THE FEDERAL DEMOCRATIC REPIIRUC OF ETHIOPIA
MINISTRY OF WATER RESOURCES
Arjo Dedessa Irrigation Project
Feasibility Study Report
VOLUME - I
Survey and Investigation
VOLUME - 1(a)
Annexure - 1
Topographic Survey
Annexure - 2
Geo morphological
Investigations
Annexure - 3
Geological & Geotechnical
WATER WORKS DESIGN a SUPERVISION ENTERPRISE
!N ASSOCIATION with
mniicmiNEm comnants ahd hcihocmts wow w ltd
p i> E>ii%         UMh                  Ldiiupcj
12 (IJI II 4 *iJ I Ml X'fi Frs 11 ? * I i I • • 1' ■ I! iIMfl ii d ’ ■ “rfki-iiin <k1 :lARJO-DEDESSA IRRIGATION PROJECT
FE ASIBJLJTY STU D Y RE PORT
CONTENTS OF THE REPORT
SERIAL
NO..
VOLUME NO.
PARTICULARS
1i
2
3
EXECUTIVE SUMMARY
MAIN REPORT
Part I - Report
Pan II - Maps 3 Drawings
ANNEXURES
VOLUME-I
SURVEY AND INVETIGATION (ANNEXURES - 1 to Ji
4
5
6
Volume -1 (a)
Topographic Survey, Geomorphologra! Studies 3
Geoktgicaa 3 Geotechn«ca>i investigation
Part I - Reoon
volume -1 (b)
Pan II - Appendices
VOLUME -II
NATURAL RESOURCES 1 ANNEXURES - 4 to 5)
Forestry Energy 3 Catchment D*y#loom<r*t Plan
VOLUME - 111
WATER RESOURCES (ANNEXURES - 7 to 11)
7
Volume • III
Meseoroiogicoi 4 Hyoroiogicai 3 Hydrogeoiogcai Studies
Dam 3 Aopurterant Works
a
a
10
Volume - III (o)
Pan i - Report
Part ll - Drawings
Irrigation 1 Drainage
Volume’ill fc)
Part I - Report
n
12
13
14
15
16
Part II - Drawings
Hydraulic Structures
Volume - III (dj
Part I - Report
Part II - Drawings
VOLUME IV
AGRlCUTLRUE (ANNEXURES - 1 2 to 18)
Volume - IV |a)
Soil Survey 3 Lartd Evaluation
Volume - IV (b)
Agricultural Planning
Volume - IV <c)
Livestock. Fisheries. Agricultural Mechanization 4
Agricultural Marteii^g
VOLUME - V
ENVIRONMENTAL ANO SOCIO-ECONOMIC ASPECTS
I ANNEXURES - IS to 25)
17
13
Volume -v (a)
Environment -Health 3 Socio-economic Aspects
Volume - v (b)
Organization 3 Management. Physical Infrastructure
Resettlement. Financial 3 Economic AnalysisANNEXURE- 1
TOPOGRAPHIC SURVEYArjo IHtm Irrigation Project
Topographic Survey
Table of Contents
TOPOC ftAPItic SURVEY
i.
L
j.
4
Sr
IflTRODUCTrON
RECO.VVAISSaNCE...----------------
11 ■■■III. -Mi.-.aa ■ — 
DA.M s FTE ™..— ------------------------------------------------- -------------------------------------------™»»—... 
RES ERVOi R AR EA------- —---------------- --------------------- ™™._-------------- —-------------- -—......
COMMA ND AREA------- - --- -------------------------------------- - ...........------------------- ----
6.      SAMPLE AR£AU..-.U— 
CONTROL POLVT 5 f  BENCHMARKS_______ 1       LSt OF SOFTWARE-------------
Water Works Design & Supervision Enterprise
fQ Assaclidati urliti Inlcrtontintflml CensultaoLK agd Technocrats pvt. LtdArjo Dedeua Irrtgsdon Project
Topographic Survey
May 2007
TOPOGRAPHIC SURVEY
1.
Introdu
ction
The
planning,
design  and
c
onstr
uction  of  water
resource
s  de
velopment
prefect
s  require
very 
detailed
knowledge
of
the
topographic  fea
tures  of
the
area  Acc
ordingly
detailed
topographic surveys have been earned curt for lhe dam 4 appurtenant works, reservoir area and 1he command area; and based on the details  of the survey  topographic  maps  on different scales  and contours  at required interval have been prepared to be used for planning and design of various components of the project. These are discussed below
2.       Reconnaissance
Prior to the start of the detaired surveys a rapid reconnaissance survey was made of the area from the previous Master Plan study document and 1.50,000 EMA base map to define the scope of work  and identify  lhe irrigable area and dam site. After thal a hand GPS Garmin 12xl instrument was used lor navigation and fixing the command area and dam site feature to be surveyed by total station al later dale
3.       Dam Site
After detailed topographic survey for the dam Site Che following topographic maps have been prepared
I)  topographic  map  of  dam  site  with  one  meter  contour  interval  on  following scales 1:500, 1;100Q and 1:2000.
ii)         Longitudinal  profile  at  15m  interval  along  dam  axis  and  also  along  lines parallel to dam axis at 2S m interval up to 500 m width on the both upstream and downstream of dam axis
iii)        Longitudinal  profile  along  the  center  line  of  spillway,  intake  structure  and proposed scheme of diversion
4,       Reservoir Area
The contour map with 1 C meter contour interval and scale 1 10 OOD for the reservoir area was  prepared by  generating contours  with Global Mapper since lhe area was  very inaccessible. The Digital Elevation Model {DEM) used for generation of one meter contour was based on 90 x 90 meter resolution image. National benchmarks were used to calibrate
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TopograpbteSurrey
Mij ZWi
Ule contpun Subsequently  detailed ground topographic  survey  w» undertaken to take cross-sedions  across  the reservoir al five representative locations  spread over eniire reservoir Cross  sections  were prepared based on the actual lopography  survey  and contours devetoped with DEM The comparison of the two sets of the cross sections of the reservoirs area shows that lhey are quite m agreement with each other This shows (hat process adopted is quite reliable
5.     Command Area
The detailed topographic Surveys (or (he proposed command area indicating all tenures & natural dramage lines have been earned ou( and the topographic maps on scale 1 10.000 wlh contour interval of one meler have been prepared. Against a lentalive command area of 16.600 ha as  given m the TOR, the topographic  Surveys  have been earned Out for 27.000 ha Altogether, ine following seven topographic maps have been prepared fpr the command area
Topographic map tof Arjo UedtaM Imgaban Shm N(i 1 to t
6  Sample Area
+
Detailed topographic surveys for sample area measuring about 500 ha have been carried out anti topographic maps an scare I 2000 with contour mterv.il of 0 5 meler have been prepared These maps will be util-zed for detailed planning of rhe distribution system
7. Control Points / Benchmarks
A total of 72 control points were established and concrete pillars were constructed al all these control points fhe topographic survey has been transferred from Ethiopian Mapping Agency  (EMP) references  and connected to all project topo-maps  and references Permanent benchmark  has  also been established al al these points  The values. of benchmark along with location of these control points are available in fable 1 I
8. Use of Software
The topographic surveys have been carried out with the heip of r«al stations The delaiFs of the topographic surveys obtained from the field have been ted directly into the computer for processing and preparai+nn of maps on desired scale and requued contour interval The encoding of lhe surveying data to computer prggr^nrwne and processing to produce topographic maps profiles iayouls and boundary mark have been done by using LIS CAD Land C-AC surfer and crv«i designer software
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In fcHtlatiioq wjib InlerrondorniiKoosulrjniti aori T«hnDcrat3 Pvi Ltd.
2Arjo Dtdtssa lirigition Project
Topographic Survey
T il'lr LI Bench Mirk Dili
Hit 2007
EAST         NORTH      ELEVATION
REMARK
24274 S      MIWOt'M                 l<3l?«
0MC
24212* 942 W 746!J  IWI.UJ
MJCtJ 943147.1124 JJ13 411
242101 943 074.94 Bl 1324.966
BAM
BM-atlNSj
BM-2AilNS)
NJ9W
94326O24B2              1391,6*3
BM-3
2441II
943364 909
13*0.864
BM-3A
242756
941903.1133
1432 625
BMB
242614
9406266436
94Q« J. 252
1345 242
1337.652
BM <3
242106
BM 1G
2439W        942055.7104 24 JAM      | 94207J 6493 242464      94J6I9MM 2432*5      94259I.M6I
I36S5I5
TP-GH
1367.355
1392 135
TP G11A
SM-MA-1
1420.167
ELM-MA-3-
243236
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1400.765
ElM-MA^
WJ22»
942539.3341
J 3*9.115
BM MA-5
243241
243200
942536. fSDft
1397.tS2
BM MA-6
942463.17*1
BM.MA-T
24J30?        942476 44J79 243272            *Jj4i4 y?jj _ 24JW       V-IJMK 9J7I
1)40 Ml
1366.661
1424 712
1429 7Hft
ELM MA-6
0M AX-2
BMAX1
242TM
24I?46 f
2426U
941931 1?25
I4J7 344             BM AX3
94BPMJ ISJ4
I4JI0F9             B MAX-5
2427.S2 ’ 2396*1 1 2P253
9BJ8&9 EH 1.2
•MIKTil 1174 94261? 72 l« 944231 5579
14(5.601
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Water Works Design & Supervision Enterprise
In Ass&eladim with iQtereonflueaul ContuELanu andTfithSfrtrats Pvt. U4
3ArJ o Dedesaa irrigation Project
