Birete Dam Project Hydrological Study Summary

MINISTRY OF MINES, ENERGY AND WATER RESOURCES ETHIOPIAN WATER RESOURCES AUTHORITY
LAND AND WATER STUDIES AGENCY
UNITED NATION DEVELOPMENT   PROGRAMME DTCD PROJECT ETH/75/OO5
* ft Co’?’’
r~ -
BIRETE DAM PROJECT HYDROLOGICAL STUDY
by
NAFIS AHMAD, UN ENGINEERING ADVISER GIRMA BEKELE, ENGINEER, LWSA
DANTEYEHUN HAILE, ENGINEER, LWSA
APRIL 1980
ADDIS ADABAI
I
r
i
i1
ar-
f.- fcs j
e
ABSTRACT
This report covoro the watir availability study for Dirct* Dam Project and ineludou tho derivation of an inflow dcsic^n flood hydro£r*aph* Flood routing Imo been carried out to fix the spillway capacity. The study is based art very scantly data and Is applicable to thia particular c&sc, Aay refer urica to this for use Ln tlxo cast of othor baains  where oncugh data is not availably could only
t
bo with caution.ITA'JLE OF CONTENTS
ABSTRACT
TA DLL OF CONTENTS
LIST OF FIGURES
1  INTRODUCTION
e
2 . U RTSS REQUIREMSNT
3. UAT5-.R AVAILABILITY
cl) Catchment am
b)     Hydrometeorological data
c)     Runoff
d)     Maes curve
HEIGHT OF DAM
5.      DESIGN FLOOD
a)     Empirical methods
b)     Peak ityintitmi for a triangular hydrograph concept.
e)     Unit graph derivation
d) Derivation of inflow design hydrograph from the unit hydrograph
FLOOD ROUTING
7.        CONCLUSIONS
8      AC KNO: JLEiXL J ft-bTT RJFLRENC&S-LIST OF FIGURES
1, Rosarvoir capacity end er cm eurv«
2* Maas cum of Inflow nnd Outflow-Draught 94 litroa/aoc*
3* Mass curve of Inflow and Outflow—Draught 60 lltrcs/spcf
Lay cut and dam axla cross-section 5,        Dincnsionlesa graph
Leg relation curve
7* Hirete River Uasin
6,        Unit hydrograph
9* Inflow design flood hydro gm ph IO, Spillway discharge elevation curve 11* Inflow and Outflow hydrograph- 4 -
SIRETE DAM PROJECT HYDROLOGICAL STUDY
1 * INTRODUCTION
The Diroto river basin is loc-itcd in Shoa, admini ts tm tive region cbout 106 kcr. North Most of Addis Abeba, It is planned tq conserve the aurfacu runoff of the basin to nuoi tha water re<;uxrcc
of the proposed cement factory, Eirctw river ia a tributory of Sodoble river which in turn drains into Mupoz- rivir. A site for Q fiil-daru has been saloctqd cn Birctc viVOF which la about TO kr.t. from the proposed cement factory. The location of th* dam site is approximately at 9 «29 N and JS^-20'E
or
2, tfATSft REQUIREMENT
According to thq information received the present vtt&r damand of tho cement factory ±9 2*000 m^/dsiy or 23*15 lltrea/see.
No projected dopiund is indicated* however the provision for incrous need in future ±b cuntnucplatad* Ta meat the water requirement the following water availability study is conducted,
3 . WATER AVAlLADILITY
□) Catchment aruo
Upatraa
ni
of
the
Boroti
Data 
site, 
tho 
catchm
vnt
area 
of
the
drainage
basin
which
would
contrib
ute
to 
the
surface
runoff
is moasurod as 37.85 kin^.
