUS HPPs
Consultancy Services for Pre-Feaslbility and Feasibility Study
of the Dabus Hydropower Project
Pmcjremcnt Retorra Nurnbw - OHPP/MoWlE/01/2014
DABUS HPPs 
PRE-FEASIBILITY DESIGN MAIN REPORT-vol. 1 of 2
July 2016
atudln placrnnQali o>-i*MUlting «Hgln»r^studio ing. g pietrangeli i.r.L
via cicerone, 26 - romt Italy
U!l: 06.32.10480htw: 06 12.27 276
e-mail: datiusi&pietrangeii.it
/U.O l J jt)
pag. 1 of 1
Ministry of Water Irrigation & Energy
Haile G/Silase Road
P.O. Box No/ 5744
Addis Ababa, Ethiopia
Rome, 01* July 2016
Att.ne:
Mr, Getu TilaHUN
Email:
oetu tilahun2005@Yahoo.com
Telephone;
+251 11 11 / 11 6637222
Telefax:
+251 11 6610710
DABUS
Consultancy Services for the PRE FEASIBILITY and FEASIBILITY StuOes of the DABUS Hydropower Project PREFEASIBILFTY DESIGN and EIA BASELINE rev. B- Transmission Letter
Dear Mr. Getu T1LAHUN,
fol lowing:
-     your comments to the pre-feasibihty design report and EIA baseline report (reference is made to the attached matrix);
-     the workshop held in Addis Abeba on 16* June 2016;
-     our meeting held at your office on 17* June 2016,
We have updated the following reports:
-     100 WHO R SP 003 B vol. 1 / 2 - Main Report;
-     100 WHO R SP 003 0 vol. 2 / 2 - Drawings;
120 HYD R SP 001 A - Hydrological Report;
130 GEO R SP 001 A - Geological and Geotechnical Report;
-     140 SEA R SP 00IB - BASELINE Study,
Yours faithfully,
Sfixto Ing, G.
Antonio P1ETRANGEH
Antonio BRASCA
Enclosures : n. 20 hard copes; 2 CD
cap. soc. €98 130 l.v. - C.C.I.IJ. roma n. *92*27 - trib. Rumn n. 2626/8 2 • c f C544G170596 p.l. 01<0i561C04DABUS hpp
___________ _______________________
WORKSHOP 16 JUNE 2016 CONSUL TANT’S RESPONSES TO CLIENT OBSER W TJONS
Page jof   gDABUS hpp
WORKSHOP 16 JUNF 2016
CONSULTANTS RESPONSES TO CLIENT OBSERVAT1ONS
Page 2 of 9
Comments by Client
Q7
Q8
5cxl?t»lJLZ  £>3QC  iunfir  »e®w  verify  deafly.  at  n/vctt  vte  tfie  Dane  survey **av ctykfucted, on HP cP-105 (DAM area) or on HP di155 (MM area)
Section J.?, Desmbe briefly the problems for fXKt processing of the PLEIADES Stereoscopic images within the test four months In onAr to compare and chouse the refined one than only using SfiTM 30*JQm as usual
A7
AB
StfCto?  ZJ;  ft  needs
further
cfanffcations  why
rhe
drone
survey
dtd  not
been competed w the Pleiades topographic database will be used, instead of Ui? SRTM for the
conduct tvi Ph ch /tW.
Feasibility Study since it has a much better accuracy, *
Q9
A9
There were problems finding suitable landing area near PH ch-loSTrhe Consultant wiH^rtarth^
the PLEIADES images which are sufficient for design purposes at th.s stage.
Qll Q12 Q13 QM
Q15
Q16
Q17
Pfease justify name whether the figure name DAM CM 150,
surveyed areas 7s correct or not.
I  Me  J.  J  J  Droned  surveyed  areas  foatiaes.  there  are  no  ground  contra! pumts shown at Dam Ait CH 155 and Alt CH IM If not necessary for analyse please give your justification dearty.__________________________________ _ Section 7 3.2: Discuss tn detait the type modei or software and method used for carrying out the Reservoir operation and energy production s/mutot/ems.
The Reservotr volume and area curves as a fijrxhan of elevation shmAd be
determined based on detailed topographic survey.
As per (he TOP section The Consultant should estimate the reservoir dead storage and dead ievcb based on sediment measurement, but the report was done based an secondary data from literature against the TOIL__________
Dabus Commant (1): Dam ubty axpart ______________________ Most  of  the  activities  art  executed  as  per  frtr  grvtn  Wft  for  Pre-feavbifty study. However most of the rryxYtanr srte investigations host  not done and completely left for  feastoUrfy  stage  investigation.  Smtterty ounny the aarvnt geotechmcaf field investigation the emphasrs grven to dam site and reservoir area  is  quite  tow.  Thts  means,  dam  foundation  reservoir  water  tightness, presence  of  weak  rones  not  cun&dereu  as  regutred  for  next  phase  work pianrimg._______________ ______________________________________________ Construction matertei avaHabto quantity and qualify shou/d be addressed to
some detaif at feast for prefeaubttiry stage.
Instead of defaced petragrafihy analysis of roc* the project area, the
consultant should g»ve more empha&s to direct or rndtreef field investigation ly structure geofopfcai investigation is more important If necessary at thu
such             • shoM be done speaficaify a certain construction material—
A10
All
A12
A13
Answer by Consultant
The drone survey was earned out on the ch-155 dam area.
This typo wtl be corrected in Revision B of the PrefeasibiUty Report.
All the topographic  activities  regarding the Pleiades  have been concluded T^uistionT^d
processing). In April 2016 a ground survey  was  carried out to acquire Ground Control Points 
(GCPs) to georef er ence and orttxxectrfy the images. Processing to obtain contour lines has recently 
Another drone survey will carried out in a future stage once good landinq areas have been identified,
The name of the figure should read ’ch 15S*. This will be corrected in Revision B oHhc TPhreefeCaosinbsuililtty anRteepxporlta. i_n_e_d_t_h_a_t _a_n_o_th_e_r_t_yp_ e__o_f _d_ro_n_e_ (R_T__K_) _w_a_s __u_se__d_that does not require GCPs. This drone Is currently under testing.
A mathematical model created by SP was used for the reservoir routing, The model i?described aF length in the report._________________________________________________________________
Agreed.
In the Feasibility Study the accuracy will be improved [ Plea ides Database).
A14     M<xe detailed modeling will be done during the Feasibility Study.
Alb
The Consistent explained that investigation planning will be done in the near future in oottaborMlon wFoithr tthheesiCtleieantt.ch - 130 the Consultant estimated the quality and the quantity of the quarry 
.., material (rcr page 86/223).
For the site at ch - 108, detailed indications were not considered necessary given the abundant evidence of easily available construction material outcropping everywhere.
The Consultant pointed out that this information was included in the report 130 GEO P SP 001 Al7 (Geok>gy Report) of the Prefeasibility Report.
160627, DABUS, Reply to OBS M PRE-FEAS Repot! (Workshop 160615)
ituebo pirtrangd:, romeNo.
Comments by Client
Answer by Consultant
At Pre feaifbttiry Atope, fhe                 route J/V pt*w rtoiusr srte the fo/fowng
r
Q18
data/ information rs rvqt-Mred SLCft .*> fnQtneerrng gecifirgrtai prtjnJe tntMing pnti&ptd /Oct hiw, ro& mass c/iarartrfidfcs. hrdrogeofotpcar' /nfcrmabon presence of wvnft rones/tectonK fault or ttpawe to construction risk jAxat be addressed t+r# Thus proper prebminary evatarfavr of afremabve routes 3«tv/ ®fes /tv appurtenant structure of the cascaded dams is reputed to dead? the rodts in a partroAar sfe are competent enough to mafie an apeneg through rt
Alfi
This will be addressed in more detail m the Feasibility Study.
The  fi€C  dam  type  select**)  atfem  j>iw  m  the  report  both  dams  are  not I £ww/?onp jrv un* j/sc> cqpws? jV fljrh other.
Q19
A19
1
The alternatives Studied at site di - 130 included ar RCC dam and a CRf dam. As may be read in the report, the cost of the RCC dam and the CRED are practically  identical w the Consuftant preferred to adopt a 'safer* solution and with fewer maintenance costs, i.e. an RCC dam.
As far as the dam at ch - 108 1$ concerned, an RCC dam was considered the only option given the general geological conditions.
The Consultant points out however tnat the two dams are completely different, so much so that the dam at ch - 130 was presented with two distinct solutions:
dam with central spillway and ski-jump dissipator;
non-overflow dam with separate spillway.
The dam at ch - 106 on the other hand can be Overtopped with a stepped spillway.
Q20
77ie A?>w4‘ rfam and appurtenant structures should be dearty indicated tn the map.
A20
Noted and agreed.
This will be corrected in Revision B of the Report.
DABUS //Mb?; Barharw Gudfasa
Q?1
/o .synthetic stream bow generation approach monthly average discharges the min value taken snot the appropriate va*je {Page 4J of 22SJ
A21
Revision B of the Prefeasibitaty Report will correct this error.
There are so many softwares are fisted in your pre- teavbtitty report that used to analyse data but the tefiaMffy and Quality of those sortware ts not defined.
A22
The software used to eiabocate the Pre-Feasibillty Design was: - either software internationally recognized, or
mathematical models elaborated by SP..
Q22
Q23
there  s  to  *$f  </  abirewatioryacronym•'  on  the  front  page  of  the  pre- fejMtMty report Therefore, ft & not easy to understand the report
A23
Noted and agreed.
This will be addressed in Revision B of the Report.
Dabm Hyt&o Pvtw Project• Conwrivnfr on PFS
—
Q24
Despite comments provided on the inception report related to the name of the Mftvstry d s not Ci.YLS*tertsJ m the preparation *x erw report. >f bears the
former name of the Mimstry whrir is htivstry of Water Jmgatwn and Energy.
A24
Noted and agreed <
This win be corrected In Revision B of the Report-
Q2S
Srm/idr to the comment Ata Z comment gnen on how the rmpact of proposed bydf cpower schemes on the trygabon schemes intended to be developed nfilf addresses. However, nothing has been said m the report related to this.
A25
The  Consultant  points  out  that  intended  irrigation  schemes  are  not  within  the  scope  of  this assignment.
Only the Impact on existing Irrioaton schemes will be consdered in the ESiA report.
Q26
J
tn the .'fs-epbon report /£ was proposed to use EBEE* done for topognxphfc dtfto coDectfon but m the pre feasibility study however Ptf/Abf^ saMNe imagery has also been used m this study. The reasons necessrtate to use both of then) is however not stated tn the report.
A26
The accuracy of die drone is greater than the accuracy of the Plea Ides
However the areas that can be surveyed by drone are in the order of a few square Ulometres. The Pieaides, on the other hand, can be usee to survey areas of several hundreds of square kilometres (over 1,000 km2 were surveyed for the Dabus project).
For this reason the drone was used to survey the main works (dam and powerhouse), while the Pleaides images were used for the topography of the icmaming parts (reservoir, quarries, etc. i.
160627, DABUS, Reply to DBS at PRE FEAS Report (Workshop 160615)
stucto p«rangeli, rwDABUS hpp
WORKSHOP 16 JUNE 2016
CONSUL TANTS #£SACW£fS TO CLIENT OSSERVA 77ONS
-——
Comments by Client
Answer by Consultant
.
The entire nver course was studied.
, J 1 ■
Q28
Q29
Q30
77kxj^ a comment given on the inception report this study xhtf focuses on the same fen- srfes- along the over course h/k/j ar? /rAv*0/7e?t/ by previous
studies.
Sefectxn of dams made by considenng different factors which had impact an the rr«f of investment and maintenance. Mrf manly depends an the distance
of the location of construction material. However. without canstderabtvi of this and other factors simply f utter conpacted concrete (R<X) and concrete surface rode fU dams are proposed_______________________________________
Some quarry sites are identified in this study but no
and volume of the material to be quarntxf has been given
related to quality
As can be seen dead, from the longrLudinal profile of the nver [ref, Fig. 2.2.2), the areas studies were the only ones feasible from a technical-economic viewpoint.
The mam factors  (geology, quarry, maintenance etc.) have been considered in this  preliminary evaluation, more than adequate for a pre-fesibility  design. More detailed elements  will De considered in the next stage (Feasibility Design).
Reference is made to quality and volume of the identified quarry sites at Page 86/723 oFthfT Prefeasjbility Report,
Dabu&PPreF&aJibiJrtyDwsfgn_ HydrutoglcaJCompcwmC: Hydrology A
Water Quality OHrdCtorM fSurafeJN) _______________
The site map diail incorporate the focation of the preposed dam sites and the rhrr networks for ease for vtsuabjatw even though shown tn ( Vol. 2
_ drawing), Page JD _______________ _________ _______________ ______ __ _____ Jn calibration processes using cunoarent data (preopriabon and flow
discharge data), you used a J years data only (196$ 1967).
jwziwiiifjy for vaidatiof) (1969 and 1972 J 9/4). Page 54 and 55.
Q3L
In    statistical    tests    (hypothesis    testing)    randomness,    independence
homogeneity and stationarify methods
adopted but not shuwn
Noted and agreed.
Revision 0 of the Prefeasibilrty Report will address this.
Periods spanning Jan 1965, Dec 1967 and Jan 1972, Dec 197-1 provide the more complete set of significant daily discharge measurements at the station of Dabus near Assosa and were therefore selected for calibration and validation purposes. Different periods  were selected to perform the calibration and the validation of the model in order to guarantee that the calibration resulting from one period was actualy efficient for the application of the model to another period, n other 'words in order to guarantee the generalization to other period of the calibration performed on a speafic period.
A table presenting the results of the performed tests in terms of the specific statistics useo by each test is included in the revised version of the report.
summanred results d each test. except trend nd stetoonanty tests Moreover, ch&e tests are inferred as neglected (trend), and conservative to use tn practice (the staticnanfy fed), &nd how* Page 66 joJ 62
Q32
near Ar/o
______ _________
The assumption of neglecting the observed trend stems  from the observation that the trend is negative, i.e. the observed maxima tend to decrease in the spanned period (1961-200B). The analysis  of the extremes  considering the sample as  a notstationary  sequence. i.e. considenng statistical models wivose parameters are dependent on time (e.g. Reiss and Thomas, 2007, Coles, 2000, Fawcett, 2013), would have produced a not conservative decreasing in time of the discharge quinbles.
Reiss,  R.D.  and  Thomas,  M.  (2007).  Smsdg*/  Analysts  of  Extreme  Valves.  With  application  to Insurance, Finance, Hydrtdogy and Other Fields. Brrkhauser. Third Edrbon.
Coles. S. (2000). introduction to statistical modelling of extreme values Springer.
Fawcett, L. (2013). Topics in Statistics. Environmental Extremes. Lecture Notes. University of Newcastle.____________________________ _____________________________________________
Noted and agreed
Q33
Please correct properly the hydrometric station name of instead of Ano throughout the documents.
_________________
-------- 1 On page 47, wh^chpM on /eft d/T^T* indicates' Similarly Fig 2 on page
Q34
49
This will be corrected in Revision B of the Report Noted and agreed.
This will be corrected in Revision B of the Report
I6O627, DABUS, Reply to DBS dt PRE-FEAS Report (Workshop 1606 IS)
stud in pirtiMMXll, romeDABUS hpp
WORKSHOP 16 JUNE 2016
CONSUL TANTS RESPONSES TO CLIENT OESERVA77ON5
Page 5 of 9
.
No.
Comments by Client
Answer by Consultant
■  ■-
1
-------- -
Q35
comment « J Support Data, /f is not J good J to
dady data as pmtedand^ or avwf pt/bfcfy cAat ft? jierw/y /aasans.
A35
Noted.
This will be removed from Revtson B of the Prefeasib4rty Report.
ftfrdnveMW' Devetopmant antf Awn                                       AinftTofate
Atffrajprt CffGE fdmifr flejdwjr £Swn fwMrn/J irrafe^ one tfftoe ™sT ^TjpLXt^ /55VW Ar ECtapto to Au*r up eflkwtWMflfa/ towdy dwwrar SrrwffJ in particular dUe to tor tort tfuf ^^qptwr U dfh? l# toe M.K5 for toe 6rn^jenta£W7 <✓ fto* strategy, it is ignored n the report Thus, the issue should have been tndiated and »W *? tot? .-c^lyT
Agreed to discuss the CRGE in the final report
LegJ prison A*r EtNopun water resource management proclamation
197/2QW and fagutfw (M&2W5X Mety association
Agreed to incorporate the p<odamat»n 197/2000 and Regulation (115/2005) in the final report
fo
Q36
*wto t wTsfricOw? t/ dam for v»*«r generation are nof /beppofated m toe report.
Q37
Due to toe tort toaf there wt# be huge amount of solid waste generated dunng ffre construction time, it it /rrpurtant to address the issue wrto regard to Solid Waste Management Prodamatxn (513/3007)
Agreed to address Proclamation 513/2007 in the final report
Q3B
Arma*  LI  ofpagel32.  ft  s  stated  that  Hippopotamus  amphibious  s  one  of the iM*f iiTWTM/s Irwip *? tot- 5fiLt/K dw fw todert?/?, to/f **7 jrwrW
m toe /bCT W tor t LXwurtdrtf re^wr dfce$ Jtratfcato tos tort. On toe otoff flaraX too? an? ton?atoned and rare odd antmats indicated in the report      Like      panfherapardus      (Leopard),      Panther*      Leo      (Lion), Geocchekonepardafis (Leopard tortoise), Niobcux Cortxody/usry/obciis (NAe Crocoiie) and it does net /nctato any rrvbgabon mechanism to protect the wtidammaLs
As  per the TOR, this  Is  an ELA Baseline Study  phase. Therefore, the resource and the potential impacts  are discussed. However, detailed mitigation measures  will be presented in the Full ESIA report.
Q39
Anne*2 (table} of page 132, There are kst of plants which are found in the direct impact area But, the Ost does not seem a*r*Artt or exhaustive for instance, mere is no a smgie drmber fife form ^.we Art /At? //st. Does that mean there is no climber in the area?
Yes,  the  list  is  not  complete  ar  exhaustive.  Detailed  sampling  and  Identification  of  plants  will  be carried out during full EIA
Q40
Ptanr status or nAnerabttty is not wef described
Once detailed sampling and Identification of plants Is done during full EIA, we will give Information
□n species vulnerability using IUCN RED LIST OF ETHIOPIA
SlophysTc*/ ^VirVfWDVJt
Q41
Regarding water guakty assessment, it is not dearly described the techniques used to collect the samples for instance, how the number of sanptes rs statically determined and other techneat issues tike samphog period (Dry/ramy season) and sample distribution are not weii addressed n the report. The 1 report should have been presented m such way to Atuslrate the statistical representatives of tor samples, the rationale behind the dstribution of the samptes and the method of analysis which insures the retebi/tty of the output
Agreed to discuss bow the sampling sites are selected and the technique used to collect the sample In the final report
1
160627. DABUS,. Reply to OfiS at PRE-FEAS Report (Wflrkstiop 160615)
studio plctrangeli, tomeNO.
Comments by Client
,.                Answer by Consultant
Q42
jt ts not dear and ws# described what standards ar?                  fo evMuto tof /*>               jjaramefevy <X wafer qua&ty (Tjrfr&ty- t^ssubed JJUk? AanSiesF efG). iMryewtX ctowJWS/ parameters Atr AX’ COO an? w* jtA#sssrtZ C¥» tor’                           ZxtV^CMCj/ issues related to water t/uaMy are totaify okerAwkw/-
Agreed to elaborate in the final report
lOfenW Managemtinf
Q43
1“
77?p wjfwjtow/ V £t*M J™ i»5 sftjcfrrt/ js untl fieyaid tfrs toe constAfanf expected tv                  in depth from sub catchment pant <✓ b*w r*h*to seems forgotten
According to CBPWDG (Community  Based Participatory  Watershed (Development Guideline), the watersheds  size ranges 6,000-20,000 ha ts used to identify communities and ranges of community where watershed planning can start at this  level. However, before implementation starts  an in depth study will be expected to do based on the Guideline,
Q44
As uvAcdtrd >n r/v report, estimation at sod toss « made by tssif Thts does not ccmder fosses from guMes and mer Araks which suppose to under estimate toe preda-tian tV the tees Then, y o reamnended to ja apaafe
L>tber technique wAito a-*m to conoder toe tose racned by pufres            over banJts
RUSLE is used for soil loss estimation. The mam soli loss part and cause for Gully formation is the untreated or unmanaged part of big watershed. So that focusing on assessing soil loss  from the first phase of erosion Is  essential at this  stage lhereforc, that is  why  this  assessment does  not consider losses from gullies and river bank.
Q45
Instead of usmy USL£ it Hwfcf /wur been better to use toe Revised universal sat lose fQUdbon fRUSi E) which is the predecessor LISLE
RUSLE is used for sori loss estimation. Please see the collected data
The current land use and land cover has already been done and shown in the document. However, assessing the dynamics or change of land use is not covered in the 1 OR and beyond the scope of this assignment.
Q46
Projecting future iana use kind cover changes of a water shade area based on toe previous trends has m massive importance for a pepper management of a nuter^hed are which is highly associated to protect a dam from sot erosion induced sediment dccumuiabun. One way or toe atotfv, understanding <✓ the rate of patterns or change of toe wafer shade area uribca/ty important to determine iffe time of me dtorn. So that, the consuRant shotAd have made the tang use iand cow change of Etebus watershed area tn a mariner to show the water shade area dynamics.
RrM/ntnary fmpaci latu/rwnt
Q47
The   consultant   M   not   strictly   fofow   the   Hydropower   Production. Transportation And Dtstnbubon godetine (MKM ERA)
As per the TOR this is an ELA Baseline Study phase. Therefore, during the next phase the Full ES1A will strictly address all the Lssues as per the EP A Guideline.
Q4B
Air portion (Degradation of air quality by dust and vehicles emissions)
T he same as above
Saari   r&fubon   TMxfrnes   Me   crashers.   concrete   miters.   dozers   etc. Increase ri ambenf nase near tt>e substations)
The same as above
Q50
Hffler poMon (Contamination of surface and undergr^ari wafer, hasanA*#
1 he same as above
16062?, DABUS, Reply toOBS at PRf’FtAS Report (Wcwkshop 160615)
studio pien'amjHi. romeiff* S'* 'x"’J
L I/"- T '■ • ‘,J
I f' ’kt' '* 
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„ ■     •:        ~. ■ ,
,.   -
,r‘
,
i’
No.
Comments by Client
Q51
Sat/ pollution (Eraston of nwrbed downstream of tfje dam, Serf compaction. Reduction in sou ferMy 5<W d^YaMjsa/Mn as a nesiX flf flrcakato?, #5* cZ soil contamnaban fiom substations) are not we* assessed.
ftegartftess a/ ffc?
mwcxmenAs/ /sues; patera^
/mpdt-T ttwf tfw arrsurtdrtf /us identified /ar the project mN cause a tremendous amount tZ r^sPuctftvj on ff> swjttwV. Wcwenx tne
/wwwritfndW/tXF and condusiun which /rnpies no envwnmencai
grounds not to precede CV ^xticwsctf
HPP seems 3 paraMa wtfr
Q52
regard to the toss identified by the consultant. Does that mean all the enwormniai Joss ts nothing) Or did the consultant conduct the study tor saJte of procedure)
Attention shoufd be grven for thrs issue and the consultant is expected to propose the proper mrtigaten measure identified on pnrfeastMty level to
►mtkA  the  negative  impacts  suppose to be  reduced to insignificant  level according  to  Envtronmentaf  pokey  of  Eth/opu  (J  99/]  Proclamation  on Environmental   impact   assessment   (/9ty2(M2l   Gmde/me   for   darns   and
reservoirs (2004)
Other comment
Q53
of the tables. graphs maps etc in the EJA report are not visible This dAumerrt wnT     usW tor furthers studies and reference tor the future.
QSd
The preknwnary environmental impacts including forest resources m the project area should be storied and incorporated m the report These may
•ndude impact on aguabc eiosystems terrestnat ecosystems fi e, Itora and
r
faunaj, forest bmOh^rsity, foods and droughts tn downstream areas due to die reduction of forest coverage, land use and /arid cover patterns (such as agriculture, settlement, forest, vegetation. rods etc].
QS5
Hydropower projects are usuatiy located tn areas Mth forests. Large forest areas have been cleared Ta use as dam construction and water reservoir. Therefore, the preliminary environmental mitigation mediums and costs for
mrbgahng forest loss and other enwonmentai issues jre- oor briefly reported in the pre-feds&Jtry- study of the Dabus Hydropower Project
QS6 .
As per the TOR of section 54} the prHmWfy social mitigation measures and compmation costs (eg. resettlement support, land and material tones, and
[60627, DABUS, Reply to DBS at PRE FEAS Report (Workshop 160615)
Answer by Consultant
Yes there will be soil erosion dunng construction and this impact will be elaborated and mitigation measures identified and recommended in the Full ESIA.
Based on this  EIA Baseline Study, there will be both positive and unavoidable negative impacts d^soculed with the Implementation of this Project. These potential beneficial as well as significant adverse impacts on the physical, biological and socio-economic environment are identified.
However, based on this  baseline survey  result, it is  concluded that there are no environmental grounds  for not proceeding the Proposed Dabus  Hydropower project to the next phase and therefore, It is  recommended to proceed with the Feasibility  Phase of the Project, This recommendation is consistent with EPA's EIA Guideline.
As per EPA's EIA Guideline requirements, a full Environmental and Social Impact Assessment will be prepared during the Feasibility  Study  and al relevant matters  related to the potential environmental impacts identified in this Study will be fully addressed.
The    significant    environmental    impacts    require    avoidance,    mitigation    and/or    compensation measures. These measures will be identified and elaborated further in the next phase.
Will be corrected In the final version of the EIA Baseline report.
Preliminary  environmental impacts  including forest resources  In the project area was  described. The existences of unique riparian and wetland vegetation were discussed and the recommendation is to avoid any damage on these resources.
However, as a compromise, it was also recommended to asses areas in the Dabus drainage system as a substitute that is going to be lost by the dam.
During  the  next  phase,  the  Full  ESLA  will  identify  and  recommend  mitigation  and/or  compcnMtion measures for forest losses and estimate all costs to implement these measures.
The TOR requires an EIA Baseline Study for this phase and a detailed and Full ESIA in the next _phase. Therefore, dunng the next phase the Full ESIA will strictly address all the issues as per the
studio pietrangeli, roneDABUS hpp
WORKSHOP 16 JUNE 2016
OCW55WL TANTS RESPONSES TO CLIENT OBSERVATIONS
Page 8 of 9
NO.
Comments by Client
Answer by Consultant
etc) 4^e nof dearly desunbed tn the preVftiaCir/rt}' report.
EPA Guideline. Thrs will Include all environ mental mitigation and management, butter zones arid watershed development environmental monrtonng capacity building and training costs.
r
The pretenrnary totat project cost wdu&ng emravnertaf and soda/ costs.s
not tnduded in the pre-feasibifity report. The term lAoj sai*
1
Q57
irast? /5 nrt* JaTC/odeJ in Ebe /xe-/taaMfr rcpcirf, TZw fernj em^nanmanfttf antf soda/ cast refers to finanaat expenses Chat are being paid antfor stotM be pMi         jcrajnfft? for me total finanaai investments ana operation of
The same as above
5<K>Q'ftWKvrwc fnwww*?f
To JdtXesf rt^trW sooat issues (page 66-7J) /ike Mteracy > education, ethnic a rv^gtouf J/m/jOfoa Mcfo tfcww?m/c C5W 2007 report is referred This holds true for physics/ disabdrty rate and health infrastructure (page 99). Was it not posstbte to refer latest references or to tete projected figure instead of ta/ung Mt dated figures ’
Q5H
None out of the five Tables and Charts below that are based on CSA tan be updated because of lack  of up to date and disaggregated data - disaggregated such as  urban/rurai, male/female, regioci/zone/woreda, etc. Such comprehensrve and disaggregated data can onty be obtained from the CSA and the next national population census is yet to be conducted In the absence of such data, another source Is  administrative data compiled by  the woredas. In addition to its  limited rdabilrty, very  often, administrative data is  either not comprehensve, or not disaggregated by gender and the rest of variables, or unavailable altogether.
Despite these limitations, efforts  wdl be made during the upcoming ESLA fieldwork  to gather, where possible, data and informabon required updating those Tables  and Charts  (1 to S below} that are based on CSA's outdated data.
1.    Table 2.4: Literacy Rates of Population of the Project Area by Sex and Urbart/Rural Residence (CSA 2007)
2.     Chart  2.1:  Proportions  of  Major  Ethnic  Groups  Living  in  the  Proposed  Dabus  Hydropower Project Areas (West Wollega Zone and Asossa Zone) (CSA* 2007
3.     Chart  2.4:  Proportions  of  Populations  by  Religious  Affiliations  in  Dabus  Hydropower  Project Areas (Based on CSA:2007)
4.     Chart 2,5: Economic Activity Rate of Population (Aged 10 Years and Above) in the Proposed Dabus Hydropower Project A/ea by UrbarvRural Residence and Sex (CSA 2007)
5.     Chart 2,9:  Physical Disability  Rales (Per 10,000 People) among  Populations in the Proposed Project Area (Based on CSA 2007)
On the other hand, Table 5.20 is  correct except that tne year (2007) rs  mistakenly  written in Ethiopian calendar instead of Gregorian. Therefore, the year 2007 wiH be converted Gregorian 2014/15.
Q59
4s per          7tW. /fa* sddo economic aspec&nsts of me Dabus Hvdrupoi^r project are not dearly stated w the pre-feastbrfay .report ftisrtve as nets as negative sbd&ecunomic impacts Mfety to be accrued due to the project are to tv fated, For instance. jemop^aphc profile. t^ater use partem like agricultural
Yes, due to field constraints the socio-economic aspects/rlsks of tlw Dabus Hydropower project are not dearly stated in this EIA Baseline Survey report.
160627, DABUS, Reply to QBS at PRE FEAS Report {Wcxtshoc 1W61S)
SixJ o pietrangek, romrDABUS hpp
WORKSHOP 16 JUNE 2016
CONSULTANT'S RESPONSES TO CLIENT OBSERVA HONS
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Answer by Consultant
1
practices. water wdkty anjfysrs forest dependent economic jrtwoes, tmparf on human health and econunw status, tnodence of water related diseases, resettlement, reiiahdttahon and hxm! response programme ofthe protect, etc.
However, delated field survey will be carried out during the full ESLA
\---------
160627, DABUS. Reply to DBS Jt PRE-FEAS Report (Wortshop 1&D615)
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PRETEASIBILTFY STUDY Report
INDEX
pagei
Vol. 1 of J - MAIN WORT
Paragraph Subject
1      CONTENT1
2       EXECUTIVE SUMMARY3
2.1            INTRODUCTION .3
2.2            TOPOGRAPHY3
2.3            HYDROLOGY4
2.4            GEOLOGY and GEOTECHNICS5
2. 4.1 CH — 130 Si! E..<................y
F3 Pe
2.4.2        CH -108 SITE..................................................................................................................................................... 7
2.5            SEISMIC ASSESMENT...................................................................................................................................0
2.6            PROJECT LAYOUT OPTIMIZATION................................................................................................................. B
2.6.1     GENERAL............................................................................................................................................................. 8
2.6.2     SITE ALTERNATIVES..........................................................................................................9
2.6.3     EVALUATION OF AL TERNA TTVES.................................................................................................................9
2.6.4     EM and HM EQUIPMENT. 10
2.7            DABUS CH-13011
2.8            DABUS CH-10816
2.9           TRANSMI SSION U NE SYSTEM20
3       TOPOGRAPHY 2
2
3.1            INTRODUCTION22
3.2            CATCHMENTS AND RESERVOIRS AREA.,..22
3.2.1     INTRODUCTION22
3.2.2     SRTM DATABASE'.2J
3.2.3     PLEIADES DATABASE23
3.3            WORKS AREA""^””....24
3.3.1     INTRODUCTION'............................................................................................................................................. ,2V 3.3.2     SURVEY.........................................................................................................................’.................... 24
4       HYDROLOGY.................................................................................................................................................................. 29
4.1            INTRODUCTION29
4.2            AVAILABLE HYDROMETRIC and METEOROLOGICAL DATA29
4.3            ANALYSIS of AVAILABLE DATA.......................................................................................... „j j
4.3.1     HYDROMETRIC DATA............................................................ ”" JJ
4.3.2     METEOROLOGICAL DATA’............................................................................................................................... 34
4.4            EXTENSION of the DAILY DISCHARGE TIME-SERIES at DABUS near ASSOSA35
4.4.1     INTRODUCTION 35
4.4.2 SYNTHETIC STREAMFL OW GENERA T1ON............................................................................................... 3g 4.4.3      TRANSFER FUNCTION
4.4.4     RAINFALL-RUONFF MODELUNG."jZ 4.4.5     CONCLUDING REMARKS
5       GEOLOGY and GEOTECHNICS
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5.1            REGIONAL GEOLOGY39
5.2            LOCAL GEOLOGY39
5.3            CH - 130: DAM SITE........................................................................................................................................... 40
5.3. J MORPHOLOGICAL FEATURES.
5.3.2      GEOLOGY OF THE DAM SITE.......................................................................................................................... «
5.3.3     GEOTECHNICS OF DAM SITE.......................................................................................................................... 42
5.3. 4 QUARR Y FOR CONSTRUCTION MA TERLALS42
5.4            CH - 130: WATERWAYS and POWER HOUSE44
5.4. J MORPHOLOGICAL FEA TURES.44
5.4.2        GEOLOGY and GEOTECHNICS........................................................................................... 44
5.5            CH - 108: DAM SITE................................................................................................... ....................................... 45
5.5.1     GEOMORPHOLOGICAL and GEOSTRUCTURAL FEATURES45
5.5.2     GEOLOGY OF THE DAM SITE46
5.5.3     GEOTECHNICS OF THE DAM SITE 46
5.5.4     QUARR Y FOR CONSTRUCTION MA TERLALS.48
5.6            CH - 108: WATERWAYS and POWER HOUSE.............................................................................. ..................49
5.6.1     GEOMORPHOL OGICAL FEA TURES.............................................................................................................. 49
5.6.2     GEOLOGY and GEOTECHNICS.......................................................................................... 49
6       SEISMIC ASSESMENT50
6.1            INTRODUCTION50
6.2            DETERMINISTIC SEISMIC HAZARD APPROACH50
6.3           PROBABIUSTIC SEISMIC HAZARD ANALYSIS........................................... ,
52
7       PROJECT LAYOUT OPTIMIZATION.............................................................................................................................. 56
7.1            INTRODUCTION56
7.2            PROJECT LOCATION AND SITE ALTERNATIVES.......................................................................................... 57
72.1      GENERAL57
7.2.2     SITE ALTERNATIVES59
7.2.3     CONCLUSIONS63
7.3            EVALUATION OF ALTERNATIVES.63
7.3.1     GENERAL63
7.3.2     DESIGN CRITERIA .
73.3      AL TERNATIVES.......................................................................................................................................~ 74
7.4            DABUS CASCADE
7.4.1        GENERAL......................................................................................................................................................... \78 74.2      SELECTED LAYOUT.'..”75
7.5            EM EQUIPMENTK
7.6            TRANSMISSION LINE SYSTEM'......................................................................................................................  g0
8       Dabus ch - 130.92
8.1            SELECTED LAYOUT
8.2            RESERVOIR.........................................................................................................................................................
8.3            DAM AND AUXILIARY WORKS..
8.3-1 SELECTED DAM TYPE............................................................................................................................................94
8.3.2     SELECTED FOUNDATION99
8.3.3     SELECTED AUXILIARY WORKS95 8.3.4     RIVER DIVERSIONjqq
8.4            WATERWAYS101
8.4.1     INTRODUCTIONJOI
8.4.2 INTAKE SUBMERGENCE
8.4.3     POWER TUNNEL102
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8.4.4        SURGE SHAFT DESIGN AND HYDRAULIC TRANSIENT ANALYSIS......................................... 103
8.45       PENSTOCKS................................................................................................................................................ 106
8.4.6      HEAD LOSSES.t08
8.5        POWER HOUSE, CIVIL WORKS.....................................................................................................................
8.5.1     GENERAL......................................................................................................................................................1°8
8.5.2     GEOLOGY.....................................................................................................................................................
8.5.3     GENERAL ARRANGEMENT........................................................................................................................ U?
8.5.4     SUBSTRUCTURE LAYOUTU3
8.5.5     SUPERSTRUCTURE LAYOUT..................................................................................................................... 113
0.6             EM AND HM EQUIPMENT........................................................................................................................... 114
8 6.1     INTRODUCTION
8.6.2        MECHANICAL EQUIPMENT............................................................................................... 114
114
863        ELECTRICAL EQUIPMENT..........................................................................................................................118
8.6.4      HYDROMECHANICAL EQUIPMENT........................................................................................................... 115
9       Dabus ch -108.................................... ........................................... . ........ .................................. . .......... ...................122
9.1            SELECTED LAYOUT..,,........................ ........................................................................................................ 122
9.2            RESERVOIR...................................................................................................................................................124
93 DAM AND AUXILIARY WORKS................................................................................................................................124
9.3.1     SELECTED DAM TYPE................................................................................................................................. 124
9.3.2     SELECTED FOUNDA TION.......................................................................................................................... 128
9.3.3     SELECTED AUXILIARY WORKS..................................................................................................................128
9.3.4     RIVER DIVERSION........................................................................................................................................ 129
9.4            WATERWAYS................................................................................................................................................ 130
9.4.1     INTRODUCTION130