Topographic Snrvoy
Kay 2007
Tahls 1.1 [Cont...J B«nch Mark Data
EAST           NORTH        ELEVATIO.* ST                AEAMAK
------ —
21391- H 951969 20) 1  1321 443
&M-1Q
21*03        I 951993 832 )            1322.904             BM ISA
JI443        2 952010.44) 8            1335 2?!              RW
21S2B      6 953472.2571t            1301741               BM-H
21522        0 9533117991              1301 043             BW-HA
JJ«?      K 94*633 6774              1389 991
BM-443A
22664          94] 876.9231
1384 721
BM-4G
224194         946721.1277
14)1 121
0M-9B
21842A           951143)039
I3CM 711
0M-1A
213004 951O6L7923
217986 952161.2236
215542 J 948)47 24)7
2155)5 94837) 2142
I3H.35I
0M-3A
uro sot
BStBr)
1395 541               HM-iG
1395 151             WKUA
224929
1 .
930733 9)6 1322 842 BM.-MA-IJ
22450? 1 950957 7891
B.M-MA44
221941 ' 954043.6531
1354.496
1541 753
rjiM.MAdJ
22192)         I 9540)9 8439
1536335
ftMMA.l1
rm 67 ' 95)980 2967
222145 95)996 1451
1442 464
BMMA-15
1453.714
BM-MA-16
21*027           931143.7*59
1)16.445
BM Bz2 KlAiTcOl stakm FC ISO
218453 j         9*121*0548
1306 2)6
BSftlZi! iMh*
224198
946783.2)22
1325 772
BM'MA-W
22.1716
2219W
■MTIfXHIJll 946^10.0293
1322 789
1)68 58 J
I5M-MA 2n
BM-MAJO
221976
946759 3177                  1367 659
BM'MA-II
224340
21*412
BM-MA 12
951172 3198
13*4 501
1307 )8|
BM BZ
216)61       WS474 4412
I2965t|
2154*4       956658 268*
1249021            |  A- 5M6
215658 >56600 646) . 12*3 421 A- 0647
T
213427 L
>56025  8$6  ;
1T
1418 931
9*4 MAfd
Water Works Design & Supervision Enierprtsa
In AsioclaJOn '*1tb Inteieondutnial GontulUnii andTtebccfriuFit. Ud
4Arja tkeduil
PToJtcl
M*; 2007
Tabl« 1  9 iCofrL...) 0«nch MM □*»
LAST         NORTH           ELEVATION                             REMAAJf
11*40$
V'S              1410 95 1
BM M«S
217724 9591,3(1603* 1)12 II*
21'MS | 95900) TOO1 11120*9
COMMON Pt BM JrGugw A B«in,. COMMON PT OM l.Gugu A
|
2194*9     W)?2 «6*4 2I9J3)        9513*1 55*^
211071 93*135 63)7
UH) 051
0M 1*
12*1 131
1299 371
6M-15A
B.M 17
2110)9
95*223 SI 12
1)02 771
BM I7A
22 W4
2297*7
235*77
■)*21<W42«
136$ M2
0MF3
Mini jm<            H*4 *27              BM F4
.. .
■14 592* 02 S2               I1I7.ITI               BM-1G
235*86       941850 8)03
1317011
BM-JG
2)7171             MiJIJ M4) 238005       945922 1101 2JAl'U        04 5422. JW
l)|6 5)1
1)19 4?1
A M14
B M14A
1)24 Ul                BM15
2J9HI 94 5554 94-76 lu'-flll BM15A
24UMJ7 *■■ 1 «5ni hM9? 1)51 4*| BM BZl
24014) 944*2? M?D2 13)1561
2M29? 0494)2 5391 1321 751
0M-BZ2
El M 0Z2
24$$JJ     ■M49II5MII _                1329J5I               e MSZ3 245256 .    9452l» 507-1                 1 US ?*,              A M 9Z4
*10199     ‘J4S09* :<?”■>             l)*0 9)1           BM F2
2409*2    ■J4.1AI.M9 M614
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UM 313               0MF1
2152*1      S5M5Sul$B                  12*1 645          6M-6C
211*35   956 HJL 4301              1281 115         BM-7C
21535'      055771 )952
1294 115              INSr At BM‘ G
L
.’1 ..'>r, OjULrJ* 0JD4 Ml’ ul* BM2 G
■      ■s*H'M LI1??
2154JO     96149D.017*
I2”4 165              H\| ial
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Waiflr Works Design & Supervision Enterprise
In AhiMlaUna *lth ImrrrjDLnfDlaJ Den ml tin ft uJ Trrltnorrits Pvt Lid.
5ANNEXURE-2
GEOMORPHOLOGICAL
STUDIESArji iMdusi Irdfuaon Project
May 2007
TABLE OF CONTENTS
TABLE DF CONTENTS
LIST OF FIGURES.....
1. INTRODUCTION. „..,
t
f 1 iSFrtP-Ai
111 M IIM I11 ■*■ 11 ■
.•■itaai.imr I
1.2 Objectives
i i mi 111 rw ■ i ■ ■ i 111
UMEhiODOLOGv 
2
a 4 Previous GEow>RPMaLO
G*t*L Works
.3
2. LANDFORM S DESCRIPTION
OF TH.E AREA
,.3
2.1 General PmvMGRAPH’y. .
i>U— UIUMU.U4I.IU ■111*11
....-■■.■-.■■.-^■■■■■■3
2.2 LAN&SCAPf EVCtiJITON
kllL&llIlAKM 11 lajlllll
3
3.
FLUVIAL GEOMORPHOLOGY OF THE DlOESA RIVER
.*
3 i General..
12 Stream TypC
3.3 LONGiTV&NAi PROFILE ...
34 EnTHEMCHMEiNT....
3 SCmannei Reaches
kAiBMAiaaaatiit ■oaiiaatvti
11 • »wiftrw rt
.III Mllliail.IIB.lllll.llll 1 10
I I lt«» • tTT'TTf
13
._.I5
16
3.6Cmannol cmstuR&ant.e aw response potential
4.
RIVER CHANNEL...
kMIL&J IMUlllBillMBJIlLMimillia III 11111 SJ11 IB 111 III11 BB
< 1 General ..
4 2 Dam On ThC O0E$a Rivffi
5 FUTURE WORK ............ . ............
5.1         StaoE’Time Relations
... 19-
Ifl
IIIMtllllMIIIMIIIH1
isttauiM 2
I
iiiraiiiMiiimiui
22
6. REFRENCES...-..,.
r»9 t^Tfr*** »r«
...
23
Water Works Design & Supervision Enterprise
!□ AHDd &{Jbd wJti iDltrcontineaLiJ CtitiiUltaiiCS Idd TcCtiiMriLs Pvt. Ltd.
rArjo DsdesM trrl gallon Project Geom&rpbDtofical [nTestt£illoDS
M*J2W7
list of figures
Fig 2.1 Type of landforms around the Arjo-Dtdesa Irrigation Project Area . ...................... 5
Fig 2 2 Geafogtal units around Arjo-Didesa Irrigation Project area.....................
.7
Fig 2 3 Major Lineaments around the Aqo-DuJesa Irrigation Project area....................        8
Fig 3.1 Photographs of Didesa River features ..
Fig 3.2 Plan view of Didesa River around the Dam sic 
......... ........................... 11
....... 12
Fig 3.3 LongHudinal) cros^secfponal and plan view of major stream types...................... 12
Fig 3.4 Longitudi&l profits along lhe Dictesa River channel......................... .......................13
Fig 3.5 River across sections across the Didesa River.......................... ........ ..... 14
Fig 3-6 Types of channel entrenchment........................ .......... ....... ............................ >...... 15
Fig 3 7 Photograph of pool-riffle channel reach of the Didesa River, downstream □(
the Dam axis . ........ ......... ................... ....... -........ .....
Fig 3 9 The spatial distribution of reach types with a drainage basm
.......... ........ ............ 16
iin^%UShnC@Siar ■ ■ ■ in- ■ m--s        ...... ...... . . .. t ■       ■ . ■     ■■
Waler Works Design & Supervision Enterprise
In Association with Inlcrcoathienlal CdfiSUJUots ud Technocrats Pvt. Ltd
I]AfJ® Dadeui Irrigation Project Geotnorphologie InvesdgttlOM
GEOMORPHOLOGIC INVESTIGATIONS
1* INTRODUCTION
1.1 General
May ZM7
An understanding of landscape type of an area, rivtr-and stream-channel response to both natural and human factors is necessary for addressing several important issues including me prelection of property  and structures; dam-brlckgfrsrle selection, design, and maintenance; protection and rehabiMatmn of riparian and aquatic habitat; channel capacity; ground-water lavefs. general aesthetics; and recreation The channel bank  erosion that accompanies natural channel migration across a flood plain represents a constant threat to property and structures located m or near th* channel Human disturbances such as dams, channelization, in-channel sand and gravel extraction. and urbanization introduce additional instability  tD which rivers  and streams  adjust by  such processes  as  channel-bed degradation (erosion), channel-bed aggradation (deposdron of material}, and 'or channel widening These adjustments, which represent 1he channel's attempt to establish a new approximate equilibrium, pose additional threats to property, structures, and habitat located m or near the affected channels.
Channel adiustmenls are ma|Cr concern for several reasons. A significant lowering of |he channel bed poses  an immediate threal to bridge pier foundations  as  well as  buried pipelines and cables In addition, significant bed lewenng increases bank height and bank instability lhal may trigger channel widening Channel aggiadarpn raises me bed elevalion. reduces channel capacity and increases the likelihood of Hooding Any channel change that occurs on the mainstream rivers and streams also may migrate upstream on the tributaries where additional property structures, and habrtal may be at risk Finally, any long-term channel adjustment presses a<so may instigate or worsen local scour problems
1,2 Objectives
The mam aim o( inis study is to present landscape types in the ArjO- Didesa Irrigahon Project area  and  fluvial  geomorphology  of  lhe  site,  and  predict  future  changes  that  mrght  be expected due to the proposed dam project through a combination of field observation data analysis and interpretation
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JIrjo Pedesu LrrlfiUoE Project
Geomorphologic [uTustlgatlotM
Haj 2007