Ij ) Hyd rojrae t» pro 1 og 1 Cal d a jg
No hydror.Letworological station exists in the catchment, The nearest rainfall stations arc located at Dor bo (09°*26'N,
38'VJ9't) and Inchlnl (c/’-lS’N, 58°-23*J3). There Ao one evaporation station at Hine in tho catchment of Avcah Basin which 1$ close to Birotu,1 ) P-lnfrx) 1
Only four yonr- inini^Xl dats is available from Derta station, Annual finnrww ar*  ruproduced in table 1*
1
_ Tablo1-
Tear
niLiiu-ii hamiax* qv                                              _______ ——■ Annual Ha infill 1 tn Ttin
1975
1976
1977
1978
f)44,2
890.8
1 f143-8
1,007.8
ll) Evaporation data
Three years annual evaporation data from Hino station is available as given in table 2,
Tabla 2 ~ Evaporation data for Ji in a
l’an Annual Evaporation
Ye^r
Actual Annual Evaporation
E            3 0,8 x E
K3Ek
act
pan
ran
1976
1977
197&
1 ,624.2
1,525.2
1,499.1
1 ,299*4
1,330,3
i i99*3
t
'Hie above values have been cd Jus tod for tho Eiisslng data for 1977-7&* P-n coefficient, K  for working out the actual
f
evaporation frora th* rwrvoir area is taken equal to 0,8#
C) Runoff
A fov dlsehargt fneasuruisents were taken on Birete rivet near Inchini which ara reproduced in table 3,
Table 3 - Digcharge of Birqtc river near inchinl
Discharge of Blr&t# river
Date of nioasuroraent
noar lnchini
28 March 1979
12 Hay  ”
26 *»
16    Dec              11
17       “
24 Litres/oec
01 “  "
19“  «
11" “
23  " n-Ab earlier mentionod there is no rainfall station within the catchment do a direct rainfall runoff corolatlon la
not possible. To havo an idea of thd water availabilityt the
rainfall at Derba station io considered to ba the* sane as for
liirc'tc catchment. Based on this assumption an attempt has
bjen ino.de to estimate runoff based on the mg-th ad dovulopod by ilnn t? S, Soil Conservation Service published in
+
Complete daily rainfall data for Dvrba is available for four years from 1975 to 1976. Huwovar it is available from May to Dceoriibsr for 197J4 and froci January tg October for 1979 which has also b^on utilized for computing runoff*
Based on the characteristics like, hydrologic soil i;roup  land use and treatmont class  and hydrologic sail cover
1
p
complex, tht: U.S, Soil Conservation Service lias developed various curve?? for direct runoff generated by rainfall storru Runoff coefficient is calculated by dividing (Struct runoff by tho corresponding rainfall senna, To estimate the runoff epafficiant
for every month, it would bo important to first arrive it the representative storm for tho relevant month against which the
direct runoff value could bo r^ad from the curve* For this
purpose the daily rainfall data has boon analysed. For every
nan th the first ton valuss of daily rainfall sterns starting
frora the maximum valua in descending order have boon Considered as partial series* The values are listed in tables U and 5*
j-v^rng&a of atoms for each nonth and stones far a return
period of 2 years have been calculated, In th* Light of these
reprint alive storms, the values of runoff coefficients for
Grach month have been calculated with the uiv of curve as given in the S>Ot of curves developed by U*S, Soil Conservation Service*
Direto river catchment above the dam jita has been considered to fall within the following characteristics,
i) Hydrological Soil Group
Since the tap soil in the catchment icainly consists of Clay and is likely to have slow infilteratian rate, it is considered to fall in Group E>,—-          —-f
treatment ar practice is clG9flififld Good for hydrologic
condition for infi1teratian*
ill) Hydrologic Soil-Cover Complex
Storm rainfall related to .-.nt^codent moisture condition, AMC II, has bL-en considorod,
For the above choracturisties curve number, ON 0$ has bwtrn adopted to md the direct runoff eorrosponding to the monthly re proaentativa atortna arrived at in tables h and 5  and
7
11) Land Use and Treatment Class
Land use or cover is taken aa Small Griln  ant the
t
the runoff coefficients have been calculated.
Tabla- Ji - Maximum daily rainfall storms„ averages, 2 years
return period 3 terms and runoff coefflcionta
JAN
5 year
data
75-79
FEB
5 year
data
75-79
MARCH 5 year
data
75-79
APRIL
5 year
data
75-79
MAY
6 year
data
-79
JUNE
6 year
data
7^-79
1
2
3
4
S
6
7
8
9
10
3c
s
*2
Runoff
|Coaff*
20,0
20.0
6.6
5.7
4.7
4.6
1.6
1.0
O.4
O.4
6.50
7.46
5.55
0.00
29.0
12.5
10.0
8.4
8.0
6.5
5.7
5.5
5.2
5.0
9.50
7.23
0*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
9.60
0,04
34,2
20*0
14.5
9-0
8.0
7.5
7.3
7-2
6.7
6.6
12,10
a. 89
in. 87
0.05
44.1
44.0
34. 1
30.9
26,0
24.3
17.5
14.0
13.9
12,0
26.03
12.03
24.55
0. so
' 50.1
29.3
27.0
2.1,2
22.0
21.7
19.9
1?.O
19.0
17.5
24.77
9.63
23.55
0*20
1L1
-
, T-TL~r-
...Tibi*? 5 - Maximum dally rainfall atoms, averages, 2 y«-irs
return period storms and! runoff coefficients
JULY
6 years
data
7^-79
AUG.
6 years
data
74-79
SEPT.
6 yuars
do t n
74-79
OCT.