9.4.2     INTAKE........................................................................................................................................................... 130
9.4.3     POWER TUNNEL........................................................................................................................................... 130
9.4.4     SURGE SHAFT DESIGN AND HYDRAULIC TRANSIENT ANAL YSIS......................................... 131
9 4.5 PENSTOCKS................................................................................................................................................... ,..732
9.4.6     HEAD LOSSES.................................................................................................................................... ",....... 135
9.5            POWER HOUSE, CIVIL WORKS.................................................................................................................. 135
9.5.1     GENERAL....................................................................................................................................................................
9.5.2     GEOLOGY....................................................................................................................................................... jjg
9.5.3     GENERAL ARRANGEMENT............................................................................................. ........................... 139
9.5.4     SUBSTRUCTURE LAYOUT..............................................................................................................................
9.5.5     SUPERSTRUCTURE LAYOUT,....... .........................................................................................................
9.6            EM AND HSS EQUIPMENT........................................................................................................................................
9.6.1     INTRODUCTION........................................................................ ......................... .................................. 141
9.6.2     MECHANICAL EQUIPMENT.................................................................................... . 141
9.6.3     ELECTRICAL EQUIPMENT............................................................................... ”           .............. 145
9.6.4     HYDROMECHANICAL EQUIPMENT .... .......................................................................................................
10     TRANSMISSION SYSTEM.............................................................................................................................................
10.1          SCOPE OF THE STUDY...............................................................................................................................
10.2          ANALYSIS of ETHIOPIAN TRANSMISSION SYSTEM DEVELOPMENT STATUS.....................................146
103 TRANSMISSION SCHEME....................................................................................................................................... H9
10.3.1   ANAL YSIS OF PREVIOUS SCHEMES..........................................................................................................149
10.3.2   DESIGN CRITERIA.......................................................................................................................................... iso
10.3.3   VOLTAGE LEVEL FOR TRANSMISSION SYSTEM..................................................................................... 757
10.3.4   NUMBER OF CIRCUITS AND LINES.............................................................................................................757
10.3.5   PROPOSED OPTIONSAND COMPARISON OFALTERNA TTVES............................................ 152
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10.4
10.5
10.6
10.7
10.8
10.9
LINE ELECTRICAL CAPABILITY................................................................................................................... 155
RURAL AREAS ELECTRIFICATION ALONG THE LINE ROUTES.......... ................................................... 159
DABUS HPPS SUBSTATIONS....................................................................................................................... 160
AVAILABLE SITE DATA...............................................................................................................................  161
PROPOSED LINE CORRIDORS..................................................................................................................... 162
PRELIMINARY GEOLOGICAL ASSESSMENT OF THE PROPOSED CORRIDOR.....................................165
11 REFERENCES............. *........... .. .................... .. .............................. .. .„•*........... .......... ........... ...... .................. ............ 168
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W. 2 of 2-DRAWINGS___________
-      100 GEN D SP 001 Project Location
100 - WHOLE PROJECT
100 GEN D SP 002 General Layout, Plan 150k
100 GEN 0 SP 003 General Layout, Profile
110 - TOPOGRAPHY
-      MO GEN DSP 001 Key Map
-      110 GEN  SP 002    Alt. ch    155, Dam, Plan 6.5k
110 GEN D SP 003  Alt. ch      155, Dam, Plan 6.5k Orthophoto 110 GEN D SP 004  Alt. ch    130, Dam, Plan 6.5k
110 GEN D SP 005  Alt. ch 130, Dam, Plan 6.5k Orthophoto
110 GEN D SP 006 Alt. ch 130, PH, Plan 6.5K
110 GEN O SP 007 Alt. ch 130, PH, Plan 6.5k Orthophoto
-      110 GEN D SP 008  AIL ch    100, Dam, Plan 10K
-      110 GEN D SP 009  Alt. ch       IOS, Dam, Plan 10k Orthophoto
120 - HYDROLOGY
-      120 GEN O SP 001 Hydro and Meteo Stations
120 GEN D SP 002 Catchment Areas
130 - GEOLOGY 130 GEN DSP 001 Plan 500k
-      130 GEN DSP 002 Legend
-      130 GEN D SP 003 DELETED
-      130 GEN D SP 004 DAM ch. -130, Plan 6k
-      130 GEN DSP 005 DAM ch.-108, Plan 6k
150 - HPPS ch -130
-      150 GEN D SP 001 DELETED
-      150 GEN D SP 002 DELETED
-      150 GEN D SP 003 Layout 35k
150 DAM D SP 001 Reservoir Area, Plan 90k
-      150 DAM D SP 002 alt. 1, DAM, Plan 5k
150 DAM D SP 003 alt. 1, DAM, Longitudinal Profile
150 DAM D SP 004 alt, 1, DAM, Typical Sections
-      150 DAM D SP 005 alt. 2, DAM, Plan 0k
150 DAM D SP 006 alt. 2, DAM, Longitudinal Profile and Section
-      150 DAM D SP 007 alt, 2, DAM, Typical Section
-      150 WWY D SP 001 WWs, Plan and Profile
-      150 WWY D SP 002 WWs, Typical Sections
150 POW D SP 001 Horizontal Section ® Generator
150 POW D SP 002 T ransv. Sect. ©Turbine
160-HPPSch -108
-      160 GEN D SP 001 Layout 35k
-      160 DAM D SP 001 Reservoir Area, Plan 20k
160 DAM D SP 002 Dam, Plan 2k
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-      160 DAM D SP 003 Dam, Longitudinal Section
160 DAM D SP 004 Dam, Typical Section
160 WWY D SP 001 WWs, Plan and Profile
160 WWY D SP 002 WWs, Typical Sections
160 POW D SP 001 Horizontal Section @ Generator
-      160 POW D SP 002 Transv. Sect. © Turbine
170 -TL
-      170 GEN D SPOOl Plan SOOk
170 GEN D SP 002 Single Line Diagram
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ABBREVIATIONS ACRONYMS
page i
ALS                   Aerial Laser Scanning
BoP                   BALANCE OF PLANT
CFRD                Concrete-Faced Rockfill dam
CRU               Climatic Research Unit
cvc                 conventional concrete
D/S                    Downstream
DEM                  Digital Elevation Model
DS HA               Deterministic seismic hazard approach, and E                       Energy
EEPCo              Ethiopian Electric Power Corporation EEP                   Ethiopian Electric Power
EHV level         Extremely High Voltage
EME                  Electro-Mechanical Equipment
GCPs                Ground Control Points
GTP                  Growth and Transformation Plan
HME                  Hydro-Mechanical equipment
HV                     High Voltage
HVAC                Heating Ventilation and Air Conditioning
IC
Installed Capacity
ICS                    Ethiopian Interconnected System
IP
Installed Power
IRR
Internal Rate of Return
ISWS
Insulated Shield Wire Scheme (ISWS)
MoWlE
Ministry of Water, Irrigation and Energy
OHTL
Overhead Transmission Line
O.L
Operating Level
PF
Plant Factor
PGA
Peak Ground Acceleration
PSHA
Probabilistic seismic hazard approach.
RCC
Roller-Compacted Concrete Dam
RP
Return Periods
SASW
Spectral Analysis of Surface Waves)
SCADA
Supervisory Control And Data Acquisition
SRTM
Shuttle Radar Topography Mission.
ToR
Terms of Reference
UEA
University of East Anglia
u/s
Upstream
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1 CONTENT
page 1 of 569
The Dabus HPPs. PRE - FEASIBILITY Study, May 2016 illustrated In this MAIN report indudes:
this REPORT (Vol. 1 of 2);
the DRAWINGS (Vol. 2 of 2)
The study has been prepared in compliance with:
the Terms of Reference (ToR) received in February 201S;
the Contract signed on 29th October 2015;
-     Inception Report rev. 6 submitted on 19 February 2016.
As scheduled in the ToR and Inception Report the following investigations were performed in order to prepare this study:
collection of the data from hydrometric and meteorological stations in the project area;
surface geology (foundations and quarries);
surface geo-mechanics;
seismic lines (SASW);
trenches;
laboratory tests (thin sections).
This Volume (REPORT) comprises the chapters covering the major sections and subsections in which the entire project has been subdivided, in particular:
EXECUTIVE SUMMARY
This chapter gives a summary of the investigations, the alternatives that were studied and the selected layout.
-     TOPOGRAPHY
This chapter illustrates the topographic investigations, particularly SRTM 30 x 30 m; drone survey and Pleiades satellite images.
HYDROLOGY
This  chapter  briefly  summarises  the  acquired  data  and  the  numerous  approaches  that  were  followed to determine run-off and flood flows.
-     GEOLOGY and GEOTECHNICS
This  chapter  bnefly  summarises  the  investigations  that  were  carried  out  in  the  project  areas  (dam, water ways and power house areas for both sites under study: ch - 130 and ch - 108).
-     SEISMIC ASSESMENT
This  chapter  illustrates  the  study  that  was  carried  out  concerning  the  seismicity  of  the  project  area using the deterministic and probabilistic approaches.
-     PROJECT LAYOUT OPTIMIZATION
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This  chapter explains  the alternative layouts  that were studied and the optimisations  performed. It shows  the reservoir regulation and energy  production as  wdl as  the main pa*ameters  of the economic analysis.
DABUS ch - 130
This  chapter  describes  the  technical  elements  of  the  ch-130  plant:  damr    spillway,  bottom  outlet, diversion works and EM and MM equipment, etc
-     DABUS ch - 108
This  chapter  describes  the  technical  elements  of  the  ch-lOS  plant:  dam.  spillway,  bottom  outlet, diversion works and EM and HM equipment, etc.
TRANSMISSION SYSTEM
This  chapter  illustrates  the  alternatives  that  have  been  studied  to  connect  the  plants  to  the  national grid.
-     REFERENCES
The list of bibliographic references are given In this chapter.
The study also comprises:
Vol. 2 of 2 (Drawings), containing 41 drawings;
-     the HYDROLOGICAL Report (120 HYD R SP001A);
-     the GEOLOGICAL & GEOTECHNICAL Report (120 GEO R SP001 A).
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2.1   INTRODUCTION
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The Dabus HPPs PRE - FEASIBILITY May 2016 is Illustrated In the following documents:
r
MAIN REPORT (Vol. Iof2)
The report gives a description of the investigations that have been carried out (topography, hydrology 
and geology) and the layout that has been studied to optimize the hydroelectric potential of the Dabus 
river.
DRAWINGS (Vol. 2 of 2)
To complete  the  report, SP has prepared 41 drawings that Illustrate the works composing the plants, HYDROLOGICAL Report (120 HYD R SPOOlA)
GEOLOGICAL a GEOTECHNICAL Report (120 GEO R SP001A)
The following subsection summarize the main resufts of the study.
2.2       TOPOGRAPHY
The foltowing topographic databases were used to develop the Pre-Feasitulity Study:
SRTM 30 x 30 m for:
a Catchment Area;
o Reservoirs Area;
DRONES Survey for:
□    HP? ch - 105 (DAM area);
e HPP ch - 130 (DAM area);
o HPP ch - 130 (PH area);
□    HPP ch - 108 (DAM area).
PLEIADES (satellite Image). PLEIADES stereoscopic  images  were purchased with an accuracy  of 50 cm pixel for an overall area of more than 1000 km? composing the reservoirs  and the entire river section affected by the works.
During the mission carried out in April 2016 a total of No. 22 Ground Control Points  (GCPs) were positioned to orthorectify  the images  and to enable us  to carry  out a topographic  restitution of the contour lines which Is currently under completion.
The drawings below show the topographical results:
-     110 GEN DSP 001 Key Map
-      110 GEN D SP     002 Alt ch 155, Dam. Plan 6.5k
110 GEN D SP  003 AR, Ch 155, Dam, Plan 6.5k Orthophoto
-      110 GEN D SP    004 Alt. ch 130, Dam. Plan 6.5k
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110 GEN D SP 005 AIL ch 130, Dam, Plan 6.5k Orthophoto 110 GEN D $P 006 Alt. ch 130, PH, Plan 6 5K
+
-      110 GtN D 5P 007 Alt, ch 130, PH, Plan 6.5k Orthophoto 110 GEN D SP 008 Alt ch 108, Dam, Plan 10K
110 GEN D SP 009 Alt ch 108, Dam, Plan 10k Orthophoto
2.3       HYDROLOGY
An in-depth! study of the Dabus catchment at the foreseen HPPs' locations was tamed out on the basis of the hydrometric  and pluviometnc  data made available during a specific  mission to Addis  Abeba. The pluviometric dataset was  further extended considering the reconstructed data by  the Climatic  Research Unit (CRU) of the University of East Anglia (U£A)r The quality of the obtained datasets was assessed both in terms of efficiency and consistency.
A time series of monthly average discharge at the HPP locations (Dabus Alt. 130 and Alt 108) was obtained on the basis  of the reconstructed time series  at Dabus  near Assosa, which spans  48 years. Four different approaches were considered to reconstruct this time series, f.e.:
synthetic  streamflow  generation  on  the  basis  of  The  observations  directly  collected  at  Dabus  near Assosa;
transposition to Dabus near Assosa of the observations collected at Dldessa near Aqo by means of a power-law transfer function;
-     transposition to Dabus near Odessa of the observations colleaed at Didessa near Arjo by means of
an artificial neural network transfer function, and
-     rainfall-runoff modelling on the basis of the data data by the Climatic Research Unit (CRU) of the University of East Anglia (UEA),
The multi-annua I average discharge at Dabus near Assosa resulting from these approaches are respectively 160,40 ni3/s, 148.59 m3/s, 146.40 m3/s  and 143.44 m3/s. Among these approaches, the one that was preferred for defining a time series  to be used in energy  production evaluation is  the ANN transfer function approach.
A standard flood frequency analysis was then earned out in order to obtain the discharge quantiles with defined return penod. For sake of conservativeness, the time senes recorded at the station of Dldessa near Aqo was preferred. The analysis was can-led out adopting the standard 'block maxima' approach, being tne blocks (as usual in hydrology) extended over a penod of one year.
Three different models  (probability  distributions) were fitted to the single sample of dimensionless  maximum annual discharge (Extreme Value Type I, or Gumbel, or EV-1 distribution, Generalised Extreme Value or GEV distnotition; Log-Normal distribution), being the parameters of each model estimated by means of the
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Maximum Likelihood (ML} method, and the goodncss-of-fit checked by means of both diagnostic plots and the Kolmogorov-Smirnov Test.
Confidence intervals of parameters' estimate were calculated by means of the Delta Method. For safety's sake the discharge quantiles provided by the EVI (Gumbel) model were selected for design purposes.
The discharge quantiles with 100, 1000 and 10 000 return penod resulted 1375 m3/s, 1769 m3/s and 2161 m3/s respectively.
2.4       GEOLOGY and GEOTECHNICS
Tlx? geological and geotechnical study was carried out In two phases;
in the first phase, an in-depth desk study and onellminary site visit were performed including:
□    analysis of the existing geological maps and precedent studies;
□    pbotolnterprEtation of the satellite images and drone aerial photos;
o preliminary geological survey and site Investigations.
in the second phase a lengthy mission was carried out to both sites under study (named HPP - 108 and HPP - 130) during which the following were performed:
o geological survey with examination of No. 45 rock outcrops (photographic documentation was coliected of each outcrop);
o collection of No. 16 rock samples;
□ analysis erf thin sections of No. 8 samples with petrographic analysis by microscope (realized by the University of "Roma Ire", Department of Geology);
o No. 25 geostructural stations, with a total of almost No. 580 pint measurements;
o No. 4 trial pits, ranging in depth from 0.6 m to 3,2 m;
o No, 6 seismic lines with SASW technique.
The main results of the investigations are summarized below;
2.4.1    CH - 130 SITE
PftMArga
Both flanks of the dam site are formed by a residual soil cover. Trial pits (Nos. 1 and 4, respectively on the nghc and the left abutment) were excavated m red silty-clayey soils, with medium plasticity, stiff and dry.
At the bottom of both pits, the soil contains fragments (milllmetric to centimetric) of vey weathered rocks, which form a sort of4 skeleton * in the soil matrix. This lower part of the excavation can be interpreted as the top side of the "C layer of the classic sol horizon, known as ’parent rock" or substratum.
100 WHO R 5POC38 (PRE-FEASIBILITY Repurl)
100 WHO R SP 0036
itudio pletnKigeli, njrnrDAflUS HPPs
PRE-FEASIBILJTY STUDY Report
cage
6 of L6Q
On  the  right  side  of  the  alluvial  plain,  the  bedrock  Is  covered  by  alluvial  soils.  Trial  pit  No.  2  (1.2  m  deep) shows greyish to light tvcwn alluvial soils, medium to highly plastic.
Trial  pit  No.  3  (an  the  alluvial  plain  at  the  foot  of  the  slope)  shows  that  very  weathered  rock  underlies  a  thin sandy sol cover.
These evidences lead to the assumption that weatherec bedrock may be encountered around a range of
4-6m with fresh (or slightly weathered) rock in the range 54-10 m of depth. This is in accordance with the
results of previous pits which encountered the thickest layer of soil in the upper part of the slopes.
Comparable results  were obtained with the seismic  tests  carried out using SASW technique on the right side, at the lower part of the slope. The results of the SASW tests confirm that In this area a soil layer thicker than 3-4m covers the bedrock.
QUARRY for CONSTRUCTION MATERIALS
Due to the presence of basaltic rocks near the dam site, a specific survey was executed during rhe last visit in order to Identify suitable quarry sites.
Three possible sites have been identified.
raiVe 2 4 Z:                        SttJ
COORDINATES
SITE No. 1
709140 m E
1085128 m N
SITE No. 2
706365 m E
1083B70 m N
SITE No. 3
709467 m E
1081341 m N
WATERWAYS anq.PQWERtlQUSE
According to the geological data, the tunnd wifi be excavated In Precambrian rocks.
Only  a few outcrops  (OC) were surveyed along the tunnel: one is  represented by  pegmatite intrusion (crystalline basement) found inside a creek  (OC No. 64) and one by  effusive lava (riolite type, OC No. 69) which presumably forms a thin cover of the crystalline basement cm the top of the hill.
More than 4-5 m of soil cover was surveyed in the intake area.
Large  rock  outcrops  were  surveyed  in  the  power  house  area  (OC  No.  69)  and  along  the  creek  which  flows from the powerhouse area to the Dabus River (OC No. 65, 66. 67).
In this  area the rock  is  bask  intrusive, dark  grey, ranging from tonalite, to diorite and gabbro. Some pods  of basic  granulite are included. Low traces  of metamorphism have been reported in this  area. Microscopic  analysis of thins  sections  on two samples  (OC No. 65), classifies  this  rock  as  a tonalite (rich In feldspars, plagiodases and amphiboles).
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The rode is  very  strong to extremely  strong (specimen can only  be chipped with a geological hammer). A moderate to low density  of joints  was  surveyed on the outcrops. The degree of weathering is  very  low in the outcropping rock.
2.4.2     CH - 108 SITE
DAM Aifia
At  this  site  the  river  has  a  rectilinear  course  flowing  NW  along  a  very  narrow  riverbed,  ranging  from  30  m  to about 50 m in width.
The dam axis  is  located m a V-shaped asymmetric  valley. The left flank  is  steep, with an average slope ol about 60%, but with two different sections: the upper part has  a gentle slope of about 15-20% (~10°), while the lower section is  very  steep, with a slope of 100% and more. Some deep creeks  (approximately perpendicular to the river) cross the bank, forming very steep scarps and isolating large boulders.
The right bank  has  an average slope of around 20-25%. The abutment is  modelled by  some very  gently  sloping areas  which separate other steeper sections  of the slope. A wide flat and very  gentle slope area is  located around level 1220 m a.s.l. (see following photo). The slope Is crossed by some creeks mased In rock.
The   bedrock   at   this   site   is   formed   by   Precambrian   rocks,   here   constituted   by   meta-granite   and   meta- granodionte. Large outcrops of rocks are visible on the plateaus, on the banks and along the riverbed.
Bedrock outcrops widely in both flanks of the dam area.
All the outcrops can be classified as meta-gramte and meta-granodiorite rocks. Differences in terms of
petmgraphic characteristics and metamorphlc/texture grade can be appreciated in the same outcrop and in
between different areas. Nevertheless, these differences appear as gradual transitions from one type to the
other, without evidence of a real dominance of one petrographic type and/or metamorphic grade
QUARRY fQf CONSTRUCTION MATERIALS
The presence of several outcrops  of rocks  in the whole area suggests  that it should be passible to obtain rockfill material and concrete aggregates  from a quarry  area to be located close to the dam site. The exact location will also depend on the work site needs, in order to optimize excavations and transportation.
WATERWAYS and POWERHOUSE
According to geological information, this area Is constituted by Precambrian rocks (meta-qramte and meta granodiorite). The morphology along the tunnel alignment is very rough, dominated by outcrops of rocks, steep elongated hills frequently rounded by scarps and deep gullies.
IOC WHO R SPOQ30 (PRE-FEAS1 BillTY Report)
iOO WHO R SP 003 B
studio plctrnjmedwjs hp^  PRE-FEASIBIL/TY STUDY Report
w5
The powerhouse will be located on a small hill on the right bank  of the river; this  area is  characterized by  a gentle morphology. No outcrops  of rocks  were surveyed in this  area, while large outcrops  are visible on the river bed.
2-5 SEISMIC ASSESMENT
The  scope  of  this  study  is  to  illustrate  the  seismic  assessment  earned  out  over  the  Dabus  project area  aimed at evaluating the Peak Ground Acceleration (PGA) at different Return Penods (RP).
In  particular,  the  following  two  approaches  were  used  In  order  to  estimate  the  value  ol  the  PGA  related  to  its return period.
DSHA : Deterministic seismic hazard approach, and
PSHA : Probabilistic seismic hazard approach.
The following table summarizes the PGA values at different Return Periods-
r*%ip 2 5.2:              A54 ukjA J' cM\rwt RP
RP                PGA/g
475
950
2500
10000
0-05
007
0.10
0.15
2.6   PROJECT LAYOUT OPTIMIZATION
2.6.1 GENERAL
A site alternatives study, an alternatives evaluation assessment, an EM equipment and a TL system study were performed in order to:
select the most promising sites for hydropower generation along the Dabus river;
perform  a  sensitivity  analysis  on  the  main  costs,  energy  production  and  economical  cost-effectiveness for a list of suitable dam type and dam height alternatives;
- provide the relevant design criteria of the electro- mechanical and hydro-mechanical equipment of the selected cascade layout;
provide the relevant design criteria for the evacuation o< the power output from the HPPs.
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2.6.2 SITE ALTERNATIVES
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p*9« 9 erf 169
A  comprehensive  study  of  the  Dabus  river  hydropower  potential  was  performed.  The  study  focused  on  the two  alternative  sites  provided  In  the  "Dabus  Medium  Hydropower  Project,  Reconnaissance  Study"  report  from the  MoWIE  (UPPER  and  LOWER  Dabus).  Two  alternatives  were  moreover  Identified  by  the  Consultant,  on  the basis  of  a  nver  profile  and  catchment  area  assessment  performed  over  the  Shuttle  topographical  database. The two Identified alternatives were at ch -130 and ch -108 km from the Dabus-Blue Nile confluence.
An assessment of the MoWIE Upper Dabus  alternative (located ch -155) showed that such alter native would be ten times  less  cost-effective than the ch -130 and was  therefore discarded. The MoWIE Lower Dabus alternative is coincident with the Consultant's ch -108 alternative.
Subsequently, the alternatives evaluation study focused on the ch -130 and ch -108 sites.
2.6,3      EVALUATION OF ALTERNATIVES
An evaluation study was performed with the aim of defining the criteria for the choice of the most promising HPP layouts among the many solutions which can be envisaged at both ch -130 and ch -108 dam sites.
The evaluation was conducted through an economic sensitivity analysis aimed at understanding the effects of the dam type selection and reservoir elevation on the profitability of the plant in its entirety, by calculating:
•     realization cost of the plant;
•     energy production and revenues;
•     relevant economic parameters such as IRR, NPV of benefit-costs cash flows, payback period.
In particular, the dam types considered were;
■    Roller-Compacted Concrete Dam (RCC)
■    Concrete-Faced Rockfill dam (CFRD)
The investigated reservoir elevations ranged, for the two locations:
■    Ch -130              from 1360 to 1385 m.asL at 5 m step Intervals
■    Ch -108              from 1220 to 1250 m.asl. at 5 m step intervals
Moreover, the following aspects were considered and mduded in the analysis:
■     natural average inflows entering Into the dam site cm a monthly basis;
■    elevation-area and elevation-volume curves of the reservoir;
■    dam dead levels due to sediment load from the river at each site;
■    losses due to ecological flow release from the dam;
■    balance of precipitation-evaporation from the reservoirs;
• hydraulic sizing of the waterways and head losses;
■    tailwater levels in correspondence to the powerhouse sites;
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•     generation efficiency;
•     road relocation;
■ BoQ and costs of the main civil and electro-rnedianlcal (EM) works;
•     economical parameters: energy selling price, plant lifetime, cash flow and discounting rate.
CH-130 ECONOMIC RESULTS
Both. RCC and CFPD dam types were investigated at the ch -130 site.
The economic  analysis  showed a substantial equivalence of the solutions  from an economic  point of view. Therefore, an RCC type dam was selected, since such solution is safer and allows for far easier maintenance operation and costs.
The following combination of dam type and reservoir maximum operating level has been selected as the most cost-effective:
■ DAM TYPE;
RCC
• RES. MAX. O.L:                   1373.7 masl
Such  alternative  is  the  one  with  the  highest  internal  rate  of  return  (approx..  18%)  and  the  lowest  payback period (app ox. 4 years).
r
CH _-10e_£CQNQMK RESULTS
The conditions of rhe ch -108 valley are ideal (or the construction of a rigid dam, sc an RCC dam type was considered. All The Investigated reservoir elevation alternatives are very profitable, since the 1RR Is always very large and ranging between approximately 30% and 33%, with a short payback period of 2 years.
The alternative with the reservoir maximum operating level at 1230 masl, Le. the lailwater level of the ch -
130 PH. was chosen as the optimal solution and best compromise between IRR index and benefit-cost
difference.
2.6.4    EM and HM EQUIPMENT
According  to  preliminary  studies,  the  total  rated  discharge  (Qrat)  and  rated  head  (Hrat)  for  each  hydropower plant are the following:
- HPP ch ■ 130:                                   Qrat= 240 m’/s                Hrat 138 m
HPP ch -108
Qrat= 250 mVs               Hrat = 215 m
From technical, economic and operational points of viewr the following turbine layout was selected as the most
promising:
♦ Ch -130:
n°2 x 152 MW Francs 
turbines
• Ch-108:
n°2 x ?47 MW Francis
turbines
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PRE-FEASISIUTY STUDY Report 2.7       DABUS CH - 130
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Oaous ch - *.30 is the first plant of the Dabus river cascade (304 MW).
The energy production and the installed power of the riant are summarized below:
/aOtoZZl; dHAuttft’W
HPP                  IP (MW)
e (GWh/y)
--------------- 1
Dabus ch - 130        304
1293
i
The ch -130 plant fully regulates the flows commc         into the reservoir (4590 Mm3/y}. The main characteristics of the plant are described below:
7a£^ 2 7.2,’                - J JO. Mar? CTWJrtmsOcr
1
Dabus- 130
1
tl
*2
Catmment Area
10,009
Awmge Net Annual Flow
entys
146
Annual Pure**
Mm’/y
4,592
nwrW
459
■7 1
FLOOD
in a.a.L
1374.1
1375.7
Mat O.L.
m a,»,l,
1373.7
Mm O.L
n a.i.l.
1357.6
DLAZ
n a.il.
1342.0
Tot wage (B> Mat a-L)
Mm’
2470
Uve floraqe (Max o.L -
DEAD)
Mm'
2335
l_2
Type
-
RCC, gravity
RCC. gravty
Dam Crest electron
n> Ltd.
1376
1376
FoundaPon irwn efevatlon
m B.t.1.
1322
1322
Mai. height
fn
55
S5
Cretf length
fn
1610
1610
Gate number, type
r : Fhted 'Wheel Gatei / n.l Radial
gates
Mai. grass bead
m
32
■
Deugn flood (TR;
ffl'/s
2160 (10,000 yr)
2160(10,000
F)
Type
overflow
Lateral Spillway
Gale number and type
-
<6 radial
Gates (12.5*6 m)
Un-gated
Energy disopatw
■
skiMnort*^
pool
sttppBj
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itutfjp pHtfangeti. FDHifDABUS HPf'j
PRE-FEASIBILTTY STUDY Report
page 12 of 169
INTAKE
L STRUCTURE
Typ^ number
-
1
Sill dewatlon
m ajJL
1342
Gates number and type
*
n. 2 Bufchwtf sate / 2 Fixed Wheel
Gate
Nurntier, ntemal diameter
f, IT
n, 1 / 9m
Length
HI
5,500
| SURGE SHAFT
He*grn
79
Da mete*
TH
16
Steed pentfixts num.
•
2
Diameter
m
7
1 engtfi
rn
2100
POWER HOUSE
type, num.
Open A#r
Si«s
IT
45 I as m
Inlet Valves, Type
-
BrpUne vahes
*, DQmeter
f, m
n.2 / 4.2
Turbines, #
f
n.2
Type
Francis
Ws WL (Frands) / CL cL
| (Pttoii
T 1.4.1.
1230
Geodetic Hoad
frt
149
Design Held (Mai. Nrl Head }
m
113
OUQn flkjw
240
Plant Factor
i Inttafec Po^er
MW
304
Average net bead
m
131
Annual f rMHTj> Prrtductlori
■GWVv
1293
In particular;
CAM
The dam will be an RCC GRAVITY DAM with a straight axis. Two alternatives have been selected:
-     alt 1 with central spillway In the dam body; alt. 2 with lateral spillway.
The main dimensiors of the dam are:
-     55        m           Max. height
•    1610     rn           Crest length
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The following drawings:
-    150 DAM D 5P 002  - alt. 1 DAM, Plan 5k;
-     150 DAM D SP 003  - alt. 1 DAM. Profile;
150 DAM D SP 004 - alt. I DAM, Typical Sections;
-     150 DAM D SP 005   - alt 2 DAM, Plan 3k;
-     150 DAM D SP 006  - alt. 2 DAM, Profile;
-     150 DAM D SP 007  - alt.  1 DAM Typical Section;
f
illustrate the darn's main features.
3
SPILLWAY
Two spillway types are being proposed.
The spillway is designed to discharge the peak of the un-routed flood tor RP = 10,000 years, Q:cce = 2160 m /s, without considering rhe routing effects of the flood inside the reservoir.
Alt. 1. SPILLWAY on the dam body
The high-flow evacuation system comprises the following hydraulic elements:
o  an  ogee-shaped  overflow  crest  with  sill  elevation  at  1368.7  m  a.sd.  and  a  total  length  of  90 m (gross of piers. net of abutments) divided into six bays controlled by radial gates 12.5 x 6 m each;
o a chute on the d/s slope drvided Into six canals through side walls;
d a flip bucket energy dissipating device, located at the end of the chute;
o  a  plunge  pool  on  the  nver  bed,  designed  to  receive  both  the  spillway  discharges  and  the middle-outlet jet.
The spillway is Illustrated In the drawings:
o 150DAMSP002 - alt. 1 DAM, Plan 5 K
□ 15ODAMSPOO3 - alt. 1 DAM, Longitudinal Profile
d 150DAMSP004 - alt 1 DAM, Typical Sections
Alt. 2. Lateral SPILLWAY
The second alternative exploits a saddle on the right abutment between 1.374 and 1300 m a.s.L. The flow drscharge system composes:
o 470 m of un lined canal excavated along the natural saddle;
o 200 m of un-gated cement overflow sill at el. 1373.7 m a.s.L;
o 2.3 km of un lined, stepped-bottom canal.
The spillway is illustrated in drawing 150DAMSP005 - alt. 2 DAM, Plan 8 K.
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BOTTOM OUTLET
57W K ffgpo/t
page 14 erf 169
The main function of  the bottom outlet will be to drawdown the reservoir In an emergency or (or extraordinary maintenance and for controlling the rate of reservoir impounding.
The overall length of the outlet, from the entrance to the exit from the dosed conduit wll be about 30 m. The invert level at the entrance is 1348 m a.s.L, being the level in correspondence to the dead volume in order to guarantee the outlet functioning dunng the overall lifetime of the dam.
RIVER DIVERSION
The nver diversion system will provide a "reasonable protection" against the flooding of the area in which the dam is built, that wll be dewatered for the period of Its construction.
The selected DESIGN FLOOD (Tr » 20 yrs) for the diversion structures is 1.100 m3/s.
The MAIN WORKS for the river diversion structures are shown in the drawing ISO DAM S 5P 002, and Include: DIVERSION STRUCTURE. composed by:
o headrace canal;
o culverts;
o tai I race canal.
-     U/S COFFERDAM
-     D/S COFFERDAM
The  diversion  structure  Is  approximately  360/460  m  long  and  foresees,  from  upstream  to  downstream,  the following elements:
-     HEADRACE CANAL
A headrace canal excavated in rock approximately 100 m tong.
CULVERTS
The  No.  2  culverts  will  have  a  rectangular  section  7  m  wide  and  9  m  high.  The  structures  will  be approximately 110 m long. Part of the culverts will then be Incorporated in the dam body.
-     TAILRACE CANAL
A tailrace canal excavated in rock, approximately 250 m long for Alt. 1 and 150 m for Alt. 2.
No. 2 cofferdams are foreseen to keep the central part of the riverbed dry during plant construction.
The UPSTREAM cofferdam will have a maximum height of about 9 m with crest level at 1335 m a.s.l., in order
to allow the design flood to pass In the diversion structure without overtopping.
The DOWNSTREAM cofferdam instead will have a maximum height of 4 m with crest level at 1330 m a.s.l., to avoid overtopping by (he tail water level corresponding to the design flood.
WATERWAYS
The waterways comprise:
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-       INTAKE.  The  intake  will  be  a  concrete  structure  that  will  divert  water  from  the  resen/olr  to  the  power waterways. The intake structure is designed to divert a maximum How of 240 m3/s.
The intake will be equipped with:
o trashracks;
a trashrack cleaning machine;
o two sets of maintenance gates (type: bulkhead gate);
o two sets of intake gates (type: Fixed wheel gate).
-       POWER  TUNNEL  The  total  length  of  the  power  tunnel,  from  the  intake  to  toe  surge  shaft,  will  be approximately  5500  m.  The  tunnel  invert  will  vary  between  el.  1343  m  a.s.1.  at  the  Intake  to  el.  1321 m  a.s.l.  at  lhe  surge  shaft,  having  a  longitudinal  slope  of  approximately  4  m/km.  The  tunnel  section will be circular with an Internal diameter of 9 m and an available flow section of 64 m2.
SURGE SHAFT. The shaft will have an Internal diameter of 16 m with a throttle of 3.4 m.
PENSTOCKS, Two steel lined penstocks are foreseen after the surge shaft. The total length of each
penstock from the surge shaft to the turbine will be approximately 2.1 km. The penstock section will
be circular with an average internal diameter of 7 m.
The   general   layout   and   main   geometrical   features   of   the   power   waterways   are   shown   in   the   following drawings:
-     150 WWY 0 SP 001, PLAN AND PROFILE
-     150 WWY D SP 002, TYPICAL SECTIONS
POWER HOUSE
The  powerhouse  will  be  located  along  the  Dabus  nver  left  bank.  Its  longitudinal  axis  will  be  n  a  NW-SE direction, approximately perpendicular to the alignment of the penstocks.
The main structure will be approximately 85 m long and 45 m wide.
The powerhouse will be divided in three bays  housing the two generating units, one In each bay, and the erection bay. The erection bay, at the 5E end of the powerhouse, will be approximately  35 m long and 45 m wide.
The  main  floor,  conrespondmg  to  the  access  level,  will  be  at  eL  1237.0  m  a.s.l.,  whereas  the  roof  will  vary between d. 1258,5 and 1259.0 m a.s.1.
In  order  to  exclude  the  n$k  of  flooding  of  the  and  possible  damage  due  to  extreme  floods  PH  mam  floor  will be Higher than maximum flood level (flood with Tr = 10000 years):
-     PH main floor elevation
- Rood level (Tr = 10000 y)
= 1237.0 m a.s.l,
= 1234.5 m a.s.l.
The  control   rooms   will   be   located  at   the  WE  end  of  toe  superstructure,  ’whereas  the  auxiliary  electrical equipment will be at the east end.
IOQ WHO R 5POT3B (PRE-FEASLMlHY Report]
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The layout of the powerhouse Is shown in the following drawings:
150POWD5PQO1, Horizontal section ©Generator;
l 50POWD$P002 Transversal section ©Turbine.
r
2,8   DABUS CH -108
Dabus ch - 108 is the second plant of the Dabus nver cascade (494 MW).
The energy production and the Installed power of the plant are summarized below:
Zf. /-■
zzw
HPP
IP (MW)
E(<JWh/y)
Dabus ch — 108
494
2140
The ch - 108 plant exploits the entire Inflow regulation of the reservoir at ch - 130. The ch plant Therefore will operate mostly as a run-of-the-nver plant.
The main characteristics of the plant are described below:
r.iWt12 A ZJ
- ZCty chfrartEYHfts
unit
Dabus “ 108
CatcNncnl Area
hnr
10.44X1
Arerage Het Annual How
mVs
1st
AnnHJjJ Runoff
Mm'/r
4 768
r
fflrtVV
458
ll-LOOO
m as 1
1231.2
1 Max O.L
m Lid.
1230.0
I Min O.L
rH
1224.1
DEAD
m Lil
1202,0
Tc*. VdZrfjgr ( tffl Max o.L)
Mm1
53
Uwe flori^e (MAX O.L - DEAD)
LB
—
1 Type
RCC, gravity
rum Cf«t elevation
in a.i.l
1232
Foundation mln. elevAton
m a.i.l,
1150
Max. height
m
02
Cretf hcr*q*r
ffi
444
Gate number, tvpe
*
Max. gloss head
m
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ca$t 17 of 169
—-—-                        11
Design flood (TO)
mVs
2160 (10.000 yr)
Type
overflow
Gate number and Type
•
Un gated
Energy dissipator
-
Stepped
P !ki f
------------- -
--- ------------------ M
Type, number
1
SJ devaban
rr. a.s.L
1205
Gate* dumber and type
-
n. 2 Bulkhead gate / 2 Fired wheel
Gate
Number, internal ckameter
rn
n. 1/9
.cngtii
m
6.200
Height
m
71
Lwameter
m
16
Sled penstocks nurr,
»
2
Diameter
m
5
Lwgth
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-
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-
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1007
_________________________
CeWtTC Head
m
230
Oesqn Head (Mast Net Head )
m
221
Design flow
m’/s
250
Plant Factor
Instiled Power
MW
494
Average net head
tn
213
Annual Energy Production
GWIVr
2140
In particular
P_AM