Knowledge  Of  geomorpholgicBl  oondlions  of  an  area  can  be  used  to  plan  and  undertake economically a subsurface investigation and design
1.3 Melhodology
The ability to recognize both landforms and fluvial geomorphology of an area on topographic maps, aenal photographs and site investigation combined with a geologic knowledge of the area, is very important when characterising a site and when tocatmg poleniiai sources for construction matenals
Land-sat imagery  and aenaf photographs  interpretation combined wtto brief on-sile inspection were used to determine 1he geomorphology of Arjo^&desa Irngation projecl area near toe dam site The fand-sat imagery available is in 7 bands, is of 1:250. GOO stale taken to 2003. The image cover the project area and Ils environs, was used m constructing lhe preacoenvri.THSnt of toe project site in general The aenal photographs are those taken in 1973 and are 1 02. MO scale Stereoscopic  interpretation of the aerial photographs  was 
done to 1race lineaments, landforms and plan view of the River channel morphology around the dam site The site inspection nciuded studying depositional landforms, sediment types and channel geomelry. While on-si1e, photographs  of the stream channel and the surrounding physiography  has  been taken Analysis  of photographs-derived landforms description and channel information 13, included in assessment of the plaeoenvimoment and Changes in bank full channel Width and shape gf the channel upstream and downstream from the dam
1.4 Previous Goonriorphologlcal Works
GeofliorjtotJi'ogicBl investigations around lhe pro|ttot area nave lo date barely scratched rhe surface. These may have ma|0r implications for a design of an engineering structure on lhe Didesa River USBR (1864) slated thaL toe mam CWesa River around rhe dam site and in toe reservoir Hows on volcanic rocks from headwaters to toe approximate area of Qdesa-Wama function,  m  toe  bend  where  toe  River  lurns  to  toe  northwest  and  then  entrenched  in melamorphto rooks to (he rest of the way to lhe Blue Nile According IO V7APCOS 11890) and BCEOM (1998) studies toe NNW flow of the Didesa Rmer around lhe proposed dam site is controlled by faults. In the upper reaches the bedrock constituting the River valley is volcanic of Tertiary age
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2Arjo Dedetsi lHle*tjon Project
May 2007
2, LANDFORMS DESCRIPTION OF THE AREA
2,1 General Physiography
The Didesa Irrigation Project area and its environ* are divided in to three physiographic regions. These are the Didesa pfane (1270* 1400m). uplands ex cham of ridges flflOD- 2500m). plateau (greater than 250Om) and pediments (FJg 2.1 a, b 4 c).
The Didesa plane is found dose to the Didesa River. The chains of ndges are rugged and dissected by  the tributaries  of Didesa River The pfataau is found on top of the basaltic ridges. The Dicfesa plane is covered by elluvisl soils. They are mainly s-ity to sandy reddish brown and black cotton sods, are products of the break down of granitoids and basalts. While the Plateau area being made of Tertiary  basalt* the uplands  are made up of Precambrian granitoids and Tertiary volcanic rock* (Fig 2.2}. Pediments are gently sloping trosional surfaces, exposing volcanic and basement bedrocks or covered with a veneer of alluvium that can be transported off the pediment surface wi|h |he next significant flood evon1
Lineaments wilh general 1rend of NE-SW, NW-SE and rarely N-S and E-W can te traced from aerial photographs (Fig 2.3) for the area Locally, the lineaments govern stream and river courses and atgimgnt of ridges They am mainly straight to curvilinear .wth variable lengths
2.2 Landscape evolution
Knowledge of geological history helps us to understand the present-day landscape of the Didesa valley. Precambrian rocks are exposed in the uplifts surrounding the plateau in the Aqo-Didesa area They  consist of highly  metamorphosed gneissic  country  rocks  intruded by  granges  Approximately  900 million years  ago an east-west compression produced a continent scale North-South trending orogenic belt and the associated Precambrian rocks in western Ethiopia Different Tertiary  to Quaternary  volcanic  rocks  1iial come th«ough the regional fault lines and fissures characterize the rest of the bedroc* geology ol the Arjo- Didesa area. Surface deposits, alluvium, residual and colluvial sculs cover ihe river valley and the sfope areas.
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M17ZMT7
Irt Ethiopia and In the Arjo-DWesa area landscape development srnca early Tertary tlM been dommaled by episodes of continent'wide uplift. The epi orogenic uptfl of Afo-AratSa. chat do« not Involve deformation, which is related to mantle plume whose decompression melting generated an enormous oulpounng of large velums of magmaP formed LM plaleau Wjlcaoic  regions  in Ethiopia The fissures  that MTMd M a condud for lhe magma are created by upraised snd up parched landmass (proto nftingj n response Io Ihase uplifts volcanic aclivity dominated tut almost nvers eroded downward, hill slopes ffitTMtad end deep fiver valleys were formed Over frme. lh«* processes han resorted m me removal Of subslanliar thickness of rocks (rom aCfMS large areas of exposing many of the formerly buried land Present day streams estabfislwd chtHr courses al this lime and because they were lifted high above base level they began to rigorously down cuf Deep entrenchment of streams and differential erosion df the plateau began at this time. The slraam courses were established on flat lying undaforoied strala. tn valleys such as 1hat of the Didesa, Ihis process &f erosion is conlmumg today. SJtodugh at 1 rate «tpeFCtprib*n during a human lifetime
As rocks nave been eroded deep river valleys have Peen formed, separated from neighboring valleys by Steep, fist-topped escarpments or hills nsing up to <3GCm above lhe adjacenl valley floors The escarpmanLs and hills are mainly formed in volcanic rocks with trachyte and phonolite ofrEn capping iheSe escarpments or forming prominent ouulder- Sfrewn ridges Soil Covers are typically thick hu1 grassy ground covers are dhlHUlve.
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Fig 2.1(a); Type erf lancHccms around the Arjo-Uidesa Irrigation Project Area
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Fig. 2.1 (a)
F.g 2.1 [c|
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Fig 2.2: Geological units abound Arjo-Dicfesa Irrigation Project area
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l'ij> 2.3: Mapi I iiwoment* around the \ijo-Didesa hrie.mou Ph»ic<i area
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X FLUVIAL GEOMORPHOLOGY OF THE DFDESA RIVER
3,1 Genera)
!Uy2007
An understanding of the channel geomorphology of a river Is important for several issues including the protection of nparian (sireamside) resources; protection of wldlrf# habitat (for example, the in-stream habitat for the threatened Hippos), and bank stabifeatron related to property loss Also, such information may be important lor the site selection, design, and construction of dams
The focus of this study is the channel of the Didesa River, upstream and down stream of the Dam axis  Throughout this  reach an anastomising. silt-bed channel characterises  the Drdesa River.
The Ddesa channel bed around the dam site » situaled on erosion-resistant bedrock.
Efiuvium in the Dicfesa River Valley is typified by Silt and clay with a basal layer of sand and
gravel The bank materials consist mostly of cohesive sift and clay that are relatively 
resistant to erosion compared to sand banks. Also the banks typically have partial-In
complete mature tree cover that may enhance or reduce bank stability al specific locations 
(Fig 3.1).
Along  its  couvse.  the  Didesa  River  is  presently  eroding  perhaps  upstream  areas  where  it's young  Pronounced  and  abrupt  changes  in  the  character  of  the  nrver  occur  where  it  flows from  one  rock  type  onto  another  For  instance.  when  developed  on  volcanic  rocks,  valleys are   characteristically   wide   and   the   river   meanders   within   broad   floodpla-ns   marked   by numerous   oxbow  and  abandoned  channels  In  contrast  on  basement  recks,  valleys  are narrower,  ihe  river  toBowS  comparatively  a  stra  ghter  course,  and  floodplain  development  is limited (Fig 2.2)
