Birete Dam Project - Hydrological Study Summary

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PROVISIONAL MILITARY GOVERNMENT OF SO
CIALIST ETHIOPIA
o A r a 'j i
ETHIOPIAN WATER
RESOURCES
AUTHORITY
BIRETE DAM PROJECT
HYDROLOGICAL STUDY
a4ft 1 ann > A. * f It f •
Addis A baba Ethiopia
kABSTRACT
Thia report omri th« mtir availability study for Biroto
Daa Project and includea the derivation of an inflow design flood hydrograph* Flood, routing hue bo&n carried out to fix the spillway capacity# The atu-ly Is baaed on very ecantiy data and is applicable to this particular case* Any reference to thio for u5C in the case
of other baainsp where enough data ie not availablOj, could only be with caution*1
abstract
Thia report cqvwb the- vaur availability study for Blrote Dara Project and includes the derivation of an inflow design flood hydrograph. Flood routing has bean carried out to fix the spillway capacity.. The study is based on very scantly data and is applicable to this particular case,. Any reference to this for
of other basins, whore enough data is not available! could only bo with c&ution,»
in the case- 2 -
TA3LE OF CONTENTS
ABSTRACT
TAULD OF CONTENTS
LIST of figures
1 , IWTRODIPCTI ON
2* WATER REQUIREMENT
j. VATER AVAILABILITY
a)    Catchment area
b)    Hydrometeorological data
o) Runoff
d) Has a curve
U. HEIGHT OF SAM
5.       DESIGN FLOOD
a)    Ebperloul methoda
b)    Poak aquations for a tri anular bydLragraph concept*
c)    Unit graph derivation
d)    Derivation of Inflow design hydrograph from the unit hydrograph.
6.       FLOOD ROUTING
T, CONCLUSIONS
8* ACKNOtJLBDBtMLMT 9. ICFZRENCESlist of figures
I „ Reservoir capacity and area curve
2.       Mass curve of Inflow and Outflow-Draught 94 litres/sec*
3.       Maae curve of Inflow and Outflow-Draught 6& 11 tree/sec.
4,       Lay out and dam axis cross-section 5.       DidonsionLess graph
6,       Lag relation curve
7,       Birate River Batin
8* Unit hydrograph
9* Inflow design flood hydrograph 10* Spillway discharge elevation curve
11. Inflow and Outflow hydrograph- * -
31RETS DAM PROJECT HYDROLOGICAL STUDY
1 * INTRODUCTION
Th* Eirete riwr baain is Located in SbM odialtd. 0 trative region about 106 kfl. North Uaet of Add! a Ababa* It is plmmad to
fonaom the surface rvnoff af th* basin to raeot the writer raquirent
of tho prop&aod totiertt factory* EiretS river it a tributary of
Sociable river which in turn drsino inro Huger river, A site for
a fiLL-dara baa been selected on Eirote river which is about 10
Hd, from the proposed coKont factory* The location of the dim
Site la approximately at 9^-29  W end j6°-2O E
2* tfAT^a requiklkeht
Aeoordiing t& the inforcztian received ths present
dttiffimd of thu coDffnt factory ia 2|000 m^/day or 53*15 litroe/aao*
Ko projected doonnd ia indlc-atutLp hovGv&r the provision for  inc redact
h
j
-
need in future is eontcEiplated   To oast tho water requir^nent
e
the following wat-ir ava.±l ability atudy la oo-nduotod.
3* MATER AVAILABILITY
o.) C-atehacnt or-ea
Upstrecra      of      ths      Oereti      Dan      site*      the      catchaont      tirea      of the      drainage      biain      which      would      contribute      to      tti«      curfato      runoff la bapcured as 37-&5 kn ,
b I E!yd roa&t ec raid gl a &1 do ta
No hydronetoereLogical station Mists in the catchmnt, The nearest rainfall stations are located at Derha (0> -26'K,
38°-39*<) and Inchini tO^-19'H* 3^3'E). There Is one waperation *tation at Hine in the aatchcant of Awaib Basin which ie close. t& Qi rote*
Z
P- 5 -
i) Rainfall data
Only four yours roinl^ll d-t- is
station# Annual figures ars* raprodutad in
available from Dcrba table 1•
T'lElnl Annual Riinfili at Derba-------------------------------------------------
Year
Annual Rainfall in nun
1975
1976
1977
1978
9^4.2
S9Q.8
i fi^e
1 tOO7.0
ii) Evaporation data
Three year* annual evaporation data froo Hine atntion is availably « given in table 2.
Table 2 - Evaporation data for Hine
Year
- --------------------------------------------1 Pan Annual Evaporation
iZiD
Actual Annual Evaporation E          n 0*8 X E
act                            pan
1976
1977 [ 1978
1,624,2
1,525.2
[                     M99.1
1,299. 4
1,220*2
1,199.3
Thfl above value* have been adjusted for the ndseing data
for 19j 7-78 + Pan coef Clcl-gtit, K B for working out the ac tual evaporation frou the reservoir area io taken equal to 0,6,
c)           Runoff
A few discharge c&ea a urgent s were t-kon on Sirate river near Inehini uhich are reproduced in tabla 3#
Table 3 -  Oischr.rfl* uf Birote rivar near Ir.chinl
Date of moasurnTnent
Discharge, of Bi re to rivor
noar Inchin!
28 March 1979
12 Hay           ■
26 *               II
16 Doc           iri
17 " ■It
Litres/aac
Sl "  1'
19 "  ”
11 h ■
23 ’■ *t) Kiinfaii dnti*
- ,         „ 
only four yvntt* r.ii«ai+
action. Annual fi ure» are « product in tablft 1.
ditc. i3 avzLi-la^l* froa DerM
S
Table 1  J .n ,»l Rainfall at Derbo.----------------------------------------
-
n
yqar
Annual Hainfall in ed
i$?5
W
1977
*978
$«.!
S9O.8
1(143*6
1,007.6
———------------------------------------------------------------
11) Evaporation data
Three years- annual evaporaticn data frees Hine Btotibn is available as given in table 2,
Table 2 - Evjpag&tiaa data far Hine
Year
Pan Annual Evaporation.
EE
Actual Annual Evaporation B          s 0.8 x E
act                             pan
can
1976
1977 1 i9?a
1.624.2
1.585.2
1,499,1
1,299,4
1,220.2
1                       1,199,3
The above values have been adjusted for the Dissing data for 1977-78* Pun coefficient! P°  working cut the actual
r
evaporation free the reservoir area is taken equal tg 0*8*
o) Runoff
A few discharge seaaunasents vara taken an Si rote river near Inchini which arc reproduced in table
Tabla J - Diech^riig of Birete river near Inchini Diocharga gf Biret-e river
Data of aaabutniEiant-
2B March 1J79
12 H^y            i*
26 u
16 Doc
17 1«
4
near Inehlnl
24  Li tres/ago
91     11                    11
19     H                      11
11
n
11     Ti
23 *1 11
IIJMES 0^ wOfiKS
GtfrEMbl PA60T
gm
aNNe
Unit Of rnea? E^
HJ
OuJ
Knr
ICS
ii
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H*
i
ccitfM r^Mi
M1
-
r-paiiHK^n? PUMH
1  M*
izr
D&5
_■ -j«
8- -
11
(imcm, rpawmli
M*
? -
Screed concrete CTflH
•?
prfifnt
TH
Pl
'— 1
L
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7mvdoam 3 oodiadm' oww • {uoow<3
vi Kv/vmids onwnNOO am wvo mr oft«3S3b ONiLVino3aX9 OH
Number d*
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—
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IS
LEGEND
3 KC FIJIM K A U.M fl
MAIN VOLUMES OF WORC
oCHOBHblE ObbEMbl PA60-4-
BI PETE DAM PROJECT HYDROLOGICAL STUDY
1 * INTRODUCTION
The Bireto river basin la located in Shea admlnlstretive region ebout 1O& kci. North Vest of Addis Ababa* It is planned to conserve the surface runoff of the basin to n$ot the water roquircc
of the proposed cement factory* Eircte river is a tributory of
Sodoble river which in turn drains into Mutfor river. A site for
a fill-dam has been selected cn Eirett river which is about 10
kru* from the proposed cement factory* The location of the dam
site is approximately at 9°-29'N and 36°-20'E
2* MATER REQUIREMENT
According to the information received the present water demand of the cement factory is 2,000 m^/day or 23*15 litroc/aoc.