The dam will be an RCC GRAVITY DAM with a straight axis.
A stepped spillway on the dam body has been selected.
The main dimensions of the dam are:
-     B2        m           Max. height
-     444      m           Crest length
100 WHO R SPO03B (PRE-FEASIBIIJTY Report’: 100 WH0R5P 0038
stuho plctrangeti, rczneI3ABUS HPPs
The results of the Investigations have been summarized in the Chapter 5 and discussed at length In the report 130 GEO R SP001 A, and may be briefly described as follows:
in  the  whole  dam  area  the  rock  strength  can  be  assessed  as"strong” to  “very  strong’.  The  weathering grade is generally low to very low.
So an average foundation depth of between 1 to 3 m has been assumed.
The following drawings:
lf>0 DAM D SP 002 - DAM, Plan 2k;
160 DAM D SP 003 - DAM, Longitudinal Profile;
160 DAM D SP 004 - DAM, Typical Section,
illustrate the darn's main features.
SPILLWAY
The spillway system, located In the central part of the dam crest, includes:
an ungated overflow crest:
c the 9ll elevation is 1230.0 m a.s.1. with a length of 440 m;
o d/s stepped chute (slope 0.75:1);
c two abutment return channels that convey the flows to the nver channel at the toe of the dam;
o an energy dissipating stilling basin.
Considenng  that  (he  elevation  of  the  dead  capacity  is  approximately  the  same  as  the  invert  of  the  intake,  a bottom outlet has not been considered necessary,
BaEBJaiffiBSIQM
The  nver  diversion  system  will  provide  ’*reasonable  protection"  against  flooding  of  the  area  on  which  the  dam is built. The area will be dewatered for the period of the dam construction.
The selected DESIGN FLOOD (Tr = 20 yrs) foe the diversion structures is 1,100 m3/s.
The  MAIN  WORKS  for  the  nver  diversion  structures  are  illustrated  In  the  drawing  160  DAM  D  SP  002,  and include:
- DIVERSION STRUCTURE, comprising: c headrace canal;
o tunnel,
o tail race canal. U/S COFFERDAM D/S COFFERDAM
1 !W WHO R SPM3B (PRE-FEASiailfTY JW/
I DC WHO R SP Q03B
■rnjdrfi pw+r^nqch, romeZABUSHPPs
W£-FEAS/SIUTY STUDY Report
PW    19 of it>9
The diversion structure Is approximately 170 m loop and foresees, from upstream to downstream, I he follow nq elements:
- HEADRACE CANAL
A headrace canal approximately 40 m tong excavated in rode.
’ TUNNEL
A circular tunnel with  a diameter of 10 m. The structures are  approximately 110 m long. Part of the culverts will then be incorporated In the dam body.
TAILRACE CANAL
A tailrace canal approximately 20 m long excavated In rode
No. 2 cofferdams are foreseen to keep the central part of the riverbed dry during plant construction.
The UPSTREAM cofferdam will have a maximum height of about 12 m with crest level at 1170 m a-s.L, In
order to allow the design flood to pass through the drversion structure without overtopping.
The DOWNSTREAM cofferdam instead will have a maximum height of 4 m with crest level at 1152 m a.$JM  to avexd overtopping by the tail water level corresponding to the design flood.
WATERWAYS
The waterways comprise:
INTAKE. The Intake will be a concrete structure that will divert water from the reservoir to the power waterways. The intake structure is designed to divert a maximum flow of 250 m3/s.
The intake will be equipped with:
o trashracks;
o trashrack cleaning machine;
o two sets of maintenance gates (type: bulkhead gate);
c two sets of Intake gates (type: fixed wheel gate).
POWER TUNNEL The total length of the power tunnel, from the intake to the surge shaft, wrll be approximately 6200 m. The tunnel invert will vary between d. 1205 m a.s.l. at the Intake to el. 1191 m a.s.I. at the surge shaft, having a longitudinal slope of approximately 2.2 m/km. The tunnel section will be circular with an Internal diameter of 9 m and an available flow section of 64 m2.
- SURGE SHAFT. The shaft will have an internal diameter of 16 m with a throttle of 3.4 m.
PENSTOCKS. Two steel lined penstocks are foreseen after the surge shaft. The total length of each penstock from the surge shaft to the turbine will be approximately I km. The penstock section will be
circular with an average Internal diameter of 5 m.
The  general  layout  and  main  geometrical  features  of  the  power  waterways  are  shown  in  the  following drawings:
-     160 WWY D SP 001 A, PLAN AND PROFILE 160 WWY D SP 002 A, TYPICAL SECTIONS
100 WHO R SP001B (PRE FEASIBILITY Report)
100 WHO R SP ODJd
studio plffrmgdL romcOA&JE HPPs
POWER HOUSE
PRE-FEASIBILITY STUDY Report
P*<K  20 of 169
The powerhouse will be a conventional surface nutdoor type with vertical axis units located on the nght bank of the Dabus river, approximately 6 km downstream the dam area.
Its  longitudinal  axis  will  be  In  a  NE-SW  direction,  approximately  perpendicular  to  the  alignment  of  the penstocks.
The mam powerhouse structure will be approximately 85 m long and 45 m wide.
The powerhouse will be divided in Three bays  housing I he two generating units, one in each bay, and the erection bay. The erection bay, at the SW end of the powerhouse, will be approximately 35 m long and 40 m wide.
The  main  floor,  coresponding  to  the  access  levet.  will  be  at  ei.  1012.0  m  a.s.l.,  whereas  the  roof  will  vary between el. 1033,5 and 1034.0 m a.$X
In  order  to  Exdude  the  risk  of  flooding  of  the  and  possible  damage  due  to  extreme  floods  the  powerhouse main floor will be higher than the maximum flood level (flood with Tr = 10000 years).
The main elevations of the substructure will be:
-
turbine axis at elevation
1000,0 m a.sd.;
-
valve roam floor at elevation
995.0 rn a.sd,;
runner removal access at elevation
995.0 m a.sd,;
foundatKMi slab al elevation
982.5 m a.s.l.
The layout of the powerhouse is shown in the following drawings:
-      160 ROW D SP 001 Horizontal Section © Generator
-      160 POW D SP 002 Transv. Sect. @ Turbine
2.9       TRANSMISSION LINE SYSTEM
The Dabus Cascade power output will oe evacuated by means of HV overhead transmission lines-
The terms of connection to the Ethiopian HV gnd have been evaluated on the basis of:
•        a review of the available existing development studies;
•         tne  objectives  of  the  assignment,  re. Identification  of  design  main  aspects  and  issues,  together with   economic   viability   of   the   proposed   scheme   for   promotion   of   regional   cooperation   and rncreasmg power trade with neighbouring countries;
•        Dabus Cascade projects preliminary components and ratings;
■   evaluation   of   adequate   transmission   infrastructures,   coordinated   with   the   existing   corndocs   for reliable and stable power transfer into the grid and Increased efficiency and electrification rates.
IDO WHO R SP0038 (PRE-FEASteiLTTY Report]
100 WHO R Sp 003B
studio peungeli, romeCUflUSHPPs
PR&FE4SIBJUTY S7W Y Report
P09< 21 of 169
The prefeasibility  study  indudes  a review of available previous  schemes  by  third parties, a technical assessment or the feasible interconnection schemes  by  considering also preliminary  economic  parameters, and the main geological and environmental constraints.
The Consultant stresses  the high reliability  and efficiency  of the schemes  under ALT_2 (400kV) and ALT_3 (SOOkV). Although they  are more expensive than ALT_1 (230kV) they  are nevertheless  tailored to the country's ongoing expansion plan at EHV level.
IDO WHO R SPCW3B (PR£-FUSIBILITY
lDOWHORSPOCia
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Wf-FEASIBILITY STUDY Report 3 TOPOGRAPHY
3.1 INTRODUCTION
page 22 of 169
This chapter describes the topographic data used to develop the Dabus pre-feaslbllity design.
The following topographic databases were used:
SRTM 30 x 30 m for:
□ Catchment Area;
:j      Reservoirs Area
DRONES Survey for:
o HPP ch - 155 (DAM area);
o HPP ch - 130 (DAM area);
o HPP ch - 130 (PH area);
o HPP ch-108 {DAM area).
PLEIADES (satellite Image). PLEIADES stereoscopic  Images  were purchased with an accuracy  of SO cm pixel, for an overall area of more than 1000 km2 comprising the reservoirs  and the entire river section affected oy the works.
During the mission carried out in April 2016 a total of No. 22 Ground Control Points  (GCPs) were positioned to orthorectify  the images  and to enable us  to carry  out a topographic  restitution of the contour lines which is currently under completion.
A brief description of the topographic databases is given in the following paragraphs.
3.2 CATCHMENTS AND RESERVOIRS AREA
3.2.1 INTRODUCTION
Two  different  databases  were  acquired  for  the  general  topography  of  the  reservorrs  to  enable  us  to  have redundancy of topographic data:
SRTM 30 x 30 m;
- PLEIADES Survey.
Al  the  moment,  the  SRTM  database  has  already  been  processed  in  the  pre-feaslbilrty  design,  whilst  the PLEIADES stereoscopic images are being processed and will be available shortly.
iOD WHO R SP003B /WE-FEASIBILITY Report)
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3.2.2 SRTM DATABASE
PXE-FEASIB/LITY STUDY Report____________________we 23 or_w
The  global  digital  elevation  model  was  adopted  as  the  main  basis  for  the  topography  of  the  Dabus  Cascade area SRTM 30 x 30 m - Shuttle Radar Topography Mission,
The Shuttle Radar Topography  Mission is  a dataset of Digital Elevation Models  covering almost all the Earth, which were acquired through Interferometric  radar technology  by  the Shuttle Endeavour during a single flight mission which lasted 11 days, in February  2000. During the mission, aimed at the production of a global model to be used for scientific  and technical purposes, the Earth was  sampled from several positions, m order to reduce terrestrial reliefs  shading effects  and increase the accuracy  by  averaging the measurements. After the acquisition, all the data was  processed by  the MASA Jet Propulsion Laboratory  in California and, to date, NIMA is post-processing the data m order to reduce errors and improve the resolution.
3.2.3     PLEIADES DATABASE
INTRODUCTION
In order to have a redundancy  on the reservoirs  area and of the works  area, SP acquired the stereo satellite imagery  covering all the project area, aimed at the creation of a Digital Elevation Model and Orthoimages using Ground Control Points,
Similarly  to the drone photogrammetry  survey  (see following paragraph), a high number of ground control points  (22 G€P) was  used ;n order to provide the necessary  ground support dunng the joint digital elevation model and orthophoto extraction from the satellite stereo-couples  Imagery; the location or the adopted GCP is shown In the following image;
IOC WHO R WC1B (PRE-FEASIBILTTV Report)
IM WHO R SP TO3B
Studo plttrargeli, rrif-pDABUS HPPj
SURVEY AREA.
PRE-FCASIBILTTY STUDY Report
page 24 of 109
The total area reaches an extension of 1022 km? and represents the area of interest for the Dabus Cascade project, within which all the further topographical investigations (GPS + DRONES) were performed.
The area was acquired through only one Pteades satellite run on the 2 February 2016.
POST PROCESSING
Post processing has  recently  been completed. SP processed the satellite survey  using PCI Georr.atica SW, a specific  software foi management of satellite photogrammetric  projects, DEM (Digital Elevation Model) extraction and orthophoto calculation starting from ad hoc collected satellite images.
Moreover a morphological filter was  applied on the point cloud reconstructed by  the photogrammetric restitution of satellite imagery, in order to filter out most of the outliers  and off-terrain points, such as vegetation, and small objects etc.
The  filter  was  applied  through  the  RiscanPRO  software,  which  Implements  procedures  appositely  suited  for management of large point douds reconstructed by photogrammetry techniques or LiDAR instruments.
3.3   WORKS AREA 3.3.1     INTRODUCTION
A survey earned out with Drone was performed do the following areas: o DAM ch-155;
□ DAM Ch -130;
o PH ch - 130;
o DAM Ch - 108,
3.3,2     SURVEY
The following pictures portray the drone dun ng some actions on sjte:
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A total area of approximately 20 km2  was globally surveyed ove*’ the entire project area, with a total number of 1380 pictures collected in 11 flights.
The following table gtves full detail of the mam drone survey characteristics divided for area:
Tj&r 7 J. 1:                    Drvnc
Dam Alt CH 155
Dam All CH 130
PH All CH 130
Dam Alt CH IOS
Area surveyed
km2
4.6
5.2
3.6
5.5
Pictures
#
430
497
31B
135
Points
About 15.5 militons
About 17 millions
Atom L? irdlions
About 8 5 mdtons
Right average height
m
334
313
344
542
Ground control points
#
0
5
2
0
Rights
3
3
2
3
The following maps show the extent of the survey limit
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PR&FEASIBIUTY STUD Y Report
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The drone survey was processed using the ALS suite, a specific software for management of photogrammetric: projects. DEM (Digital Elevation Model} extraction and orthophoto calculation starting from ad-hoc  collected aerial pictures.
Moreover a morphological filter was  applied on the point cloud reconstructed try  the photogrammetnc restitution of drone imagery, in order to filter out most of the outliers ano off-terrain pants, such as vegetation, small objects, and moving objects such as vehicles etc. Hie filter was applied through the ALS software for all the areas except for the alt. ch -130 Dam site where a filter was applied by hand.
100 WHO SKIDj 0 (PRE-Ff ASJEJILTTf Report)
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page 29     169
4 HYDROLOGY I
4.1   INTRODUCTION
Thu  chapter  summarises  the  results  of  the  hydrological  analysis  carried  out  for  Datius  HPPs  project.
For the detailed description of the hydrological study reference should be made to the following:
•      120 HYD R SPOOlA (HYDROLOGICAL Report)
The  Dabus  HPP  project  Is  located  on  Oabus  river,  a  north-flowing  tributary  of  the  Abbey  Rwer  in  southwestern Ethiopia.
Dabus  river drains  an area of about 14,400 square kilometres  and its  course defines  part of the boundary between Bemshangul-Gumuz  and Orcmsa Regions, It originates  in the high volcanic  mountains  that border the drainage area to the south, and flows  generally  northwards  In a large and flat basin known as  the Dabus swamp, then continuing northwards up to its conjunction with Blue Nile River.
According to the modified Kopen system of climate classification, the basin falls  within Tropical Climate Ilr beinq characterized by  a mean annual rainfall around 2000 mm with a mono-modal distribution along the year.
The  proposed  layout  envisages  a  cascade  with  two  power  plants,  being  the  two  dams  located  at  the  following coordinates:
r</. T
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LOCATION
AREA
SITE
___  _________ DABUS Alt. Ch.0 + 130 DABUS AlL Ch 0+108
WGS&4
UTW Projector 36 M
Ew?        Sowtn
(IWJ
705605
1057644                  10W9
700957         [       1103040
10-400
The  catchment  area  and  the  location  of  the  hydrometrical/meteorological  stations  are  shown  in  the  following drawings:
o 120GENDSP001 - Hydro and Meteo Stations o 120GENDSP002 - CATCHMENTS AREA.
4.2       AVAILABLE HYDROMETRIC and METEOROLOGICAL DATA
A  specific  mission  to  Addis  Abeba  (February  15-19,  2016)  was  carried  out  In  order  to  collect  the  data  for  the hydrological   study.   In   particular,   the   official   available   hydrometric   (dally   discharges)   and   pluviometrlc   (daily rainfall   height)   data   were   requested   and   collected   at   the   Hydrology   and   Water   Quality   Directorate   of   the Ethiopian Ministry of Water, Irrigation and Electricity, and at the Ethiopian National Meteorological Agency.
100 WHO« SP003B (PW-fEASlBIUlY Report)
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Table 42 l and 4.2.2 report the list o' the hydrometric  and meteorological stations  within and close to the catchment of interest for which data were made available. Figure 4.2.1 repo'ts  the location of these stations also reported in drawing 130GENDSP002 - CATCHMENTS AREA.
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4.3       ANALYSIS or AVAILABLE DATA 4.3.1    HYDROMETRIC DATA
page 31 of
Table 4.3.1 reports the annual efficiency of the considered hydrometnc stations (Table 4.3.1, Figure 4.2.1). The efficiency was evaluated as the ratio between the number of the valid daily observations and the number of days within each considered year.
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Among the hydrometric  stations  located within the Catchment of Dabus  nverr the discharge observations collected at ST 5002 (Dabus  near Assosa) are clearly  the most significant for the project. Unfortunately, the available time senes observed at this station extends over 17 years, from 1963 to 1979.
Table 4,3.2 reports  the monthly  average discharge observed at stabon Dabus  near Assosa (ID 5002) wlthm the available recorded penod, while Figure 4.3.2 shows  the mulb-annual monthly  average discharge. The average multi-annual discharge (not rigorously calculable due to gaps) is close to 160.0 m3/s.
JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
Time (months)
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Of particular interest appear the Observattons recorded along DkJessa river ana its butanes. It is to be noticed that Odessa Is the closest to Dabus of the Mbutaries to the Blue (Abbey) Nile. The fart that the catchment area of the Dldessa rtwr Is hydrology simter to that of Dabus nvar in terms of both land features and meteorological disturbances (storms) makes rhe discharge orations Elected at its stations quite interesting for the scope of the present study.
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page 33 y 169
?
The  catchment  area  drained  at  the  station  of  Di  dessa  near  Arjo  (1D4001)  is  equal  to  9981  Kmz,  Ze.  quite similar to that drained by the station of Dabus near Assosa (10067 Km ).
Fig. 4.3.3 compares  the concurrent daily  discharges  collected at the stations  of Dldessa near Arjo (ID 4001) and of Dabus  near Assosa (ID 5002) In the years  1965-1%8. The assumed hydrological similarity  appears  to be confirmed by the observations. Two important features are evident by this comparison:
(a)  The  overall  wet  season  hydrograph  observed  at  Dabus  near  Assosa  appears  generally  shifted  in  time with   respect   to   that   observed   at   Didessa   near   Arjor     therefore   showing   that   the   meteorological disturbances   that   cross   the   region   tend   to   affect   the   Odessa   catchment   more   than   the   Dabus catchment
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page 34 of 169
(b) The occurrence of a succession of floods during the wet season is evident in the DIdessa's record while the record observed at the considered station along Dabus  appears  much more smooth, therefore showing that the presence of flooding-land areas  tends  to moderate the flood flows  upstream the station of Dabus near Assosa.
4.3.2    METEOROLOGICAL DATA
Table 4.3.3 reports the annual efficiency of the considered pluviometnc stations, being the efficiency evaluated as the ratio between the number of the valid daily observations to the number of days within the considered year.
studio ^IcrrjingciL rgmc0ABU5 HPPs
PRE-FEAS/BIUTY STUDY Report
page 35 of 1&9
Table  4,3,4  reports  the  annual  cumulative  precipitation  at  available  pluviometnc  stations  characterised  by  an annual efficiency > S0%.
4.4 EXTENSION of the
4,4.1     INTRODUCTION
DAILY DISCHARGE nME SERIES at DABUS near ASSOSA
The time senes of daily discharge actually observed at the station of Dabus 17 yea's.
near Assosa (ID50O2) extend over
100 WHO R SP003B (PRE-FEASlBUJTy fappt)
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PRE-FEASIBIUTY STUDY Report ____ ____________ p^c 36 at lot
Three different approaches were adopted in order to extend this time series:
synthetic streamflow generation;
use of Odessa near Aqo observation by means of a Transfer function; and
- rainfall-runoff modelling.
4.4.2     SYNTHETIC STREAMFLOW GENERATION
Figure 4.4.1 shows the annual average discharges resulting from the application of the autoregressive model.
4.43 TRANSFER FUNCTION
Two kinds of transfer function were applied in the present case:
a power-law transfer function; and
an Artificial Neural Network (ANN) transfer function.
POWER-LAW TRANSFER FUNCTION
Figure  4.4.2  shows  the  annual  average  discharges  resulting  from  the  application  of  the  power  law  transfer function.
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PRE-FEAS/B/LFTY STUDY Report
37 of 169
AHTTF[GLAL NEURAL NETWORK TRANSFER FUNCTION
Figure 4.4,3 shows the annual average discharges ’■esultlnq from the application of the artificial neural network transfer.
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4.4.4    FtAINFALL-RUONFF MODELUNG
Figure 4.4.4 shows the annual average discharges resulting from the application of Rainfall-runoff model,
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4.43 CONCLU DING REMARKS
As shown In the previous paragraphs, three different approaches were adopted in order to e-teod the actually observed time series  of daily  discharge at the station of Dabus  near Assosa (1D5002) to a period of time spanning the years from 1%1 tn 2008, ie.:
synthetic streamflow generation;
-     use of Odessa neai Arp Observation by means of a transfer function; and
-     ramfall-runoff modelling.
Tab.  4.4.1  recalls  the  multi-annual  average  discharge  at  the  station  of  Dabus  near  Assosa  resulting  from  the application of each applied approach.
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38 of 169
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As already observed, the extended time series resulting from: the first approach should not be used for energy production evaluation as  it ts  based on data recorded in the decade 19654974 and could not account few possible trends in the following years,
The multi’annua I average discharge resulting from the other approaches vanes from a tn ml mum of 143.4 m3/s (ramfall-runoff modelling) and a maximum of 148.6 m3/s (power law transfer function).
Between these approaches, the one that was  preferred for defining an extended time senes  to be used in energy  production evaluation is  the ANN transfer function approach. In-fact, this  approach Is  based on actual observed discharges (observation at Didessa near Arjo) and provides the best resutts in terms of com pan son between predicted (or simulated) and actually observed discharges.
103 WHO R SP003B (PRE-FEASIfllLnv Report)
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S GEOLOGY and GEOTECHNICS
5.1 REGIONAL GEOLOGY
39 erf 169
Three main physiographic  provinces  characterize the Ethiopian region: the highlands  {represented ny  the northern and southern Ethiopian plateaus  and the Somali plateau), the Afar depression, and the Mam Ethiopian RrfTMER"
The  studied  area  is  located  in  the  Southern  Ethiopian  Plateau  formed  by  Precambrian  metamorphic  rocks  and associated Intrusive igneous rocks.
The Ethiopian Precambrian shield is  formed by  dominantly  north-trending linear belts  of low-grade volcano- sedimentary  rocks  and mafic-uttramaflc  rocks, sandwiched between medium- to high-grade gneisses  and migmatites. The high-grade gneisses  and rmgmabtes  are referred to as  Lower Complex, which is  part of the Mozambique Orogenic  Belt and generally  consist of amphibolites  fades  orthogneisses, paragneisses, migmatrtes, and amphibdite with bands of marble.
Referring to the project area, the tract of western land along the Ethiopian-Sudanese border is  underlain by gneiss  and migmatite known as  the Bare Group. It is  flanked on the east by a large trad of land underlain by the relatively low grade volcano sedimentary succession known as the Birbir group.
I^I^T 5 2 2 topped Qrotogtca/ rntp ar Ftfwfnt *wn TrAr^ et at (1996.2, mod fed by Abbatr ef M (Ml5J
5.2       LOCAL GEOLOGY
According  to  the  geological  maps  published  by  the  Geological  Survey  of  Ethiopia,  the  following  lithologic  units constitute the project area.
1-50 WHO R SPOCie (PRE-FfASlBlLTTV Reportj
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SOIL COVER fOal). This consists of alluvial and laterite soil cover. The fine-grained soils are found in flat laying plains and along the river meanders. Alluvial deposits have several gradings, from cobole-pebble size to sandy- grave! to slit size. This unit affects. In particular, the proposed site at Ch. 130.
TERTIARY BASALT (Tib). The lower flood basalt unconformably  overlays  the basement composed by Precambrian rocks. Thickness  and extension of lava flows  vary  depending on local morphology  and tectonic factors.
This  unit,  although  it  is  widespread  in  the  study  area,  does  not  directly  affect  the  works.  It  outcrops  on  the plateaus and forms a large part of the reservbr area of the Ch. 130 - Dam and the relative catchment area.
PRE-TECTONIC INTRUSIVES fPodt Potlk According to the geological map NC 36-12 MGimbiw, two units were Identified in the project area. The first one (Pgdt) is  granodiorite-tonalite with minor gabbrexe pods. This  unit represents the bedrock of the dam site at Ch. 130 and the reservoir area of the dam at Ch. 108.
The  second  unit  (Pgtl)  is  described  as  granite-granodiorite.  This  unit  has  been  mapped  in  the  northern  par. of the project area and constitutes (almost in part) the bedrock of the dam site and power house at Ch. 108.
The geological map NC 36-16 (which covers the power house site of the dam at Ch. 108) describes a slightly different interpretation because the rocks  are reported as  metatonalite (tn) and metagranodionte (gd), corresponding to the unit MPgdt’ of the Geological sheet NC36-16, and as  metagranite (gti), corresponding to the unit ’ PgtlT The prefix -meta indicates that a metamorphic process has been identified in these rocks.
The survey earned out on site confirmed this latter interpretation: strongly deformation, iso-oriental Ion of
crystals, foliation and metamorphic alteration of the original structure have been recognized in most of the outcropping rocks, particularly in the dam-site at Ch. 108. This evidence was confirmed also by petrographic 
analysis conducted on thin section derived from some samples collected on site. For this reason the prefix -
meta is used m this report where a metamorphic process has been recognized.
5.3  CH - 130: DAM SITE
5.3.1     MORPHOLOGICAL FEATURES
In this stretch of valley the river flows with a straight course, from SW to NE. The nver bed has a width of
250-300 m and splits into various channels separated by elongated islands covered by vegetation.
The dam axis is located in a very gently U-shaped symmetnc valley. Both the flanks have two different sections: the upper part has  a slope of about 15% (<10°), while the lower section, at the foot of the slope, lias a very genlle morphology with a slope of 2-3%.
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53.2     GEOLOGY OF THE DAM SITE
During the site visit, several activities  were conducted: geological survey, collection of rock  samples, thin sections  with petrographic  analysis  by  microscope, geostructural stations, trial pits, seismic  lines  with SASW technique. The results (photo documentation, stereoplots, pit logs, etc.) are Included in Annex A.
From the results of the on-site activities, the following geological situation can be summarized.
Both the flanks  of the dam site are formed by  a residual soil cover, constituted by  red sllty-dayey  soils. The results  of tnal pits  and the geological survey  may  lead to the hypothesis  that on the abutments  weathered bedrock  should be encountered around a range of 4<*6m, while the fresh (or slightly  weathered) rock  in the range 5-rlO m erf depth. Comparable results  were obtained by  the seismic  tests  earned out with SASW technique on the right side, at the lower part of the slope.
On  the  right  side  of  the  alluvial  plain,  the  bedrock  is  covered  try  alluvial  soils.  Large  outcropping  of  rocks  in the riverbed suggest that bedrock in this area should not exceed the depth of some metres.
Outcropping  of  meta-granltes  (with  a  strong  foliation  and  gneissic  structure,  as  shewed  by  thin  sections)  has been surveyed along the creek on the right bank, Just downstream the dam axis.
On  the  alluvial  plain,  left  side,  the  bedrock  (granite  and  meta-granlte  to  meta-granodiorite,  as  indicated  by  the microscopic analysis on thin sections) is frequently outcropping In scattered or wide areas.
Several outcrops  of rock  were surveyed on the river bed. All these outcrops  are made or meta-granodionte and meia-tonalite with several mete-lavlc  intrusions, in form of dykes. The Intrusions  form very  inclined layers (dykes) which cross almost transversally the nver bed.
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page
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Fqur* 5.5.3: Left: nets ntn&ons, riwvbad left (OC50). Right: grwtorff. n*ft*dnght (OC fo. 56)
5.33 GEOTECHNICS OF DAM SITE
Rock  outcropping In the dam site is  slightly  weathered or very  slightly  weathered (almost fresh in the meta- lavlc  intTusmns). In some places  a higher level of weathering can be reported, particularly  where the grain is medium to coarse and where the metamorphism created a strong lamination structure.
The rock is strong to very strong up to extremely strong In the fine grained meta la vic intrusion.
Due  to  the  small  extension  of  rock  outcrops,  joint  spacing  is  not  evaluable  from  a  statistical  point  of  v.ew  A moderale to low density of joint can be estimated at this stage of the study.
Regarding the reservoir‘s  watertightness, at this  stage of the study, considering the presence of a broad silty- dayey soil cover on the Hanks of the reservoir, a low risk of water loss can be assumed. More in-depth analysis will be developed to evaluate a potential water loss through fractures and lineaboos in the bedrock.
Regarding  slope  stability,  no  significant  landslides  on  the  flanks  of  the  reservoir  have  been  reported  in  the previous studies, nor have been identified by photo aerial analysis and recent surveys.
53.4      QUARRY FOR CONSTRUCTION MATERIALS
Suitable material, for rockfill and granular materials  should be constituted by  strong, not weathered/weatherable rock. Both the basaltic  and the Intrusive rocks, which form the bedrock  of the project area, should be suitable for this purpose. Basaltic rocks are preferable for concrete aggregates.
Due  the  presence  of  basaltic  rocks  near  the  dam  site,  a  specific  survey  was  executed  during  the  last  visit  in order to identify suitable quarry sites to be investigated in depth dunng the next phase.
Three  possible  sites  have  been  identified.  Site  No.  2  is  the  most  evident  and  suitable  quarry  area.  The  site  is located 2.5 km south of the dam site, corresponding to a basaltic elongated hill.
The rock is widely outcropping In an area of almost 650x250 m, forming rock walls up to ten metres high. The rock  is  dark  grayish, fine-grained, very  strong aphanitic  basalt, which could be suitable for the embankment dam and also for concrete aggregates. This site represents the first quarry choice.
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TFe volume of basaltic rock available In this area is estimated m some hundreds of thousands cutxc metres, to be directly excavated without soil covering.
100 WHO R SPOWB (PRL-FEASmrLTrr Report)
JOO WHORSP 003B
studio p*rtrange|i, romeDABUS MPfX
PR&FEASISIU7Y STUDY Report______ 5.4   CH - 130: WATERWAYS and POWER HOUSE
44 of tes
5*4.1 MORPHOLOGICAL FEATURES
A 6.5 km long headrace tunnel will be excavated along a senes of genhe hills on the left bank of valley. The intake will be located in a gentle slope, about 1 500 m upstream of the dam axis, while the exit of the tunnel will be about 1340 m a.s.L; from this polnl, a 1,9 km long penstock will link the tunnel to the power station.
The power house will be located at about 1250 m a.s.L, dose to a creek. This creek has a very irregular course, with many curves,, and flows into the Dabus River at about 1235 m a.sl.
S.4.2    GEOLOGY and GEOTECHNICS
The following activities were conducted during the Site visit: geological survey of waterways and power house site, collection of rack samples, thin sections for petrographic analysis by microscope, The results of these activities are ‘nduded in Annex A.
Only  a  few  outcrops  were  surveyed  along  the  tunnel  which,  according  to  the  geological  data,  should  be excavated in Precambrian rocks. More than 4^5 m of sml rover was surveyed In the intake area.
Large rock outcrops were surveyed in the pc.ver house area and along the creek which flows from the PH area to the Dabus River. In this area the rock is basic intrusive, dark grey, ranging from tonalite, to diorite and gabbro. Some pods of basic granulite are included.
The rock is very strong to extremely strong. A moderate to low density of joints was surveyed on the outcrops. The degree of weathering Is ver/ low in the outcropping rock.
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5.5.1    GEOMORPHOLOGICAL and GEOSTRUCTURAL FEATURES
In this site the river has a rectilinear course flowing NW along a very narrow riverbed, ranging from 30 m to about 50 m in width.
The dam axis is located in a V-shaped asymmetric valley, The left Hank is steep, with an average slope of about 60% but with two different sections: the upper part has a gentle slope of about 15■ 20% (—10°^, while
r
the lower section is very steep, with a stope of 100% and more. Some deep creeks cross the bank, forming very steep scarps arnj isolat-ng large boulders (see following figure).
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The right bank has an average slope of around 20-25%. The abutment is jelled try some very gently areas which separate other steepty sections of the slope, A wide flat and very gentle slope area >s located around level 1220 m a.S.L [Seefollowing pbnto). The slope Is crossed by some creeks Incised in rock.
Spur? IT 2; Rignt MittHm ste Ch. 108 ftfyf rt,f
I220 m * s/j
The  bedrock  of  this  site  is  formed  by  Precambrian  rocks,  here  constituted  by  meta-gramte  and  rneta- gra nodlorite. large outcrops of rocks are visible on the plateaus, on the banks and along the riverbed.
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46 °r 169
The project area is affected by some tectonic hneations wdl discernible on aerial photographs. These IIneations haxe a strong effect on the river course geometry: in many stretches the direction of the river changes abruptly as to De "captured" by tectonic alignments. More gerwrally, the satellite images show many evidences of these phenomena known in literature as "river elbow capture".
5.5.2    GEOLOGY OF THE DAM SHE
The following activities were conducted dun ng the site visit geological survey, collect no of rock samples, thin sections tor petrographic analysis by microscope, geostructural stations. The results of these activities are included in Annex A.
All the outcrops  can be classified as  meta-g^amte and meta-granodiorite recks. Differences  in terms  of petrographic characteristics and metamorphK/texture grade can be appreciated in the same outcrop and in between different areas.
hQt/fr 5 5 j
in tithotogy j/xf structure rt the bedrpdt at Darruutt' Ch. 103
In  almost  all  the  outcrops  a  strongly  deformation,,  iso-orientation  of  crystals,  foliation  and  metamorphic alteration of the original structure have been recognized. This structure has been identified and confirmed by microscope analysis conducted on thin sections.
5.5.4. T^r? stxtKjns        {tctrograpttic
5.5.3    GEOTECHNICS OF THE DAM SITE
During the last visit, several Joint sets were measured. The results are shown In Annex A (two representative stations are shown in the stereopfols of the following figure). In almost all the geostructural stations, three mam joint sets were identified.
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JOG WHO P. SP DO3B
stud a pierranoeh. rwnrOABUS HPPs
PRtr-FEASIB/LJTYSTUDY Report
47 of i&s
figure 5.5.5 Stereopkrts of jrtscfrurfL'ra/ s&fiwra £M JT £ jnd £Mfi JJ
The first set Is measurable in ail the stations: plunge (dip direction) is in the range from 190° to 220° with a
dip of 2S°-r40 . The second set
fl
Is less frequent than rhe first but often measurable; it is almost “conjugated’
to the first set, having a similar strike but an opposite dip direction (dip Is mainly in the range 65-80°). A third set is mainly sub-vertical, with a strike of about 20-40°.
The first joint set plays a role in the morphological and stability feature of the valley. This joint orientation Is effectively  against slope stability  on the right bank  and the opposite for the left bank. The first joint set provokes some sliding phenomena on the right bank and, in some cases, when associated with other joints, some toppling on the left bank. This geo*structure is also the reason of the different morphology of the two banks, with the nght one less steep than the left one.
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figure 5. Ss-n? totfte tfree main j&nf sets
The rock strength can be assessed as "strong’' to “very strong,r (according to Hoek. & Brown critena for practical estimates of rock mass strength). The weathering grade ks generally low to very low.
Different spacing anti joint characteristics have been reported for the three joint sets. In genera I, a moderate grade of fractciratlon can be evaluated for the rock mass.
Regarding slope stability, no significant landslides on the flanks of the future reservoir have been identified, al this stage of the study, by photo-aerial analysts. Local sliding on the right bank and some toppling on the left bank of the dam site are provoked by the particular geo-structural condition above commented. These slides are, in any case, of relatively small dimensions and they should not affect the feasibility of the dam; they will have to be taken into consideration in the following design stage.
5.5.4   QUARRY FOR CONSTRUCTION MATERIALS
The presence of several outcrops of rocks in the whole area suggests that it should be possible to obtain rockfill material and concrete aggregates from a quarry area to be located dose to the dam site. The exact location will also depend on the work sate needs, in order to optimize excavations and transportation works,
A specific Investigation campaign will assess the quality of the rock for Doth concrete aggregates and granular material.
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■Audio pietrarvqeli, romcDABUS
WE FEASrerUTY STUDY Report
5.6 CH - 108: WATERWAYS and POWER HOUSE
49 of lfr>
5.6.1    GEOMORPHOLOGICAL FEATURES
A 6 km long headrace tunnel will be realized on the right bank of the nver; a 0.9 km long penstock will join the tunnel to the powerhouse, located on the right bank of the river.
The morphotogy along the tunnel alignment Is very rough, dominated by outcrops of rocks, steep elongated hills frequently rounded by scarps and deep gullies.
The powerhouse will be located on a small hill on the right bank of the river; this area is characterized by a gentle morphology. No outcrops of rocks were surveyed in this area, while large outcrops are visible on the river bed.
From a qeostructural point of view, the same features described for the dam site are recognizable also In this area.
5.6.2    GEOLOGY and GEOTECHNICS
The following activities were conducted dunng the site vist: geological survey of waterways and power house site, geostnjctural stations along tunnel and penstock.. The results of these activities are included in Annex A.