Traditionally the channel pattern of a river has oeen classified into straight, meandering and braiding types. However, a more recent classification incorporates ana branching (including
anastomosing) as a recognizable pattern The diversity in the channel patterns is caused due to variably of water discharge sediment load, bedrock outcrops ground slopes 
human achnratoes and tectonic influence. Tectonic movements change the gradient of a river
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and modify  the valley. Alluvial rivers  are not significantly  affected or constrained by  bedrock and flow through sediments  that have been eroded and deposited by  them or their ancestors.
3.2 Stream Type
From   the   Figs   3.1a,   3.2   the   Didesa   River   can   be   classified   into   Stream   type   DA   - Anastomlsing stream (FFg 3.3} based on the following characteristics:
Qenrtfjrh   Anastomising   narrow   and   deep   with   expansive   welt   vegetated   Pood   plain   and associated wetlands Very gentile relief with highly variable sinuosity. Stable stream banks
Landform/soJI/featurcs; broad, tow gradient valleys  'with fine ahuvujm/lacustrme. Anastomising geological control crealmg fine deposition wrih wen vegelated bars  that are laterally Stable with broad wetland flood plains
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Fig 3.1: Photographs of Didesa River fealures a) anastomising b) bank vegelahon
cove* ano c) silty bank material
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Fig 3.2: Plan view Of Didesa River argund the Dam site
Fig 3.3: Longitudinal cross-seclionai and plan view of major stream types (Modified after Rosgen. 199-a j
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3J Longitudinal profife
May2W
Plol of lhe relative elevation of a streambed from headwaters  to mouth is  called rts 'longitudinal proWe’. The longitudinal profile of a stream i» generally  concave upward which is in accordance with the steady  downstream decrease in slope. The longitudinal profile and
river  cross  section  of  Lhe  Odesa  Rrver  channel,  which  is  inferred  from  global  mapper  5
served  as  breaking  the  stream  reaches  into  slope  categories  that  reflect  profile  morphology (Fla  3,4  and  Fig  3.5).  Fig  3.3  represents  types  of  dominant  slope  ranges  of  major  stream types   (Rosegen,   1994)   The   Dldesa   river   longitudinal   profile   can   be   classified   into   slope range  of  E,  with  a  dominant  stope  range  of  <  2%,  and  belongs  to  those  of  gentle-gradient types.  Bed  features  generally  are  related  to  channel  slope.  In  such  rivers,  there  ■$  a  little change  in  the  elevation  of  the  riverbed  along  lhe  river  channel  They  are  also  chracterised  by lack   of   high   derbis   transport   (Supply   limited)   and   absence   of   waterfarts,   bed   forms   are relatively   stable.   Longitudinal   profile   also   mdicaits   a   stream   reaches   (Fig   3.4)   related   to slope  categories  that  reflect  profile  morpholpgjes  Thai  is  to  say  Ihat  riverbed  features  are related  to  a  channel  slope.  Alluvial  bar  development  requires  a  large  width-fo-depth  ratio  and smaU  gram  easily  mobilized  and  aggraded  by  flow  Gradient  and  bed-  feature  relationships are ntegral to the delineation of Stream type caleganes
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From Pos: 2421 19.67. 942S7B.49
■870m
To Pos: 243620.14
943141J J
1400  m
1380  m
1360  m
1340  m
1320 m
From Pos: 243102.00. 942300
1380 m
1360 m-
1340 m
ream
1320 m
0m
670m
To Pos; 243605.00, 942809 00
Fig  3.5:  River  across  sections  across  the  Didesa  Rivera)  up  slream  of  the  Dam  axis  b)  at the dam axis c) down stream of the darn axis Note Distances in between are indicated
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3.4
Entr
enchme
nt
An
important
element
of
delineation  4
relationship  of
a  hW  to
its  valley 
or/and
landform
features. This interrelationship determines whether the river is deeply i nosed or entrenched m the valley  floor ex  in the deposit feature. Entrenchment is  defined as  the vertical containment of r™er. the degree to which ti Is  incised in the valley  floor. This  makes  an rmportant distinction of whether lhe surface adjacent to 1he channel is a frequent flood plain, a lorrace or is  outsda of a flood prone area The flood-prone ares is defined as lhe width measured at an eievabon, which is determined at Twice the maximum bank-full depth The entrenchment ralid «S the ratio of flood prone area to the t»nk-ful surface width of lhe Channel. Campanng the lealures  of River Dodessa as  given in Fig 3.4 and Fig 3 5 with chose in Fig. 3.6 (Rosgen, 1594), it can be concluded that the CWesa River belongs  to moderately  entrenched river with entrenchment ratio ol 1.41 - 2 2, typified by  a well developed floodpfain
Fig 3.6: Types of channel enirenchmem tRosgen 1994)
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3.5 Channel Reaches
May 2007
Channel reaches  exhibit similar bed farms  over stretches  of streams, across  channel widths and its  length The Didesa River is  characterised by  poot-nffle reaches  (Fig 3.7) in the length under study  H is  unconfined by  valley  wall and consists  of oscillating sequence of bars, pools  arm nffles  resulting from oscillating cross-channet flow that causes  flow convergence and scour on alternating banks  of the channel Concordant downstream flow divergence on the opposite side of the channel results  tn local sediment accumulation in discrete bars  Bed form and grain roughness  provide the pnmary  flow resistance in pool- nffle reaches. Bed forms  m many  pool-nffle reaches  are relatively  slable morphologic features  even though the matenal forming the bed is  transported annually  Pool-riffle reaches  are commonly  armored exhibiting a near bank  fuEl threshold for general bed surface mobility
Fig 3.7: Photograph of pool-nffle channel reach of the Didesa Oam axis
River downstream of the
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3.6 Channel disturbance and response potential
Response   to   environmental   change   varies   tor   different   channel   Reach   morphologies   are associated   with   physical   morphologies   and   the   environment   that   limit   the   range   and magnitude  of  possible  channel  responses  to  changes  in  hydrualic  discharge  and  sediment Supply.   The   specific   response   of   a   particufar   channel   reach   reflects   the   intensity   of disturbance  or  the  magnitude  of  change  as  well  as  the  conditions  cf  the  reach  Table  3.1 iltugtrale*  patterns  of  potential  response  for  Didesa  Rrver  channel  reaches  Lower  gradient pooto-rrtfle    reaches    become    progressively    more    responsive    to    altered    discharge    and sediment   supply   with   decreasing   ratios   of   transport   capacity   Io   sediment   supply,   smaller grain  sizes,  and  less  Ghannel  confinement  Pool-nfflB  reaches  as  ih  Gumera  River  project area  snows  potentially  greater  response  of  bed  surface  torture,  scour  depth  and  generally have   significant   potential   for   channel   geometry   response   to   perturfautatton   m   sediment supply   and   discharge   Changes   in   both   channaf   and   valley   storage   are   also   likely responses, as well as channel roughness due to alteration of channel sinuosity.
Interprjjjed Didesa River poor - riffle reach level channel response Potential to moderate changes in Sediment supply and discharges (» «likely to charge : = unhkely Io change) Table 3.1 (Modified from Montgomery and Bufingfon, 19971
Grain
Width De
pth Roug
hne
ss Scour
depth size
Sediment Stope Storage
Response
+
+
Reponsts 
segments 
consist
of
tower
gradient
more
transport-limited   reaches   m   which
significant morphologic  adjustment occurs  in response to increased sediment supply. The spafial distribution of source, Iransport and response segments  generally  reflects  the distnbution potential of impacts  and recovery  times  DHtribution of these segment types define watershed - scare patterns  « sensitivity  to altered discharge and sediment supply (Fig 3.7) Sediments  delivered to transport segments  rapidly  propagales  to downsUeam response segments, where sediment accumulates  Consequently, locations  in (he channel network  where transport segments  flow into response segments  indicate places  particularly susceptible to rmpacis  from accelerated sediment supply  Because response segments  aw sensitive to increases m sediment supply, mey are excellent sites for monitoring the effects