No projected domand is Indicated, however the provision for incrous nocd in future is contemplated* To moot the water requirement
the following wat^r availability study is conducted*
3* MATER AVAILABILITY
a)           Catchment area
Upstream of the Beroti Daa site, the catchment area of the drainage basin which would contribute to thi surface runoff
2
is measured as 37*85 km *
b} Hydrometeorological data
Na hydrometeorological station exists' in the catchment* The nearest rainfall stations are located at Derba (O9°-26'Nt 36%39'E) and Inchini (O9°-l9'N  38°-23’E}, There is one
t
evaporation station at Hine in the eatchnent of Awnah Basin which is close to Sirota,-6-
BIRETE DAM PROJECT HYDROLOGICAL STUDY
l f INTRODUCTION
The Blrete river basin is located in Shoa adciinistrativa region about 106 kra» North West of Addis Ababa. It ia planned to conserve the surface runoff of the basin to ae&t the water roquironn of the proposed cement factory* Bircto river is a tributary of Sodoblq river which in turn drains into Huger river. A site for
a fill-dae has been selected on Birete river which is about 10 kid, free the proposed com-ent factory. Tho location of the dam alto is approximately at 9°-29*N and 38°-20 E
2  WATiR REQUIREMENT
l
e
According to the information received the present water demand of the cement factory is 2,000 m^/day or 23,15 litros/soe.
No projected docuind is indleatjd, however the provision for increase need in future la contemplated. To moot the wat&r requirement
the following water availability study la conducted.
3* WATER AVAILABILITY
a) Catchment area
Upstream of th© Beroti Dam sitOg the cntchsont area of the drainage basin which would contribute to thq surface runoff
2
la measured as 37.85 km ■
b)          Hydrompt»orolcgicql data
Nd hydrouqttiorploglcal station uxlata in the cotchinont* The noaru^t rainfall stationa arc located at Barba (0>9°-2;6■]£,
38°-35’E) and Inchinl
There la eno
evaporation station st Hlnc in the c&tchncnt of Awash Basin which is close to Dirato,station.
i) RiinfjLll data-
Only four yaars rainfall data is available fron rerba Annual figures arc reproduced in table 1.
Tn.hi * 1 Annu-1 hainnii
------- ----------------
Year
Annual Rainfall In ma
1975
197*
1977
1978
890.8
1,1*0.8
1,007.8
U) Evaporation det a
Three yo^rs annual evaporation dets fro*n Fiine station is avallabia as given in table 2t
Table 2 - Evaporation dets for Hine
Year
Pan Annual Evaporation
ran
Actual Annual Evaporation
E    = Q 8 x E
act                            pan
Mt
nn
1976 I 1977 [ 1978
1,*ZU,2
1,525.2
|                     13990
i,2993
1,220.2
|                        1,199,3
Thn abtivfi valutas havn biun adjusted rax* tho taisain^ data Cor 1977-78, Pan coefficient  K, for working out the actual
p
evaporation froc the reservoir  area is taken equal to O*Bt
8
c)           Sunaff
A few discharge □ojiurvQ&nts were taken on Sirote river near In chi til which art reproduced in table 3,
Tablo 3 - Dischr.rftu of Birote river near IrehlnA
Date of noasuroraent
Discharge of EJirete river
near Indbin!
2B March 1979
12 Hay ”
26      «
16 Doc "
17 "
2^ Litres/see
81 ” *
19"
11 * »
23 „0   Rainfall data
Only four yo^rs rai
_ j -K f", n X. J. :i Li C j—
availn'3!®
table l*
froo      Derba
stationB
Annual figure, nr® repred»«d
Table 1- Anm&l
Rainfall at Derb& _—-
Year
Annual Halnffcll i» nfi
19?5
197*
1PW
1970
$44*2
1 J143.8
1,007*9
■ ■
i±) EvapWaticn data
Three years annual t-vaporation duta froo Hine station la available as given in table 2*
Table 3 - Eviporatkrt d-jta for Hina
—1
Yeor
Pan Annual Evaporation
nn
Actual Annual Evaporation
E _ s O.a s E
act                             pan
1976
1977
1978
1,624,2
1,525*2
1,453.1
1,299.4
1,230,2
1.199.3
The above values have been adjusted for the raiseIng data
Ccr l^TT-TSa Pdn, coefficient  Kg. for working out the actual
t
evaporation rrqra the reserveir area i± taken equal to D fi,
B
c) Sunoff
A IW discharge raeaBuraMints waro taken on 31 rate river ne±r Inchi.nl which are reproduced in table 3.
?abl« ? - Diatzh-r^ of Birute riv.or near Tnchlnl -—Iltfar incnint
Data of muaBuroEent
PiachargG of Birute river
notr Inchlni
2fl   March   197$ 12     May      ”
26        ■         h
li       Dec       “
17      ■■       «
24 Litrui/ooe
Si 11  1
T9 "  ’i
11"  1
23      «As ec.rlior m
........a «.*. as -
direct rainWU
coral.tic. 1.
within the
O° *
not poeaibl-,  To  M**  ™
rainfall dt Durba station
Birflto c^cchalting Ba»ud eft thia asa
id., or t». W.tcr  ™il.bilH7. th“
a
1.       con.ldsrsd       to       bo       «>•       ““       Ea up.pti.prt nn nttoFnpt has
baon , „d6 t. 
runorf W- ™ *- -“«* 71”**
thw
for
May
U.S. Soil CMsr-=H»" s«t‘“ published In 1964.
co-pl.t. dour rnlnrnll dot. «• D„ta Is evall.bl.
four poors fr™ 1975 « 1978-
il 19
r ™°
to December for 1979 .nd Cron Jonnery to October for 1979
d
which, haa also boan utili^ad f?r computing runoff«
Bated art tho cbaractorieties like, hydrologic soil group, land uso and crp=trt&nt cl MB, O“  hydralogic sail cowr complux, the U.S. Soil'Conservation Service tULB developed various curves for direct runoff genorated by roinfili storm. Runoff coefflcient 1« calculated toy dividing direct runoff toy tho corresponding rainfall aterm, To estimate the runoff eeifficlint
for every month  it would be important to first arrive nt the
T
r&prdsL'rttitivu ntarq for the relevant month acalnet which tn# direct runoff value tould be rend froc tto curvt*, For thio purpnstr the doily rainfall data has been anclysod, For uvory month tho flrat ten values of daily rainfall 5t  m  starting
Q3
tram the naxinura value in doicending order hive been ccnsidarad as partial si*rX«», The values are listed in tables U nhd 5, Averages of atorcm, for each month and stores for a return period Of 3 yvnra have been calculated. In thv hight of representative *t«M, the values of runoff colfriulenta
th C a g far
MCh      month      have      b on
v
calouln^      wi         th      tho      uee      of      ?lirv&         ns
in th* set cf curves developed by Vis. Soil
Hirsts river catchment ab  v  the dam
given Ssrvico
&e
fl ito has b.en
^uMderod to fall within tht, followllifi chirMtl ria
,
l) HyciroloHlcal Soil Group
““ h°P "°11 l" “* “«““»« „ .iM®
sen.itieT-s,a lo roil in Croup D. 
-       < it i»6
which ha« tlw been utilized for computing runoff,
Baaed. on the (tai'acteriztlea like   hydrologic soil
r
,£rDi±p  land uss and troatment elos&  and hydrolofflc soil povot*
(
r
temple*, Che tLS, Sail'Canservatlon Service has deVolopod various
curves fpr direct runoff ganoratfid by ra inf nil
Runoff
cooffitiwHt is cnlcylatod by dividing direct runoff by the
corresponding rainfall ntqnn, Tq eatiffijats the runoff coofrielout
for every month | it would bo lrapoptcnt to first arrive st the
ropreiontntivw storm for the rolov^nt month againa t which the
direct runoff value could bo rw&d from th  curve. For this
o
purpose the doily rainfall date has be on analysed* Far every
□ anth the fl vac ten valuta of dally rainfall storms starting
from th<. iDaxinum value in dog sending order have boon canslderodtruitncmt
conditlon
ill)
Trea traen t C1 as *
Land uae or cover
or practice is cla
for infilteralion -
Hydrologic Sall-Cover Complex
Storm rainfall related to antecedent moieLure conditl
AHO Up has been considered.