According to the geological data, the tunnel will be excavated In Precambrian rocks, mainly meta-granlte and meta granodiorite. Several outcrops  were surveyed, recording similar conditions  (from a geological and geomechanical point of view) comparable with those of the dam site.
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Very similar joint sets were recorded also in these sites (at some km distance from the dam site) suggesting that the three main joint sets are dominated by regional tectonic features. Small differences are recorded in terms of dip and dip direction.
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6.1       INTRODUCTION
The scope of the present chapter is to illustrate the seismic assessment, carried out over the Dabus project area aimed at evaluating the Peak Ground Acceleration at different return periods.
In particular, the following two approaches were used in order to estimate the value of the PGA related to its return period.
OSH A: Deterministic seismic hazard approach^ and
PSHA: Probabilistic seismic hazard approach,
In the following are summarized the main results of the seismic assessment according the above mentioned report, whereas the detailed results are shown in the following report:
•     130 GEO R SP 001- Geological report
6.2       DETERMINISTIC SEISMIC HAZARD APPROACH
In the deterministic approach (DSHA), the seismic parameters are estimated from a definite seismic scenario, assumed to occur at the dosest possible distance from the site, and for the maximum credible earthquake (MCE).
DSHAs were performed adopting the following two seismic zoning of the region:
•      zoning denying from the Global Seismic Hazard Assessment Program (GSHAP) project, (see Figure 6,2.1);
•     zoning deriving from the work of Adbalia 2001 (see Figure 6.2.2),
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Tables 6.2.1 and 6.Z2 list the ground accelerations computed with the classical DSHA for lhe two seismic zonings shown above according to the attenuation relation ships
ln(—J = -3.864 * 103 M - 1,351 In U - 0.0006 tt ff
-     Inf—) - -4.057 - 0.866 M — In N - 0.0025 ft
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-     Inf^) = —6.539 + 1.43 M - 1.5 4- ln(ft + 1)
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Jonathan (1996) Twesigomwe {1997) Mavonga (2003)
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fOvrtts
Source
R ed Sea
Mavonga      Average
0.001              0.001
Yemen
Afar
MmiJi
6.7
7.1
6.8
Rm I n
906
970
506
Twesigomwe
0.001
0.001
0.003
Jonathan
0.001
0.001
0.003
East Ethiopian Rift
6.8
419
0.005
0.005
0-001              6.001
0.002              0.003
0.003              0.004
Western Rift
7.8           529
0.007
0.009            0.008              0.008
East Kenya Rift
7.4           587
0.004
0.005            0.004              0.004
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Twesigomwe     Jonathan      Mavonga      Average
0.019                0.014            0 007
0.013
3: SW Sudan
6.0
260
0.010
4: South Sudan
7.2
403
0.013 0010 0.006
0008 0 008 0D05
6: Central Eth opia                6.8          191
0.020                0016             0 009
7: Afar & GiHf of Aden          7.5          542
0.005                0 006            0 005
0 007
0.015
0 006
The seisms  zoning From Abdalla et al. [2001) is  more conservative since it locates  the seismic  source boundaries  closer to the Dabus-sTe than the GSHAP seismic  zoning. The maximum average ground acceleration is O.OlSg denying from the Central Ethiopian setsmic source (Region 6 of Abdalla et al.).
6.3   PROBABILISTIC SEISMIC HAZARD ANALYSIS
The following maps show the seismic risk according to the Abdalla study, expressed in terms of peak ground acceleration for some relevant return periods:
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„ rWi^ £P =
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I CM WHO R. SPOO30 (PRE-f EASIBiL JTY Report)                      JOO WHO 6 5P 0030
Kudio pietrangek rome□ABUS MPPs
PRE-F&lSiSILJTY STUD Y Report
page 55 of 169
The following table summarizes the PGA values which can be read hnm the above maps:
r-flS4 WrrWV
RP              PGA/g T/ws;.'
___
475
950
1900
2370
0.05
0.08
0.09
0.10
The fallowl ng figure portrays the PGA/g vs Tr interpolation curve based on the PGA values read from the above maps.
The PGA values follows a time-depending law according to an expression of type:
o PGA/g = A • LN (T-bl) “ + C
By interpolating the above values by means of the least squares Atting procedure, the following A B and C
r
parameters can be found:
o A = 0,00046
o B = 2.62
□ C = 0.0007
The following table summarizes the PGA values at different Return Period according to the above mentioned relationship:
to (U,2              a*4         ar rfflterwr w3
RP
PGA/g
jwsl
<-)
475
o.os
950
0.07
2S00
0.10
10000
0.15
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7 PROJECT LAYOUT OPTIMIZATION
7.1 INTRODUCTION
The present chapter provides the information regarding the layout of the Dabus cascade, in particular:
•     Project LOCATION and SITE ALTERNATIVES;
•     ALTERNATIVES EVALUATION;
•     DABUS CASCADE;
•     EM and HM Equipment;
■ TRANSMISSION LINE SYSTEM
PRQjjCLLOCATIONS SITE ALTERNATIVES
The chapter provides a general description of the project location and the identification of the most suitable sites for construction of the HPPs.
ALTERNATIVES EVALUATION
A study on the possible and feasible alternatives was carried out, In order to perform a sensitivity analysis on the mam costs and energy production for a list of alternatives.
The alternatives' evaluation study was done considering several options for dam type and dam height at each site, and assessing the related energy  production by  means  of a reservoir operation study; the chapter therefore shows in detail the related design criteria and the major findings In terms of benefit/costs of each solution.
BABU5 CASCADE
The alternatives’ evaluation study ended with a suggestion of the best choice on the various components of the cascade m order to achieve the greatest economic benefits; in the Dabus Cascade chapter, a complete description of the selected layout is  provided, together with the relevant energy  production outputs  and reservoir regulation study.
TRANSMISSION LINE SYSTEM
The prefeasbllity study indudes a review of available previous schemes, a technical assessment of the feasible interconnection schemes by considering also preliminary economic parameters, and the mam geological and environmental constraints.
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page57 of tfi9
7.2.1 GENERAL
The  cascade  will  be  located  In  the  central  and  western  area  of  Ethiopia,  along  the  Dabus  over  in  the Benishangul-Gurnus region, over an area bounded by the foilowirg coordinates (geographic WGS84):
rjtfr ZZ /,• 
LATITUDE
^r.Tcf rrftT'f t&onfinat&s
LONGITUDE
’ (MPT,
(MAX)
(MIN) "
(MAX)
8.87° N
10.03° N
34.55° E
35.65° E
The Dabus River flows m a south-north direction, over a course of approximately 300 km before delivering its waler into the BJue Nile River, at a point located approximately 100 km south of the Ethiopia-Sudan border along the Blue Nile path.
The project location Is illustrated in the drawing: 100 GEN D SP 001 - PROJECT LOCATION and is described herein:
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tfudld piMiangrU, mmrDABUS HPPs
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P*ge
58 of it?
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7.2.2 SHE ALTERNATIVES
PRE-FEASJB/tm STUDY Report
59 or hw
Several   alternatives   were   investigated   and   their   feasibility   checked   as   potential   sites   for   hydropower generation.
Ttie starting point on which the study was based was the reconnaissance study of the Dabus river, produced by the Ethiopian Ministry of Water, Irrigation and Energy and reported In the following document:
• MoWIE, 2002, ZteZwj
Sft/t/y
The above study identified two potential sites far the construction of a cascade of hydropower plants, named respectively UPPER DABUS and LOWER DABUS.
In order to univocally define the positions of the mam works  along the Dabus river, we have adopted the naming convention of the progressive distances (Z.e chamages) of the sites along the nver.
The chamages were defined as negatrve kilometric distances progressively increasing in the upstream direction ard originating from the Dabus River - Blue Nile River confluence.
Accordmgly, the Upper and Lower Dabus HPPs are located approximately on the following chalnages:
■ UPPER Dabus - Chainage -155 km (ch -155);
• LOWER Dabus = Chainage -130 km (ch -ISO).
The above sates were Initially proposed In the ToR during the tendering phase of this assignment.
The alternative at Oi -15S km has readily proven to be not economically convenient, due to reasons that will
be danfied In the following, while the ch *130 km alternative has been seen to be really cost-effective.
Furthermore, a thorough and comprehensjve study of the Dabus river topography was done, confirming the effectiveness of the ch ■ 130 alternative and allowed us to identify an additional site (ch *108) with an even
greater hydroelectric potential
The major considerations on the sites are:
Qt-JMn
The ch *155 site allows for a dam with a reasonable height of 404-50 m. The nver follows a very low longitudinal slope (approximately  equal to 0.5 m/km on average) for a long stretch Involving the 15 km immediately downstream the site. The solution allows for a dam with heights comparable to the ch -130 alternative but. considering the river slope, the only conceivable solution foresees a powerhouse located at the dam toe. where the geodetic head is provided by the dam height only.
Compared  with  the  ch  -130  scheme,  the  only  advantage  of  choosing a  powerhouse  at  the  dam  lue  would consist m eliminating the construction cost of the waterways.
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Moreover, the main disadvantage of the ch -130 solution is that It would partially submerge the road connecting Ascsa to Mendi, while the ch -155 alternative, being located upstream of the road nver crossing, would not.
However, the Following observations have to be taken into consideration-
•  the  incidence  of  the  waterways  of  the  ch  -130  scheme  would  be  approximate^  28%  of  the  whole construction cost, but it would produce three times the energy that could be produced by the ch -155 scheme;
■  the  road  Interrupted  by  part  of  the  ch  -130  reservoir  could  easily  be  restored  by  rehabilitating approximately 30 km of an existing alternative road, which could cross the Dabus over on the crest of the ch -130 dam or immediately downstream (see drawing 150DAM SP 002). This is described in more detail in the the design criteria chapter
A quick economic assessment was performed for the considered site, in order to understand rts feasibility and economic profitaWity, by performing a reservoir operation and energy production si mutation and estimating the main costs  for the dam, PH civil works  and EM equipment. The construction costs  of the HPP were estimated calculating the approximate quantities of the main construction material (f.e. concrete, excavations, reinforcements) and by  applying the relevant unit prices, extracted from similar works  in the Consultants cxpenence,. to each item. Because the quantities and unit prices are gross values, the plant cost estimate s purely indicative and should not be taken as definitive, but It is still useful for comparison purposes among different solutions.
According to the following input data:
-E
• IP
■ cc
GWh/y               250 MW                    60
energy production installed power
M$                         139                main construction cost
Q Coaffl.Rtt
M$                     86
O ClM                                                                        27
dam construction cost EM equipment cost
O C-H
o Ccrwi
Ml
B                        PH civil cost
M$                        18
contingencies {additional 15% over sum of
Dam, PH and EM cost)
• Cmh
M$/y
0.9
operational  and  maintenance  cost  (03%  CC
+ 1.5% EM cost)
- EP
c$/kWh
8
energy selling price
• I
%
8
discount rate
• lifetime
y
50
pant lifetime
following economic parameters would    anse from the assessment:
• [RR
130 WHO fl SPOO30 (WE-FFASfBIUTV Report)
%
11.6
internal rate of return from the investment
100WHOHSP 0033
5TudbO p^ctrangeb, rorreCABUS HPPs
PR&FEASJSIUTY STUDY
Report
P*9C
1&9
• pg
y 
7.3
payback period
■ B-G*v 
M$
84
sum of discounted benefit-cost at
end of plant
lifetime
The alternative at ch -155 is characterized by a negligible economic interest, since Its IRR (return rate of the Investment) Is  low and the sum of the discounted benefit-cost over the plant lifetime is  only  8-1 M4, /.e approximately ten times lower than the ch -13D solution.
For this reason, the alternative with dam at ch -155 and powerhouse at the dam toe was discarded with no further study.
DABUS River HYDROPOWER POTENTIAL ASSESSMENT
Further to the ToR sites, in order to find the best sites' location for the development of the cascade, and maximize the potentially exploitable head (and related energy production) the Consultant performed a detailed study of the river profile and related catchment area along the Dabus river up to the confluence with the Blue Nile River, and in particular in the stretch ranging from chainage -80 km to -160 km where several potential and promising sites for HPP installation were found.
As can be seen on the graph, the river profile shows a sudden slope change starting from chainage -130 from the confluence and has a remarkable slope (approx. 12 m/km on average) between the chainages -130 up to -99 km with a slight slope variation between chainages -119 to <08 km approx.
r
At the same time, the catchment area at each point of the river grows almost linearly without encountering relevant tributaries.
190 WHO R STO3H (PRETEASlBIUTY Repoit)
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The profile shows the possibility for a development of a HPP cascade by exploiting:
*     a first geodetic head of approximately 100 m between chainages -130 / - 119 km.
•     a second geodetic head of approximately 150 m between the chainages -108 / -99 km.
As a result of the findings of the topographical analysis, the following sjte alternatives were identified and studied as suitable locations for the development of a classic scheme with a cascade of two plants in series’
•    01-130
o DAM
• Ch
km
-130
chainage from Blue Nile
■ IL
m
1327
invert level at dam site
■A
km3
10009
catchment area
• Q«g
ftP/s
145.6
average flow
o PH
- Ch
km
-119
chainage from Blue Nile
• TWL
m
1230
tailwater level at powerhouse site
This location coincides with the findings of the Dabus Reconnaissance Study Report from MoWlE.
o DAM
• Ch
km
-10B
chainage from Blue Nile
- IL
m
1155
invert level at dam site
■A
knV
+ 391
residual catchment area
(downstream to ch -130 dam)
*
mJ/s
+ 5.7
residual average flow
(downstream to ch -130 dam)
□ PH
■ Ch
km
-99
chainage from BJue Nile
- TWL
m
1230
tail water level at powerhouse site
The following table gives the coordinates of the dam and powerhouse locations for the selected sites;
Ar.*ri         PH stef CDfflfrMtiX f WGW tTM J£> N)
Ch -130
EASTING NORTHING
— —-L^Jl         ____ £mj
Ch-108
EASTING NORTHING
!jT
| ifTl'l
DAM
PH
705475 1087430
705 IBS 1094415
7OB045                  1103160
705450
1107570
Both sites are suitable for dam construction.
ICO WHO R SP003B (PRi-fEASlBJUTY Report)
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page 63 c 16'i
7.2.3 CONCLUSIONS
Along  the  Dabus  river,  the  following  three  alternatives  stes  were  identified  and  studied  in  order  to  find  the best location   for    hydro-power plants development;
•     Dam @ Ch -155;
•     Dam     Ch -130;
■     Dam @ Ch -100;
from the Dabus river - Blue Nile confluence.
The alternative with dam $ Ch -155 has  shown not to be profitable and was  abandoned; the study  was focused Instead an the alternatives  at ch -130 and ch -1OB which have the highest expectations  to be very profitable.
For them, a further sensitivity  analysis  changing the dam type and dam height was  prepared, searching for the mast convenient solution on the economical point of view, the alternatives  evaluation study  is  further discussed and explained in the next chapter,
7.3 EVALUATION OF ALTERNATIVES
7.3.1     GENERAL
An  evaluation  study  was  performed  with  the  aim  of  defining  the  criteria  for  the  choice  of  the  most  promising HPP layouts among the many solutions which can be envisaged at both ch -130 and ch -IOS dam sites.
The evaluation of the scenanos  was  conducted through a sensitivity  analysis  aimed at understanding the effects  of the dam type selection and reservoir elevation on the energy  production, on the costs  of construction and subsequently on the profitability of the plant in its entirety,
In  particular,  the  sensitivity  analysis  was  performed  by  considering  a  series  of  plausible  alternatives  for  dam type and dam height and, for each alternative:
L by calculating the main realization cost of the HPP;
a.    Dam and hydraulic structures (spillway);
b.    Waterways and surge shaft,
c.    PH civil works;
d.    E.M. and HM equipment;
e.    Road relocation;
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2.    by performing a reservoir operation s-rnulation and energy production assessment. Including available
heads, head losses and water losses for spilling and ecological flow;
3.    by performing an economic analysis and calculating the:
a.    discounted Benefit-Cost difference obtained over the plant lifetime;
b.    Internal Rate of Return (1RR);
c payback period of the Investment.
The  following  paragraphs  deal  with  the  adopted  design  criteria  for  the  evaluation  of  the  alternatives,  and  the obtained results.
7.3.2      DESIGN CRITERIA
The  Dabus  river  cascade  has  been  conceived  as  a  sequence  of  two  hydropower  plants,  each  characterized  by the following general layout:
■    a DAM
■    a WATERWAY system, composed of
a a headrace TUNNEL (low-pressure)
o a SURGE SHAFT for mass oscillation and water hammer effects mitigation
o a double PENSTOCK (high pressure), which deliver the water to two turbines housed Into
■    a POWERHOUSE
■    a TAILRACE CHANNEL for the final delivery of the water to the river
In  the  present  evaluation,  several  alternative  scenarios  were  assessed  by  consrdenng  a  list  of  suitable  options for:
a DAM TYPE
□ DAM HEIGHT and subsequent RESERVOIR REGULATION.
Changing the above parameters  allowed us  to perform a sensitivity  analysis  of their effect on plant costs  and energy  production wrth related revenues  from energy  sates  according to the workflow described in the previous chapter.
Further  to  the  incidence  of  dam  type  and  height,  the  following  aspects  were  considered  ana  included  In  the analysis:
•      natural average inflows entering into the dam site on a monthly basis;
•      elevation -a rea a nd elevation-volu me curves of the reservoi r;
■ dam dead levels due to sediment load from the river at each site;
•     losses due to ecological flow release from the dam;
•     balance of precipitation-evaporation from the reservoms;
•      hydraulic sizing of the waterways ana head losses;
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■    tail water levels in correspondence to the power house Sites;
■    generation efficiency;
•     road relocation;
•     BoQ and costs of the main civil and electro-mechanical (EM) works;
■    economical parameters: energy selling price, plant Irfetime, cash flow and discounting rate.
IMLQ¥£
The resavoir operation and energy  production simulations  were earned out by  considering the multi-annual monthly average discharges at Dabus Ch -130. already defined in the hydrological chapter and obtained from the 48-year long historical senes  of recorded monthly  average discharges  at Odessa and transferred to the Dabus site.
The  resulting  multi-annual  monthly  average  discharge  adopted  during  the  reservoir  operation  routing  is surnmarved in the following-
rWjtfr 7 J. J.'              Daduy £> eft -J JT? fl/flxJ/ monthly dSsctorpes JAW    F*3        MAR     APR      MAY     JUN       JUL          AUG          SEP OCT NOV
DEC       Mlh      MAX          AUG
mSjs    m3's      ml/i       mj/t       m3/i      m3/s      mj/s          rn3/i
47.9 32.4 262 26 f 34 7 83.4 784.0 305 9 395 8 331.5 177 1 82 4 26.1
m3/$     m3/s      mj/t      mj/i            m3/s
29SrB     144.0
The Mowing table lists the senes of natural discharges due to the residual catchrrent between the two HPPs:
rdswe* z, jz-                 # f?*? -Ji3S .wiva/
JAN FEB           MAR   APR       MAY JUN            JUL         AUG        SEP          OCT         NOV       DEC      MTN    MAX         AVG
Jifl/s        ml/*      hi3/i
m]/i        m3/i         mJ^         rrBA          mj/s          m3fr       m3/s 
1        m3/s         m3/i m3, 5
1.9 J 3         10      1.0     14     3 3      7 2       11.9     155      129       6.9      3.2     1-0      15.5
The natural discharges of the residual catchment are added to the flows released by the ch -130 alternative, and In particular:
■    monthly turtiined discharges;
■    monthly spilled discharges;
■    ecological flow on monttMy basis,
in order to obtain the average hydrograph of the incoming flows at Ch -108 site and carry on a proper reservoir routing and energy production assessment.
The relevant inflow values and reservoir routing results are discussed in the paragraph below.
ELEVATION-AREA and ELEVATION-VOLUME curves
The reservoir volume and area coves as a function of its elevation were prepared in order to be used as input data to the energy production study.
The curves were prepared on the basis of the SRTM 30 x 30 m - Shuttle Radar Topography Mission already introduced in the related Topography chapter.
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The area and volume curves  were calculated at 2 m steps  of elevation interval adopting a GIS (geographic information system) software and fitted to simple mathematical curves  with least-squares  procedures  for an easier management during the computations.
The calculation was performed with Math cad software and provided the following results:
■ Ch -130 km:
-HMmr AfBsw
creflofftf - C.
ZWr 7,3. J; m -13$. L^s^w An?J ftxw
£Xpvw0wt A/w fitting, Regrns/cm axff>oent - O 961791
The  following  table  summarizes  the  coefficients  (from  A  to  F)  of  the  fitting  equations  used  to  model  the reservoir volume and area for the ch -130 reservoir:
We 7.3.3:                       Da6u* & cfi -130 T#rw Arra IWw ctxvw fitting prameters
RESERVOIR VOLUME
RESERVOIR AREA
Vto -
■(—J
Afe)
/Mra.7¥fer
va/uL-
parameter
wa/ue
A
8.13051977
D
175.0465016
B
0.25609085
E
0,06380322
C
1313.59722709
F
1132.0960603
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Mgr 67 or
• Ch ’108 km:
m-ie1
LM- IO'
*
■*
+ z
4
1 *OJW*L‘
/
ij>i?
■/
z 
/t
iS-Kr
*•
J
.1 un.
t
fl
II
*»
.11 *"-
/ Ark at/on V&tJft* fifbry ,
= 0.9999//
EJnebcn Arfa fitting J XegrtxaQH CDetfowr J 0.969C53
nyur* 7-J-Z G7 /£$ S-cvL-me and Arr* lltvcs /frCrap
The  falfowng  table  summarizes  the  coefficients  (from  A  to  F)  of  the  fitting  equations  used  to  model  the reservoir volume and area fo* the ch ’108 reservoir:
£\arvi -S cr - W nrsmrw- 4/ed-Lirfvrs /Sm^ pjraTOtefs
RtSEHVOIR VOLUME
RESERVOIR AREA
/i-CvLt'
rz-F?1* AW - fy-)
parameter
pawitfrf
A                 29,95859762 0                  0.21450335 C               1159.75569113
0                  44.62511428
E
F
030079548
1164,64523136
RESERVOIR DEAD LEVELS
In order to estimate the effective regulating volume and capacity of the reservoir over the whole plant lifetime, a solid yidd assessment was performed considering the specific rates for the area.
The  specific  sediment  yield  was  taken  from  literature  and  In  particular from the  already  oted  document from MoWIE “2002, Dabus Medium Hydropower Project Reconnaissance Stud/'.
In the above report, some values of specific yield are given based on observations from stations located close to the project area. Except for an outlying station with an exceptionally high sedimentation rate (approx. 2500 t/krrr/y, which is  one order greater than normal values  and with no physical meaning) the remaining stations provide values  inside the normal ranges; therefore, the maximum specific  sediment yield from the remaining Stations, equal to 500 t/km /y, was adopted.
Assuming  a  precautionary  sediment  density  of  B00  kg/m\  and  a  plant  lifetime  of  35  y,  the  following  dead volumes could be envisaged at both Dabus 130 and 108 alternatives;
J
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T&e 735;
kV*?
^A/rne
DABUS 130    DABUS 108
E
A
T
r
VkrrH/y                      500
km*
10009
y                                   3S
ka’m3
1300
500
391
35
1300
specific sediment yield
catchment of Interest
plant lifetime
sediment density
sediment volume at end of plant
>VQIifetime
Mm'       135
5                     lifetime
The  dead  volume  was  used  to  compute  the  lower  level  reachable  by  the  reservoir  during  operation  and  for the setting of the hydraulic works Intakes.
ECOLOGICAL FLOW LOSSES
An  ecological  flow  release  was  foreseen  for  the  energy  production  simulations  ,  in  order  to  avoid  the  river stretch between the dam and PH becoming completely dry if all the available discharge is turbinated.
As can be found in the reference document:
cj 2003 World Bank, Water Resources and Environment, Technical note C. 1
in the present reservoir simulations, the ecological flows  were considered to be released according to the
folk)wing rule;
IF
IF
Qu. > AAF
10% AAF < Qin < AAF
>>>
>>>
>>>
Qr« = 20%
Qico - 10%
AAF
AAF
IF
0 < Qis < 10% AAF
Qku = Qw
where;
o Qrco Is the ecological flow to be released;
a Qin Is the inflow entering into the reservoir;
c AAF is the Average Annual Flow ol the river, equal to 145.6
m3/s at ch -130 and 151.3 at ch -108.
E VAPQRATiON-PREO PITA R ON
The  open-water  of  the  exposed  reservoir  surface  is  subject
to  direct  evaporation  and  to  the  contribution  of
The rainwater which, for large reservoir extensions such as tho
se found In the Dabus alternatives and especially 
ch -130, are not negligible ana could vary the regime of the Inflow
s by up to 2O%/25% in one month.
The following table lists tiie average preapitatton/evaporation bala
nce on a monthly basis:
Preaptiboq/FvKmtian
over cwt
FEB    MAR   *PR
MAY
i rm'
JUN      JUL (rr-TiJi
AUG
SEP Tin)
iJnW
OCT
lirrn i
DEC
min    MAX
YEAR
RAJN
R'APORATIGN
JAN
(/nnl
3
3
i™j_
&l
NOV
'“r'r
nrr
124       130
23
1S2
I’M        115
187      303       327       331   ’ 32!            148        24          3
96
a?
87
90
93
105       115
BA1ANCE
421       42/        -129        03        72        207       240       246       231         55        ■81       412
i™1! -
3
87
332 1737
152 1337
100 WHO R SPO039 (PRE-FEASIfllUT* Report)
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cage69
WATERWAYS HYDRAULIC SIZING AND HfAD LOSSES
Head losses In a water conveyance system are the result of energy dissipation as a consequence ot the flow
□f   water   across   sudden   geometnc   variations   (concentrated   losses)   and   friction   along   the   waterways (diSnbuted losses),
Since the losses are related to the flow speed, they depend on the waterways size and discharge., the hydraulic calculations and waterways sizing and resulting construction cost are further explained in the related chapters m the following.
IMWATEP, LEVELS
A  preliminary  assessment  of  the  tailwater  levels  at  the  powerhouse  locations  for  ch  -130  and  ch  -100  was done.
The evaluation was needed In order to define the:
■    available geodetic head for each plant, allowing for a correct energy production assessment;
■    vertical setting of the turbines, once the submergence requirements are calculated.
The tailwater levels were assumed based on:
■    river elevations, deduced from the Shuttle topography;
■    downstream hpp (ch -iOS) reservoir regulation;
■    turbined discharge.
The following operating taihvater levels were assumed:
■    Ch -130 normal TWL:                   1230                   masl
■    Ch -108 normal TWL:                   1007                   masl
GENERATION EFFICIENCY
Generation  efficiency  takes  into  account  the  product  of  the  efficiencies  of  the  various  items  of  equipment involved »n energy production, /.ft:
• turbine efficiency;
■ generator efficiency;
« transformer efficiency.
Turbine  efficiency  vanes  slightly  with  the  variation  of  the  head  and  turbined  discharge  with  respect  to  the nominal value of the ctesign point. Normal turbine efficiency values are around 0.95.
Generator and transformer equipment, being electro-mechanical equipment, are characterized by  almost constant and dose to unity  efficiency  values. In particular, the values  0.985 and 0.995 can be assumed as efficiencies for generator and transformer respectively.
k
130 WHO R SPOWfl (PR£ FFASI03LTn Report]
100 WHO R SP 0030
pirtwigefi, rorrw*MfiuiHPft________________ PRE-FEASIBILJTYSTUDY Report____________ _______ page 70
During  these  simulations, a constant efficiency value of 0.92 was conservatively adopted to  take  Into account global generation efficiency involving both turbine, generator and transformer efficiencies.
ROAD RELOCATION
While the reservoir at ch - IDS would have a limited impact on the surrounding environment due to Its extremely reduced area (approximately  3.6 km2} the alternative at ch *130 would have the major Issue of partrally inundating the national road connecting the cities  of Asosa and Mendi, which crosses  the Dabus  over at chainage 144 km.
The reservoir affects  a road stretch of approximately  2 km m length; the Asosa-Mendi connectKin could be restored by  rehabilitating an existing 30 km lone way  consisting partially  of an unpaved road, but in good cordition (on the left side ol the river) and partially by off-road vehicle tracks.
r
The following map show the inundated road and the proposed relocation far the ch -130 reservoir;
733; & / Jt?
flft |X4 T7CW PfWPOSAi
The cost far road retocation/rehabilitabon has  been assumed to be 0.75 M$/km; such value should be considered only as a gross index cost in order to allow the companson of the alternatives as explained in the foltowing,
BoO miGOSTS
In order to allow the sensitivity analysis t0 carr-ed
3 and cdst ewaluat W was
|
for
main elements directly or indirectly involved In the HPP cascade, Ze:
IDG WHO ft Sr-:- 't
P'SZHTlTT’-- ppp™-.
10OWHORSP D03B
itvd»c pteirangeh. fumeDAfS/5 HPPs
PffF- FEASIBILITY STUD Y Report
71 ol 169
■    Dam and spillway
•     Waterways-
o Headrace tunnel
q Surge shaft
o Penstocks
•     Powerhouse civil works
■    Powerhouse EM equipment
-     Road relocation
The BoQ estimate was  done considering the main items  and/or construction material for each segment of the cascade excavation, concrete, reinforcement steel and formworks  for the concrete works) as  better defined below; the cost assessment was  done by  applying the relevant unit prices  from Studio Pietrangelos extenave database and selected on the basis of the similarity and magnitude of the works.
In addition to the main cost estimation, a lump-sum miscellanea stem was Included in the order of +10+15%
over the costs summary. Moreover, contingencies were induded as an additional 15% ever the whole cost
Because the quantities were estimated as gross values and the unit prices were taken from statistics from
similar projects, the plant cost estimate Is purely indicative and should not be taken as definitive, but it is still
useful for comparison purposes between different solutions.
In  the  following,  a  bnef  list  of  the  preliminary  BoQ  main  item,  detailed  far  each  element  of  the  cascade,  Is provided:
RCC alternative dam
•     foundation excavation
-     main body RCC
■    u/s + d/s formworks
•     concrete reinforcement sled
•     miscellanea
CFRD ALTERNATIVE DAM
•     foundation excavation
•     tout-venant rockfl II unit price
■     selected rockfill
•     upstream concrete facing
•      plinth and wave-wall concrete
•     concrete reinforcement steel
■     miscellanea
SPILLWAY
■    foundation excavation
■    concrete
100 WHO R SPQ03S (PRE-FEAS1KLJTY Report)
lOOWMO R SP0038
itudio pietJingeb, romeDABUS UPPs
PR&FEASIBIUTYSTUDY Report
page72 of i69
■ concrete reinforcement steel
• miscellanea
HEADRACE TUNNEL
• underground excavation
■    concrete
■    concrete reinforcement steel
■     miscellanea
SURGE SHAFT
■    underground excavation
■    concrete
•     formworks
■    concrete reinforcement steel
•     miscellanea
PENSTOCK
■    foundation excavation
■    basement concrete
•     concrete reinforcement sted
•     miscellanea
POWERHOUSE CIVIL WORKS
■    mdec cost (as detailed below)
EM EQUIPMENT
■    index cost (as detailed below)
ROAD RELOCATION
• index cost (as entailed below)
The following table moreover lists the adopted unit prices adopted as relevant values for the above items:
EXC_UP
7+125
i/m3
excavation unit price
UNDEREXC UP
40
S/m3
underground excavation unit price
CLS_UP
150+175
$/m3
concrete unit pnee
CLSJtEINFJJP
1300
i/ton
concrete reinforcement steel
FORMJJP
55+60
$/m7
form works unit price
RCC-UP
90
$/m’
RCC unit price
100 WHO R SPOOLS (PRE^FEASianJTY Reoort)
1MWHORSPOOJB
s^xlio pietrang^, romeDABUS HPPs
PRE-FEASIBILITY STUDY Report
rckf_tout_up                             12
RCKF.SELECTEDJJP                    29
CLS_FAQNG_UP                           216
PSTK_STEEL_UP                          2500
PH-CIVILJJP                                  30
$/m3 Wm2 $/ton %
EM_ EQUIP JJP ROAD_UP
ECONOMICAL ANALYSIS
1.1948 * Ipo-^MS
page 73 of 1^4
tout-venant rockfill unit price
selected rockfil! unit price
upstream  concrete   facing  unit  price (CFRD DAM)
penstock and lining sted unit price powerhouse avil works
(percentage over EM equip, cost) Electro*  mechanical  equipment  cost (IP - installed power in MW)
D.75                   MS/km                 road rehabilitation unit price
Th.-s paragraph presents the preliminary economic analysis criteria adopted for the Dabus alternatives.
The evaluation of a certain investment ?n any economic approach envisages the comparison of income and
expenditure at drfferent dates by discounting the relevant amount to a certain date in accordance with a
normative interest rate called the "'discount rate".
The value obtained is called the Present Value (P.V.).
For  a  discounting  rate  r,  the  cost  Cr   disbursed  or  received  in  the  year  is  discounted  to  the  year  0  by  the equation:
The evolution of inflation dunng the project's  lifetime and its  effect on each component of the cost estimate is  practically  impossible to establish. To overcome this  problem a common and simple economic  approach, In this  preliminary  phase, based on a constant market price system has  been applied and it will have the same effect in any monetary flux,
In order to determine the profitability of each project the following indices have been computed:
-      the  INTERNAL  rate  of  RETURN  (1RR)  that  is  the  discount  rate  rf   at  which  the  present  value  of  the penodic  benefits  is  equal  to  the  present  value  of  the  initial  Investment.  In  other  words,  the  method calculates  the  rate  of  return  an  Investment  Is  expected  to  yield.  The  criterion  for  selection  between different   alternatives   is   to   choose   the   Investment   with   the   highest   rate   of   return.   A   project   is convenient when its IRR is higher than the opportunity cost of capital in the country.
-      the  payback  period,  consisting  in  the  number  of  years  required  for  the  Invested  capital  to  be  offset  by resulting  benefits.  However  the  payback  period  does  not  allow  the  selection  from  different  technical solutions   for   the   same   installation   or   choosing   among   several   projects.   In   fact   It   does   not   give consideration  to  cash  flows  beyond  the  payback  period,  and  thus  does  not  measure  the  efficiency  of the investments over Its entire life;
the  NET  PRESENT  VALUE,  which  is  (be  difference  between  revenues  and  expenses  discounted  at  a fixed, periodic interest rate.
100 WHO R 5POC3B (PRE-FEASIB1LTTY Report}
100 WHO R SP 0030
srud*c pi^tf.irwj*|t , romcDWUSHPPs
PRE-FEASieiLfTY STUDY Report
74 erf 169
The formula fex calculating net present value, assuming that the cash Hows occur at equal tme intervals and that the first cash Hows occur at the end of the first period, and subsequent cash How occurs at the ends of subsequent periods, Is as follows:
-(/ + 0i 4 MJ 
t
(l+ry
where:
*     Il - investment in period I
■    Ri = revenues In period I
■    01 - operating costs in period i
■    Mt = maintenance and repair costs In period i
•     Vr = resadual value of the investment aver its lifetime, whenever the lifetime of the equipment Is 
greater than the assumed working life of the plant.
■    r = periodic d:scount rate
■    n = number of lifetime periods
The economic study was carried out for a 50*year operation period.
The annual operation and maintenance expenses  for the plant have been computed based on the following construction costs  percentages., according to a statistical international experience reached in similar projects around the world:
■    0.5% of the civil works construction cost
■    1.5% of the EM eQuip.
For the analysis, the following energy selling pnee and annual discount rate were considered;
EP
c$/kWh
8
energy selling price
r
%
8
discount rate
7.3.3 ALTERNATIVES
The methodology and design critena desenbed above was used to compare the cost-effectiveness Df different solutions varying the dam type and dam height.
Jn particular, the considered dam type were
•     Roller-Compacted Concrete Dam (RCC)
•     Concrete-Faced Rodcfill dam (CFRD)
The investigated reservoir elevations ranged, for the two locations.
- Ch -130
from 1360 to 1385 m.asl, at 5 m step Intervals 
■ Ch ’108
from 1220 to 1250 m.asl, at 5 m step intervals
LOO WHO R hPn03B(P<?E-FEASlB- LrTY
r
100 W^IO ft $P O03B
studio pietrangplir romewjs hpps ____________  __________ PPE-FEASI81UTY STUDY Report __________  p*^75on^
An additional solution was found for the ch -130 Site, with reservoir maximum elevation al 1373.7 masl, since this level is the first which allows a full regulation of the incoming discharges, with no spillway losses and while keeping a constant turbimng discharge. Reserve*' levels  below such value provide an insufficient reservoir volume with the consequence of having some spillway losses.
The following tables lists the investigated alternatives at ch -130 and ch -108:
Ch -13d ALTERNATIVES
TMfrZXZ*              23J7 A/tarrMfivus-
ALT,
DAM TYPE
RES. MAX&L
1AX
BX
CX
DX
EX
FX
GX
fem) </wjs7
1360
1365
1370
11737
B75
1380
IMS
1
A
X
1360
“ B
X
1365
C
X
1370
0
X
1373.7
E
X
1375
F
X
1380
G
X
1365
Ch 408 ALTERNATIVES
rj£»ip 7 ? tf               Ch -108 A/tem#font
KIT DAM TYPE RES. MAXO.L
1
A
X
X
X
122 D
1225
1230
X
X
X
1235
1240
f
5
1245
1250
Bw Slewing table and graph sumranze the results of the economical analysis for both the sites, showing the:
■    operating minimum and maximum reservoir levels and tailwater level (TWL);
■    dam height and type;
■    resulting turbined discharge as from the reservoir routing simulations;
■    produced energy;
■    plant cost;
joo who a 5.P003B (PRE-FEASIBILITY Report'
100 WV«ORSP 0030
Wucto pie&angell, ron’#dabus hpos_____________________ PRE-FEASI81UTY STUDY Report_____________________ 76 of its
* economic parameters like internal rate of return (IRR), the payback penod (PB} and the sum of the benefit-cost net present value (B< NPV).
7.1                t7? - J ffCC A*.T? fa™™- AtjVj# .<c,<sd.'&*
Ch -130 RCC
ALT.          OLl-2N
tm)
Ol_M*J
Ln>)
Hdam
t«T>)
DAM
W