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of  upstream  actions  and  can  be  considered  locations  of  critical  importance  in  watershed monitoring programs
Fig 3.8; The spatial distribution of reach
types with
distribution of potential impacts and responses
D»desa River
drainage basin influences the to disturbance
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4. dam HYDRAULICS AND RELATED GEOMORPHIC EFFECTS ON DIDESSA RIVER CHANNEL
4.1 General
Majr 2007
A  River  modibcabon  by  a  reservoir  will  alter  significantly  the  flow  characteristics  the  channel cross-sections. and the longitudinal profile of a river.
Dam construction changes  both the discharge regime and sediment supply  downstream channels  resulting In channel incision, constriction or widening and changes  rn channel substrate. Studies  document channel incision and bed surface coarsening immediately 
down st re am of dams  in response to sediment impediment (Williams  and Wolman. 1984) Tributanes  channels  also may  incise in response to mainstream channel incision through upstream knick  point propagation from lhe<r confluence Decrease In discharge below a dam may  cause narrowing of actrve channel width. Tributaries  sediment inputs  downstream of dams  may  cause channel aggravations  and accumulation (Alien cl al, 1989) and accumulation of fine sediment because at decreased sransport capacity  resulting from dam construction Differences  in channel form and function affect the probability  of specific response to a given perturfoatran. Channel classification can aid interpretation and assessment of response potential by grouping similar physical environments
4.2 Dam on the Didesa River
The Didesa Rver channel is  a stable river channel, naturally  anastomises  across  <ts  valley over bme while maintaining approximately  the same aoiS-sectional shape It has  shown Ihai it has  a moderate gradient Therefore the expected changes  in channel geometry  due to damming of Ihe river may be used to infer channel instability
Reservoirs  can be thought of a$ permanent floods  that affect ihe river system upstream from ihe dam in a quite different way  than the annual floods, which lasted far only  a lew weeks  or perhaps  a mcrnlh. The planned reservoir will cover many  of the pools, riffles  and rapids across the reach of the Didesa River.
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Dants crap sediment in the upper reaches of reserve** where tnbirtartes enter the relatively quiet water of lhe reservoir and deposit sediment Io create deltas and wetlands, which can have effects on the environment that are both positive and negative
Downstream changes in the stream flow regime atlnbuted to che operation of a dam might be expected due to a decrease in the magnitude of peak  flows  and an increase in ihe magnitude of low flows. Also, post-dam suspended-sediment concentrations  could get substantially  reduced rmmediaftely  downstream from the dam. Farther downstream, several tnbutanes chai contribuie unregulated flow to the Oidesa Rnrer will moderate ihese effects
Dams  result  m  upstream  and  downstream  changes  In  a  river  channel  Upstream  from  (he dams a backwater pool is created, the depth and extent of which depend on lhe height of the individual dam and she slope of the channel Wflhiri thus backwater reach of the nver. lhe decrease in flow velocity may result in some deposition of material on the channel bed As lhe  water  flows  over  the  dam.  its  velocity  and  erosive  power  increase  The  potential consequences    of    these    effects    include    increased    channel   bed   and   bank   erosion rffimediately  downstream  from  the  dam  as  the  nver  expends  the  extra  energy.  Most  □(  lhe erosion occurs during hrgh flows when the nver's erosive power is the greatest Bed erosion may  create  a  plunge  pool  at  the  base  of  lhe  dam.  with  the  formation  of  companion depositional  bars just downstream as lhe river loses its ability to transport the scoured bed load  The  depositional  bars  may  divert  the  flow  toward  or  away  from  lhe  banks  Bank erosion  results  m  channel  widening  and  also  may  contribute  to  the  formation  01  the deposflponaJ bars The form and downstream extent of these effects vary from site to Site as affected  by  human-caused  and  natural  conditions  that  may  either  promote  or  inhibit  lhe nver's response to 1he overflow dams
Effects of damming w«ll change the character of the Ctdesa Jt wifl cause lhe water-surface area, the channel area and lhe sand bars to decrease The change m channel-surface area rs relatively easy to determine from maps and aerial photographs However, the thickness of sediment on the flood plains is not as easy to determine This stored sedimeni also contains targe amounts  of slcxed carbon, which is  of current concern to many  research scientists interested 4n rhe global carbon oubget (Stella rd 19^0)
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5- FUTURE WORK
Various  fcmns  of  channel  activity.  Including  widening,  tank  erosion.  meantar  movement, and  degradation,  have  been  identified  in  the  downstream  pad  of  the  Dldesa  River  basin. Because the channel spdlvrty wourd be intensified during the period following impoundment Of  stream  now  and  sediment  designed  reservoir.  Il  would  be  powuMe  that  part  of  the channel  changes  is  the  result  of  changes  in  hydrologic  regimen  by  the  reservoirs.  It  is desirable  to  investigate  this  possibility,  as  well  as  to  explore  feasible  methods  of  reducing the economic losses and enhancing Brie anwonmenlal character resu 1i ng from the channel changes
A  future  geomorphologic  investigation  should  be  able  to  provide  sediment  and  channeL geometry  data  and  interpretations  of  those  data  pertinent  to  1he  bank-stabilization  project. Therefore,  it  would  include  description  of  the  analyses  of  sediment  data  and  would  provide data and interpretations of the channel geometry of the Oidesa River.
An objective of the investigation might be directed to determine whether significant alterations in sediment size and supplies to lhe downstream pad of the 3 des a Rrver basin mpght occur in the future if changes  would occur, an additional objective would be to describe those changes  in quantity  with time The nformation may  be relaled Co man's effects  on land use and constnjctKin of the ma|or reservoirs  in Che Odesa basin and to natural effects  of topography, geology, climate, and physiography  To permd reasonable evaluations  of the area and temporal variations  in sediment yield, sediment stze. and channel characteristics. Che future work  should consider the entire Oidesa River tasm in parts, as  well as  from adjacent parts  of other drainage basins. The study  resutt would be able to report fluvial- sediment transport and channel morphology  data for the pe riod of extensive stream flow regulation in the bas«n and for the period -mmediately  preceding regulation All of Lhu data and generalizations  to be presented might be intended to represent controlled hydrologic  conditions  or uncondoned conditions  of a generally agricultural rather than a pristine watershed
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5.1 Stagg-TJme Re Fl Mons
[JhjuigeS in lhe elevation of a stream ch-an net wtth time provrifl an addili&nal mean 3 of evaluating channel Injtahilrty. Degrade hen commonly occura <1 channels downstream from reservoirs. The water rebased from a dam 1$ largely sadimwit free and thus ha a a tendency to re glare me sedimanl bad 1C» an amount similar to lhat Of unregulated Condition* The KCtfrenre  and   airtent  of  charmrf  degradation  is  a  measure  of  the  imbalance  that  a reservoir  h3S   imposed  on  a  stream  Changes  slagt  (water,  level  erevatlpn)  with  time  rate largely  are  rotated  Lo  datum  changes  of  the  channel  bed  (aggradation  or  degradation) Changes of stage elevation for np^tJuraUon rale result bcm from changes of bed Elevation and  from  changes  m  charmal  cohvayante  a1  discharge  Mb  exceeding  those  equivalent  to the  act  rye.  channel  stage  An  explanation  of  Lhasa  trends  would  be  that  aggradalions  or degradation  Of  [he  Charmer  twd  U  nql  occurring.  but  that  me  relatively  targe  rebase  rates from the upstream reservoir are causmg erosion  of the channel sides be ow Lhe ban* full stage