For       the       above       chnractvristics       curve       nvmLci       p       Ch has        been       adopted       to        rend        thu       direct       runoff       correspcridint: monthly       r^proaent?-tivc       ^tarnss       arrived       at       in       table*       h       and th<j runoff" coefficients have been cclCulated»
95
to tho
5 T and
Tabla - Maximum daily rainfall storms,
return period storms and runoff coefficients
av«rsgLH  2
t
years
JAN
5 yecr
d-ta
75-79
FEB
5 year
da tu
75-79
MjtriCH 5 year
data
75-79
APRIL
5 year
da,ta
75-79
MAY
6 year
data
r -79
JUNE
6 year
data
7^-79
1
2
3
t|
5
6
7
8
9
io
X
S
V
X2
Runoff
Cfloff,
20.0
20*0
6.6
5.7
4-7
4,6
1,6
1 .0
0.4
o.4
6,50
7.46
5-55
0,00
39.0
12.3
10*0
8.4
8.0
6.5
5.7
5.5
5*2
5.0
9.3S
7.23
8.66
0.03
17.2
13.6
12.5
12*0
9.2
7.5
7-4
7.4
6.9
6*8
10.05
3.58
9.6o
0.04
34.2
20.0
14.5
9.0
8.0
7.5
7-3
7,2
6.7
6.6
12.10
B.89
1",87
0,05
44 * i
44*0
34,1
30.9
26.0
24.3
17.5
14 ,0
13.3
12.0
26.08
12,03
24,5 5
0,20
50*7
29,3
27-0
22.2
22. Q
2 • .7
1 -9
P.O
1 '.O
1 ',5
2  .77
« .63
* >55
0.20
_________Tabla 5 - Ma*iciu™ dtiil
return period atoroa -ind runo
..i    11 Sf
JULY 6 years
data
7ft-7 9
kUG»
6 years
data
74-79
SEPT.
6 y*ar
data
74-7
1
2
3
4
5
6
7
3
9
10
X
s
X2
Runoff Coeff*
59.£
40,5
40.0
35,0
33.7
29.3
23, 8
26*7
24,6
24,4
34,26
10.36
32.95
0.27
67,0
37.0
31.9
30.4
29.1
29,1
28.9
28.0
27.7
24.3
33*34
12.27
31,78
0,26
!■ i-
34,
30
21r
1 f 7
21.5
18,6
18.5
18.3
23.53
5.31
22.86
0, 1B
/r
/»•
r jo.
17
1
7
J.5
8.3
20.80
14,53
18,96
0,12
L
To calculate the runoff, rainfall data for the year
1976, which happens record, has b%ren adopted*
d) Haaa curve
to be the driest year in
thv feur yuars
Maaa curve study Is carried out to establish vatur availability. Other parameters to carry out the study are the base flow, evaporation from the reservoir, infiltration and
ac-apatfC through the dan, Re^radin^ base flow, tabl^ 3 ravanls that the Binlmuci flow recorded is 1'1 litr^s/sec. It has boon learnt that a ‘V notch’ Wuir is conteaiplatwd downstream of tho dam sit* to record the base flow* however no further data other than listed in table 3 is available ao far. As suek it has-a-
T±ble 5 - Maximum dully rainfall storns, sversfiso, 2 ye ire
ruturn poriod >torQfl and runoff cowrfiei^n
JULY 6 yenrfl
data
74-79
AUG. 6 years
data
7^-79
SEPT. 6 years
data
74-79
0C7
6 ye
dat
7I1-
1
2
3
4
5
6
7
8
9
10
X
s
X2
79
45, 6
45. 4
30. O
17, 6
IB, 0
14, 2
11. 0
9« 4
8. 5
8, 3
20 J80
14. 53
18,!96
O. 12
23.0
12.0
12,0
11.0
10.9
4.1
4.0
2.0
1.3
0.6
8.O9
7.00 7.20
0.03
4
9.7
3,6
2.8
1.0
1.0
0.2
Runoff
Caeff*
59,6
40.5
40,0
35.0
33.7
29,3
28.8
26,7
24,6
24,4
34.26
10.36
32.95
0,27
67,0
37»O
31-9
30.4
29.1
29,1
28.9
28.0
27.7
24. J
33.34
12*27
31,78
0.26
34.5
30.4
24,8
24,0
23.0
21.7
21.5
18.6
18,5
18.3
23.53
5.31
22,86
0.18
S I B
1
3.05
3.50
2.61
0.00
9
1
To calculate the runoff, rainfall data for the year
197^1      which      happens      to      be      the      driest      year      in record, has br&n idoptsd.
d) Maas Curve
Mass curvo study is carried out tc establish vator avallability« Other parameters to carry out the study Qt& the base flowp evaporation from the raservoir^ infiltration and seepage through the dam.. Rueradinij base flow, tabla 3 rovoals that the rrinimura flow mwl d is 11 litr^s/aec. It has bot>n
the four- ytrnrs
v
l&srnt that a notch  vwlr is ccntut&plated downstroan of the
1
dam site to rue ord the base flow, however no further data other than listed in table 3 is available 0O far. A* such it has- 9 "
be ,n assumed that 10 litres par second would b« avails - •
In the driest month. It has fur th ar been assumed that ©«
IQ litres per second base flow, 5 litres per second would bo lost duo to inf 11 torat±on and seepage through the dan. For evaporation from th* reservoir* date for the year 1?76 (tabla 2 is adopted. Monthly figures of evaporation are given in tabla 6
Tabla 6 - Monthly evaporation data for Hine
Month
1
Pan Evaporation
cm
Actual Evaporation E                    „        E
act 3 0 + 8 x pan
nra
January
1 February March
April
Kny
June
July
AugUAt September
October Movernb^T | Deccebcz
168
153
163
176
120
108
102
137
?  106
129
1  125
138
139
122
130
1M
96
86
82
110
85
103
100
110
dfiti is
January
qn-J  for
year**
fur the estimation of the cnnthly runoff* monthly rainfall analysed in table 7, Rainfall data for tK) nontha of
to April* November and Dooonib.r is available for J y„arB the months from May to October it 1, available for 6
-verase rfilnfcll is calculated for each nouth.
Total
annual rainfall far the four years for which
WrnllnblG     and     tho     total     or
<'fttpleti. data is
thu last column.  of
data  la  available,
annual rainfall Of
average monthly rainfall le shown ln
totals* Out of the four years fcr l4llch C4iJI|plate year 1976 has boon th  driest with a total
0
690*8 tan, which is
890 ,R
’ 1OC = 67.78#
1019*310
of the total of average monthly rainfall. To reflect a dry y«oar within the scopfc of data available  the aonthly average rainfall
p
la reduced to the same 87■ 7B$. Final figures of monthly rainfall are shown in the last line.
The above approach of arriving at the representative monthly rainfall for * dry year has been adopted because the pattern of monthly rainfall is not regular. Perusal of the
monthly rainfall data would show that there is a largo variation in the rainfall for the same month in various years and this variation is much more in th* dry months. As such the figures of monthly rainfall adopted in further study are considered to
I give a bettor picture in which large variations witbin the scanty data are absorbed to & certain extent.
■r.iifTnble 7 - Monthly rainfall fit D rba
o
1r
Yea
I 197*
1975
1976
1977
1978
1979
I Jan,
4 = = •• = 0.7
6.2
6.8
0.4
52.3
Feb.
|«ar.