Qt’.m twl      E
C-3wr/¥)
COST
(Mi) _
IRR
TO
PB
Wl
B-C_NPV
mti. □roll
AIL 1A
1352,9
1360.0
40
RCC
100     1230
483
23?
13.2
6.2
211
AIL IB
1354.6
1365.0
45
RCC
150     1230
755
297
15.7
5.1
413
A11.1C
1356.2
1370.0
50
RCC
190     1230
1023
359
17.3
4.5
616
Alt ID
1357.6
1373.7
54
RCC
240     1230
1293
422
18.4
4.2
819
Alt. IE
1360,4
1375.0
55
RCC
240     1230
1311
431
18,3
4.2
827
Alt IF
1369,8
1300.0
60
RCC
240     1230
1378
467
17,8
4.4
851
AIL1G
1377.0
1305.0
65
RCC
240     1730
1426
511
17,0
4.6
845
X J <              Or -/Jiff jff£7C frwxw And/i^ A\e£l*sf qm&h
& J30 CFRD Diim Eronwvc Anafy&sflesj/fo
Ch 130 CFRD
ALT. OLmjn OLmv Himm DAM Qrjia TWL E COST IRR PB B-C_NPV
(m)
(*n)
fni)
i-)
(m3/s)
[r-jnd'i
(GMvri
— V "J
lYJ[Mil—
Ait. ? A
1352.9
1360.0
40-
CFRD
100
1230
483
229
13.4
6J 
214
AJC2B
1354,6
1355.0
45
CFRD
150
1230
755
293
15.9
5.0
419
100 WHO Cl SP003B (PRE-FEASJHlLihr Repot*}
1 OCi WHO ft Sl> 00JB
STUfltO pletranyrii, ronvDABU5 HPPs
PRE-FEASZBILTTY STUDY Report
page 77 cf
A|t,2C
1356.2      1370.0
50
CFRD      190     1230      1023
351
17.6      4.4
625
Alt. 2D
1357.6      1373.7
54
CFRD     240     1230     1293
•112
18.7       4J
832
AR.7E
1360.4      1375.0
55
CFRD     240     1230     1311
420
18.6      4.1
840
AIT.2F
1369.8      1380.0
60
CFRD    240     1230     1378
453
18.2       4.2
869
AJt.ZG
1377.0      1385.0
65
CFRD    240     1230      1426
497
17.5       44
869
7_J_5; O? -J 30 CTRD SAM Fcpfwmic Ana^w ^estiVily graofi
fjf^e Z J. /Jr
eft Jffl? jffciC iCtjjriu favx’htm;- Ana^ksrj’ flesu/tr
Ch -10B RCC
ALT.          OLhp,        OLmax     Hqam     DAM                 TWL          E          COST         1RR        PB     B-C_NPV
L™1
(H]
■ ->j
II
[masO
ISW-.Y)
(M»
(%j
(v)
-- -- ■(M- VTJ
Alt.lA
1217.5
1220.0
75
RCC
250
1006
2036
311.8
32.7
1.9
1739
A1L1B
1220.4
1225.0
80
RCC
250
1006
2087
324.1
32,3
2.0
1776
Alt.lC
1224.1
1230.0
85
RCC
250
1006
2140
337.3
32.0
2.0
1815
Alt. ID
1230.5
1235.0
90
RCC
250
1006
2194
351.1
31.6
2.0
1855
Alt. IE
1235.1
1240.0
95
RCC
250
1006
22S0
367.2
31.2
2J
1393
AIL IF
1240.0
1245.0
100
RCC
250
1006
2308
384,4
30.8
2J
1932
All.lG
1245.0
1250.0
105
RCC
250
1006
2367
403.4
30.2
2,2
1970
IDO WHO R SP
OOja [WLk^tASl
BiLITY Report
}
IDO WH
ORSP 003
0
S^U
d’D plWTHliJCb.DABJS HPPi
78 of 169 page
-
7.4 DABUS CASCADE
7.4-1 GENERAL
In the previous chapter, a comparison based on economical parameters was done for selected dam type and height alternatives at the sites. The assessment has helped in the choice of the most promising and profitable layout.
By looking at the above graphs and tables, the following considerations can be remarked:
Ch -130
The  ch  -130  dam  site  is  characterized  by  a  wide  valley  with  fairly  good  geostructural  conditions,  which  are adequate for both a RCC or CFRD dam type. Both the alternatives were therefore investigated.
The economic analysis showed a substantial equivalence of the solutions on the economic point of view, with negligible differences on the plant profitability. Therefore, a design with a RCC dam was selected, since such solution is more safe and provide far easier maintenance operation and costs.
The alternatives  with reservoir elevation below 1373.7 masl, given the incoming flows  vanability, cannot be fully  regulated with a constant output discharge at the turbine and are characterized by  progressively increasing spillway losses (and loss of revenues) as the reservoir max O.L. Is lowered. Such alternatives have a low IRR index  and a low benefit-cost difference. The cost-effectiveness  of the HPP rapdly  grows  with the reservoir max O.L up to the first solution which allows enough reservoir volume for the full-flow regulation.
100 WHO « 5POO36 (PRE’FtAStBk rrv Report)
100 WHO ft 5P 003 B
studio pieirargoh, fameDiets MPPs
PRE-EEASiSJUTY STUDY Report
79 of W
Such alternative is the one with the highest internal rate of return (approx.. 18%) and the lowest payback 
period (appro*.. 4 years). Increasing the reservoir level would entail an increase in the initial investment costs 
with limited economic benefits (slightly increasing payback period and lower IRR even if the sum of the
discounted benefit-costs, /.* net present value, is slightly greater at the end of the plant lifetime).
For such reasons, the following combination of dam type and reservoir maximum operating level has oeen
selected;
■ DAM TYPE:                         RCC
* RES. MAX. O.L:                   1373.7                 masl
QtUJM
The ch -108 dam site is  characterized by  a deep and narrow valley  with outcropping bedrock. The conditions are ideal for the construction of a concrete dam. The sensitivity  analysis  of the plant cost-effectiveness  as  the O.L max vanes shows that such HPP u very profitable, for several reasons
■  The  available  geodetic  head  is  relevant  (approx..  220  m  considering  the  head  provided  by  the  dam height);
•        The   discharges   are   already   regulated   from   the   upstream   HPP,   allowing   for   reduced   reservoir oscillations and a constantly high head;
•     The construction costs are reduced, despite the large amount of installed power.
The internal rate of return for all the investigated alternatives  are very  large and ranging between approximately  30% and 33%, with a short payback  period of 2 years; the highest IRR Is  obtained with the lowest dam height but more revenues  are produced during the plant lifetime if the maximum operating level is raised.
For this  reasons, the alternative with the reservoir maximum operating level at 1230 masl, Le. the tapwater level of the ch -130 PH, was  chosen as  the optimal solution and best compromise between IRR index  and benefit-cost difference.
7.4.2     SELECTED LAYOUT
A  layout  with  both  RCC  dams  was  chosen  and  further  developed  after  completion  of  the  economic  sensitivity analysis.  While  at  ch  -106  an  inline  spillway  In  the  main  dam  body  was  the  only  convenient  technical  solution, at  ch  -130  two  different  spillway  alternatives  (Ze.,  inline  spillway  Into  the  dam  body  as  alternative  1,  and  a lateral  spillway  into  an  adjacent  valley  as  alternative  2)  were  studied  from  a  technical  point  of  view  only,  since the costs are quite similar.
The selected layout can be found In the following drawings, furtherly shown hereafter for convenience: 100 GEN D SP 002 General Layout, Plan ISOk
100 GEN D SP 003 General Layout Profile
'too WHO R 5P0U3B (PRE-FEASIBLIITY Rfipon:-
LOO ’WHO R SP QC3B
SLudns pIc’Lrangel'. rompDABU5 HNS
PRE-FEASIBIUTY STUDY Report
^ag<* 80 cf 169
■
■
II
II
■
■
■
■
Z< J.           Germ/ bxwrf-
100 WHO R SP003B (PRE FEASI8(LITV Repot)
IDO WHO R SP 0013
studio ptfcrangeii, rome
■
■
■
■
II
h
h
MDAfiUS HPPs
PR&FEASI8/LJTYSTUDY Report
page  81 of 169
A*W1? Z <2-               ijenea/ J>>wf PQGfTLf
10G WHO H 5POO2B (PR.E-FEASIBJUTY Report >                    iOC WHO R 5P 0016
p.etrangclr, runeMAUS *iPPi
PRE-FEASIBILTTYSTUDY Report
pw    82 or 169
Pie plant's main features are briefly summarized in the table below and described In the following paragraphs o' the next chapter:
TadW                         daficzs cmum/p. Man Charxtemtm
Dabus ’ 130
Dabus’ 108
aft. 1
aft. 2
unit
1
1
HYDROLOGY
10,009
10,400
»t |C
j
dtchment Area
146
1S1
mtys A
verage Net Annual Hew
4,592
4,766
Mm’ty A
mnual Runoff
4M
450
’runty
13744
1375.7
12312
m iii
WOO
1373,7
1230.0
m a.s.L
4m 0.1.
1357.6
1224,1
t a.s.l.
4«n O.L
1342.0
1202.0
m svsJ.
DEAD
2470
53
Mir*
Tot. storage (IIP Maa c.L)
2335
IB
Mm'
Live storage (Max o.L, - DfAD)
DAM
RCC, gravity
RCC gravity
RCC, gravity
-
Type
1376
1376
1232
m a.5.L
Cam Crest elevation
1322
1322
1150
•D fl,3d.
FoundatKxi min. elcv^tiar’'
55
55
02
m
Max, height
1610
1610
444
fh
Crest length
BOTTOM OUTLET
n. 1 F«ed Wheel Gates / n4 ftadial
ptes
-
Gate number, type
32
mi
Mil. gross head
SPILLWAY
2160 (10,000 yr)
2160 (10.00D yr)
2160 (10,000 yr)
m’/s
Design flood (TH)
overflow
Lateral 5{>Hway
overflow
-
Type
rwriral
Gate? i12.5x6 m)
Un-gMrd
Un-gated
-
Gate number and type
ski jump +- plunge
POO*
Stepped
Stepped
-
Lnergy dissipator
i INTAKE STRUCTURE |
1
1
Type, n«nb*<
IM2
1205
m a.s.1.
Sill elevation
n. ) Bulkhead gate7 Fixed Wheel
Gate
n, 2 Bulkhead gale i 2
Fixed Wheel Gate
Gates number and type
POWER TUNNEL
rt l/9m
n. 1 / 9
*, m
Number, internal diameter
5.500
6,200
m
Length
SURGE SHAFT
79
73
rn
Height
16
16
FT
Diameter
100 WHO R SP003A (PRE-FEA5JB1IJTY Report)
:00 WHO R 5P Q03E
soidJD pietrangclir ramrDABUS HPPs
PR£~FEASI8JLJTY STUDY Report
page 83 of 169
1
PENSTOCKS
2
2
-
Sled penstocks num.
7
5
m
Diirrwter
2100
970
rn
Length
POWERHOUSE
Own Air
Open Air
-
Type, num
45 x 85 m
4S x B5 m
m
Sue
Biplane Vafaev
& plane Vaim
-
Inlet Valves, Type
r_2/ 4.2
n.2 / 4.2
£, m
B, Diameter
n.2
Francis / n 2
■s
Turbines, *
Frinris
•
Type
1230
1007
rt-L
tf/s WL (Francs) / CL el. (Pelton}
POWER
149
230
hi
Z-ecdetic Head
143
221
ffl
Design Head (Max Net Head )
240
250
nt1.'*
Design flow
Plant Factor
304
494
HW
"nfuled Power
131
2D
ffl
Average iwt •’wead
1293
2140
GW* , f
j Annual Energy Productxjn
ENERGY PRODUCTION
As explain^ previously, an energy production and reservoir operation simulation was earned for each studied alternative.
In the following tables and graph, the energy production assessment of the selected layout for the ch -130 and ch -108 alternative is reported:
100 WHO R SPCCJH (PRt FEASIBILITY Report)
100 WHO R SP DD3B
stud*- oirtrangeli, rwnpDABUS HPPs
PRE-FEASIBILfTY STUDY Report
page 84 bF 169
1’5
eM
s
> I I ■<
1
WHO R SPM3B (PRF^EASIBILFTY Reports
Z < J; l?, -JJ0 Erwij) powvclw
100 WHO R SP (XBE
studio pctrangell, fameDABUS hop.
PRE-FEASIBILTTf STUDY Report
*
n MtH8WH
r? *M J-
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-
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7,5 EM EQUIPMENT
TURBINE SELECTION
The  reservoir  operation  studies  determine  the  plant  capacity  and  indicate  the  head  at  which  the  capacity should be developed.
According to preliminary studies the total rated discharge (Qrat) and rated head (Hrat) for each hydropower plant are the following;
HPP ch - 130:
HPP ch - 1 OB
Qrat= 240 ma/s 
Qrat- 250 m3/s 
Hrat = 136 m
Hrat = 215 m
The rated head of the hydropower plant usually determines the selection of turbine type to be adopted. The graph below shows apphcabon range of the turbines, depending on head.
spec Ilk ipr pd
/v-r 751
Appbcatwi ranges of different types of water turbines, Source Votth Hydro, Inc
As may be seen from the above graph:
- in ch - 130, Francis turbines are the most suitable
in ch - 106, either Francis or Pelton turbines are suitable
However, in ch -108 hpp the specific speed (nq) of the Pelton turbines will be quite high (low head application) with respect to their range, implying that design of the units will not be optimi2ed. On the contrary, the units of ch - 106 hpp will be in the optimum range for Francis turbines.
In  our  opinion  the  most  suitable  type  of  hydraulic  turbines  for  ch  -130  hpp  and  ch  -103  hpp  are  Franas turbines.
Plant RaSng
The preliminary selection of the turbine is very important because it affects the efficiency of the hydraulic units (qt), over the whole operating range.
The preliminary total Installed Capacity (IC) of each plant can now be determined, as follows:
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IC = p g Qrat * Hrat • Tjt • rjg • qtt
IC= 297 MW for ch - 130 hpp
B7 0/ 169 P*J«
IC= 482 MW
for ch • l08 hpp
where:
p = density of water (at 25°C),
99/ kg/m3
g = acceleration due to gravity,
9,804 n?/s
Qrat = rated discharge in mVs,
ch - 130: 240 m Vs 
ch - 108: 250 mJ/s
Hrat rated head <n m
ch - 130: 138m
th - 108: 215 m
qt - turbine effkiency in %,
qg “ generator efficiency in %,
94.0%
98.0%
qtt = transformer efficiency in %r
99.7 %
Plant Factor
The Plant Factor (PF) Is defined as the railo of the average yearly generation of the plant to the total Installed Capacity of the plant.
PF= EN (GWh] / ( 1C [MW] * 8760 [h])
PF = 0,5
PF ■ 0,5
for ch - 130 hpp
for ch - 108 hpp
where
EN = electricity generated in
GWh for a gaven penod (one year).
From the simulation of the
reservoirs' operation, each power plant will generate the following yearly average
energy:
ch - 130 hpp:
EN= 1293 GW year
ch ■ 108 hpp:
EN= 2140 GW year
The PF of each power plant usually defines the plant's role in the interconnected system. A PF of about 0.5 usually means that the plant will contribute to the necessity of the intermediate-load.
Number of Units
To select the most appropriate number of generating units for the given head and total flow conditions, the following has to be taken into account:
□    operational flexibility, the availability of the installed capacity and the efficiency at partial loads increase with the number of installed units;
□    the cost of equipment and power house civil works increase with the number of units;
1
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o the unit capacity should not exceed that capacity which, if lost from the system due to an unplanned shutdown, would cause destabilisation of the network;
o transport limitations may Impose some restrictions on the S2e of the equipment.
in accordance with specfic technical parameters of ch - 130 and ch - 108 (head and flow variations, characteristics of the waterways, usual load etc.), a tendency towards fewer units should be investigated Installation of only one larger unit is not recommended (or reasons of lower operational flexibility.
In our opinion the most promising alternatives in terms of number of units are the following:
-     Alternative A;
2 units fev each hpp
-     Alternative 0:                               4 units for each hpp
As already argued, the capital cost per kW for a given hydropower plant generally decreases with fewer units. Moreover, fewer units can result in lower civil works cost and higher unit efficiency implying higher revenue during the economic life or the power plant.
The preliminary comparison between the two alternatives are given In the tables below.
It is worthwhile recalling that the major technical and economic parameters of the Electro-Mechanical (EM) equipment have been calculated and reviewed together with a leading manufacturing supplier.
fjZw Z£ /; t/i - iefrCEKvr .Vurnter of Un/fr
ch - 130 EM PRELIMINARY STUDY ABOUT THE SELECTION OF NUMBER OF UNITS
ch ■ 130, EM Technical Parameters
Item
Alt A - 2 units
Alt B - 4 units
Description
T\pe_
Francis
e
Type of turbine
Layout
Vertical
<-
Arrangement of turbine (horizontal/vertical)
QPH Rat
240 rr^/s
e
Total Rated Discharge through the PH
Qu Rat
120 m^s
60 m3/s
Rated Discharge of each unit
I
H Rai
H des
131 m
131 m
Design Head
136 m
138 m
Rated Head of each unit
nt Rat
94,09%
94,06%
Rated Efficiency of each unit
1C PH
30. MW
304 MW
Installed Capacity in the PH (Turtxnes)
IC
152 MW
76 MW
Rated Capacity of each turbine
In -                -
187,5 rpm
272,7 rpm
Rotabonal speed
na
S3
55
Specific speed (nq=n Q!-5/ Hdes*,,J)
D3
3,7 m
2,6 m
Runner diameter
He
■3.6 m
-4,1 m
Submergence (unit below the TWL)
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1
20 m
15 m
Unit centreline distance
Ch ■ 130, E
MEconomy Parami
eters
Item
Alt A - 2 units
Alt B-4 units
Description
EM Cost
Ref
+ 21600 k USS
Additional   Budget   costs   for   EM   Equipment foreseen in PH
cv; cost
Ref
- 100 k US$
Decrease of Civil Works Costs for PH.
EN sell.
Ref
-20 k US$
Decrease of yearly Income coming from energy sell (hp tariff - 0.05 US$/kWh)
TiJn? Z5.Z-
eft JOR SetertJprt of Number of Uruts
CH 108, EM PRELIMINARY STUDY ABOUT THE SELECTION OF NUMBER OF UNITS
CH 108 - EM, Technical Parameters
Item
Alt A - 2 units
Ait B - 4 units
Description
Tjype
Francis
4-
Type of turbine
Layout
Vertical
e
Arrangement of turbine (horizontal,-'vertical )
QPH Rat
25Q mLs
<-
Total Rated Discharge throuqh the PH
Qu Rat
125 m’/s
62,5 mJ/s
Rated Discharge of each unit
H des
213 m
213 m
Desiqn head
H Rat
215 m
215 m
....
Raied Head of each unit
qt Rat
94,12%
93,99%
Rated Efficiency qf each unit
IC PH
494 MW
494 MW
Installed Capacity in the PH
IC
247 MW
123,5 MW
Rated Capacity of each unit
.n
200 rpm
300 rpm
Rotationaljipeed
nq
40
43
Specific speed (nq-=n O°' / Hdes?   !
s                    74
D3
38m
r
2,6 m
Runner diameter
Hs
-55m
-6,4 m
Submergence (unit below the TWL;
I
20 m
16 m
Unit centreline distance
CH 108, EM, Economic Parameters
Item
Ait A - 2 units
All B - 4 units
Description
EM Cost
Ref
+23800 k USS
Additional Budget costs for EM Equipment foreseen In PH
CW Cost
Ref
•22B k USS       Decrease of Civil Works Casts for PH.
EN sell
yearly
Ref
+■ 135 k US$ Increase of yearly Income coming from energy 
1 sell (hp tariff = 0.05 U5$/kWh)
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From a technical point of view, it can be noted that the difference in terms of specific speed (nq) between alternative A and alternative B is very small. This fact implies that turbine efficiency and submergence between alternatives are very similar
From an economic point of view, In the case of Alt. B with 4 units, the decrease in the cost of the power house civil works and the additional Increase in terms of income from the sale of energy (if any) are very small If compared to the additional Investment for the EM equipment.
For each plant foreseen, the performance of the generating units of Alternative A (2 units) and Alternative B (4 units) are very similar: the design of the units Is well optimized for either alternative. However, as expected, the alternative with the higher number of units has higher investment costs, more than 21 M US$ for each project.
It is  worthwhile recalling that the alternative with a higher number of units (4) will assure more operational flexibility- If the Client prefers  to have such flexibility  from ch - 130 hpp and ch - 108 hpp, there will be additional investment costs mainly due to the EM supply.
In our opinion,  we believe that  operational flexibiirty could be achieved through GERDp HPP  operation (16 units, 375MW each), about ISO km from ch - LOB plant.
With the tentative date of first operation, say 2021, Dabus Cascade would be connected to the national grid providing an average power output of about 400 MW. According to the development plans and considering the recently implemented HPPs, the national grid will have more than doubled Its generating capacity from about 2100 MW in 2012 to more than 12,000 MW in 2020, disregarding minor and dispersed plants.
Assuming conservative availability rate values (outage and programmed maintenance rates included) of 0.65. and a spinning reserve of 0.15 (corresponding to about 1170 MW, say  15% of the on-line power), the frequency primary regulation will be supported by roughly 6,600 MW. Thus, the selection of Dabus Cascade unit splitting does not seem constrained by load rejection issues, since the loss of the bigger unit rate ~250 MW corresponds to less than 4% of the system regulating capauty (
At  this  stage  of  the  study,  the  loss  of  the  bigger  plant  (say  less  than  8%)  of  the  two  does  not  seem  to compromise the static regulation asset of the system.
Our  preliminary  calculations  and  considerations  have  shown that  from Technical,  Economic  and  Operational points of view, the most promising number of units for each PH is two (2).
7.6 TRANSMISSION LINE SYSTEM
Dabus Cascade power output will be evacuated by means of HV transmission lines, overhead type. Terms of connection to the Ethiopian HV grid are evaluated on the basis of:
* a review of the available existing development studies
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•
the objectives of the assignment Ze. identification of design mam aspects and issues, together with economic  viability  of the proposed scheme for promotion of regional cooperation and increasing power trade with neighbouring countries
•           Dabus Cascade project's preliminary components and ratings
•             evaluation  of  adequate  transmission  infrastructures,  coordinated  with  the  existing  corridors  for reliable and stable power transfer into the grid and increased efficiency and electrification rates
The prefeasibility study includes a review of available previous schemes oy third party, a technical assessment of the feasible interconnection schemes  by  considering also preliminary  economical parameters, geological and environmental main constraints.
The Consultant highlights the high reliability and efficiency of the schemes under AIT_2 (4OOkV) and ALT_3 (500kV)r even though more expensive than ALT_1 (230kV), nevertheless  tailored to the country's  ongoing expansion plan at EHV le^cl.
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8 Dabus ch -130
8.1   SELECTED LAYOUT
pnge 92 of 169
Dabus ch - 130 is the first plant of the Dabus river cascade (304 MW).
The energy production and the installed power of the plants in the cascade arc summanzed below:
HPP                   IP (MW)
E (GWh/y)
I
Dabus ch - 130
Dabus ch — 108
304 1293
494 2140
L
The ch - 130 plant fully  regulates  the (lows  coming in to the reservoir (4590 Mm3/y); consequently, the downstream plant (ch - 100) will have a much smaller reservoir volume (2470 Mm3 - V_130 vs  53 Mm3 V 108) and will operate mostly as a run-of-the-nver plant.
The following figure show the general layout of HPP - 130.
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111? Jj
1- -
t
*
5fft/rr tf. J. 1:                  ch *130 GftterJ t ayout
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0
8.2   RESERVOIR
The reservoir formed by the dam at chainage - 130 has the following main elevations:
cf\ 13Q r^ervo/r Atm Volume
EL
Area
Volume
m.a.s.L
Km2
Mm3
1374.1
170
2530
Flood
1373.7
166
2470
MAX oL
1357.6
63
730
MIN oL
1342
132
Dead
The Area - Volume curve 15:
MnJ
7M0
&M.n
iaa.n
Kc-a
3041
UM
tn
uu
T
IJ21
Ajurr* £.2.1;
- Volume Curve
8.3   DAM AND AUXILIARY WORKS 8,3.1     SELECTED DAM TYPE
The dam will be an RCC GRAVITY DAM with a straight axis.
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Two alternatives have been selected:
-    alt 1 w*th central spillway in the dam body;
alt. 2, with lateral spillway.
The other options Investigated and then discarded, were:
-     DAM MATERIAL TYPE
□ a ROCKFILL DAM TYPE was studied (see previous chapter) and discarded since the cost of the rockfill solution was com para We to the RCC solution, the latter (gravity dam) is preferable given that the rockfill dam is much more vulnerable in the event of overtopping, thus requiring a very safe spillway system.
o CONVENTIONAL CONCRETE DAM TYPE was discarded In respect of the RCC type because of the: higher cost of CVC with respect to RCC, In particular the cost of the cement;
- RCCs higher placement rate.
DAM GEOMETRY TYPE
o ARCH DAM TYPE was not considered because the geology and geomorphology of the valley are not adequate for an arch dam.
The following drawings:
-     ISO DAM D SP 002 - alt. 1 DAM, Plan 5k;
150 DAM D SP 003 - alt 1 DAM, Profile;
-     150DAM D SP 004   - alt 1 DAM, Typical Sections; 150         DAM D SP 005 - ait. 2 DAM,    Plan 8k;
-     150DAM D SP 006   - alt 2 DAM, Profile;
ISO         DAM D 5P 007 — alt. 1 DAM,  Typical Section; illustrate the dam's main features.
100 W HO ft 5POa,10 (PRE-f EASIBCUrr Repot)
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■
MIl
£ J. Jr & -j j0 j
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Fgwrt ftXZ tit - J10 m 2 fam fan A*
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DABUS UM'S
B.3.2 SELECTED FOUNDATION
PRE-FEASIBIUTY STUDY Report
PHQC
In this initial design phase (Pre-Feasibility Study), the dam foundation has been Identified on the basis of the following:
• geological survey with examination of No. IS rock outcrops (photographic documentation was collected of each outcrop);
- collection of No. 8 rock samples;
thin  sections  of  No.  5  samples  with  petrographic  analysis  by  microscope  (realized  by  the University of '‘Roma Tre*, Department of Geology);
No. 7 geostructural stations, with a total of almost No. 130 Joint measurements;
No, 4 trial pits, ranging In depth from 0.6 m to 3.2 m;
No. 6 seismic lines with SASW technique.
The results of the investigations have been summarized in the Chapter 5 and descnbed at length In report 130 GEO R SP001A (GEOLOGICAL Report) and may be briefly described as follows:
- from 2/4/6 m Weathered Bedrock;
3/5/10 m Fresh Bedrock.
So an average foundation depth of between 5 to 7 m has been assumed.
B.3.3 SELECTED AUXILIARY WORKS
SPILLWAY
Two spillway types are being proposed.
The spillway is designed to discharge the peak of the un-routed flood for RP = 10,000 years, Q   xj  = 2160
]W
m /s, without considering the routing effects of the flood Inside the reservoir.
J
- alt. 1. SPILLWAY on the dam body
The high-flow evacuation system composes the following hydraulic elements:
o  an  ogee-shaped  overflow  crest  with  sill  elevation  at  1366,7  m  a.s.l.  and  a  total  length  of  90 m (gross of piers, net of abutments) divided into six bays controlled by radial gates 12.5 x 6 m each;
o a chute on the d/s slope divided into six canals through side walls;
° a flip bucket energy dissipating device, located at the end of the chute;
o  a  plunge  poo!  on  the  river  bed,  designed  to  receive  both  the  spillway  discharges  and  the middle-outlet jet.
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The spillway  will be divided In six  channels, with a rectangular cross  sectional shape that can be operated separately  allowing therefore remarkable Flexibility  for operation, inspection and maintenance.
Each section has a radial gate 12.5 m wide and 6 m high (with stop logs and gantry crane). The external abutments and the piers will be nose-shaped In order to limit flaw contraction and optimize the hydraulic efficiency of the spillway.
The stop logs will be moved by means of a gantry crane with travelling hoist beam. The gantry crane will mn on dedicated rads along the horizontal curve of the crest, following the crest bridge alignment. The stop legs will be sufficient In number to allow one gate to be out of service.
The spillway structure ends with a flip bucket designed to:
o partly dissipate the energy in the air,
□ impact the over bed f plunge pool*) downstream of the dam toe far enough to protect agarnst
scouring effects.
The spillway is illustrated m the drawings:
g 150DAMSP002 - alt. 1 DAM, Plan 5 K
o 15QDAMSPOO3 - alt. 1 DAM, Longitudinal Profile
d 150DAMSP004 - alt. 1 DAM, Typical Sections
- alt. 2. Lateral SPILLWAY
The second alternative exploits a saddle on the right abutment between 1374 and 1380 m a. si. The flow discharge system comprises:
q 470 m of unllned canal excavated along the natural saddle:
o 200 m of united cement overflow sill at el. 1373.7 m a.si;
o 23 km erf un-lined, stepped-bottom canal.
The spillway Is Illustrated in drawing 15ODAMSPQO5 — alt. 2 DAM, Wan 8 K.
BOTTOM OUTLET
The  main  function  of  the  bottom  outlet  will  be  the  drawdown  of  the  reservoir  in  an  emergency  or  for extraordinary maintenance and for controlling the rate of reservoir Impounding.
The overall length of the outlet, from the entrance to the exit from the closed conduit will be about 30 m. The invert level al the entrance is 1348 m a.sl, being the level in correspondence to the dead volume in order to guarantee the outlet functioning during the overall lifetime of the dam.
The inlet will be horizontal with a hydrauheaify shaped roof, bottom and lateral walls in order to avoid flow separation and risk of vortex and introduction of air into the flow causing vibration and cavitation problems. Trash-racks are envisaged at the entrances respectively to prevent accidental entrance of large submerged debris and for safety reasons.
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The entrance will have a rectarxjuiar section 4.4 m wtde and 6 m high. The Het section will be reduced with a triton 2.4 m long from rectangular to a 4,4*4.4 m square section. A 3.6 m long transition has  been foreseen from the square to circular section. The bottom outlet conduit will be about 18 m long with a diameter of 4,0 m.
The last stretch will be about 10 m long and will host the wheel gate and the radial gate, moved by means of an oleo-dynamlc servomotor. The flip bucket, with jet impacting into the plunge pood, has been chosen as a dissipation device.
8.3.4 RIVER DIVERSION
The nver diversion system will provide a "reasonable protection" against the Hooding of the area on which the Dam are built, that will be dewatered for the period of their construction.
The selected DESIGN FLOOD (Tr = 20 yrs) for the diversion structures Is equal to 1,100 m3/s.
The MAIN WORKS for the river diversion structures are sketched in the drawing 150 DAM S SP 002, and indude:
DIVERSION STRUCTURE, composed by:
o Headrace canal;
c Culverts;
o Tailrace canal.
-     U/S COFFERDAM
D/S COFFERDAM
The  diversion  structure  is  approximately  360/460  m  long  and  foresees,  from  upstream  to  downstream,  the following elements;
-     HEADRACE CANAL
A headrace canal excavated In rock approximately 100 m long.
CULVERTS
The No. 2 culverts have a rectangular section 7 m wide and 9 m high. The structures are approximately
110 m long. Part of the culverts will be then incorporated in the dam body.
TAILRACE CANAL
A tailrace canal excavated in rock, approximately 250 m long for Alt. 1 and 150 m for Alt 2.
No. 2 cofferdams are foreseen to keep the central part of the riverbed dry during plant construction.
The UPSTREAM cofferdam will have a maximum height of about 9 m with crest level at 1335 m a.s.l., in order to allow the design flood to pass in the diversion structure without overtopping.
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The DOWNSTREAM cofferdam instead will have a maximum height of 4 m with crest level at 1330 m a,$l, to avoid overtopping by the tail water level corresponding to the design flood.
8.4   WATERWAYS
8.4.1      INTRODUCTION
The present paragraph Illustrates the general layout of the Dabus ch 130 power waterways system. The waterllnes are located on the left side and rhe system comprises the following structures:
r
Intake with trashracks;
power tunnel
-     gates with related shaft;
surge shaft;
-     penstock;
manifolds.
The  general  layout  and  main  geometrical  features  of  the  power  waterways  are  shown  In  the  fallowing drawings:
-     150 WWY 0 SP 001. PLAN AND PROFILE 150 WWY D SP 002 TYPICAL SECTIONS
r
Here below are illustrated the preliminary calculations and results concerning:
-    intake submergence;
power tunnel;
hydraulic transient analysis and surge shaft design;
-     penstocks;
head losses calculation.
8.4.2    INTAKE SUBMERGENCE
The geometrical features of the intake are illustrated In in the drawing;
150 WWY D SP 002 A, TYPICAL SECTIONS
The sill elevation of the intake has been fixed in order to assure a good submergence with the minimum water level and to avoid vortices that may:
produce non-uniform flow conditions;
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- Introduce air Into the flaw, potentially treating non ideal operating conditions for hydraulic machinery 
(e$. vibration, cavitation);
Increase head losses and decrease hydraulic efficiency;
produce high internal pressure into the tunnel or in correspondence of the approach walls,
draw debris into the intake.
Several formulas have been developed that define the submergence required for the intake. ASCE (1995) indicates a criterion based on Fraud number and on internal diameter of the waterway.
Referring to the figure here below and considering a symmetrical approach flow, the following formula has been used to define the minimum Intake submergence:
V
S = 1.7 -== D
where
• D - 9,0 m, is the tunnel diameter;
■ V - 3.8 m/s, is the velocity related to Che design flow of 240 m /S.
3
The minimum intake submergence obtained is 6.1 m
B.4J POWER TUNNEL
The total length of the power tun nd, from the intake to the surge shaft, will be approximately 5 SOO m. The tunnel invert wiN vary between el. 1343 m a.sl at the intake to el 1321 m a.sl at the surge shaft, having a longitudinal slope of approxrmately 4 m/km, The tunnel section will be circular with an internal diameter of 9 m and an available flow section of 64 m\
The maximum flow velocity is 3.7 m/s (with the max discharge of 240 m /s}.
The layout of the power tunnel and transversal section are shown in the following drawing;
-     150 WWY D SP 001 A, PLAN AMD PROFILE 150 WWY D SP 002 A, TYPICAL SECTIONS
2
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H.4.4 SURGE SHAFT DESIGN AND HYDRAULIC TRANSIENT ANALYSIS
The Hydraulic waterway system has been provided witi> a surge shaft, with the aim of reducing the amplitude of pressure fluctuations by reflecting the incoming pressure waves and providing stability to the system in respect to the small load variation.
A preliminary analysis has been performed in order to evaluate maximum and minimum water level In the surge shaft and to assess the proper orifice size, connecting the surge shaft to the tunnel.
The size of the surge shaft has been chosen In order to maintain the water level oscillations within reasonable limits under transient events.
A throttled type surge shaft has been considered in order to improve the efficiency m damping oscillation due to load rejection or toad acceptance.
An orifice, placed in the T junction between the final section of the power tunnel and the shaft itself, can increase the damping effect of friction losses.
The geometrical features such as dimension and shape of the orifice are to be selected In ocde^ to maximize the damping effect of the shaft without excessively increasing (filling) or decreasing (emptying) the pressure in the tunnel.
A preliminary transient analysis has been performed In order to determine the inertial effects of the tang water column of the waterways an the plant, for the design and operation purpose.
The worst case dunng operation in generation mode is  the load rejection. The turbine runaway  must be controlled by dosing the wicket gate m a relatively short time. By dosing the wicket gate, a hydraulic transient is created at the turbine inlet, with an upsurge wave \hp on the penstock.
+
Upsurges on the penstocks have been preliminarily evaluated through the classical Alliezi-Michaud formular
Where:
p
water density
L         (m)             penstock length
v»       (nVs)        water speed
T         (s)               dosing time
The following graph shows the overpressures obtained at turbine inlet for different flow vetodty and relevant penstock area.
200 WHO k SPOO30 (WE-FEASTHIlTTY Report;
100 WHQRSP 003B
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PRE-FEASIBILITY STUDY Report
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The selected design overpressure of the penstock is 90 m w.c. corresponding to about 60% of the gross head.
Duong the dosing manoeuvre that can occur for sudden Interruption of power generation, the most onerous mass oscillation phenomenon occurs in the shaft reaching the maximum water level.
The maximum oscillation obtained in the surge shaft has been evaluated through the classical formula (water losses neglected) ( ref: D. Otnn/ G. Noseda, Idrauhca, 1987}\
f
where:
V&
(m/s)
water speed in the tunnel
Ag
1
Cm )
tunnel area
Lfl
(m)
penstock length
Ao
3
(m )
surge shaft area
AHq
<5)
head losses In the tunnel
The  following  graph  illustrates  the  variation  of  the  shaft  diameter  with  the  maximum  oscillation  varying the power tunnel velocity without throttle orifice and neglecting the water losses.
100 WHO R SP003B (PRE-FEASIBILITY Report)
IM WHO R SP 00?B
studio pi«4r«ngd< ronifDAflUS hpp,
PRE-FEASJBILITY STUDY Report
peg? 1 OS Bf I'*?
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The maximum oscillation obtained In the surge shaft with optimum throttle onto, has been evaluated through the classical formula (water losses neglected) (ref: D. Gtrinf, 6. Hosed#, Irfrax/to, /9£7):
where:
Vo
(m/s)
water speed m the tunnel
Ag
(m )
1
tunnel area
U
(m)
penstock length
Ap
(m )
?
surge shaft area
AHq
(5}
head losses in the tunnd
The optimum throttle onflce design has been established considering that the pressure in the tunnel quiddy reaches 2?*. Therefore, in the tunnel a pressure equal to the hydrostatic level plus rhe head losses through the orifice (£*) is established.
Taking  aJso into  account the head  losses  In  the pressure tunnd, a further decrease  of is obtained, as per the following formula (rtf' tx de Madefy Idtaufica, 1961):
where:
AHtt (s) head losses In the tunnel
IX WHO R SP00J6 (PRE4EA5IBILTTY Report)
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PRE-FEASIBILTTY STUDY Report
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In order to obtain the optimum balance between volume of the shaft excavation and a feasible diameter of the shaft structure, the maximum water oscillation has been established as 35 m.
The required shaft diameter is 16 m with an optimum throttle of 3.4 m.
8.4.S PENSTOCKS
Two  steel  lined  penstocks  are  foreseen  after  the  surge  shaft.  The  total  length  of  each  penstock  from  the  surge shaft to the turbine will be approximately 2.1 km.
The  penstock  inverts  will  vary  from  el.  1321  m  a.s.l.  at  the  surge  shaft  section  to  el  1225  m  a.s.l,  at  the turbine section. The penstock section will be circular with an average internal diameter of 7 m.
The  diameter  has  been  selected  with  the  criterion  of  keeping  the  overpressure  not  much  greater  than  60%  of the gross head. The maximum flow velocity Is 3 m/s for the maximum discharge.
During  the  next  phase  the  penstock  design  will  be  optimized  with  a  diameter  varying  in  relation  to  the  Increase of the hyarostabc head.
The layout of the penstock (Ze. plan, profile and sections) Is shown In the following drawings:
-      150 WWY D SP 001 A. PLAN AND PROFILE
150 WW D SP 002 A, TYPICAL SECTIONS
The figures in the following pages show the water system plan, longitudinal profile and transversal sections.
100 WHO R SP003B (RRLTEASISILTTV Report)
100 WHO R 5*0036
Studio CM'trangeli. ronwOABUS
PRE FEASIBILTTY STUDY Report
PJQt 107 rj
!MWK)R SPOOtt rPfiE-FEASJBJLTTY Report^
3.3.J; Ch-130.WWfPtirlM
100 WHO R 50 0C3B
shjdjCi ptartngHi.page 108 of 169
DABUS HPPs
8.4.6 HEAD LOSSES
PRE-FEASIBILTTY STUDY Report
The  preliminary  head  losses  in  the  power  waterways  system  and  relevant  formula  adopted  are  shown  here below.
’jM- S.4 ! Wtienvav* Head Lasses
discharge
Qmax      m3/s
240
INTAKE
AHlnt        m
Trashrack
Inlet
Contraction
0.08
0.02
0.02
0.04
va
AH = K —
fr           t
V2
AH  - —
ln
_ K k_d
f  ho zj
TUNNEL ( D =■ 9 m/L = S.5 km)
Gates
Contraction
Distributed
AHtun      m
3.89
0.02
0.03
3.84
V1
AH, = S T-
2
" I2*? 2H
Dig
PENSTOCK ( D = 7 m, L = 2.1 km)
AHpen     m
Bifurcation
AH^, = K  jL
u
Distributed
D2jf
Bends
V2
AHh.r =
VALVE
AHval       m
Machine valves
1.58
0.25
LIB
0.15
0.02
0.02
va
A^L. = Jf„ -
OUTLET
AHout      m
Draft tube
Outlet
0.15
0.09
0.06
IJ J
A "° - “■ z£
p3
AH     s Kjjjjf —— 
flu£
TOTAL HEAD LOSSES
AHtot       m       ,5.7
----------
J
---------------------- I
8.5 POWER HOUSE, CIVIL WORKS
8.5.1 GENERAL
The powerhouse wilt be a conventional surface outdoor type with vertical axis units located on the left bank of the Dabus nver, approximately W km downstream the dam area.
I JO WHO R SP003B fPRE-FEASTBlLTTY RepCKE)
100 WHO R SP 3O3B
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PRE-FEASIBILTTY STUDY Report
109 * w
This chapter describes the powerhouse civil works mdudlng the following preliminary aspects:
• Geolog/
-     General arrangement of the powerhouse
Substructure layout
-     Superstructure layout
The arrangement of the electromechanical equipment and the hydraulic steel works is described in the relative chapters.
The layout of the powerhouse Is shown in the following drawings: 150POV/DSP001 Horizontal section ®K3eneratoc
r
15QPOWD5P002, Transversal section ^Turbine.
IQO WHO R SPOOJla (PRE FEASIBILITY Report)
too WHO R SP DC3B
sfjdio p^var^H, romeDA&USHPPs
PPF-FEASISIUTY STUDY Report