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6. REFRENCES
Allen.   P  M.,  Hobbs  R   .  MMf,  N
1359-  Downstream  impacts  of  a  dam   on  ■  bedrock 
fluvial syslem Bulletin Qf the association of Engineering Geofogisfl 26. 165-189.
ECEOM. 1996. Abbay River basin inlegralod development master plan project
Httllay.   R   F   19S7   PodimwtS   and   pediment   fwmmg   processes   Jouma/   of   Geologtcai Education. 15  63-09.
P
Montgomery.    DR.    Buffington.    J.M.    1997.    Channel    Reach    morphology    in    mountain range drainage basi ns Geological society of American Bulletin 109. 596-611
Qsierkamp, R E, Crowther H .
1962 Sediment and channEl-geamelry
invesligaiions  far the Kansas  Riverbank'-slabihzation study  Kansas. Nebraska and Cciarado U S. Geological Survey. Water-Resources  I nvestigalions. Open. Fite Report 01-120
Rpsgen, D I 1994 A classification of natural rivers, Catena 22, 169-199
Mengesha,  I  Tadrwps.  C.,  WorkiHeb,  H.,  1996.  ExpJanabon  CO  ihe  geolOgruat  map  of Ethiopia, Scale 1.200090, 2nd edition Geological survey ofEttiiopia
USER. 1994. Land end waler resources of 1he Blue Mile basin
WAPCOE. 19M Prel.mmary waler resources development master plan f&r Eth+opia Williams. GP Wolman. MG 1984 Downstream effects of dams on alluvill rivers Lfmled Stales Geofogicgt Survey professional paper 1206 US Government Printing office. Washington DC. USA
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23ANNEXURE- 3
GEOLOGICAL AND
GEOTECHNICAL INVESTIGATIONSAfj® Dedessa Irrigation Project
Geological & Geotechnical Investigations
TABLE OF CONTENTS
Table of CONTENTS...... -.....
list o f tables.
LIST OF FIGURES_______ __ - LIST OF PLATES-______ __ — 
U5TQF APPF-VEHCES........... .... .■ -■ i■ 11■ -—•!111rrr^tft■ ii" ■?»•»• i
LIST OF AfiRFVjATIOXS. ...
1.    INTRODUCTION—--------------------------------- ---
2.       INVESTIGATIONS             ____________ — 
2r (ifNiRAi
............ ............................................
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Geological fc Geotechnical JnvestlgatloiLS
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4. CONSTRUCTION MATERIALS
Introduction
4.2           Clay Materials
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III
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43
li
Water Works Design & Supervision Enterprise
to AsMdiUon witfi WrconttoBotal Coiwoltinu and TechiMmiu Pn Ud.Ar]o Dtdew* IrrtEadN) Project
Geotog fell & Geotechnical Investigations
JU J 2007
4.J
■I 4
4.1
46
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LIST OF TABLES
Table 2.1                  HOftEHOl.r ;*ILL1NC DETaELS ............................................................................................................ .............................................. 5
T^Lt 1 i
5UMMAR ¥ OF Dtt. (£[K»£ LA YERS AT fflKDAM ANfr-SADDLE AXIS FROM THE DRILLING
...............6
TABLE 2.3:               DfPTH TO THE WATER TAJLt_________,___________ ___ __________ __________ __________________________ ________ ft
Table 2.4
Details of cLaV rurrow areas_____________________________________________ ,____m_______.________________ ___ 7
Table 13:                 Fjcmated Quantt roi O BdUWW A NO rjl 'AJfH V MATTRIAL
............................................... ........................ 9
Table 2.6                 Lakmatdrt iefts COHDUC1K-------------------------------- ------- -------------------------------------- ---------------- ------------------------------ -------- |]
T*SU! Il
Table 12.
Table 3.3:
GRWJNDWATU KJIMT data OF THE PROJECT AREA-------------------------------- -------------------- -------------------------- ----- ------- 1«
VESruirmAT Ajuo-Dfomsa site.................................. ............ ...... ..................... . ...»........ „..................... ................................... „4|
Summary nr THE YESMJftrn ................................................... ................................................................................. .................... <2
Table 5.1 Summasv of clay DtsEN parameters ________________________________________ _______________ ________________ 47
TABLt S 2: BJMHARV Of RtsT; DESIGN PARAMETERS__________________________________ _ _____ _______ _ _________ _______*9
unit J y
Fa*U J,*
Location df Clay wusce areaf   ................................................. ....................................... ............................................................... 54
sblicted deskjh mm
____ _______ _ _______________________ _____ _______________ 53
Table 3 3:                SVWA* V of FALLING head PEF.MEaR:i J TV 1BT RESULT ------------------------------------------------------- -------- ------ ----- -- ------- _ 54
Table 3.6:
Table 5.7;
Summary c* the LugednyaLue ........................................................................................ ............................. .
..
53
SIJMMaR Y IJF PfeAKIEABIIJITT '(EIT RESULTS FOR BOTH TEST TYPES
....... ............
lii
Water Works Design & Superrtsloo Enterprise
In Association with InttnzftDQiiMiULl Consul LinLs ud Tecluwcnls PtL Lid.Arjo Dedessa IrrlgitlM Project
Geological & Geo tee tuilcal Investigations
M*j2W7
LIST OF FIGURES
Fh i«BJ1        Ekjfti3®X 1 I a’MJATIQN MAP — ------------------------------------- - --------------m------- ..hi.i-.-hiii;. Uiiii—IHII-.UI................................................................................................ -.Hi.—;!;.. -.m; ri 4
Figure 2 2
Hl iRROW AREAS t CKAT1DN MAT., .tiniraiiiuwsiuii
inm-
1-1 ; mm i; m. ill ;m- ■!!!■
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Figure J 1 Geological ma^ui- the dam a-ndresfrwmr area----------------------------------------------- - —---------------------------------------------------- -...20
F(GUR£3.2            CjBDUUOICAI MAPCU THF Dam. SADDLE AKD 5KL! WAY AREA................................................................................... 21
FIGURE 3.1*         GEOLOGICAL lRCSS SECTION OF THE v*AM AXIS ALONG A M Figure 3.4             GEOLOGICAL CROSS S&C i I'jM r.+ 1 i^F fJAM jTTfJ aLONQ Al j i
........ ............. 22
23
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F«UMl 1 7             STtSfOnbTOf anil HUNlOW^miiN IN rut ALKAUbT BASALT Of RIGHT ABUTMENI [N*35j .... .... 32
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FlGLUE J 10a: JttMWC DIRECTION Of THE A- X.AI. 3NI BASALT A r “HE MJVUt COURSE IN THE RESERVOIR AREA. I N-iU| 15 PTL.il.TCJf. 1 1011 Ii hkiWm WK'WWDf DIE APHANmC BaSaLT (LEFT RESER YcxR]. iHl MAJOft DIP DIRECttW IS N251*AT
33
......... .. 34
AM AVERAGE DIF AMSU OF Jf*C *<0> ________ ____ ——...._______________________ _______________ _________  W
c
FhilrttE  1-  IOC  bflT3lkfiLlli  K4<.Q  JCKNT3TXF  THE  ALKALIK  BaSaL  T  AU  OvFM  rUF  ARtA  I  DAM  SITE  AND  RESERVOIR!  THE
MAM 4XT DIRECTV*! IS Ml I &‘aT Ah' AVERAGE DCF ANGLE OF TZ*(Sr-1 35I
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FIGURE 5 1 SAND FILTER DESIGN ...._________________ __________ _______________________  ____ ___ _______________________ „„,S3
LIST OF PL ATES
Plate 1             Mcxpholog*/ o3 tom site iooking d/s
Plate 2            Fraclure condition of Che alkahne basalt as observed in (he river course
Plate 3            Jointed aphanitic basalt exposed an the surface on “he ten side Of i.r e rwer outsito
the reservoir level
Place  4  Alkali  basalt  difce  at  Ketkelo  spring  ctoe  Lo  the  confluence  of  Wama  wrth  Dedessa river
LIST OF APPENDICES
Appendix A Appendix 6 Appendix C Appendix 0 APPENDIX E
Borehole logs
PlKHograpM of ccxe
Field Pe/meabihiy Teit resuiis Test ptli tog sheets 
l*hoh*tc«¥ Tests result
Appendix F           Geophysical survey results
Appfndix G>        Piezometer Insiallatiqh Detail
Waler Works Design & Supcrrislon Enterprise
In Asia cJitlon wlW JnterconiintDULi ComuJtuits uid Tnchnncny; Prt Ltd.
IVAi>D< dess* Jjvlfstlca Prtjed
Ewtogfcai & GEQtwhnfca) Invejtig&tions LIST OF ABRE VIAT1ONS
ADi