3 1.9
22.3
48.9
10.9
14,0
25-4
43.4
74.6
30.6
23.7
1 Apr.
76.3
51.0
19-9
17.7
29-5
May
165.0
38.3
14.3
62,6
0.7
132.9
June
104,4
133-7
107,2
115,4
115.7
154.7
July
288.4
268.0
219.5
258.2
330.5
202.9
Aug,
r — => a _ 168.1
202,3
285.7
199.7
298.8
232.8
Sjpt,
■             -■ 128.7
164,2
97.2
146.7
139.3
207-2
Oct.
' E — —■ —• 12.8
3.4
12*0
183.8
27,8
35,0
Nov.
—
0.0
0*0
31.3
24.3
27.1
Doc,
: 9 = — — 
0,0
0,0
0.2
8.9
14.3
Total
c a K c = = e= ■ ■
944.2
890.8
1143.8
1007.8
TOT^L
Average R*p, dry
year
66.4
13-3
11.7
128.5
25.7
22.6
197.7
39.5
34.7
194.4
38.9
34. t
413.8
69.0
60.6
L
731.1
121.8
106,9 |
1567.5
261 .2
229.3
1387,4
231.2
202.9
883.3
147,2
729.2
274.8
45.8
40*2
82.7 16.5 1 14.5
23.4
4.7
1014.8
890.6- 12 -
Table 8 shows the computations of monthly volume of
flow which ist
* 1 * Vr * Vb
Where V* ’ monthly volume of inflow
V     = monthly volume of runoff
■ Runoff caeff. x Catchipent Area x monthly rainfall
depth
b   • monthly volume of bfte^ flow calculated nt the
V
rate af 5 litres/aec*
'Table B - Monthly Voluss of Inflow
h
Runoff
Co+ff,
Monthly Rainfall Depth
Monthly        Vol, of Runoff
V
Monthly        Vol, of Base flow
Monthly Vol,
of Inflow
V. ■ V +v
1        «r b
2
3
4
5
6
i           0,12                    40,2                             102,590
■ 0,03 14,5
i D.00 4,1
l6 46o
(
t           0,00
31.7
-
-
■          o,aj                    22,6                               25,66o
ch         0,04
34.7                               52,540
11         0,05                    34,1
64,530
0,20                    60.6                            458,740
• 0,20 106,9
y 0,2? 229.3
!■ 0,z6 202*9
609,230
2,343,330
1,996,740
13,390
12,96°
Ut39o
13,390
12,100
13,390
12,960
13,390
12,960
’3,390
13,390
195,930
29,420
13,390
13,390
37,760
65,930
77,490
472,130
822,190
2,356,720
2 010  1JO
t#
Il ; °*i&                  129,2                            880,240
.          893,200
In tabla &, evaporation losses from tba roservair have not bflen accounted for. For calculation of evaporation losses,
the reservoir surface area relevant to tho reservoir level in
the particular month is estinatod with ttu help of reservoir
staeo-volumo and reservoir staje^araa ourvoo( capacity and area13
1
curves) • RafarertOS is made to Fig. b
Fig*   arc attached as App&ndix B« Fo
pf the reservoir surface area relevant
it ii assumed that the reservoir is full at the
computations
for               plotting
initiate the computations
to thu particular month
— > end of Septenfeur,
which is the end of rainy season■
starts from October,
have been conducted^
----------- b ThareTore the mass curve study
-----------
For the reservoir full stage two studies one for the condition when the live storage
1* 2 nil Hi oa eubic metros with one million cubic metros
as dwad
Storage
and a constant draught (withdrawal) of 9^ litres/soe|
Mid the other for tho condition
when the live storage ia 1*5
million
eubic metrea with the same dead storage oi one million
cubic Hietrea and a ■conatniit draught of 68 litres/aec,
Tables 9
and      10      shew      the      computation      for      the      first      case      and      tables      1      T
and       12       for       the       second.       The       basis       for       adopting       tha       dead       storage as one million eubic metre is explained below.
i) Sediment inflow
There is no aadiuwn't flow data available. No sediment sampling has been done. In view of the topographic features, vegetation cover  rainfall intensity and other characteristics
t
of the catchment, it is asauned that 1 mm depth of soil over the catchnniut area would flow as a^dinont annually. Tn view of tho variation cover in th& catchment it may he an ussujuption an the conaamtlve side, but in the absence of data it has been adopted* Furthur it has been assumed that tti* unit weight of the sediment trapped in the reservoir and deposited would be 00* of the unit weight of thu loose soil including organic matter eroded from
thu cauha&nt,
Regarding trap efficiency of thq reservoir, the live atoragw and the drainege area ratio i*w. 2 r 10^
gives a value of 86* as read from the graph given In th* Blue Mlle Bn,in, Hydrology report, prepared by U.S.B.R. in
37.85x1“ °*O53t
the prflS&ncg of M
±t lfl assumed that the trap
•fflciency      may      reduce      to      80*      and      20*      of      the      sediment downs tr&Qin,
may paaa14- 15
Therefore, yearly BedijpaDt vplun* S
r     yT
eroded fro*
.he Cfttchmant and retained in the reservoir,
B ■ A x h i IO3 i O.fl n3
Y  ’ 2
Where A * catchment area In m
h ■ sediment height eroded in oni»
Volume of the
S
yr
deposited sediment in the reservoir, 3^t
A x h x IO3 X 0*0 
= a- -■ -—-r
O
. 37.B5 i i i io ■ 37,950 a /year
-
"
3
3
It ia proposed to provide a d»ad itomfo Mw of the reservoir which givas a reasonable life hofore the jcdiiticnt oneroachca upon the lira storage* Encroachmont on live atcrags is slow because pro-grosalvnly the sediment passing downstream incrcaaoa and ultimately a balance la reachedv
Considering a dead storage ■ 1 x IO0
Life of the dead storage
■ ———12— o 26 years,
37.850- W -
Tilde 9 - Reservoir surface area relevant to each raonth^ Live storage, L " 2 x W* a l Dead Storage, « 1 X 10 
□
Draught D ■ 9*4 litr&s/sec*
3
» 1
Accuniulatad
Vol- of Inflow 1
th
|
Acc, Vdl. of
Inflow + live
Storage
V ♦ L
1  n? ’
AOC* vol. of
Uraught
D
0?
Available Storage
V +L +0 “0 1     * 3s
nr
2
3
4
5
f
 ■
J,
1.
195,980
225,400
238,790
252,100
289,940
rch                355.870
Til
433,360
905,490
1,727,680
ily ^.oeu.uoo
11? ♦ '     6,094,530
bpt> 6,987,730
2,195,980
2,225,400
2,238,790
2,252,180
2,209,940
2,355,870
2,433,360
2,905,490
3,727,680
6,084,400
8,094,530
8,987,730
251,770 495,420 747,190 998,960
1,226,360 1,478,130 1,721,780 1,973.550 2,217,200 2,468,970 2,720,740 2,964,390
2,944,210 2,729.900 2,491,6oo
2,253.220
2,063,500
1.877,740
T.7H.58O 1,931,940 2,510,480
3,000,000
3,000,000
5,000.000
R&servbir Surface Area
2 
03
6
592,000
570,000
538,000
506,000
4B0,000
452,000
424,000
460,000
542,000
600,000
600,000
600,000
The accumulated valum of inflow (runoff) estiMatud in tabla :? la 6 987 7JO n\ According to Mr.. C*A,Munoz*a not* attnchud
tf
aa Appendix A. tho runoff baaed an the tainiaucs flow per Kn^ of Kuger rlvnr at Chancho for the driest year 1971 works out
= 0,3399938 x l(J6 x 37,85
* 12,860,76a tP
The i t ff calculated in table 9 is leas and 1. adopted for
o
further study.17
Table 10 - Het accurculated volume of Inflow + live storage (continued from tabla 9)
1
ii
*|
1
>
Rasferwgir
Surfaco Area
n'
■w
H
Monthly
act, «vtp,
rfim
Monthly
Vol, of Evap.