W 110 of 169
100 WHO R SPD038 (PRf-FEASIBILITY
100 WHO RSP Q03B
Studio plrtrgngell.
■
■
■
■
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ii H
II
I
I□ABUS HPPs.
0.5,2 GEOLOGY
PRF-FEASIBILITY STUDY Report
112     169
The  geology  of  the  site  was  investigated  during  the  survey  earned  out  in  April  2016  during  which  large  rock outcrops were surveyed in the power house area.
The  results  of  the  survey  are  summanzed  in  the  chapter  5  and  described  in  length  In  the  report  130  GEO  R SP001 A.
In  this  area  the  rock  is  basic  intrusive,  dark  greyr   ranging  from  tonalite,  to  diorite  and  gabbro.
The rock is very strong to extremely strong. The degree of weathering Is very low In the outcropping rock.
Pie excavations will be In sound rock.
Systematic  reinforcement  (mesh,  spntz  beton  and  bolts)  is  foreseen  on  the  excavated  slope  and  spot  bolting where necessary.
The geological situation Is favourable for the construction of a surface powerhouse.
8,5.3 GENERAL ARRANGEMENT
The powerhouse will be located along the Dabus river left bank.
Its   longitudinal   axis   will   be   in   a   NW-SE   direction,   approximately   perpendicular   to   the   alignment   of   the penstocks.
The steel penstocks will provide the connection between the waterway and the surface powerhouse.
The  layout  Is  illustrated  In  the  following  drawings  enclosed  iin  *  Vol.  2  of  2  -  DRAWINGS
iSOPOWDSPOOlf Horizontal section (©Generator;
150POWDSP002, Transversal section ^Turbine.
The main structure will be approximately 85 m long and 4S m wide.
The  mam  floor,  corresponding  to  the  access  level,  will  be  at  el.  1237.0  m  a,si,  whereas  the  roof  will  vary between ef. 1258.5 and 1259.0 m a.s.l.
In  order  to  exclude  the  risk  of  flooding  of  the  and  possible  damage  due  to  extreme  floods  PH  main  floor  will be hrgher than maximum flood level (flood with Tr - 10000 years):
PH mam floor elevation
= 1237.0 m a.s.l.
100 WHO Ft SPW.1B (PRf -FEASIBIl 1TY Report}
100 WHO R SP M30
jrudlo
rtwtK-□abus
- Hood level (Tr = 10000 y)
PR&FEASIB/UTYSTUDYReporl
- 1234.5 m a .si
[hFjr 113 erf
The powerhouse will be divided In three bays  housing the two generating units, one in each bay, and the erection bay. The erection bay, at the St end of the powerhouse, will be approximately  35 m long and 45m wide.
The  control   rooms   will   be   located  at  the  WE  end  of  the  superstructure,  whereas  the  auxiliary  electncal equipment will be at the east end.
The two step-up transformers (175 MV A ) will be located outdoor in the backyard.
0.5,4 SUBSTRUCTURE LAYOUT
The  substructure  of  the  powerhouse  basically  comprises  the  part  of  the  structure  that  Iles  below  the  main  floor (1237.0 m a.sl) with the deepest point at el. 1207.5 m a.s.l.
It will be a massive concrete block which will house the valve rooms, the spiral cases and the shafts.
The substructure will also Indude rooms  and galleries  needed for the auxiliary  mechanical and electrical equipment and for turbine maintenance including cooling water pumps  and compressors, runner removal access, etc.
The mam elevations of the substructure will be:
Turbine axis at elevation - Valve room floor at elevation
1225.0 m a.s.1.; 1220.0 m a.s.l;
Runner removal access at elevation            1220.0 m a.s.l;
- Foundation slab at elevation
1207,5 m a.s.l
As illustrated In the previous chapter the powerhouse will be founded on sound rock.
8.5.5 SUPERSTRUCTURE LAYOUT
The superstructure of the powerhouse basically  indudes  that part of the structure that Kes  above the ma.n floor at elevation 1237.0 m a.s.l. inducing the exterior walls, services  and control rooms, and the roof at devatwn 1258.5 to 1259,0 m a.s.l
The  height  of  the  superstructure  is  determined  by  the  maximum  clearance  height  required  for  moving  and dismantling the major items of equipment.
This structure will be constructed entirely in reinforced concrete
1 on WHO * SPCOJB (PRE-rtASIBIUTY Report)
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The control rooms will all be concentrated at the end of the structure, minimizing cable lengths.
The electrical equipment will be distributed all along the structure, in steel structures accessible from both sides.
The auxiliary equipment will be distnbuted In the space available all along the power house at the lower elevations (generator, service, turbine, shaft level).
8.6 EM AND HM EQUIPMENT
8.6.1     INTRODUCTION
The preliminary concepts of the Electro-Mechanical (EME) equipment and Hydro-Mechanical equipment (HME) foreseen m ch • 130 hpp are described here below.
The aim of this preliminary design study is to define the main features and dimensions of the major equipment required for safe and economic operation of the hydropower plant.
The dimensions and layout are preliminary and will be confirmed at a later stage of the design.
8.6.2    MECHANICAL EQUIPMENT
(1)   TURBINES
jumper of units (NJ
As already discussed at Chapter 7, a two-unit installation has been selected for the ch • 130 HPP as being the most appropriate solution from a technical and economic point of view.
DfiSflnHfidSinifeJ
The design head is the Net Head at which the turbine efficiency Is maximum.
In ch - 130 hpp the design head has been established equal to the weighted average head (from energy generation simulation):
Hdes = Hwgen = 131 m
Rateo Head fHrat)
The  rated  bead  Is  the  Net  Head  at  which  the  full  gate  output  of  the  turbine  produces  the  generator  rated output.
In ch - 130 hpp the rated head has been established equal to the maximum head: Hrat = Hmax- 138 m
109 WHO R SP0C3B (PRF-FEASffiriTTY Report)
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studio jn.ji'%, rvr-eDABUS HPPs
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Design Discharge (Odes)
The design discharge foe ch - 130 can be achieved as follows:
Qdes = Qtot / N = 120 rn’/s where
Qtot = total discharge of the hpp, 240 rrP/s
N = number of units, Turbine SdectKin
2
The selection of the turbine and its  arrangement Is  very  important to ensure the success  of the hydropower plant from an economic  and operational standpoint The diagram below, courtesy  of Vatech, Indicates  the type of turbne depending on head, discharge and turbine power data.
In case of ch -110 hpp: Qdes- 120 rrr1/!
Hdes= 131 m
1
A A A  Seferttw? of fi/rtwe f>pe Artemy ft? iVrf /ftwtr m Dtstfwrye  Courtfery £/ kJ rfTWJ
r
The plant parameters are suitable for Francis turbines.
For  the  head  range  and  power  output  of  the  ch  -  130  hppr   large  numbers  of  installations  similar  tn  size  have been designed, constructed and commissioned by various manufacturers worldwide.
Turbine Characteristics
The table below summarizes the major technical parameters of the hydraulic units foreseen In ch - 130.
100 WHO R SP0C3B fPRf-FEASIBILITY Report)
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Studio fwtrangdi, romcmbushw____________ _____ pf!E-FEASiBiLirrsn!PYR_eport_
ch -
130, MAJOR TECHNICAL TURBINE PARAMETERS
Type
Francis
Type of turbine
layout
Vertical
Arrangement of turbine (horizontal/vertlcal)
Qu Rat
120 mVs
Rated Discharge of each, unit
H des
131 m
Design Head of each unit
H Rat
138 m
Rated Head of each unit
qt Rat
94,00%
Rated Efficiency of each unit
ICu
152 MW
Rated Capacity of each unit
n
lB7 5 rp-m
r
Rotational speed
nov
272 rpm
Over speed
nrun
334 rpm
Runaway speed
nq
53
D 5               0 75
Specific speed (nq=n Q - / Hdes - )
Hs
*3j6 m
Submergence (unit below the mini mum TWL)
PD2
1200 rm2
Runner Rywheel
ch - 130, MAJOR TURBINE DIMENSIONS
1
Turbine
D3
3,7 m
Runner diameter
11
.— -J-T- — 
‘ -T-J
L
_,
Spiral Case
I
A
4,2
Inlet
1
IDO WHO SPOOJB (^FEASIBILITY Report}
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P4<?e 117 of is9
(2)   MAIN INLET VALVES
Each unit of the ch ■ 130 HPP will be provided with a shut-off valve installed at the spiral case inlet to isolate the turbine and stop the unit If the turtune wicket gates  fad to dose. Furthermore, the shut off-valves  will allow inspection and maintenance operations without dewatering the penstock.
The  available  head  of  ch  -  130  hpp  Is  suitable  for  the  selection  of  the  butterfly  valve,  biplane  type. Because  of  their  compact  and  simple  design  and  low  cost  (2Q%-30%  less  than  Spherical  Valves},  butterfly valves are usually used for projects where beads reach about 250 metres.
Des/gri Data
o   Rated  flow
o   Rated  diameter
o   Static   pressure
o   Maximum pressure (including overpressure due to water hammer)
(3)   GOVERNORS
Each turtune will be equipped With a digital, microprocessor type PID governor.
i jo
4.2 m
b4 MPa
2.3 MPa
The  turtune   governor   system   will   be   with   pumping  units  and  pressure  accumulators,  to  operate  the  wicket gate in the event of electricity blackout
130 WHO R SPOOJB (PRE-FEASTBILTTY Report)
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page   118 or IM
(4)    BALANCE OF PLANT (BOP) MECHANICS
The ch ‘ 130 hpp will include the following auxiliary mechanical equipment: Coding Water System
Drainage and Dewatering System
Heating Ventilation and Air Conditioning (HVAC) Systems Pre Detection and Suppression Systems
-     OH Handling system
Compressed Air System
-     Power House Machine Hall Cranes
any other equipment necessary to meet the functional requirements of the power plant
8.6.3    ELECTRICAL EQUIPMENT
(1)    GENERATORS
The two (2) generators  will be of the 3-phase, synchronous, vertical shaft type directly  coupled with the
Pranas hydraulic turbines. The major, preliminary features of the generator are the following:
Ch ’ 130, MAJOR TECHNICAL GENERATOR PARAMETERS
Type
Salient pole,
Suspended
Type and layout of the Generator
Ph
3
Number of phases
h  Wye (six terminals)
Connection diagram
ICu
152 MW
Rated Turbine Output
A
175 MVA
Rated Generator Output (RGO)
V
__
1
0.85 lagging
15 kv*
Rated voltage
P.F
Rated power factor (RPF)
f
50 H2 ±2%
Rated frequency
N poles
32
Number of poles
n
T
157,5 rpm
Synchronous speed
qt Rat
98,0 %        1
Minimum efficiency at rated conditions
never
272 rpm |
Overspeed
---------- ------------------------------------------------------------- 1
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119 of 169
nrun
334 rpm
Runaway speed
GD2
not less than 15000 tm2
Mininum Required Generator Ry-wheel
■ If the Client wishes to standardize the Medium Voltage of ch - 130 hpp to ch ■ 108 hpp, the rated generator voltage could be increased (£e 16kV),
Each gerrerator will be equipped with static excitation systems and automatic voltage regulators.
(2)   STEP UP TRANSFORMERS
The ch - 130 HPP will have step up transformers from which High Voltage (HV) lines will depart to link the PH with the HV Switchyard. The major technical parameters  of the step up transformer (type, if 3-phase or a bank 
□f   single  phase  transformers.   cooling   medium,   winding  connection,  etc,}  will  be  defined  during  the  further stage of the design, as tailored to the selection of the transmission scheme.
It  Is  worthwhile  recalling  that,  presently,  three  different  alternatives  are  proposed  for  the  transmission  system, based on 230 kV, 400 kV and SOO kV voltage levels.
Once the Client selects the desired voltage level, SP will optimize the design of the step-up transformers.
(3)   BALANCE OF PLANT (BOP) ELECTRICS
The ch - 130 hpp will include the fallowing auxiliary electrical equipment:
Two (2) auxiliary service transformers
Two (2) lots cf generator Isolated bus ducts
Two (2) generator switchgears, including circuit breakers
One (1) Emergency Diesel Generating unit
-     LV AC auxiliary power supply
DC power supply system
Unintemiptabte Power Supply (UPS) system
protection, control and monitoring equipment
-     telecommunication and control systems
one (I) distribution tone to the dam premises, where both auxiliary system and a
any other equipment necessary to meet the functional requirements of the power plant
8.6.4 HYDROMECHANICAL EQUIPMENT
(1)    WATERWAYS
Two  steel  penstocks  wit  feed  the  two  turtles,  Each  penstock  will  have  a  nominal  d.ameler  of  7.0  m  and  it Will be about 2100 m long.
100 WHO R SP003B (PQE-FEA51BILirr Repot)
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rompDABUS HPft
PRE-FEASIBIUTY STUDYReport_
P*9*    120 of 169
The  maximum  design  head,  including  allowance  for  transient  behaviour  of  turtxnes,  will  correspond  to  2.3  MPa
approximately.
The maximum flow through each penstock will be 120 m’/s.
A minimum of thickness of 18 mm for the Hning will be required.
(2)    SPILLWAY
In the present report two alternatives of Spillway are proposed for ch - 130 project:
•     Alternative 1: Gated Spillway (shown In drawings 150-Dam->SP-002, 003 and 004)
•     Alternative 2 Ungated Spillway (shown in drawings 150-Dam-D-SP-005 and 007)
If the Client selects Alternative 1, the spillway will be equipped with the following hydro-mechanical equipment:
-     Six (6) radial gates 12.5 wide and 6 m high, to maintain the highest level m the reservoir and to discharge a maximum flow of 2160 m3/s dunng the flood event (QlOOOOy).
-     Two (2) sets of stoplogs, that have been foreseen to carry out maintenance activities on the gates.
One (1) Spillway Gantry crane, which will be used for erection, operation and maintenance of the
spill way stop logs.
(3)  BOTTOM OUTLET
The bottom outlet will be located in the dam body on the left of the spillway blocks.
The  bottom  outlet  has  been  designed  to  drawdown  the  reservoir  water  level  from  1373.7  to  1342.0  m  a.s.l. m case of emergency.
The bottom outlet will be equipped with:
One (1) set of Steel Lining, including transitions
One (1) set of Emergency  Gate (type: Fixed Wheel Gates), which shall be capable to dose under the full flow of the bottom outlet in case of any  failure of the downstream service gate. The gate will be operated by oil pressure servomotor.
One (1) set of Service Gate (type: Radial gates), which shall be able to operate in unbalanced condition and to regulate the flow through the bottom outlet. The gate will be operated by  two oil pressure servomotors, one for each arm.
(4)   INTAKE
The  intake  of  the  ch  -  130  will  be  a  concrete  structure  that  will  divert  water  from  the  reservoir  to  the  power waterways. The intake structure is designed to divert a maximum flow of 240 m /s.
The intake will be equipped with:
Two  (2)  sets  of  Trashracks.  Each  trashrack  is  foreseen  to  be  cleaned  with  a  cleaning  machine  installed on the plateau at an elevation of 1375.0 m a.s.l.
- one Trash Rack CJea n I ng Machine
Two (2) sets  of Maintenance Gates  (type: Bulkhead gate). The main purpose of the bulkhead gates will be to Isolate the turbine unit and tunnel from the reservoir and allow the inspection of the waterways, intake gates, shut off valves and turbines.
J
100 WHO R SPO03B (PRE-FEASIBILITY R«p<xt)
100 WHO R 5PO030
tf'Jdi? plrlrangeii, romcDABUS MPPs
S7W K Report
W*      121 of 1W
* Two (2) sets  of Intake- Gates  (type: Freed Wheel Gate), These gates  will also allow the maintenance and inspection of the headrace and to act in case of emergency and shutoff the flow In case the wicket gates  rail to close under the maximum head and maxi mum power (typical case, load rejection under maximum output,
-  One  gantry  crane  structure  which  will  be  designed  for  the  lifting  equipment  of  the  wheel  gates  and bulkhead gates.
(5)  DRAFT TUBES EQUIPMENT
The powerhouse draft tubes will be equipped with following hydro-mechanical equipment:
Two (2) sets  of Draft Tube Siding Gates  far each unit. The main purpose of the sdide gates  will be to allow the maintenance and inspection of the draft tube. Each sliding gate will be operated by  oil pressure servomotor.
’ Two (2) sets  of Draft Tube Stoplogs, to allow the maintenance of the draft tube, draft tube gates  and the unite. The stop logs  will be made of multiple elements. Each element will be designed tn be lowered and raised by suitable lifting beam operated by the Draft Tube gantry crane.
-  One  (1)  Draft  Tube  Gantry  crane,  which  will  be  used  for  erection,  operation  and  maintenance  of  Che draft tube stop logs.
100 WHO R 5P001B fPRE-FEAS[03JTY Repart}
!GO WHO R SP QQ3B
stud® plrtrar^i, rorrw*Dabus ch - 108 is the second plant of the Dabus river cascade (404 MW),
The energy production and the Installed power of the plants in the cascade arc summarized below:
7jjWf0J,r
HPP
If» (MW)
E (GWh/y)
Dabus ch - 130
304
1293
Dabus ch — 108
494
2140
The ch - 108 plant exploits the entire inflow regulation of the reservoir at ch - 130. The ch plant therefore will operate mostly a$ a run-of-the-river plant.
The following figure show the general layout of HPP ch - 108.
100 WHO R SPO03H (PRE-FEASIBlLlTY Rewnj
100 WHO R 5P 003 H
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PRE-FEASIBILITY STUDY Report
9.2   RESERVOIR
124 Of 169
Th® reservoir fonned by the dam at chainage — 108 has the following main elevations.
rjftteAZJ;
cff iOS. At?j -
EL
Area
Volume
m.a,s.l.
Km2
Mm3
1231.2
3.8
57
Flood
1230.0
3.6
53
MAX oL
1224.1
2.6
35
MIN oL
1202.0
0.6
5
Dead
The Area - Volume curve is:
9.3    DAM AND AUXILIARY WORKS 9.3.1    SELECTED DAM TYPE
The dam will be ar RCC GRAVITY DAM with a straight axis.
As spillway a stepped spillway on the dam body has been selected. The other options investigated and then discarded, were:
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DAM MATERIAL TYPE
o  a  ROCKFILL  DAM  TYPE  was  studied  (see  previous  chapter)  and  discarded  since  the  cost  of  the rocfcfill  solution  was  comparable  to  the  RCC  solution,  the  latter  (gravity  dam)  ts  preferable  given that  the  rackfill  dam  is  much  more  vulnerable  in  the  event  of  overtopping,  thus  requiring  a  very safe spllway system.
o CONVENTIONAL CONCRETE DAM TYPE was discarded In respect of the RCC type because of the: higher cost of CVC with respect to RCC, in particular the cost of the cement;
RCC's higher placement rate.
The following drawings:
-     160 DAM D SP 002 - DAM, Plan 2k;
160 DAM D SP 003 - DAM. Longitudinal Profile; 160 DAM D SP 004 - DAM, Typical Section.
illustrate the dam’s main features.
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9.3.2      SELECTED FOUND AH ON
In this Initial design phase (Pre-Feaslbillty Study), the dam foundation has been identified on the basis of the following:
geological survey with examination of No. 30 rock outcrops, 14 of them located in the dam and intake area (for each of them a photographic documentation has been collected);
-     collecting of No. 8 rock samples;
-     thin sections of No. 3 samples for petrographic analysis by microscope (realized by the University of "Roma Tre", Department of Geology);
-     No. 18 geostructural stations, with a total of almost No. 450 Joint measurements.
The results of the investigations have been summarized in the chapter 5 and described at length in the report 130 GEO R SP001A., and may be briefly described as follows:
In the whole dam area the rock strength can be assessed as "strong" to "very strong". The weathering grade is generally low to very low.
So an average foundation depth of between 1 to 3 m has been assumed.
9.3.3    SELECTED AUXILIARY WORKS
SPILLWAY
The Spillway system, located in the central part of the dam crest, Indudes:
- ungated overflow crests:
c the sill elevation is 1230.0 m a.s.l. with a length of 440 m;
o A d/s stepped chute (slope 0.75:1);
o Two abutment return channels that convey the flows to the river channel at the toe of the dam;
o A Stilling Basin, the energy dissipating device.
The main geometrical characteristics are detailed in the previous figure.
50HQMQUILEI
Considering that the elevation of the dead capacity Is approximately the same of the invert of the intake, in this particular condition SP has not considered necessary to foresee a Bottom Outlet
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9.3.4 RIVER DIVERSION
PRE-FEASIBILJTY STUDYReport
The  river  drversion  system  will  provide  a  'reascnaWe  protection’  against  the  flooding  of  the  area  on  whtch  the Dam are built, that will be dewatered for the period of their construction.
The selected DESIGN FLOOD (Tr = 20 yrs) far the diversion structures is equal to 1 JOO ml/5.
The  MAIN  WORKS  for  the  nver  diversion  structures  are  sketched  in  the  drawing  160  DAM  D  SP  002,  and Indude:
-     DIVERSION STRUCTURE, composed by:
a Headrace canal;
o Tunnel;
o Tailrace canal.
U/S COFFERDAM
D/S COFFERDAM
The  diversion  structure  Is  approximately  170  m  long  and  foresees,  from  upstream  to  downstream,  the  following dements:
-     HEADRACE CANAL
A headrace canal excavated In rock approximate^ 40 m king.
TUNNEL
A  circular  tunnel  with  a  diameter  of  10m.  The  structures  are  approximately  110  m  long.  Pan  of  the culverts will be then Incorporated in the dam body.
-    tailrace canal
A tailrace canal excavated in rock, approximately 20 m long.
No. 2 cofferdams are foreseen to keep the central part of the riverbed dry during plant construction.
The  UPSTREAM  cofferdam  will  have  a  maximum  height  of  about  12  m  with  crest  level  at  1170  m  as.I.,  in order to allow the design flood to pass in the diversion structure without overtopping.
The  DOWNSTREAM  cofferdam  instead  will  have  a  maximum  height  of  fl  m  with  crest  level  at  1152  m  a.s.l.,  to avoid overtopping by the tail water level corresponding to the design flood.
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9.4   WATERWAYS
130 al IM pay
9.4.1     INTRODUCTION
This section illustrates the general layout of the HPP ch 108 power waterways system.
The walerlines will be located on the nght side, and the system will comprise the following structures: intake with trashracks;
power tunnel
-     gates with related shaft;
-     surge shaft;
penstock;
manifolds.
The general layout of the power waterways Is shown in the following drawings: 160 WW D SP 001 A PLAN AND PROFILE
r
160 WWY D SP 002 A TYPICAL SECTIONS
r
Here below preliminary calculations and results are illustrated, concerning:
-     intake submergence;
power tunnel;
hydraulic transient analysis and surge shaft design;
penstocks;
head losses calculation.
9.4.2      INTAKE
The following formula has been used to define the minimum intake submergence:
where
D - 9.0 m is the tunnel diameter;
(
V = 3.9 m/s, is the velocrty related to the design flow of 250 m /s.
n
The minimum intake submergence obtained rs 6.4 m
9,4.3         POWER TUNNEL
The total length of the power tunnel, from the intake to the surge shaft, will be approximately 6200 m. The tunnel invert will vary between eL 1205 m a.s.L at the intake to el. 1191 m a.s.l. at the surge shaft, having a
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longitudinal stape of approximately 2.2 m/tan. The lurinel section will be circular wW an Internal character of
9 m and an available Haw section af 64 m-.
The maximum flow velocity is 3.9 m/s ( with a max discharge of 250 irr*/s).
The layout of the power tunnel and transversal section are shown in the following drawing:
160 WWY D SP 001 A, PUN AND PROFILE
160 WWY D SP 002 A, TYPICAL SECTIONS
9.4,4    SURGE SHAFT DESIGN AND HYDRAULIC TRANSIENT ANALYSIS
As  with the upstream plant (ch - 130) described previously, the hydraulic  waterway  system will include a surge shaft, to reduce the amplitude of pressure fluctuations  by  reflecting the incoming pressure waves  and providing stability to the system in respect to the small load variation.
A  preliminary  transient  analysis  has  been  performed  In  order  to  determine  the  inertial  effects  of  the  long  water column of the waterways on the piant, for design and operation purposes.
The  following  graph  shows  the  overpressures  obtained  through  the  dassrcal  Al  11  evi-Michaud  formula  at  the turbine Inlet for different How veJootes and relevant penstock areas.
The selected design penstock overpressure Is 90 m w.c. corresponding to about 40% of the gross head.
During the dosing manoeuvre that could occur due to a sudden interruption of the power generation, the
most onerous mass oscillation phenomenon occurs in the shaft reaching the maximum water level.
The  following  graph  illustrates  the  variation  of  the  shaft  diameter  with  the  maximum  osdllabon  Z™  varying the power tunnd velocity without throttle orifice and neglecting the water losses.
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Figure 9 4.2 - Mix osolition vs flow vrfoctty (V^) md r&rvirt pcnstodt irei )
The following graph shows the variation of the shaft and throttle diameter with the maximum oscillation Z^
•i
i
o «- i •
-.
1
.............. ~i - -
-.
o
M» Jl J» U M n A »»
/• Ft M <1 U II M n M >> M M
•
41 U O M 1% 44 •» IB 4»
»
i— H
Ftgurv 9.4.3 - Surge Shifted ttvvttie dtamet^ vs
The required shaft diameter is 16 m with an optimum throttle of 3.4 m.
9.4.5    PENSTOCKS
Two steel lined penstocks are foreseen after the surge shaft. The total length of each penstock from the surge shaft to the turbine will be approximately 1 km.
The penstock inverts will vary between el. 1191 m a.s.l. at the surge shaft section to el. 1000 m a.s.l. at the turbine section. The penstock section will be circular with an average internal diameter of 5 m.
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The  diameter  has  been  selected  to  keep  the  overpressure  not  much  greater  than  40%  of  the  gross  head.  The maximum flow vHodty will be 6 m/s far the maximum discharge.
During  the  next  phase  the  penstock  design  will  be  optimized  with  a  diameter  varying  in  reiat’on  to  the  increase m the hydrostatic head.
The layout of the penstock (Ze. plan, profile and sections) Is shown in the following drawings*
-      160 WWY 0 SP 001 A, PLAN AND PROFILE
160 WWY D SP 002 A TYPICAL SECTIONS
r
The figures In the following pages show the water system's plan, longitudinal profile and transversal sections.
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9.4.6   HEID LOSSES
The  prd»mrnary  head  losses  in  the  cower  waterways  system  and  the  respective  adopted  formtila  are  shown below.
9 4 /-■
Msad fiMSSrt
DISCHARGE
Qmax       m3/s
2SO
INTAKE
Trashrack
Inlet
Contraction
AHrnL      m
0.0®
0.02
0.02
p2
l' 1
=a —
c2j
0.04
^Zcantr
_ K               2*1 ‘ |z,9      ^1
TUNNEL ( D = 9m, L = 5.S km)
1 Gates
Contractron
Distributed
AHtun       m
4.71
0.10
t?2
AH      |
C4J n F
0.03
4.59
-K                 2ll '              2s|
rnjt
PENSTOCK ( D = 7 rn, L = 24 km) Bifurcation
Distributed
Bends
AHpen      m
[4.21
0.24
3.17
.vr-/
0.81
AH   = K, ~
iJf
VALVE
Machine valves
AHval       m
0.14
a.i4
t?2
77
OUTLET
Draft tube
Outlet
AHout       m
0.18
0.03
0.15
^put   Kpur jT
=
TOTAL HEAD LOSSES
AHtot        m         9.3
9.5  POWER HOUSE, CIVIL WORKS
9.5.1 GENERAL
Tne  powerhouse  will  be  a  conventional  surface  outdoor  type  with  vertical  axis  units  located  on  the  right  bank of the Dabus rlvef approximately 6 km downstream the dam area.
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This section describes the powerhouse ovll works including the following aspects:
Geology
General arrangement of the powerhouse
Substructure Layout
Superstructure layout
The arrangement of the electromechanical equipment and the hydraulic steel works are descnbed in the relative chapters.
The layout of the powerhouse is shown in the following drawings: 160POWDSP001, Horizontal section ^Generator; 160POWDSPQ02, Transversal section @Turbine.
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'     ,------- tr
L
L
9 5. J. ch !0S. PH. frannma/ station @ Turtle
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9*5.2 GEOLOGY
The geology of the powerhouse site was Investigated dunng the survey carried out in April 2016, during which
no outcrops of rocks were surveyed in this area, whereas large outcrops were visible on the nver bed.
The results of the survey summanzed In Chapter 5 and described In detail In the report 130 GEO R 5P001A.
The  powerhouse  will  be  located  on  a  small  hill  on  the  right  bank  of  the  river;  this  area  is  characterized  by  a
gentle morphology.
According  to  the  geological  information,  this  area  Is  constituted  by  Precambrian  rocks:  mela-granite  and  meta-
granodiorite.
The excavations will be m sound rode.
Systematic  reinforcement  is  foreseen  a  (mesh,  spritz  beton  and  bolts)  on  the  excavated  dope  and  spot  bolting
where necessary.
The geological situation is favourable for the construction of a surface powerhouse.
93.3      GENERAL ARRANGEMENT
The powerhouse will located along the Dabus river nght bank.
Its   longitudinal   axis   will   be   in   a   NE-SW   direction,   approximately   perpendicular   to   the   alignment   of   the
penstocks.
The sted penstocks will provide the connection between the waterway and the powerhouse.
The  powerhouse  layout  is  illustrated  in  the  following  drawings  enclosed  in,L   Vol.  2  of  2  —  DRAWINGS  '■
-     160POWDSP001, Horzontal section @Generator;
16QPOWOSP0O2, Transversal section ^Turbine.
-me geological and morphological characteristics of the area dictated the general arrangement,
The main powerhouse structure will be approximately B5 m long and 45 m wide.
The  mam  ftogr,  correspond^  to  the  access  level,  will  be  at  el.  1012.0  m  a.U,  whereas  the  roof  will  vary 
between d. 1033.5 and 1034.0 m a.s.l.
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The powerhouse will be divided in three bays housing the two generating units, one tn each bay, and the erection bay. The erection bay, at the SW end of the powerhouse, will be approximately 35 m long and 40 m wide.
The control rooms will be located at the SW end of the superstructure, whereas the auxiliary electrical equipment will be at the east end.
The two step-up transformers (285 MVA) will be located outdoors on the backyard.
9.5.4 SUBSTRUCTURE LAYOUT
The substructure of the powerhouse will basically compose the part of the structure below the main floor (1012.0 m a.s.1.) with the deepest point at el. 982.5 m a.s.1
It will be a massive concrete block housing the valve rooms, the spiral cases and the shafts.
The substructure will also include rooms and gallenes needed for the auxiliary mechanical and electrical equipment and for turbine maintenance Including cooling water pumps and compressors, runner removal access, etc.
The mam ele/ations of the substructure will be:
Turbine axis at elevation
1000.0 m a.s.l.;
-    Valve room floor at elevation
995.0 m a.s.1.;
-    Runner removal access at elevation
995.0 m a.s.l.;
-     Foundation slab at elevation
982.5 m a.s.l.
As illustrated in the previous chapter the powerhouse will be founded on sound rock.
9.5.5 SUPERSTRUCTURE LAYOUT
The superstructure of the powerhouse will basically include that part of the structure above the main floor at el. 1012.0 m a.s.l. Including exterior walls, services and control rooms, and the roof at el. 1033.5 to 1034.0 m a.s.1.
The height of the superstructure is determined by the maximum clearance height required for moving and dismantling the major equipment items.
This structure will be constructed entirely in reinforced concrete.
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The control rooms will all be concentrated at the end of the structure, minimizing the cable lengths.
The electrical equipment will be distributed all along the structure, in steel structures accessible from both sides.
The auxiliary equipment will be distributed in the space available all along the powerhouse at the lower elevations (generator, service, turbine, shaft level).
9*6 EM AND HSS EQUIPMENT
9.6.1 INTRODUCTION
The preliminary concepts of the Electro-Mechanical (EM) equipment and Hydro-Mechanical equipment (HM) foreseen in ch - 1 QB hpp are described here below.
The aim of this preliminary desgn study ts to define the main features and dimensions of the major equipment required for safe and economic operation of the hydropower plant.
The dimensions and layout are preliminary and will be confirmed at a later stage of the design.
9.6.2 MECHANICAL EQUIPMENT
(1) TURBINES
Number otlfrlts (N)
As already dtscussed at Chapter 7, a two-unit installation has been selected for the ch - 108 HPP as being the most appropriate solution from a technical and economic point of view.
Design Head (Hfcs)
The design head is the Net Head at which the turbne efficiency is maximum.
In ch - 108 hpp the design head has been established equaf to the weighted average head (from energy generation simulation):
Hdes = Hwgen = 213 m
pared Head cHraiS
The rated bead ts the Net Head at which the full gate output of the turtle produces the qmerator rated output
In ch - i08 hpp the rated head has been established equal to the maximum head: Hrat = Hmax- 215 m
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The design discharge for ch -108 can be achieved as follows.
Qdes = Qtol / N = 125 mJ/s 
where
Qtot - total discharge of the hpp, 250 m’/s
N - number of units, Turbine Sdecbon
2
The selection of the turbine and Its arrangement is very Important to ensure the success of the hydropower plant from an economic and operational standpoint. The diagram below, courtesy of Vatech, Indicates the type of turbine depending on head, discharge and turbine power data.
In case of ch -108 hpp1 Qdes- 125 m'/s
Hdes= 213 m
QWM —*
Cigtxe 9 61: Selection of Turtle type acrorting ft? Net Head and C^harge (Courtesy of VA TfCM i
The plant parameters are suitable for Francis turbines.
For the head range and power output of the ch -108 hpp, large numbers of installations similar in size have been designed, constructed and commissioned by various manufacturers worldwide.
The table below summarizes the major technical parameters of the hydraulic units foreseen in CH 109.
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Ch - IOaj MAJOR IECHNICAL RIP BI NF PARAMETERS
Type
Francis
Type of turbine
____________  -
layout
Vertical
Arrangement of turbine (horlzontal/vertral)
Qu Rat
125m3/s
Rated Discharge of each unit
H des
213 m
Design Head of each unit
H Rat
215 m
Rated Head of each unit
Hl Rat
94,00%
Rated Efficiency of each unit
ICu
247 MW
Rated Capacity of each unit
n
200 rprn
Rotational speed
nov
290 rpm
Overspeed
nrun
338 rpm
Runaway speed
nq
40
5               12
Specific speed (nq=n Q°- / Hdes *)
Hs
-5,5 m
Submergence (unit below the minimum TWL)
PD2
1600 tm2
Runner Flywheel
Ch ■ !08, MAJOR TURBINE DIMENSIONS
Turbine
03
3,8 m
Runner diameter
r-                    -d”
-i,  :—
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L
•1
t:
f|
-
________
Spiral Case
A                    12
Inlet
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DABUS HPPi
M2 B + C
Width
E+ D
13,6
Lenqht
Draft Tube
N
12,0
Height
S
21
Length
R
Height at outlet
3**6
?*V+U
1
112
Width
(2)   MAIN INLET VALVES
Each unit of the ch - 108 hpp will be provided with a shut-off valve installed at the spiral case inlet to isolate the turbine and stop the unit if the turbine wicket gates  fail to dose. Furthermore; the shut-off valves  will allow inspection and maintenance operations without dewatering the penstock.
The available head of ch - 108 hpp is suitable for the selection of the butterfly vah/e biplane type.
r
Because  of  their  compact,  simple  design  and  low  cost  (20%-30%  less  than  spherical  valves),  butterfly  valves are usually used for projects where heads reach about 250 metres.
o     Rated flow
125
o
Rated diameter
4.2 rn
o
Static pressure
2,2 MPa
o
Maximum pressure (mcfudrng overpressure due to water hammer)
3.1 MPa
(3) GOVERNORS
Each turbine will be equipped with a digrtal, microprocessor type RID governor.
The turbine governor system will be with pumping unrts and pressure accumulators, to operate the wicket gate in the event of electricity blackout.
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(4)   BALANCE OF PLANT (BOP) MECHANICS
The ch - 108 hpp will include Lhe following auxilrary mechanical equipment.
Cooling Mate- System
Drainage and Dewatering System
Heatrng Ventilation and Air Conditioning (HVAC) Systems
-     fire Defection and Suppression Systems
-     Oil Handling system
-     Compressed Air System
-     Power House Machine Hall Cranes
any other equipment necessary to meet the functional requirements of the power ptant
S.fiJ ELECTRICAL EQUIPMENT
(1) GENERATORS
The two (2) generators will be of the 3-phase, synchronous, vertical shaft type directly Coupled with the Francis turbines. The major, prehmmarY features of the generator are the following:
ch ~ 108, MAJOR TECHNICAL GENERATOR PARAMETERS
Type
Saltent pole,
Suspended
Type and layout of the Generator
Ph
3
Number of phases
Wye (six 
terminals)
Connection diagram
ICu
24? MW
Rated Turbine Output
-----------
A
285 MVA
P^xed Generator Output (RGO)
V
16 kV
Rated voltage
P.F
0.85 lagging
Rated power factor (RPF)
f
SO Hz *2%
Rated frequency
N poles
30
Number of poles
n
200 rpm
Synchronous speed
r|t Rat
98,0%
Minimum efficiency at rated conditions
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290 rpm       Overspeed
338 rpm       Runaway speed
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JABUS wPPs
nover
nrun
1—          — GD2
not less than 23000 tm2
. Mininum Required Generator Fly-wheel
Eadh generator wHI be equipped with stabc excitation systems and automatic voltage regulators.
(2)    STEP UP TRANSFORMERS
The ch - 106 HPP will have step up transformers from which High Voltage (HV) lines will depart to link the powerhouse with the HV Switchyard. The major technical parameters of the step up transformer (type, if 3- phase or a bank of single phase transformers, cooling medium, winding connection, etc.) will be defined dunng the further stage erf the design, and tailored to the selection of the transmission scheme.
It is worthwhile recalling that, presently, three different alternatives are proposed for the transmission system, based on 230 kV, 400 kV and 500 kV voltage levels.
Once the Client selects the desired voltage level, SP will optimize the design of the step-up transformers.
(3)    BALANCE OF PLANT (BOP) ELECTRICS
The ch - 108 hpp will Indude the following auxiliary electrical equipment:
Two (2) auxiliary service transformers
Two (2) lots of generator isolated bus ducts
-    Two (2) generator switchgears, Including circuit breakers
-     One (1) Emergency Diesel Generating unit
IV AC auxiliary power supply
-     DC power supply system
-     Uninterruptable Power Supply (UPS) system
protection, control and monitoring equipment
telecommunication and control systems
one (1) distribution line to the dam premises, where both auxiliary system and a
-    any other equipment necessary to meet the functional requirements of the power plant
9.6.4     HYDROMECHANICAL EQUIPMENT
(1)  WATERWAYS
Two steel penstocks will feed the two turbines. Each penstock will have a nominal diameter of 5.0 m and be about 970 m long.
The maximum design head, induding allowance for transient behaviour of turbines, will be approximately 2.3 MPa. The maximum flow through each penstock will be 125 m’/s. A minimum of th.ckness of 13 mm f.T the lining will be required.
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(2)  INTAKE
The intake of the ch - 108 will be a concrete structure that will divert water from the reservoir to the power
waterways. The intake structure is designed to divert a maximum flow of 250 nrP/s.
The intake will be equipped with:
-      two  (2}  sets  of  trashracks.  Each  trashrack  is  foreseen  to  be  cleaned  with  a  cleaning  machine  Installed on the plateau at an eievabon of 1232,0 m a.s.L
-     one trashrack cleaning machine
two (2) sets  of bulkhead-type maintenance gates. The main purpose of the bulkhead gates  will be to isolate the turbine unit and tunnef from the reservoir and allow inspection of the waterways, intake gates, shut off valves and turbines.