Ana-Dedcssa BorehoTe one ar dam site
AOCAO             Ar^ Dadtissa Coaese Aggregate Quarry s-ile ADQ1                Arjo-DedessJ Borehole one aE rack quarry site ADS1                 Aij>Dedeaaa Borehole one ar saddle dam sJe ADSPt               A/jo-Detfessa Borehole ore ar spillway site
AtSTPt
BCEOI/I
Are Borrow area Teat Phone
C                       Cohesion
CD                     Confined droned
CDSCO Construclion Design Share Company
CJhBTPI cmi’31
Chewo Bcxfow area Tesl Pit one Centimetre per second square
CU                     Confined untrained
OhBTPI Dhtndessa Borrow are* Test Pit one
0
K/A/ KrVm3
Gravniahonal Acceleration Potassium Argon
Kilo-Newton per meter cube
LL                      Liquid limit
ffi
metre
M0TP1               Manbo Borrow area Test Pit one
MDD
N13tf
Max Dry Density
130 degrees from north
NE                     North east
MMC                    Natural Moisture content
NW                    North west
Ob6TP!              OtXMiB Borrow area Test Pd one Ohm-m               Ohm meter
OMC                  Optimum moisture content
PI
PL
Plasticity index Plastic limit
SE                      South east
SSD                   Soaked dry sand SSWIN               SpftenStat for Windows SW                       South west
Water Works Design i Supervision Enterprise
In AilUcilUon wl(i lultrnnUneiilil CotwaHui Li tnd TetluitcrtU Plrt. tHLAr jo Dedessa Irrigation Project
Geological & Geot
echnical Investigations
May 2007
ucs
Unconfined compressive strength
USBR
United States Bureau of Reclamation
UTM
Universal Transverse Mercantile
uu
Unlra*ned unconfined
VES
WAPCOS
WWDSE
Vertical Electrical Sound ng
Water Works Design and Supervision Enterprise
XRD
X-Ray Diffraction
VI
Water Works Design & Supervision Enterprise
In Association with tntercnnUneoud Consultants and Technocrats Pvt LtdArjo-I)etes93 Irrigsiltw Project
Geological & Geotechnical Investigations
May 2W
■
■
■i
■
M
M
1.
Intro
ductio
n
7hii
report
relates 
to
the
geological
and
geolechnical
investigations 
conducted
in
relation
to
feasinlily study of Ano-Dedessa Irrigation project. Il also discusses the interpretative part on some aspects of the study
The Arjo-Dedesw irrigation project development consists of
■    Main dam
■    Saddle dam adjacent to |he main dam
*    Primary Canals running on lhe left and right banks of Dedessa River towards 1he command area
*     13665 ha irrigated area
The Report h» been organized in the following manner
Introduction Investigation Regional Geology
Construction Materials
Geotechnical Design Consideration
Section   i   provides   the   tntroductiari.   ft   introduces   the proied component
Section   2   oullir-as   the   investigation   methodology   and scape al the field investigation works conducted
Section    3    discusses    the    Regional    geology    of    the project area
Section 4 presents  results  of ihe various  construction malenals  mveshgated and studied for use in the dam section and elsewhere during implementation ot the project.
Section       5       derails       the       geotechnical       design Gons*darations m the design of the dam
Tre Appendices  conlam supporting documents  as  Follows  Borehole logs  (Appendix  A}, Geophysical InvesUgabon Results. (Appendix  F); Field Permeability  test Results  (Appendix  Q; Photographs  of cores  (Appendix  B). Test pus  fog sheets  (Appendix  D), Laboratory  test results summary  and interpretative charts  (Appendix  E), and Piezometer installation details  (Appendix S)-
1
Water Works Design 4 SaporrUion Enterprise
In LssoeisdOd with Intertontlnenu) CottsuILinU md T«tui»crats Pvl Ltd.Arjo pedewa irrlcstlen Project Geological & Geotechnical Investigations
2. Investigations
2 J     General
MayZM7
The aim of the field mvestigations  was  to address  all issues  necessary  to present the technical and economical feasibility  of the proposed structure? with sufficient fevel of investigations. At the beginning all relevant information found from previous  studies  of the protect area was reviewed and analyzed
Based on the avaifabfe information ground condtfions  characterization program was  developed giving attention lo how the various  conslruction parts  are affected by  lhe character of 1he ground Consequently, major probJems  were defmed and the gap of information related to feasibly of the project structures was identified and acted upon
Rotary  core drilling? was  used for investigation at the dam sue, saddle dam sdes  and quarry sites. The drilling program was  constantly  reviewed during the fiefd work  and revised in order 1o gel optimum result from the drillings  The final program, including adjustment based on continuous  •valuation of results  obtained during drilling is  shown on Taele 2.1 The Notary  core drilling work was conducted by the Construcilon Design Share Company (C&SCO).
Vertical eledncal sounding work  was  sub contracted to Ethiopian Institute of Geological Survey by  1he contractor; COSCO The of this  geophysical Survey  was  to get additional information m between the exploratory  boreholes  along the dam axis, saddle dam axes  and the spillway area A total of 10 vertical electocal sounding (VES) were conducted along the above mentioned profiles.
2.2    Previous investigation in the area
Trie pjfljeci site had earlier been studied by  the USBR (1953-64} as  part of Lhe land and Water Resources  Study  pt lhe Blue Nile easin, by  WAPCOS (1939 -90) as  part of [he Preliminary Water Resources  Development Master Plan of Ethiopia and by  BCEOM (1909) a& part of the Abbay River Basin Inlegrated Development Master Plan Project
A   detailed   review   of   the   previous   worKs   had   earlier   been   reported   m   the   Inception   report   All the   above   sfodras   were   general   desX   siwjias   and   did   not   include   investigations   consisting   of drilling,   mapping   or   laboratory   testing   me   current   dam   site   investigated   is   a   new   site   proposed by WWDSE So no previous investigation had been conduced al this ute
2
Water Works Design i Supervision Enterprise
in AssodsUon wltu InUrttmtlnrinr*! C-onsulLintt ud TttliEitttms m. LidArjoOedessa Irrigation Project
Geologic al & Geotechnical Investigations
2.3   Objectives of the present study
The obiwtives of Che gecXechnical and geological investigations conducted are
■    To Study Sub-Surface conditions at major structure sites including the dam srte
•     Ideality suitable construction materials for use in lhe proposed dam
•    Study the property of the construction materials
■   Assess foundation permeabilty properties
•     identify ihe geology of the project srte el feasibility revel
•    Prepare preliminary design Jewel to enable 10 conduct the feasibility of lhe project
2.4    Methodology
The methodology followed for geo#ogical and geolechnical inveshgalions wei e as follows:
*    Geolog>cal mapping of the dam see including lhe dam axss *    Borehole drilling a1 dam site spillway and quarry sites
Mi; ZW7
*    Permeability teslmg within the boreholes for lhe dam site and installation of piezometers
*    Geophysical investigation to augmenl the core drilling works
*    Test pitting at construction material borrow areas and sampling
*    Laboratory testing of samples collected from borehores and lost pits
2.6 Geological mapping
Geological mapping of reservoir area has  been done making use of aerial photograph topography  maps  and field work. Outcrops  of rocks  were mapped along with joint orientations spring areas etc. Details of the mapping conducted are reported under section 3
2.6 Borehole drilling
2.6.1 Location of Borohote
12  boreholes  were  dulled  with  a  total  depth  of  405m  Out  of  the  12  boreholes  7  were  drilled  along the  darn  axis  [Named  ADI  to  AD7),  1  at  spillway  (ADSpl),  2  at  rock  quany  sites  (ADQl  and  ADQ2) and  two  along  the  saddle  dam  (ADS  I  and  ADS2).  Except  two  holes  dnlled  along  the  dam  axu  at 60^  to  fine  horizontal,  the  remaining  boreholes  were  vertical  Drilling  depth  vaned  between  3m  1o 92m  The  borehole  locabon  map  4  presented  in  Figure  2.1  and  details  are  available  in  Table  2  1
3
Water Works Design A Supervision Enterprise
In Association with Inttrrontineril*! CeniultajiU md Teclinocr*ts Kt Ltd.941000
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Summery