6 years
data
74-79
NOV .
' years
da tn
’ - 8
EC.
5 years
data
74-7 8
1
2
3
4
5
6
7
8
9
10
X
s
X2
Runoff
Coeff.
59-<5
40.5
40.0
35*o
33.7
29.3
28, B
26.7
34.6
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
.1 B . .f.
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
45.6
43,4
30.0
17.6
18.0
14.2
11.0
9.4
8.5
8.3
20.80
1*1.53
18.96
0.12
23.0
12^0
12.0
11,0
10,9
4.1
4,0
2.0
1.3
0.6
8.09
7.00
7.20
u 0.03
9.7
3.8
2.8
1.0
1.0
0.2
-
-
-
-
3.05
3.50
2.61
0.00
To calculate tho runoff, rainfall data for the year I97^  which htipp^ns to bo the driest year in the four years
r
record  has been adopted,
T
d) He..is curve
Mass curve study is carried out to establish vator availability  Other parsjnoters to carry cut the study are the
#
beso flow, evaporation from the m^rvair, infiltration and suopcnjc through the dara» Regrading base flow, tabid 3 reveals that ths rainlaws flow recorded is 11 l±trus/sec  It has betn
+
learnt that a V notch  weir is contemplated downstruasn of the
l
1
dan site to record the ba.se flow, however no further data other than listed in table 3 is available so far. Ab sucl; it hasbeen assumed that 10 litres per soconu would t>« available •van in the driest month, It has furtln-r bfreii assumed that out of
10 Litres per second br.sa flow, 5 litres par second would be
last due to lnfiIteration and seepage through the dam. For evaporation Irani ths reservoir  dal- - for the ysar  97  (tabla 2)
T
is adopted. Monthly figures of evaporation arc ^iven In table 6.
Table 6 - Monthly evaporation data for Hino
-
1      ft
For tho estimation of the monthly runoff, monthly rainfall d = t?. is analysed in tabla 7. Rainfall data for the month* of
January to. April* Novc-mbfrr and December is svailablo for 5 years mnd Tor the months from May to October it ia available for 6 years. Avorago rffinfcll ia calculato-d far each month. Total
annual rainfall for thJ four yearn for which template data la availably and tho total of average monthly rainfall is shown in
tho last column of totals, Out of thu four years for which complete data is available, year 197^ has been tho driest with a total animal rainfall of 390.8 mm, which isof th® total of nver^ff^ monthly rainfalls To r^fRct a dry year within t];t 3 cope of data availably th*r monthly average rainfall
is reduced to the same 87-7®$* Final figures of monthly rainfall are shown in Liio last line.
Ths above approach of arriving at the rvprosantativa tenthly rainfall for a dry year ha* boon adopted beciLuso the pattern of monthly rainfall is not regular, Perusal of th«
monthly rainfall data would show that there is a lar^e variation in th® rainfall far tho same month In various yaars and this variation is much rnaru In the dry months. As such the figures of monthly rainfall adopted in further study aru considered to -jive a bettor picture in which lar^u variations within the scanty data are absorbed to a certain extent*
-II
I
<1X
-
-- - -- ------------,UII y ,,
HL
Tablet 7 - Monthly rainfall at Omi-I&q
Yu:ir
Jan .
Fub,
Mar,
Apr.
May
1974
1975
>976
1977
1978
1979
J
= _“ — — — — -b ■ d
TOTaL
Avuragt R*ap, dry
ytar
0,7
6.2
6.8
o,4
52.3
66.14
13.3
11*7
3 1.9
22.8
48.9
10.9
14.0 Bfi=-=a
128.5
23-7
22.6
25.4
43.4
74.6
30.6
23.7
= = —=•■ 197.7
39.5
34.7
76.3
51.0
T?*9
17.7
29,5
x»,=»w=^-=
194.4
38.9
34.1
165.0
38.3
14.3
62.6
0.7
132.9
■ =.= 'K3 3 413.8
69.0
60*6
Juno
104.4
133.7
107.2
115.4
115-7
154.7
 =
731.1
121,8
106.9
July
288.4
268.0
219.5
258.2
330.5
202,9
nB2»««==: 1567.5
261.2
229.3
Avg.
® fai = 3 s
168.1
202.3
285.7
199.7
298,8
232.8
t» 1387.4
231,2
202,9
s*pt.