two (2) sets  of fixed-wheel type intake gates. These gales  will also allow maintenance and inspection of the headrace and to act in case of emergency  and shutoff the flow in case the wicket gales  fall to dose under the maximum head and maximum power (typical case, load rejection under maximum output.
one gantry crane structure which will be designed for the lifting equipment of the wheel gates and
bulkhead gales.
(3)   DPjD.Fr TUBES EQUIPMENT
The powerhouse draft tubes will be equipped with following hydro-mechanical equipment:
■  two  (2)  sets  of  drart  tube  sliding  gates  for  each  unit.  The  main  purpose  of  the  slide  gates  will  be  to allow  the  maintenance  and  inspection  of  the  draft  tube.  Each  sliding  gate  will  be  operated  by  oil pressure servomotor.
two (2) sets of draft tube stoploqs, to allow the maintenance of the draft tube, draft tube gates and the units. The stop logs will be made of multiple elements. Each element will be designed to be lowered and raised by suitable lifting beam operated by the draft tube gantry crane,
one (!) draft tube gantry crane  which will be used for erection, operation and maintenance of the
4
draft tube stop togs.
300 WHO A SPW3B (PR£-FEA5IBlLnT Report]
1D0 WHO R SP W3U
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~loTRAN
SMISSION SYSTEM
J
10.1
SCOPE OF THE STUDY
-me   objectives   of   the   prefeasbdity   study   are   to   identify   and   define   the   design   main   aspects   and   issues, together with the economic viability for Dabus hpp Interconnection to the national gnd, bearing In mind:
✓    evacuation of rated power output from Dabus HPPs
✓    interconnection to the national grid at suitable voltage level
✓    availability of generated electricity support to the promotion of regional cooperation
✓    increasing power trade with neighbouring countries
J supporting the Ministry of Water. Irrigation and Energy (MoWlE) and the Government in achieving the
major goals given by Growth and Transformation Plan (GTP) of Ethiopia
With  these  alms  in  mind,  the  Consultant  started  gathenng  key  information  and  parameters  helpful  in  defining and proposing the most viable schemes.
The mam project components are:
•      an upstream hydropower plant, Dabus ch.-130, with its HV station
•     a downstream hydropower plant, Dabus ch.-108, with its HV station
•      interconnection lines between the two stations, about 15km long
•      interconnection lines from the stations to the HV network nodes (two alternatives options)
10.2    ANALYSIS of ETHIOPIAN TRANSMISSION SYSTEM DEVELOPMENT STATUS
The   Ethiopian   Interconnected   System   (ICS)   links   the   major   generation   plants   to  load  centres  via  transmission lines at 500 kV, 400 kV, 230 kV and 132 kV and sub-transmission lines at 66 kV and 45 kV.
In this  decade, the ICS is  facing an extraordinary  development. The committed and constructed power stations connected to the ICS are increasing passing from a total Installed capacity  in 2012 of 2124 MW to lhe planned 12,000 MW in 2020, most of which (12,000 MW) generated by Gibe Ill, GERDp and Koysha HPPs.
The increase of the specific generating capacity with large units (e.g. 37S MW in GERDp) and high splitting 16 units in the same GERDp or 10 units in Gibe III) imposes routing of power ooutput through higher
voltage level over large distances. We refer to the recently implemented 500 kV corridor to Hcleta via Dedessa, -600 km long. Consequently, important transmission infrastructures (lines and substations) are under construction or already built, among them we can mention the 400 kV lines Gibe III - Gibe H and GERDp - Beles, the Addis Ababa area ring Sululta - Holeta - Sebeta several lines at 230 kV iKoka -- Hurso Melka Wakena to Gode and Wolkite, Gilgel G.be - Jimma - Agaro - Bedele - Metu - Gambela), etc.
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PRE-FEASIBlLTTy STUDY Report
p»ge   149 of 169
Several international nterconned-ans for hydropower energy tr*n«mi5S®n wheeling are also under development, namety
•     HVDC overhead line from Wolaita Sodo s/s (Ethiopia) to Logonot substation (Kenya)
•     Ethiopia-Sudan SCO kV interconnection
•     Ethiopia-Djibouti 230 kV Interconnection
The study has therefore considered the aforementioned development plans, looking at different high voltage levels si ready applied m the country.
10.3    TRANSMISSION SCHEME
10.3.1    ANALYSIS OF PREVIOUS SCHEMES
At the tjrrve of edrting this report, the Consultant has familiarised itself with previous studies and ongoing projects from the following sources;
[lj 2002 MoWlE, DABUS, Reconnaissance Study
[2]
2013_Parsons, Ethiopian Power System EXPANSION Master Plan; Interim Report Volume 4 -
'Transmission Planning
[3]
Africa CEC session 5 JRENA_Ndhlukula_23O613
[4]
Final Interim Report Module IB Transmission interconnection Code
[5]
http;//www eepco.gav.et/project
r
Further information on the actual status of ongoing projects and detailed data relevant to the high voltage transmission grid are expected to be received from the Client, to be considered In the next stage of the design.
The following transmission schemes have been acknowledged in the available previous studies.
MIXED 230kV*132kV
The connection of the Dabus power plants was envisaged m [1] by considering the existing HV substations nearby, namely 132 kV in Asosa and 230 kV in Ghetto, which also dictated the transmission voltage selection.
The power plants were linked to evacuate the following expected outputs;
■    99 MW and 67 MW respectively via 132 kV OHTL to Asosa with lines length ranging 54 to 65 km
•     326 MW and 199 MW respectively via 230 kV OHTL to Ghedo with lines length of about 350 km Use of AAAC conductor was proposed.
Use of the 132 kV voltage level, probably chosen In respect of the lower Installed capacity of the HPPs ns 
r
unsuitable for several reasons, not reflecting the transmission development plans in the project area.
130 WHO R SPOOJB (PRE-EFASIBILITY R  rt)
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itiicHc pletringeli, romrDABUS HPPs
PRE-FEASIBILITY STUDY Report
150 at 169
On the other hand, 230 kV level would be a suitable "entry  level” for the prospected transmitted capacity (about 750 MW) of the current project layout. However, notwithstanding this, the scheme is  not functional m terms of stability and losses.
Z3QJ&
The Dabus  plants  were accredited with an overall installed capacity  of 250 MW (Lower Dabus) and 326 MW (Upper Dabus) respectively, with plant factors  of 0.29 (635 GWh/y) and 0.51 (1455 GWh/y). The interconnection to the national grid Is  realised via 230 kV OHLs  leading to Mendl, very  dose to the project site. The station in Mendi is  reportedly  planned to be upgraded from the existing 132kV up to 230 kV and provided with facilities  to receive a twin 'ASH* AAAC, 30 km long, double circuit OHL incoming lines  from Dabus  HPPs. From there, power evacuation would be assured by 200 km long, 230 kV lines to Dedessa station.
The prospected transmitted power or 580 MW Is certainly within the 230 kV system capability, thanks to the
low plant factor (0,29) and line lengths (150 km from Mendi to Dedessa). Anyhow, some of the aforementioned
concerns still hold.
10.3.2     DESIGN CRITERIA
There  Is  a  number  of  topics  and  issues  to  be  considered  In  defining  the  transmission  scheme  options  for  Dabus Project Interconnection, among the major we mention:
■     selection of the most suitable connection point, /.e., the nearest HV station, also taking into account the expansion plan of the country;
•      selection of the number of Interconnecting lines and their configuration;
•      transmission stability (both angular and voltage);
•      advise on senes and/or shunt compensation, if needed, to achieve an optimal voltage profile in all the
operating conditions, even in case of (n—1) contingency perturbation;
• 
line charging features (noload energisation) of Dabus interconnection;
• 
voltage regulation capability of Dabus HPPs generating units;
• 
reduction of overvoltage caused by receiving end load loss or Intermediate station outage;
• 
transmission efficiency selection,
• 
environmental maximum electrical and magnetic fields evaluation;
• 
corona losses evaluation;
• 
Ime tele-protection scheme;
■
reliability of toe selected scheme in terms of transmission system availability aid expected outage
rate.
In the present design stage, xef pre-feasibility, the Consultant focused the selection of the scheme on the most important topics, leaving toe others  to the next stage of toe design, as  dealing with a detailed steps  in the definition of the selected scheme performances. In doing this, the scheme has been planned tailored to:
•      the firm average power output of Dabus Cascade;
•      maximum peak power output;
100 WHO R SP003B rPRE-FfASIBlirrv Report)
l£>0 'WHO R SP MJD
5Tud»o fHfitrangeli, rcmePRE-FEASIBILTTf STUD Y Report
• existing transmission voltage already applied »n the country.
c+je 151 cf_lW
The  cntena  for  the  design  of  the  transmission  system  is  governed  by  the  need  of  complying  with  safety  and reliability parameters an the one hand while achieving a high degree of reliability at the lowest possible cost.
10.3.3  VOLTAGE LEVEL FOR TRANSMISSION SYSTEM
The  choice  of  the  voltage  level  for  the  connection  of  the  Dabus  Project  to  the  HV  system  is  subject  to  several constraints, among which;
capabilities  and harmonization with the existing transmission facilities* e.g, in order to uniform the transmission design with existing and planned projects  and avod the creation of botde necks  which would compromise the functioning of the interconnected network;
- distance to nearest interconnection station;
minimization of power losses;
- reactive power Issues.
With reference tn the highest voltage level already operated by  EEPCo/EEP in the country, all the three levels, namely  230 kV, 400 kV and 500 kV have been preliminarily considered.
The   foilawing   chart   depicts   voltage   selection   in   respect   to transmission power per circuit and distance.
Dabus  Cascade output, by  assuming to be transmitted to Dedessa* about 200 km far from the project sate, justifies the selection of voltage levels above 230 kV.
It has been thus included in the analysis also tne option at 400 kV and 500 kV levels, besides the 230 kV
10.3.4   NUMBER OF CIRCUITS AND LINES
It Is  worthwhite noting that, a single circuit line cannot assure suffident operating continuity* since it is  subject, apart from routing scheduled maintenance, to permanent outages  requiring maintenance for operation restoring.
Construction of double circuit lines is normally accepted and implemented in Ethiopia as common practice. Anyhow, the construction of a double circuit overhead line over high distance is sometimes not acceptable in view of probable outage of both circuits due to lightning (bn -25% of the events), thus greaity reducing the availability of operation. On the contrary, for voltage levels higher than 400 kV, a double circuit Ilr>e cost is likely comparable to (hat of two single circuit lines built at the same time on the same way-leave. Preliminarily, we propose linking each HPP to the ending station with double circuit lines.
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DABUS HPP*
10.3.5    PROPOSED OPTIONS AND COMPARISON OF ALTERNATIVES
The configuration proposed at this Pre-Feasiblhty Study level foresees a cascade with two hydropower plants,
respectively:
>    DABUS ch.-130 HPP, 287 MW installed power, 0.5 PF
>    DABUs ch. 10B HPP. 468 MW installed power, 0,5 PF
The total Installed capacity will be about 775 MW, and assuming a power factor of 0.92 at the HV level, the
rated power output to be evacuated will be about BSD MV A.
Based on the above common assumptions, the Consultant has evaluated the feasibility of three (3) possible
alternatives for the interconnection of the Dabus Cascade to the national grid, in terms of voltage level:
•     ALT_1, at 230 kV
•     ALT J, at 400 kV
•     ALT_3 at 500 kV
f
The number of overhead lines  would be adequate to evacuate the rated power and allow safe operation In (n- l) contingency, regardless  the selected option. The pictures  on the next pages  show the topology  of each scheme (eastern part of the grid only partially shown), briefly described below.
ALT  1  230  kV  t  High.Voltage).  The  scheme  is  partially  derived  from  what  is  reported  in  (2],  Each  plant  will  be linked  to  the  Mendi  substation  via  double  circuit  230  kV  OHTLs,  about  35  km  in  south-east  direction,  which will  reportedly  be  upgraded  to  230  kVr    In  addition  to  that,  a  15  km  long  single  circuit  OHL  will  link  the  two power  plants,  thus  dosing  a  ring.  Mendi  Substation  will  then  be  connected  to  Dedessa  Substation  by  230  kv OML>  already  planned  In  the  transmission  development  plan,  thus  excluded  from  the  scope  of  the  project. Evident benefits of this scheme are:
>     lower initial investment cost, by assuming that an equal number of 400 kV circuits alternative is provided;
>     transmitted power 2 times the surge impedance level (SIL, ~180 MW), and therefore optimized kWh cost in respect to line overall capacity;
>     whatever (n-l) contingency occurs, the scheme is still robust in evacuating the power towards Mendi station;
>     the scheme still allows power delivery to Mendi In (n-2) events, Ze in case of outage of one circuit of any double circuit line, and, during transient event, with any tkxjble circuit line tripping.
However, the transmitted rated power per circuit would be higher than the thermal limit power (*-340 MVA). Drawbacks lie in the bottleneck represented by transformation in Dedessa from 230 kV to 400 t V nr 509 kV. and stability-limited margins if compared to 400 kV and 500 kV schemes.
W0 WHO ft SP0C3B (PRE-FfAS|0ILJTY Report/
100 WHO R 5P 003B
studio pictrangck, romcDA0US HM*
PRE-FEASIBILITY STUD
»qe 153 d*
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1 KJ WHO R5PQ01B
’.'^■0 pietrarigd.. romcPRE-FEASIBILTTY STVDYRepof^
[age 154 or 169
SJABUS HPPs
This scheme is affected by:
>     low efficacy ** to large power losses and volta^ drop, Ze 2.1% (no corona) and 6% respectively 
per 100 km, carrying 37 5 MW per circuit;
>    lower flexibility in contingency event.
ALT 2; 4QQ kV (Extra High Voltaoek  This  alternative foresees  double circuit 400 kV OHLs  to connect Dabus ch.-108 to Dabus  ch.-130, and Dabus  ch.-130 to Dedessa. Higher initial investment would be balanced by higher flexibility  and reliability  of the 400 kV scheme, even though the step-up transformation in Dedessa would still be needed if power flow is diverted Into the 500 kV.
This scheme Is the most attractive, even more so if a 400/500 kV switching station were to be constructed in
very dose proximity to project site, allowing stepping-up into the GERDp SOO kV OHLs. In addition to this, we
mention other advantages:
750  MW  transmitted  power  would  still  be  above  the  SIL  of  a  typical  400  kV  overhead  line  (say,  -530 MW);
higher operation stability limit margins;
*■
>
reduced   losses   and  voltage  drop,  r.e.  1.3%   (no   corona)   and  3.9%   respectively  per  100  km,  carrying 750 MW with one circuit;
>
thermal limit power of about 1200 MVA per circuit.
A^un? 10.3.2; Aftem&tn/r 2 400 kV sttterne from Dtbus
ft? /Vctosa
Among the drawbacks, we mention:
>
greater right-of-way width;
greater line charging absorption needs at line energisation at Dabus HPP;
since  the  power  per  circuit  would  be  below  the  SIL  of  a  typical  400  kV  overhead  line  (say,  -530  MW), the second circuit would be considered as redundant for backup purposes;
need for coordination at Dedessa in case of GERDp lines contingency.
Ite WHO ft SP002B (Pfif-FFASIBTLITY Report)
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studio pietrangellr fameD*BU5 HPPs
P^FEASIB/Lrr^S7]JD^eport_
E*y 155 of Jt?
ALT 3; 500 kV (Extra High Voltage). The scheme finds  ground on the relative vicinity  of the aforementioned 500 kV overhead lines  corridor linking Dedessa station to the GERDp station, situated in the north. Although this  voltage Bevel has  only  recently  been applied m the country, It is  largely  Justified by  transferred power ratings and distances (aver 500 km). The proposed scheme would bring the following Identified benefits:
>    strengthening the SOO kV level allowing new generation to participate to frequency regulation without any interposed stepped-up transformation, as for 400 kV scheme;
>    very reduced losses and voltage drops, f.e 0.8% (no corona) and 2.4% respectively per 1OO km, carrying 750 MW with one circuit;
>    cost very similar to the 400 kV scheme;
>    thermal limit power about 2500 MVA per circuit
Iff. J. J:        ArtrrnaGw J, 5(70 Av vcfreme /rum £U£ms CJSCdtfe /u fledraw
The drawbacks would be similar to those few the 400 kV scheme.
The  electrical  performances  of  the  proposed  scheme  have  been  prelim  Ina  nty  deduced  and  depicted  in  the following paragraph.
10.4    LINE ELECTRICAL CAPABILITY
The  transmit  performance  against  voltage  drop  and  losses  is  drafted  on  the  basis  of  the  abwe  assumption, also considering the conductor already in use In the country as per ref. [2], listed here bdow:
230 kV equipped with:
2
twin bundle "ASH AAAC 150 Mm , 17.4mm diameter, 0,1830hm/krn @2Q*C
r
x =0.31B4Ohm/km (In double circuit configuration), SIL = 173MW
2
■a single * REDWING" ACSR, 362mm . 27,43mm diameter, D.08Ohm/km @2Q°C
r
100 Wl*0 ft SP003B CPftE-FEASlHILJTV Report}
100 WhORSp 0030
rtudo pietraAgcii, romePRE-FEAS1BIUTY STUDYReport
o single "YEW" AAAC, 400 Mm’, 28.4mm diameter. 0.069lOhm/km @20°C o single “MALLARD" ACSR, 403.8mm’, 28.96mm diameter, 0.07170hm/km @20°C
□ single "ASTER 570" AAAC, 570.2mm’, 31.05mm d.ameter, 0.03950hm/km @20°C, x'=0.4354Ohm/km, SIL = BO MW
156 «f
DABUS HPPS
-     400 kV equipped with:
o twin bundle "ASTER 851' AAAC, 850.86mm^ 37.95mm diameter, 0.039Ohm/km @20°C,
x'-0.30250hm/km (in double circuit configuration) SIL = 526MW
500 kV equipped with:
a quadruple bundle ACSR (data not available)
TRANSMISSION LOSSES
The conductor selection for Dabus  Project will be made at the Feasibility  Design stage. Meanwhile, a tentative, indicative comparison is  performed to give an idea of transmission efficiency  in respect to losses  and costs. Since investment costs  decrease with transmitted power while loss  component increases, the transmission specific  cost exhibits  minimum values. The following charts  provide losses  and transmission costs  (specific costs, given in dJ5D/kWh*100 km), preliminanly  based on the specific  annual costs  minimization, basic economic  assumptions  (50 years  equipment lifetime, energy  unit generation costs  as  given in Chapter 7 of this Report) respectively with 0.50 plant factor and 0.92 transmission power factor.
Investment cost is  evaluated according to similar projects  of corresponding magnitude recently  procured and constructed in the country, e.g. 220 kUSD/km for 230 kV double circuit OHL, 320 kUSD/km double circuit 400 kV OHL and 400 kUSD/km for 500 kV single circuit line.
ZaCfcV OHL twin ASTER 570 - Specific trarummlon costs per Circuit per lOOkm
230 k V overfed tone rpupped               nm ASTER 520, iOO km, specific trjnsmispon co-r is. ,-vwr-
The  230  kV  lines  are  equipped  with  a  twin  bundle  ASTER  570,  while  400  kV  and  500  kV  alternatives  with  twin bundle ASTER 851 and quadruple bundle ACSR 400 Mm’, respectively.
100 WHO R SP0O3B (PR£-FEASIBILITY Report)
100 WHO ft SP nO3B
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^sge 157 ar 169
44MkV OHl twin ASTER BSl - Specific transmission costs per cifcwt per lOCkm
rtwrj 45W? #51, /W cm. sjpftrtfc MWnw rnn* re. Aiwrr
We can observe that:
' 230 kV twin bundle ASTER OHL carries 400 MW cewer rating per drcuit at minimum cost of
0-12rUSDi'kWh“ 100 km, wltb about
current density;
400 kV (Win bundle ASTER OHL has slightly cheaper specific cost, Za O.lQcUSD/kWh* 100 kin with O.^A/mrn3;
500 kV quad bundle ACSR OHL is almost equivalent to the 400 kV alternative.
The exerase has is for comparison purposes only, as these figures will be updated at the Fealty Design stage based an more detailed technical and «crxm>cal parameters.
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Studio plcVangrll. nbmeDA3US
PRE-FEASIBILTTY STUDY Report
page 158 of 169
The capability of the proposed interconnection to the grids has been preliminarily evaluated with respect to the critical voltage versus transmitted power of each circuit configuration, since it strictly depends on load power factor and distance. The following graphs show the voltage profiles and relevant critical voltage of 230 kV and 400 kV circuits (base length 100 km for the sake of comparison). Acceptable operating voltages are those above the critical voltage, besides corresponding to higher efficiencies and lower current.
Dabus - Transmitted Power vs coscp; Critic Voltage
Vs ■ 1.07*Vn; twin bundle ASTFR 570, 230kV, 100km
fws W 1.1             230 4Vprofiles md cnbai eolUge, 1OO km tese length, cos tp 0.82+1
It«. be,«,«.hat ft, trosp»«t
pn>file *«*>>-<> ». ftftft, any
relate short length a ft, l,«, ft„s
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kv 0B|(jn
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PRE^EASIB/L/TY STUD Y Report
10,5 RURAL AREAS ELECTRIFICATION ALONG THE LINE ROUTES
ptgr 159 rf 169
Development  of  small  towns  or  areas  may  be  encouraged  by  dectnfymg  the  rural  areas  along  the  line  routes. At the time of writing, only 26% of the 97 millions of Ethiopian people are served with electricity (source:
j.Stwtwl        With        almost        85%        of        Ethiopians        living        in        rural        areas, there  Is  a  significant  bias  between  the  power  supply  of  urban  and  rural  population;  only  2%  of  the  rural  but 86%  of  the  urban  residents  has  access  to  electricity.  The  Government  of  Ethiopia  and  EEP,  the  Ethiopian Electric Power, are therefore engaged In an effort to expand rural electrification.
For this  reason, the Consultant wishes  to draw the attention to the possibility  of adopting the innovative Insulated Shield Wire Scheme (ISWS). it is  necessary  to emphasize that the adoption of the 15WS allfiWilQ supply  LV leads  along the HV transmission line route In a very  simple and low-cost wav. This  scheme Is recognized worldwide to be highly  bankable and viable for Its  undoubted return in terms  of benefits  to the local communities. We therefore propose evaluating with the Client the possibility  of extending the project in the arEa increasing household access  to electricity  in small towns  and rural areas  along the existing transmission line routes.
It Is  worthwhile recalling that this  scheme was  developed by  Prof. Iiiceto, a consultant of Studio Pletrangell for thE past 40 years, and successfully  implemented in several countries, e,gr in Bumbuna Falls  Hydroelectric Project Sumbuna - Freetown !61kV transmission line, Sierra Leone (shown in the figure).
10, 5.J.-
i rrw Slftf « oewatfthi awp
World  Bank  recommends  using  SWS  .nctodlng  ISWS.  where  possible,  as  a  means  of  reducing  investment costs which are a major obstacle to rural dectrificatton In regions with load toad density and low income.
In fact, with the aim of strengthening the tow load density  rural areas' conventional distribution by  upgrading the 15 KV to 33 kV, EEP has  recently  applied the SWSs  for |»wer supply  to Ullages  and small towns, to a tew Large farms  and to te^offw-frult processing plants  touted along some 132 kV and 230 kv  transmission lines or at a distance of up to 30 kilometres from the HV line route.
The  first  ISWS  application  in  Ethiopia  was  In  the  GhedoNekemp^e-Ghimbl  200  km  long,  132  kV  line  The Single-Phase Earth-Return SWS was implemented and was commissioned in 2003.
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r
□ABUS HPPs
The second, more extensive, app^ * **
•» *•"»
JT*£
lines- Glbel Glbe-WaikJte 230 kV single-circuit line, 71 km long; Jlmma-Bonga 137 kV ou e-circu
xm (cog: Booga-M^n 132RVdoubl^lrcu.t line, 89 km long. T*e Jlmma-Bonga-Mizan radial lines cross W«l regions, partly at high aitrtude (up to 2300 m), in areas of high keraunic level, they have therefore been effectively protected by two SWs with very low shielding angle. The ”3-Phase" SWS (Fig. I D) has been applied in all these new lines.
• Sum uF wTsjiiAnecurfi peak lc«9i H 5WL y«' 3032 * tW
•  Tolll            of ■fiWL 71 wn
■ Tot*1 artjtirf taiirwl lines 39
a
10 5.2           Stftylr Ar-h? ifcaj^am of 34.5k V 3-pfvHe 5WS #5 GtAye? &fw to IrVaMjft? stretch. ttfitOftt 2005
In mountainous  regions  the construction of HV lines  is  far less  penalised than the construction of MV lines. Contractors  prefer MV lines  only  when the routes  follow winding roads. They  end up being much longer than the point-to-point distance and therefore more expensive. Moreover, local wood poles  in Ethiopia do not have the mechanical strength adequate to support MV lines  with economically  designed span lengths. The ISWS overcomes these problems by providing an MV drcult along an HV circuit at very little cost.
In the Dabus project, the ISWS could be applied If the 230 kV option is selected.
10.6    DABUS HPPS SUBSTATIONS
The Dabus Cascade transmission system will also include HV facilities al each power plant.
Consequently   by   defining   the   most   viable   scheme,   the   substation   equipment   such   as   step-up   transformers, switchgear, SCADA will also be defined to match the operating requirements.
Preliminarily,  the  substation  scheme  could  reflect  what  has  already  been  adopted  in  several  HPPs  in  operation or under construction in the country, namely Gibe n, Beles, Gibe HI, GERDp and Koysha.
The selection of the system voltage is mainfy dictated by:
its  suitability  in  linking  the  two  power  plants  to  each  other  as  well  as  to  the  grid,  allowing  design  power transfer capability;
availaDilriy in the existing electric system operated by the national utility,
A preliminary single line diagram is given in "170 GEN D SP 002A_Single Line Diagram - ALT_1, ALT 2, ALT_3 which shows the plant main diagram of each alternative, as well as a typical unit (generator plus step-up transformer) bay diagram.
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The scheme selection and equipment characteristics will be confirmed, on the basis of the following criteria.
>    operation continuity;
>     flexibility;
>    safety;
>    harmonization to existing practice in the country;
>    economy evaluation.
10.7   AVAILABLE SITE DATA
The preliminary  corridors  of the transmission lines  cross  an area characterized by  altitudes  ranging between 1000 m and 1900 m. The project s»te of Dabus ch ■ IDS, which represents the starting point of the corridors, is located at an altitude of about I DOO m a.s.l. Mendi substation, with an elevation of about 1700 rn a.s.L, Is the arrival of Alt. 1, while Odessa substation, with an elevation of 1270 m a.si is the arrival of AIL 2 and Ah, 3. The terrain profile Is characterized by gentle slopes without significant mountainous areas.
Two types of zones can be mainly found along the corridors. The first type is an area wrth low vegetation such as  grass  and Isolated obstacles  (trees, buildings). The second type is  uniformly  covered by  vegetation or buildings (such as villages, suburban terrain, permanent forest).
Information about the existing structures Is collected through free satellite imagery exploration.
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It  is  worthwhile  mentioning  that  the  prelim!  nary  corn  dors  have  been  drafted  by  avoiding,  as  far  as  possible, crossing of swampy areas that characterize some zones of the region.
Temperature data from the hydrographic  basin are reconstructed by  the Climatic  Research Unit (CRU) of the University  of East Anglia (UEA). The following data, obtained from the model for a period from 1901 to 2014, can be assumed, initially, as  representative for the line since a significant part of his  corridor is  inside the hydrographic basin:
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""Manure yearty temperature (March)
26*C
Minimum ycariy temper Jlkire (August)
17,6'C
Mean ywh temperature
2l,2*C
The   corridor   reaches   elevations   above   1900   m   a.s.l,   In   some   Stretches,   therefore   we   expert   to   consider   and investigate lower temperature designs at the feasibility stage.
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As   f^r   as   the   keraunic   level   is   concerned   (figure   above),   the   estimated   average   level   Is   30+40   days   of thunderstorms per year.
10.8    PROPOSED LINE CORRIDORS
The transmission schemes proposed at this Prefeasiblllty Design Icvd envisages the following alternatives:
>
Alt 1, routing of the lines from Dabus Cascade to the existing station in Mendi, located about 3Q km
from the project area;
>     AJt. 2, routing of the lines from Dabus Cascade to the 500 kV existing station in Dedessa, located
about 190 km from the project area.
The
proposed  corridor  is  shown  in  the  drawing  170  GEN  D  SP  001.  Dedessa  Substation  is  the  receiving  end  of
tv/o
double  circuit  500  kv  OH  lines  recently  erected  to  evacuate  the  power  from  the  GERD  HPP,  located  in  the
West region of the Country, about 140 km north of the Dabus project.
The  two  lines  GERD-Dedessa  share  a  right-of-way  of  about  200  m,  100  m  each.  This  corridor  has  been  chosen to run, as far as possible, alor-g the existing Mendi- Dedessa road.
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Another existing transmissJon line has oeen KJentified in the area, Ztt, the Asosa Mendr Ghlrnbi 132 kV
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Alternative 1
According to tins alternative, two OHLs would depart from Dabus ch-108 leading to Merrii. The same applies for Dabus ch-130. While a single circuit OHL would link the two power plants to form a ring. Mendi is a town about 35 km south-east of Dabus ch-108, whose substation is planned to be upgraded from the existing 132 kV to 230 kV.
The proposed corridor would not cross  densely  populated cities  or townships, thus  reducing the need for expropriation and resettlement. However, it is to be noted that the following developed townships are present along the comdor:
>    Menda
>    Ato Gondas
>    Nedjo
>    Hena
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Moreover, huts and lodgings are  regularly spread along the Mend, - Dedessa road, suggesting that the corrdor be kept at least 1.5 km from the road.
The area In the vicinity of the reservoir lacks access roads due to the wilder environment.
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Alternatives 2 and 3
These alternative foresees the connection of Dabus ch-108 to Dedessa, via double circuits 400 kV or 500 kV OHls- The new OHLs  corndor would be conveniently  located parallel to the existing GERD-Dedessa 500 kV lines  in order to reduce the environmental Impact of the new lines. In this  case, almost the entire length of the lines  would be adjacent the GERD-Dedessa corndor. The lines  would run south of the existing lines  to avoid crossing.
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Access to the corridor would be guaranteed both by the already built roads and the nearby Mehdi - Dedessa road.
Optionally, it would be possible to choose a corndbr outside that described above, either In the north or in the south, provided that a new environ mental impact assessment is made and new access roads are provided. From the point of view of interferences (obstacles, urban areas, etc), this option presents a very similar terrain morphology to the proposed corridor, as well as lightly populated areas.
An accurate route selection results in noticeable benefits to the line availability. Therefore, further optimization will be camed out in respect to the mountain areas.
10.9 PRELIMINARY GEOLOGICAL ASSESSMENT OF THE PROPOSED CORRIDOR
The power generated by the Dabus Cascade will be evacuated through two different schemes, namely:
t, the link between the two Dabus HPPs. about 15 km long, plus the line from Dabus ch-130 to
Mends, about 30 km tong
2- the link between the two Dabus HPf’s, about IS km long, plus Dabus ch-130 to Dedessa. about
190 km long
The comdor of the transmission lm« connecting the Dabus to the national grid in Dedessa Is supported with 30 m to 45 m high lattice towers (depending on the etwee of the voltage level) for a total length of about 190 km, crossing a heteraqeneous  lithology  chronically  dated from Precambrian to Ollqocene - Miocene with NW-SE direction.
In order to nave a sufficient knowledge of the lithology to be etcounte^d along the designed corridor, and to foresee the postal constraints  for the towers' foundat.ons, a quick  assessor based on the available bibliography was performed.
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hqwr JO, 9.1           NW in W of ttw Geotogmji!           of Ethiopia and Somafe (out of State..!
The investigation was carried out through the analysis of the NW sector of the Geological Map of Ethiopia and Somalia 1:2,000,000 by  Mena Gtm, Abate E., Conti P.r Segari M.Jacconi P. (1973), and reported in the figure above.
The main formations (code in brackets) encountered along the corridor from NW to SE are:
•            Granodionte (gd);
•            Makonnen Basalts: Hood basalts, commonly directly overlaying the crystalline basement
(PNmb);
•               &rbir   Group:   Meta-basalt,   rneta-andesite,   metarhyolit,   phyllite,   graphitlcschlst,   marble, quartzite,   meta-conglomerate,   green   schist,   metasandstone,   metachert   and   amphibolite (PR2b);
•            Pre - tectonic and syn - tectonic granite (gtl);
•            Syn - tectonic granite (gt2);
•            Post - tectonic granite and syenite (gt4)>
The  geo-chronolog  leal  units  of  the  different  segments  where  the  towers  will  founded  are  summarized  In table below, starting from Dabus site (km 0,0)*
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SEGMENTS
LITHOLOGY
tan
Forma bon
0-43
Granodiorite
43 >8
Basalt
58-78
Granodiorite
78-93
Basalt
93 - 95
Granodiorite
95 - 98
Blrblr group
98-116
Alternate granodiorite and basalt
116-131
Granodiorite and Birbwr group
131 -135
Granodiorite
135- 139
Pre-tectonlc and syn-tectonlc granite
139 - 144
Granodiorite
144- 151
Pre-tectonic and syn-tectonlc granite
151 - 160
Alternate granodiorite and Pre-tectonic and syn-tectonic granite
160- 170
Granodiorite
170- 179
Syn-tectorvc granite
179 ’ 189
Granodiorite
189 - 390
Post-tectonic yanite and syenite (End of the line)
The alternative scheme connecting Dabus HPPs to Mendi develops 'or a total length of about 30 km in the
first stretch of the above described corridor. The geology of the corridor Is fully represented by Oanodionte.
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------------------- -------- —---------
j-—----------------
[1] C2]
[3]
[4]
[5J f6]
quimpo, R. G. (1967)- Static model of daily nver flow seq
Paper No. 18, Fort Collins, **«*>' 1 _ StoctlaStK description of daily over Wows, Proceedmgs
11 REFE
.  Hattv  river  flow  sequences,  Colorado  State  Univ.  Hydrology 
Qulmpo, R, G. and
International. Hydrology Symposium, 1. A.S.H., Fortin
Co)lln5, Colorado, 1967.
Reddy,  P.  I.  (1967).  Stochastic  Hyd'y*?9y„***  Recognition.  Oxford  University  Press,  Oxford.
&shop, CM. (1995). f^'/***^ Lippmans R.P, (1907). An inro
computi^th neural nets. IEEE ASSP Magazine 4 (2), 4-22.
(1986). Learning Internal representations by error
~ ln -—" “ 1‘
T7]
[3]
(1944). A method for the solution of certain problems in least squares. Quarterly of Applied
MaSt^Cl1^    algorithm    for    least-squares    esPmation    of    nonlinear    parameters.    Journal    of    the Society of Industnal and Applied Mathematics 11 (2), 431-441.
[9]   Searcy, J.K. (1959). How duration curves. Manual of Hydrology. Part 2. Low-Flow Techniques. USGS. [10]   Vogel,  R.  M.  and  Fennessey,  N.M.  (1995).  Flow-duration  curves  I:  New  interpretation  and  confidence
intervals. Journal of Hydraulic Engineering, 120 (4).
[11]   Vogel,  R.  M.  and  Fennessey,  N.M.  (1995)  How  duration  curves  II:  A  review  of  applications  in  water resources planning. Water Resource Bulletin, 31 (6).
U2]  Harris,  1,  Jones,  P.D.,  Osborn  TJ.  And  Lister  D.H.  (2013).  Updated  high-resolurion  grids  of  monthly dimatic observations - the cru ts3.10 dataset- Int. Jour. Climatology.
[13] New, M., Hulme, M. And Jones, P. (1999). Representing twentieth-century  space-time dimate variability, part I: Development of a 1961-90 mean monthly  terrestrial climatology. Journal of Climate. American Meteorological Society.
f HJ New, M., Hulme, M. And Jones,  P,  (2000). Representing twentieth-century space-time dirnote variability, part II: Development of 1901-96 monthly gnds of terrestrial surface dimate. Journal of Climate. American Meteorological Society.
[15J USAGE (2000). Hydrologic Modeling System HEC'HMS. Technical Reference Manual. US Army Corps of Engineers Hydrologic Engineering Center.
[16] USACE (2015). Hydrologic Modeling System HEC-HMS. User's manual v. 4.1. US Army Corps of Engineers Hydrologic Engineering Center.
Sim     r     O^T7.^?LOrgan,Mb0n        <2WGL.ide     to     Hydrological     Practices.     Vol.     II.     WMO-N’168.
ri91Leadbetter m p r
and processes. Sphn^^'^00^'
121] LTanu’
detectln9 o^ng observations in samples. NTIS-BRL N°1713.
£) ^ffmes and
ted properties of random sequences
appfcatjons.    John    wifey    and    sons    K'    ’’    TeU9e,S'    1      <2004>-    Statistics    of    extremes.    Theory    and [22] Castillo, L, Hadi, A.S., Balakrishnan, N. Sarabia J M nnf«\
2 °°5)‘
.
in engineering anq soencn lkh uri ' L^lKouegoda. N T.,
e^?/c*eerx MacGraw HilL [24] Smith, R,L. (1989). Extreme
'
^alue and related models with
&nd retiaMity for cm/ and en vironmen tai
«7 sr
n
mwtai on,e An e“m’,e °OTe
1
test for
'       6?-
' norrr|3lity with mean and variance unknown.
100 WHO R SPOOJB (PRE-FEASIBILITY
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studio pietrangeti. romcDABU5 HPP$
FEASIBILITY STUD K Report
169 of 169
[27] Helsel. D R. and R.M. Hirsch (1992). 5£j£!$£acj/ Methods in Water Resources, Elsevier. NY.
Pr
[28] Parzen. E. (1979). Non-parametrlc statistical data modeling. Journal of the American Statistical Association. 74 (365),
[29]      Mavooga T., (2010). Seismic hazard assessment and -/olcancjgenlc seismicity for the Democratic Republic of Congo and surroundings areas. Western Rjft Valley of Africa;
[30]      Chocowicz J. (2005), The East African rift system;
[31]      Abdalla J.A., Mohamedzen Y. E-A., Wahab A.A. (2001). Probabilistic Seismic Hazard Assessment of Sudan and its Vidrity,
[32]      Mldzj. V, et al., (1999). Seismic hazard assessment in Eastern and Southern Africa.
[33]      Twesigomwe E. (1997). Seismic hazard In Uganda. Journal of African Earth Sciences
[34]      Jonathan F. (1996), Some aspects of seismicity in Zimbabwe and eastern and southern Africa. [35]      1999. "Manual EM 1110-2-6053* by USAGE (U.S. Army Corps of Engineers);
[36]      1989. "ICOLD BULLETIN n. 148' by International Commission of Large Dams;
[37]      2003. Eurocode 8 (Design of structures for earthquake resistance) by Technical Committee CEN. [38]      2002 Howie. Dabus Medium Hydropower Project, Reconnaissance Study
[39]      2013-Parsons. Ethiopian Power System EXPANSION Master Plan: Interim Report Volume 4 - Transmission Planning
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[41]      Final Interim Report Module IB Transmission Interconnection Code [42]      http://www.eepcQ.gov.et/project
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I