Acc. Vai. of Evap.
m3
9-
10)
2
3
4
5
1
L
■
593.000 5?0P000
53&,OOO
506,000
48a,000
452,000
424,000
,  1<60,COO
542,000
600.000
600,000
600,000
103 iao 110 134 122
130 141
96 66 B2 no
65
60,960
57,000
59,160
67,800
50,560
58,760
59,780
44,160 46,6lO
49,200 66,000 51.000
60,960
117,960
177,160
244,960
303,520
362,260
622,060
466,220
512,630
562,030
628,030
679,030
Net Ace. Vol. of Inflow +
Live storage (Col. 3 of table col, 5 of table
■?
6
2,135,000
2,107,420
2,061,630
2,007,220
1,986,420
1,993,590
2,011,300 2,439,270 3,214,850
5,522,370
7,466,500
8,306,700
1
Mass curve is shown l  Fig. 2. It would be so«n f
n
rq>n the
•i*i COTYJ and the tabl«», 9 and 10 that at the end or April (2.U11.3OQ , 1,721,780) w 269,520 cubic metres of live storage
left in the reacrvair which would provide enou h cushion for
6
a very dry year or would add to the life of the project.fl*I
I
1
I
1
I
I
I
i
1■
a«iij a FiTable It - Live storage
- 19 -
Reservoir surraee area r-o levant t& each month
i = 1.5 x 10^
D = 68 litres/acc.
Dead storagef *>s = 1 K 1:0
Accumulated Vol. of Inflow
VL
.3
Acc. Vol. of Inflow ♦■ Live
storage V. ♦ L X3
*3
Acc, Vol. of Draught
D
Availablo Storage
VLD-
i+    e+    3  D
Reservoir Surface Area
2
tn
n
3
»a
2
3
4
5
6
195,950
225,400
1 ,695,980
1,725,400
1S2,t30
358,390
2 500,000
2,367,010
540,000
524,000'
238,790
1,735,790
540,520
2,193,270
500 aoo
t
252,180
1,752,180
722,650
2,029,530
472fGOO
239,940
355,^70
1,789,940
1,855,870
887,l60
1,069,296
1,902,780
1,786,580
456,000
440,000
433,360
1,933,360
1,245,540
1 667,820
:
4' ,000
905,400
i,?e?,68a
4,084^00
094,53c
6.987,730
2,405,490
3,277,6BO
5,58' ,4O0
1,427,00
1,603,930
1,786,060
1,968,190
2 144,450
1,977,820
2,JCC 000
f
r
1
7,594,530
8,487,730
t
2,500,000
2-500,COG
2,500,000
464,000
540,000
540,000
540,000
00020
Table 12 - Nat aeeumuluccd volume of Inflow ♦ live storage
(continued from, table 11 )
Reservoir Surface
Aren
2
m
Monthly Act. Evap.
Monthly
Vol of
Evaporation
m3
Ace. Vol. of Evapor^. tian
n?
Nat Acc, Vol
of inflow + Live
storage
(col* 3 of table 11
- col* 5 of tab*12)
2
3
4
5
6
54U|Q0O 524,COO 5GQf0W 472,000
456,000
44a,aoo
420,000 464,ODO
540,000
540,000
540,000
540,000
103
100
110
134
122
130
141
96
86
82
110
85
55,620
52,400
55,000
63,250
55,630
57,200
59,220
44,540
46,440
44,280
59,400
45,900
55,620
1GBt020
163,020
226,270
281,900
339,100
398,320
442,860
489,300
533,580
592,980
638,880
1 ,640,360
1,617,380
1,575,77°
1,525,910
1,508,040
1,516,770
1,535,040
1,962,630
2,738,380
5,050,820
7,001,550
7,848,850
Maea curve is shown In F£g. 3* It would be- obaerved from the mass curve and th* tables 11 and 12 that nt the end of
A-pri.1 (11535iG^D * 
a 2&9 500 cubic metres of live
r
“tore go l  left in tho roatirvoir which would provide a cue hi on for
a
° very dry yenr or add to the liTo of th? project»
j he present demand of the comient factory ig >23.15 litroa p^r sec, The accumulated draught by the end of April at that rat® starting from October would be,
= 23.15 x 3600 x 24 x 
not Ace. Inflow froo tables 1C and 1i ■ 433,360 _ 398,320
B  424,034 H*
- 35,040 m3
Storog. required = 424,034 - 35,040 ■ 3B8 994
rI
4
1
F ■■22
Considering a margin for a very dry yt^r, a minimum liver stonge of 500,OUC                         may be necessary.
HEIGHT OF DAM.
The dead storage for a reasonable life may bo 3
1,0 x 10^ m\ For the same dead storage, the three cases of
total storage are listed in table 13. Reservoir elevation against the total storage is read from the capicity curve. Fig. 1*
Cross-section of river at the dam axis, Fig. 4, shows that tho cafTipct&nt bed rock may be at elevation 1G7.5 (with reference to an assumed local bench oark).
Table 1J - Alternative heights of dam
Draught lit/sec
Total
Storage
Reservoir
Elevation
Bed rock
El. nt deepest
point
Maximum
Height of dam including  3  m freeboard
m
68
23.15
i
3.0x106
2.5*10
y
1.5*10
125
124.2
122
1
107.5
107.5
107.5
20.5
19.7
17.5
The difference in the height of the dam for 0.5 x 10^cn^ of llv© storage and for 2.0 x 10^tn^ of live storage is 3 metros, whereas tho draught (withdrawal) is four times* For economical utilization of the available water and to provide for projected demand, it is recommended to adopt a20,5 metre high dam with a
total storage of 3.0 x 10 sT.
5, DESIGN FLOOD
u) Lmpcrical methods:
6 1
1) Mr, C. A.Munoz in his note, Appendix A, has computed tho *lood peak by tho two empirical methods, the values ©re given
bolow.
By Fiddes method, flood peak =
ft
111.2 nr/see.
By modified Myer s method, flood peak = 104.6 nr/soe.
1Considering a nirjin livw gtorago of JOO , QUO m" nay
HEIGHT OF DAM.
22
for a  very  dry  y*ir*
bo necessary*
a, Qiplmu®
1,0 x 10^
Tho dead storage for a rntsonable life may ba -
For the snnie dead storager the three cases of
total storage are listed in table 13* Reservoir elevation against the total storage i» road from tho capicity curve, Fig, 1 » Cross-section of river Qt the data axis  Fig, , shows that the
f
tompotent bed rock may be at elevation 1G7-5 (with reference to an assumed local bench ssrk)a
Table 13 - Alternative heights of dan
Draught
11t/sec
Total
Storage
Reservoir
Elnvatian
Bed rock
El, nt deepest
point
Maximum
Height of dam including 3 m
fr ft aboard,
a
60
23.15
3.oxioc
2.5*10°
1.5x10
125
12U.Z
122
107.5
107.5
107.5
20.5
19.7
17.5
_____________________ The difference in the height of the dam for 0 5 x lO^nP
#
of live storage and for 2.0 x 10^3  f live storage is 3 metres,
o
whereas the draught (withdrawal) 1$ four times. For economical utilization of the available water and ig provide for projected demand  it is r&coimended to adopt a20 5 satre high dam with a
p
#
total storage of J,.O x kAb .
3
5. DESIGN FLOOD
a ) Lmpg r i c al iaa thody 1
i) Mr, C.A.Munos in his note. Appendix A  has computed the
f
L lood peak by the two onperlcal methods, the values are given
By Fiddes method, flood peak =
1
111.2 to /sec.
By modified Myer's method, flood peak s 1OA,6 m^/goc.- 23-21*
According to the Bln* Wil* Ba»id rep
ort. 
■• *
r
Figure TIT - fc6P PvMc fl*«* valu* for «<=» in
_ 
j, tv veiri franiietec-7
/’ r
rend from the curve ■ 3 J a^/sec.
Mr* Munoz hne rocommenced tt proTlde -
_-.
flood - H8 rt'3/s0C.