Arjo-Dedessa Irrigation Project Summary

Arjo-Dedessa Irrigation Project Feasibility Study Report

Project Overview

The Arjo-Dedessa Irrigation Project is a major development initiative by the Federal Democratic Republic of Ethiopia Ministry of Water Resources. The feasibility study was conducted in May 2007 by Water Works Design & Supervision Enterprise in association with Intercontinental Consultants and Technocrats India Pvt. Ltd.

Project Components:

Main dam and saddle dam
Primary canals (left and right banks)
13,665 ha irrigated area
Spillway and hydraulic structures

Report Structure

The comprehensive report is organized into multiple volumes:

Volume	Content
Volume I	Survey and Investigation (Topographic Survey, Geomorphological Studies, Geological & Geotechnical Investigation)
Volume II	Natural Resources (Forestry, Energy & Catchment Development Plan)
Volume III	Water Resources (Meteorological & Hydrological Studies, Dam & Appurtenant Works, Irrigation & Drainage)
Volume IV	Agriculture (Soil Survey & Land Evaluation, Agricultural Planning)
Volume V	Environmental and Socio-Economic Aspects

Key Findings

Topographic Survey

Detailed surveys conducted for dam site, reservoir area (27,000 ha), and command area
Maps prepared at various scales (1:500 to 1:10,000) with contour intervals of 0.5m to 1m
72 control points established with concrete pillars and benchmarks
Software used: LIS CAD, Land CAD, Surfer, and Civil Designer

Geomorphological Studies

Three physiographic regions identified: Didesa plain (1270-1400m), uplands/ridges (1400-2500m), and plateau (>2500m)
Didesa River classified as an anastomosing stream type with stable banks
River channel features pool-riffle reaches with gentle gradient (<2%)
Dam construction expected to cause upstream sedimentation and downstream channel adjustments

Geological & Geotechnical Investigations

12 boreholes drilled (total 405m) at dam site, spillway, and quarry sites
Geological mapping revealed volcanic rocks (alkaline/trachy-basalt) and alluvial deposits
Construction materials identified: clay, shell material, concrete aggregates
Seismic hazard assessment conducted with recommendations for dam design

Conclusions and Recommendations

The feasibility study provides comprehensive data for the Arjo-Dedessa Irrigation Project, confirming the technical viability of the proposed structures. Key recommendations include:

Implementation of appropriate filter design for dam construction
Monitoring of potential geomorphological changes post-dam construction
Further detailed studies during the design phase
Consideration of seismic factors in structural design