Summery

Dabus Hydropower Project Summary

Dabus Hydropower Project Pre-Feasibility Study Summary

Project Overview

The Dabus Hydropower Project involves the development of a cascade system along the Dabus River in Ethiopia, consisting of two main plants:

        Key Project Components
        Dabus ch-130: 304 MW capacity, 1,293 GWh/year production
Dabus ch-108: 494 MW capacity, 2,140 GWh/year production
Transmission system connecting to Ethiopian grid
Total investment with significant economic returns

    

Technical Specifications

Dabus ch-130 Plant

Feature	Specification
Dam Type	RCC Gravity Dam (55m height, 1,610m length)
Reservoir Capacity	2,470 Mm³ (live storage: 2,335 Mm³)
Waterways	5.5km tunnel, 16m diameter surge shaft, 2.1km penstocks
Turbines	2 × 152 MW Francis turbines

Dabus ch-108 Plant

Feature	Specification
Dam Type	RCC Gravity Dam (82m height, 444m length)
Reservoir Capacity	53 Mm³ (live storage: 18 Mm³)
Waterways	6.2km tunnel, 16m diameter surge shaft, 970m penstocks
Turbines	2 × 247 MW Francis turbines

Key Studies and Findings

Topography

Comprehensive surveys conducted using:

SRTM 30×30m database for catchment areas
Drone surveys for dam and powerhouse areas
Pleiades satellite imagery (50cm accuracy)
22 Ground Control Points established

Hydrology

Key hydrological findings:

Multi-annual average discharge at Dabus near Assosa: ~146 m³/s
Design flood (10,000 year return period): 2,160 m³/s
Sediment yield: 500 t/km²/year

Geology and Geotechnics

Main geological characteristics:

Dam sites founded on Precambrian meta-granite and meta-granodiorite
Rock quality generally "strong" to "very strong"
Quarry sites identified for construction materials

Seismic Assessment

Return Period (years)	Peak Ground Acceleration (PGA/g)
475	0.05
950	0.07
2,500	0.10
10,000	0.15

Economic Analysis

The project shows strong economic viability:

Plant	IRR	Payback Period
Dabus ch-130	~18%	4 years
Dabus ch-108	30-33%	2 years

Client Comments and Consultant Responses

The document includes extensive comments from the client (Ministry of Water, Irrigation and Energy) and detailed responses from the consultant (Studio Ing. G. Pietrangeli). Key areas addressed include:

Hydrological modeling approaches
Dam safety considerations
Environmental impact assessment requirements
Transmission system design
Social and economic impacts

The consultant has committed to address all comments in Revision B of the Pre-Feasibility Report.

Next Steps

The project will proceed to the Feasibility Study phase, which will address:

More detailed site investigations
Refined engineering designs
Comprehensive Environmental and Social Impact Assessment
Final transmission system design