Li) Hosed on basin cfctritt.rlJ’.iti —< ndJEf &luo Hilo Daeln for the Ribt md 3zs=._r-_ 
if
'-' =
following formula has been develaperd ty . f-SitUt
%ax * C-°°76 h1*3 C a~’,T
where H = Maxiausi IOC yi— 
—T
C Basin eiuiacii” 
Jilt
in
According       to       the       Blu«       Nil* 
Msizs
S<pcrx 
Ui
n&xa'su
100    year ans day rainfall ■ 124 sz
Therefore Q - O.OD76 r
x C,7 x 37,f
q 96,9 h^/mc,
b) rur.k U Ml 8 f c rat rjftftKM 1 ar h vd rOdTF-irh ■ _C C n C -~ pt
Un irts
hydregraph ,
the -general p>jak equations fcr a tr xrm.-anil^r
0.385
(Reft
Dwsljrn of Snttll Dtp»
by USBR)
Uhitru
ro
L Length of
■ Timo of concentration - from most distant paint
travel tin*  of
t* dam si to in
watwr
Hours
stream In ralles
H
Elevation          difference          between          the          upper and the dm site of tho stroam in ft»t
point
The
length of the striicu la 11.91 km and the olovutlon
<im
erenao between the upper point end the clam situ of rho
•treem i  (231,65 _ 10?,B3) . 123.02 metre*. (Thia LnCoraaticn
s
"trvatn lun^th and elevation diCfervnco is supplied by
»». TiOga)
V
*25
To
2
.59
123,82 x 3.2808
hours day = 2.6 houri
T
P
I*
0.6 T
c
Where D
 « T =
P
Rainfall excess period which
£a
taken as U Moura
Time fraff start of rise to peak
rate.
T
— + (0.6 x 2.6) = 3*56 hours
P
Tb
2,6? T = 2.67 k 3,56 = 9,3 hours
P
Time bELBtf of hydrograph
n
Where q
A
Q -
T=
P
the peak discharge in hlj/soc
2
Drainage area in id
Depth runoff in metres, 5^^ ° one day rainfall of 12*+ mm is Tine from start of rise to pc
q n  0,749 x 37.05 * 10  * 0,0^
6
p
3.56 x 3600
“ 137-2 d3/ sec.
e) Unitgraph derivation.
To develop unit hydrograph, far Bi rote unpaged area , the dimensionless graph. Fig, 5 and lag relation curve Fig. 6 developed for Blue Nila Basin are used. Curves cro taken from
niuo wile Bnaiiid Roport, Appendix III - Hydrology prepared by
USSR.
The pcLroEotera used aret
Lug ■
frou   centre of period of e£t««B mln ba C»ntJe &f voluna of runoff in aeconda.
L       =■       Length       of       longest       witertourso       TrOtf       dt»       site       to watershed divide in metre* ■>*5
-
0*305
T
0
11 ,S>[1 .91 S 0><ja U.7)J
123^82 X 3*2808
81,59 hours day « 
li&ura
T
P
§*.
r
Where D = Rain-foil oxoo&a period which i3 tnkeia as k hours T = Timo fro® start Of rise to peak rate.
P
T =
p 2
r B 2.167 
b
q pF T
s
+ (0.6 x 2*6) = -3*56 hours
= 2.6? x 3.56 = 9,5 hours-
p
Tfc a Time base of hydro graph*
G-7^9 AQ
P
Where q a the peak discharge in ir/aec
P
A s Drainage ureu in m
2
Q     ■     Depth     runoff     in     Batres,     jOji     of     the     max*     100     year one day rainfall of 12ft ion la taken as runoff*
T     ■     Timo     fro'D     start     of     rise     to     peak     rate     in     seconds     * q  0.7*19 * 37.85 * IO  * O.Q6B
6
P
3*56 x 3600
e 137*2 nP/sec.*
e ) Dn a. tfira ph de rd va t i n n.,
To       develop       unit       hydrograph       for       Blrate       ungaged      area 
p
the       dimensionless       graphp           Fig*       5       and       lag       relation       curve       Fig,       6
developed for Blue Nile Basin
used. Curves ere taken frarn
blue Nile Basin Report  Appendix III - Hydrology prepared by
f
USER,
The parameters used area
Lug « time from centre of period of excess rain to
centra of volume of runoff in seconds,
L       =       Length       of       longest       watercouroa       from       dara       site       to watershed divide in xpotres.I7
Z1-1ft-
FIG ?30
Table 1U - Ordinates of unit hydrograph
Time
# of
D
Laff +
JOO-   U 4
Unit q
m^/acc,
eel, 4
w
2.25
Ordinate
(From Fig, 5 )
Unit q
cfi
n _.  U rd- 393   . x r.
2.25
35.314
1
0.5
1.0
1.5
2.0
2,5
3.0 3.5 4,0 4,5
5,0
5,5
6.0
6.5
7.0
7.5
5 , O ? .0
2
3
4
5
22 f 2
44.4
66.7
33.9
111,1
133.3
155.6
177.8
200
222,2
244,4
236,7
260.9
31 1.9
333.3
355.6
4oo
3
12
19
20. S
16,2
12.3
7.0
4*7
3*1
2.13
1.52
0,97
O.65
0.45
0.31
0.21
0.1
525
2095
3320
3635
2830
2150
1225
820
540
370
265
170
114
79
54
37
IB
i4.86
59,32
94.01
102.93
80,14
60.88
34 .69
23.22
15.29
10.48
7.50
4,81
3.23
2,24
1.53
i ,05
0,51
The unit hydrograph 1, plotted in Figure 8,twrr hvdrograph
*          1 lte*r vnif hv'drpgrcpfi- 31- 3*
narlvqjion -
inflov doaifin_hydro<raph frorp the unit hydrograph
—---- -------
the catchment
sr" *
n
used
flOf- °r
the
for
Slnco there la no record Ing rula^augo vl thin 1 day 100 year probable precipitation
tho design mtarm. Tho storm rain is i
loss syniu
trical tins distribution Fig*
I of 12k nun 1b arranged in a
9, with the greatest
worst possible
casuniud in the
e
ortdi-tlsn
roiioviIia
in the middlta of the atorm to give the for flooding. Tho rainfall excema are
manner.
iHt   hour      net       10pt af 2nd      M          ■i      do^ ■■ jrd      ■I          ii      75*       "
^+th      II           tt
rainfall
IF
IP
The unit hydrographs
Of    1
far rainfall oxewa of )nm  Ittini, $M, 12nus, 22.5hbd , 
t
are derived
1 Jnnn , lawn,
ond
using the principle of unit hydrograph superposition
tho        design        inflow        flood        hydrograph        i>        obtained        both        graphically (Pig, 9) and analytically in tablo i;t
The unit hydrograph la computed for 1 inch rainfall
Mhlch in metric unit* la* for 25.t* mm or 1^ w 25,t«Hiin.
The rainfall ojccasb. from one day 1OO year probable precipitation la distributed into -J- hour unit rainfz.lL durations
Mid is tarrmod by i^
Tharifore,
Ordinate for each {• hour unit w Ordinato of unit iTraph * 
s34 -
p-iblo 15 “ Computation of inflow flood hydrogr-iph
LSB
*2”             ‘a-1
OrdinatD Tor
i ,6 i -12 3 -22.5 1  = 15
2a
2
2                     *2^
± a-2.4
Q
Total
*□
>
5 g5 j
5E j
s J
1
►
i
i
1
i
1 i 1
•
1 r
,
0.0
0.5s
2.34
3*7°
Cl.05
3.15
2.39
1.36
0.92
0,60
0.41
0.30
0.19
0.12 O.OB
0.06
0.04
0.02
0.0
0, SB
2.34
3-70
4.05
3.13
2.3?
1.36
0.92
O.60
0.41
0,30
0.19
0, 12
0»0B
0.06
0,04
0.02
0.0
3.48
14.04
22.20
24,30
18*90
14.34
6.16
5*52
3*60
2,46
1.60
1.14
0.72
0.46
O.36
0.24
0.12
0.0
6.96
28.08
44,40
48.60
37.80
28.68
16.32
11,04
7,20
4,92
3.6
2,26
1.44
0,96
0.72
0.48
0.24
OjO
13.03
52.63
83.25
91.12
70.07
53.77
30.6
20.7
13.50
9.22
6.75
4*27
2.7
1.8
1.35
0.9
0.45
0.0
8.7
35.1
55.5
60.75
47.25
35.85
20.40
13.80
9.00
6,15
4*50
2,85
1.8
1.2
0,9
0.6
0.3
0.0
1.74
7.02
11.1
12.15
9.45
7*17
4.OB
2*76
1.80
1*23
0.90
0,57
0.36
0,24
0.18
0,12
0.06
1
0,0
1.39
5.62
8*88
9.72
7*56
5-74
3.26
2,20
1.64
0.93
O.72
0.45
O.29
O.l?
0.14
1
0.096
0,048
0,0
0,5a
2.92
9.52
28.75
70.53
135*59
191.34
209.45
136,70
144.90
IOO.97
65*98
44.15
29.18
20.04
13.46
8,81
5*87
3.96
2*57
1.42
0.56
0.136
0.048
t 6llriut'* U d<,alu'1’ ri&od tiydroffT'-ph is plotted, in Fig, 9. Th*
**Uth y "
1   115 lh°* that th* Peak
 Of the inflow flood 1h 2C9,45m /j«c
3
td
Opt(Jd
’
' na"imum
compared to all the, method* 10 it la
Ca P4city r rOUtin^ through the reservoir to fix the spillway35
6,
FLOOD H0VT1NG
of
f X  -
a
i,h  * «>••*
to       bo       computed       for       which the head above the dreat to 1,5
of tho chin, above the full e<______________
a apiiiVny wld*h °utfLo*  Wrograph ha.
ie asflujned limiting
metree sc that the total height
economic ronaona the creet i8
keeping c straight width of 20
omorvation level is reasonable. For provided in the shape of r* soniclrol
20 metres,
metres*
the semicircle length
So, for a diameter of
computing the rating curve of th
would be 31,4 metres. For
crest is taken ns 31
is given by formula,
metroa.
Q ° c B H1 ■ 5
whure Q
® spillway,
di *charge
> the length of th©
J over the spillway
■i
c
i
Jt
'ii
B ■ spillway discharge In nr/sec*
■   coefficient of dischcrge which la taken qb 2
S Crest length of spillway width, which is token as 31 metros
■ Head of water above crest in
metres.
Table t6 valuea of head, H#
shows the values of discharge for various Spillway rating curve is plotted in Fig, 10.■y
i-JT , TH?N i.-Uav£_
-36
z 37
Table 16 - Spillway rating curve computations
H
m
Q0 ■ C B H1,5
T.
mJ/*
0*2
04
t
0,6 l
o.fi
1 *o
1.2
1,4
1*3
i.«
1.8
2,0
5*54
15,68
28*81
44,36
62,00
8l .50
102.70
113.90
125.48
145*70
175.36
With the help of spillway rating curvo fie, 10,
■eservalr capacity curve Fig, 1 , and ths inflow flood hydrograph lg, 9,flood rovting computations are carried out toy trial and irror method as shown in tablas 17 and 1{J, The crest level of he apillway for the full conservation level is taken as 125,
trlier indicated in table 13*- 36
,, - Flood routing computations,
&!• T7 '
J1n| Croat lovui ■ T2J
de
Sec
low at
timo t
Average rate of inflow q. far dt fn^/sic
Inflow
Trial Rooervuir Storage slovotlon
9
Out flow
at tltau T
m^/iec
3
4
5
6
7
1600
18QO I SOO 1000
l $00
1000 1B00
1800
1000 1800
1000
1800 1000 i$oo taoo 1800
1800
1800 >000 1800 1800 1800 1000
1800
0.0
0.56
2.92
9.52
28.75
70.53
135.59
T 91.34
209*45
186.70
J44.90
1D0.97
65.98
44.15
29.18
20.04
13.46
8.81
5.37
3.96
2.57
1.42
O.56
O.156
0.048
0.29
1*75
6,24
19.35
49*64
103,06
163.46
200.395
198*075
165,8
122.935
83,475
55.065
36,665
24.61
16.75
11.135
7.34
4.915
3*265
1*995
0.99
0*358
0.102
522
3150
11232
34443
89352 185508 29422a 360711 356535 298440 2212BJ
150255 99117
65997
44298
30150
20043
13212
8847
5B77
3591
1782
644
184
125.01
125.01
125*01
125.08
125.20
125.46
125.65
126.22
126.45
126.57
126.58
126.5
126.38
126.23
126.09
125.98
125.86
12S.75
125.65
125*59
125.52
125*45
125.39
125.35
0.5
0.5
0.5
2.0
6.0
20,0
48*5
84.0
107.5
121.5
122.5
113
100
87
70
60
ite
40
32*5
28*5
24,0
20.0
16.5
m.o3B -
[ ‘‘>utll1£ a imputation*. Id ■ 3Tn| Crest level ,
Inflo* tlmo *
>1
Avora^o rate of inflow q for dt m^/a&c
Inflow
Trial Rooorvoir
Storago uIqvo tlan
Qi
Out Flow
at time T
nP/aoc
4
5
6
7
1800 1800 1800 1800 FfiOO 1000 1800 1800 1000 1800 lBOO 1000 1800
1800 1800 1800 >800 1800 1800 1800 1800 1800
1800
Q> 5B
2.92 9*?2 28.75
70*53 135.59
191.34
209*45
186.70
144.90
100.97
65.98
44.15 29.18
20.04
13.46
8.81
5.87
3.96
2.57
1.42
0.56
0.156
0.048
0.29
1.75
6.24
19.35
49.64
103.06
163,46
200.395
198.075
165.8
122.935
83.475
55.065
36 *665
24.61
l6.?5
11.135
7.34
4.915
3.265
3.995
0.99
0.35a
0.102
522
3150
11232
34443
89352 185508 294228 360711 356535 298440 221283 150255
99117
65997
44298
30150 20OU3
13212
8847
5877
3591
1782
644
184
125.0T
125.01
125.01
125.08
125.20
125.46
125.85
126,22
126.45
126.57
126.5fi
126.5
126.38
126,25
126.09
135.98
125.86
125.75
125.65
125.59
125.52
125.45
125.39
125.35
0.5
0.5
0.5
2,0
6,0
20,0
48.5
84.0
107.5
121.5
122.5
113
100
87
70
60
48
40
32.5
28.5
24.0
20.0
16.5
14.0-1 
- 39
computations, b > Jim: Croi t livtl =■ t£5
Out fto*
n.
3
Inc^eniQDtnX
Storag*
d.3 ffl j
Total
Storage
in?
[ Reservoir
elevation
ond of dtf M
Remark
9                         10
11
15
13
4>0
72
900                        2250
9Qfl
10332
2250 32193
7200 B2152
2j4iO0
162108
61650                      23237a
t19250
1T735O
2414^1
1841B5
206100                         923<t0
219600                             1603
211950                           61695
191700                          92583
16G3OO                 102303
littJOO
97002
11925O 89100 97200 77157 79200 65968 65250 56403 54900 49023 <17250 43659
4G95O                         39600 32850                          32206 2/450                        27266
3000072 3002322 3012654 3044847 3125999 3288107 3520685 3762146 3946331 4038671 4040354 397E659 3886075 3783772 3686770 3597670
352050
34 54525 3398122 3349099 3305440 3265040 3233634
3206368
125.01
125.01
125.015
T25.075
125*21
125*47
125.86
126*21
126,45
126.57
126*56
126,5
126*36
126*25
126*09
.73 93
125.86
125.75
. 125.66
125.59
125.51
125.44
125.39
125.35
o*co
0,00
0.00;
0.005
0.01
0,01
0*01
0,01
0,00
0*00
0*00
0,00
0*00
O.OQ
0*00
0.00
o.w
0*00
0.01
0*00
0*01
0*01
0*00
0.004 fc 10
Flfl II^2 -
H EF£ft5WCES
p .«ign of Small Dang
by United States BUro(LU f
Land and Water Haaourcea or
Appendix III - HyaroiOffy by * B1U° Wlle Baa±n United Staten Bureau or b„ ,
Roclamatifln.
Et Mopia- *3
APPENDIX A Mr, C.A* Munoz 3 Note
fNovember 5th, 197?
Hydro mo to urology cal data
 for Pirate Rlvqr,
for the design of n small dam on Dirote River which will
No hydrometeorological stations exist in the catchment
eBly published maps at a scale of 1:250,000 which are not
juatof the tributary area his been determined by Dr. Taego from
Lliblfi aerial photographs at scale 1:50,000 and has found to
, □ value of 37*^5 s*!■ knits.
The area was visited with Mr, N* Ahmed and Dr* Tsega rid]/ November 2nd, 1979 &nd field data were collected for
poMible peak flood design. Xt appears that the catchment bten properly located on the aerial photograph# and the
Blatl&aa have been based on the value of the area as computed
T - Tsagn*
Tho nearest evaporation station is located at Hine with ;1  ’duos in mm or 1624,2, 1525,2 and 1499. » obBorvsd in Nation pa   class A, installed in December 1975* those values
rt
Observation,, nf '- mr'.iT   +. 
n                  rt
COD  26 fi) and Inchine
0r^5 -
Values of runoff have b OO computed froin data Qvalloblo
.
(jn                  River wi*h a tributary area of 484 sq, kBta, Blroto
|Jivtfr la a tributary of Mugor.
Enclosed herewith please find the detailed available Dr- Tsega, EWRa46 
on of peak_fload for Bireto River at poasibla dam s_ite (Based on * tributary area of 37*05 eq, kats  calculated by
#
pr . Tsega from available aerial photographs 1(50*000)
j firJdws Mg tbpd
field datas Length of stream 11,?1 knits*
Elevation at upper parts
” 
" dew sites
Difference of elevation:
Slope of main atroau;
(Informntlon supplied by Dr. Tsega)
ftrtnd-rd         contribution         area         coefficient         Cs         for ilope and soil typo with impeded drainago Cs » 0,5
231.65
127-7J
123.82
123.32           - O.OIO3963
11910
Q              rolling         catchment
atchoent       watne^a       factor       Cw       for       w,t           zone       and       perennial
stream
,'w = 1. OO
,= d use factor Cl (Intense cultivation) . 1.50
fl
A! ■.  Cs x Cw x Cl = 0.5 X 1.00 x 1.50 = O.75 inichment log time K * 3,00 (cultivated land)
□ luae of runoff RO = C  (P - Y) A X 10^
A
• 0.75 (60-0) 37.85 X 103
- 1703.25 X 10  (m )
33
verage flow Q ■
ass Time;
r B
•      —12 
~tb  QbH ’
t
t d
 ■ T» * *-3K ♦ Ta t a * 
* U-JU .         0.3331*8
ss
? 
r
0
 =
rt |                                       1
<r x 0.0103963*
+ 2.3 x 3 + ta a 7.86 + 3_.2706238
3.2706
0.10196220^
Q*- k7 -
trial and error.
, 
3.22O613B            7 .86 + 3.270625B
^  0 
0i
2.659147             1.229952
l^B.4          2.^7614               1.239993
ifl.35 2.^6933 
1.2403135
9.089952 
9,099993 
9,100313 
111.20 m^/see
44a
Q
8/80
9,0909
9,10031 ox
=z 2*3 Q - 2.3 * 48.35 ■
y HnX
Frtjni Blu* Wilp Studio, Appendix III, Figure HI-46:
Frequency
One* in x years
5
Peak value in m /see
Tor 37.85 sq. krata
40
"i
IO
49
25
64
50
75
100
83
Dy the modified Myer's method
Q - 17 X
for a C coefficient of 1?
= 17 x 37.85^ = 104.6 m^/soc.
From the above values nnd to satisfy the rasults of the Fiddes method it is advisably to design the spillway with
a capacity of 75 m^/aoc with a freeboard of 20 m^/sec and an eifiLirgoncy spillway of 15 ffi^/sec  The total capacity for
-
evacuation of tho poaR flood would be:
75 + 28 + 15 e
The station installed at Oerba (09° 26’M end JG°39 E}
1
Pril 1974 gives the following values:
Year
Annual Precipitation ram
’975
1976
J977
944,2
890.8
1143.8
1978
1007.8Ih<? station. lQCstnd at Incbine (O9 19 N and 3B°23'£)
48 -
Pr
indicQtcs highor values due
f va
.  ilabl® tire quJstionablo,
of th<J Land nnd Water Studi
ea
to higher altitude but the data Details are nvailablfi In the office
Agency of Ei.HA far those two
sta tion3.
;
Observations havg boon raado at Hino
sinto Juno
22, 1975
in ci evaporation catchment of the
elose to Dirote*
pan class A; howwvor Hina is located on the
Awash basin end not on the Nil© basin, but r*laUvjli The adjusted values would ba:
Yuar
1976
1977
1978
Annual Evaporation in nna
1624.2
1525.2
1499*T
Daily and monthly valuta uro available ia the Lund and Vater Studies Agency of EVRA.
itunofE. valu».g»
Xo hydrological station is existing on Bireta River* Continuous data of runoff are available for Muger River at Chantha for the period 1971-19761 tho tributary area is 484 sq, crats. The valuos of runoff are as follows:
Tear         Runoff ra"^ x 10
Runoff par sq. kmt* in 
0,3399938
0.4257851
0.5682500
0.431626
"1             6
x IP
1971
1972
1973
1974
164.557
206.00
275.033
208,907
1975
303.99T
0.6280805
J 976
207,195
0,4280888
Av
er-
ee
227*627
0,670304- 49 -
APPENDIX B Area and Volume Computation
for plotting Aroa and Capacity Carvos50
Area 4 Volump computation
Av
no
111
112
113
114
T36O
7160
906,6
4260
906^6 4260
1668O                       11920
3368O
50600
£5180
42140
(
1 1920
906.fi 5166.6
1 7O86 42266
84406
115
116
117
118
68040
87080
118000
159080
59320
77560
2518O
42140
59320
77560
143726
102540
138540
221286 323826 462366
119
120
200680
256400
12      1 12t           321880
13       122       1 392480
t4    T23            465440
15     1 124          533200
t6 j     125          603600
179880
228540
289140
357180
428960
499320
568400
102540 138540
179880
228540
289140
357100
428960
642246 870786
1159926 1517106 1946066 2445386
3013786
17
126 1
669400
6365OO
499320
3650286
18 1
127
740600
705OO0
568400
4355286
19
128
812200 I
776400
6365OO
5131686
705000
7764001

Summery

        Project Overview: The Birete Dam Project aims to conserve surface runoff from the Birete River basin (37.85 km²) to meet water demands of a proposed cement factory (current need: 2,000 m³/day). Located 106 km NW of Addis Ababa at 9°29'N, 38°20'E.
    

Key Findings

Water Availability

Data Limitations: Study based on scarce hydrometeorological data from nearby stations (Derba, Inchini, Hino)
Runoff Estimation: Used U.S. Soil Conservation Service method with CN=85 (clay soil, small grain land use)
Mass Curve Analysis: Two scenarios evaluated:
- Live storage: 2M m³ (draught: 94 l/s)
- Live storage: 1.5M m³ (draught: 68 l/s)
Sedimentation: Estimated 37,850 m³/year sediment deposition (80% trap efficiency)

Dam Specifications

Parameter	Value
Recommended Height	20.5m (including 3m freeboard)
Total Storage	3.0M m³ (1M m³ dead storage + 2M m³ live storage)
Projected Lifespan	26 years before sediment encroaches live storage

Flood Design

Peak Flood Estimates:
- Fiddes method: 111.2 m³/s
- Modified Myer's method: 104.6 m³/s
- Unit hydrograph method: 209.45 m³/s (adopted)
Spillway Design: 31m semicircular crest (max head: 1.5m), capacity: 113.9 m³/s at 1.5m head
Flood Routing: Results show max reservoir elevation during flood: 126.58m (1.58m above crest)

Conclusions

The 20.5m dam with 3M m³ storage is recommended for economic water utilization and future demand
Design accounts for data limitations through conservative assumptions
Spillway capacity designed for worst-case 209.45 m³/s flood scenario
Study notes findings are specific to Birete basin due to data constraints

        Note: The report emphasizes that all conclusions are based on very scant data and should not be applied to other basins without caution.
    