128,7
164.2
97.2
146.7
139.3
207>2 sai«==s
883.3
147.2
129.2
Oct,
B ■ * VS! 12.8
3,4
12. D
183.8
27.8
35.0
274.8
45.8
40.2
Nov, BS« ® M W
0,0
0,0
31,3
24.3
27,1
= ■ ? a 1
82.7
16.5
14.5
Doe,
 • d1 ■ kJ 
0.0
0.0
0,2
8.9
14.3
Total
— —
944.2
890.8
1143.8
1007-8
23.4
4.7
4*1
___
1
1014.8
890.83 J■
iTablfl 8 
th* computations of monthly volu»
Inflow which l*t
v i -       % * vu
WhaTQ             ■ monthly Yolum-j of inflow
V ■ monthly voluimo of runoff
r  > fiLinorf coaff, k Catch^nt Arw s monthly rainfall
dwpth
V. = aonthly vajHmfir of b»O flow ealCUlatoJ at th*
b
rate of 5 lltrws/SOC.
/
Tnblu & - Monthly Valura<? of Inflow
anth
Runoff
Gooff*
Monthly
Rs. info. 11 Dt-pth
mm
Monthly           Vol. of Runoff
Monthly           Vol, of Baso flaw
Monthly Vol*
of Inflow
V 1 -
b
£
3
4
5
6
jet.
lev*
i»c+
Ian.
r*b. Larch Iprll
P
Jun® Jyljf
few-
$opt»
0*12
0*03
0*00
0.00
0*03
0,04
0,05
0*20
0*20
0.27
0.26
0*18
40.2
U,5
n.7
za.6
3<i,T
3*i.1
60,6
106.9
229.3
302,5
182,590
l6,^6o
-
*
25,660
52.540
64,530
458,740
809,230
2,343,330
1,996,740
880,240
13.390
12,960
13,390
IStJS’O
12,100
13,390
12,960
13>390
12,960
13,390
13,390
IS,960
195,980
29,420
13,390
13,390
37,760
65,930
77,490
4?z,13O
822,190
2,356,720
2,O1Qf130
893,200
In table 3* miration loaann fron the rw**T¥®lr hnv« n&t btoti apevmtMd for. rflr cqlcuiation QT swap*nation loatoi,
th« ftiiLTVfllr JtirfacB ozw relevant to th& romTvpir ltvtil In th. pzrticuUr month la ..tinat*! with tn* help of r4iU
rv ir
fl
mgo-rcluM owl r&aer
VQ T
i  • ta#».apM cnr™.( capacity it»d
I
!
ri■
eurvirt). RsfsrtttS 1J madft tu Fitf„ 1, ownpoWiioiia  °  plotting
rr
F1C, i Art MWflhbd as Appc.Jdlx 
To initial tho conpuUtions
of Hid tcscrvolr ivrfacc aros roIvVfint t* the plttlcular (ttonth
It is assumed thal th* ro»fr«lr !• xhich 1* tin end of rain? 
h         dr
■* * » «'  °  Scpteobai-, Th = r*roro the mass our** study
tbuta from October, For ttic ro^rvolr full
two otudiee
t
have W*A condueteSi fliw for the condition whan the UW ntora(«
Xe 2 Billion cubic metres with One million cubic DfltroS is dead
sttfTUfe and a enn^tont draught (w£tto*r*Wol) *r Xitrco/seej
and the other fez tb4 condition vhen tii« live storage is 1,5
■ Milan cubic sretrea with the eama dead stcrcffa of ons million
cubic rnetr&S and a constant draught of 6S lltFOs/o»c. Tables 9
and 10 shuw the computation far tho firat taau and table* 1 1
b
and 12 far the iGconch Thu basis for adopting the defld itcroffft
I
1
F
I
DJi an* Rill Ilan cubic metre is c Kpl & lifted- bolov.
i) Sediment inflow
I
l
Thors in hq sedimont flow dtta availablo. No d-Cid iffion t
has Wfi du-no« In view of th* topographic fonturob+
vB-cetation cover, rainfall intensity and other charactoris tic a
.I
er thw catchmont, it lj C8*uni«d that 1 Ittt depth *f S&il over the
t-ttchmant area would flow is Sudlmunt unriually. Xn view of the
vegetation tovor in the catchniont it may bo an n?i.tunyp11-cn on the
eenscrvutlvn side, but in the absence of data it has buon adopted.
Further it has botsn a*i9HD»d that tin unit weight of the ssdinont
treppsd in the reservoir and deposited would ba eO$s of the unit
1
weight of the loose soil including orgenio mutter eroded froiw
Che catchment
a
J
hei&rding trap efficiency &r the rfisarv  ir  the live
&t
storage and th* draiftagn area ratio l.e, 2x 1,
6
jT.taxto  ’
Elv»» a Value of &&> a9 read frail the Kruph divert In th,
■ ' J GF
Elm Jtlit Batin, Sydnlafy t-aperfe, pz-apar*d by U.S.B.R, in
,ilj UiJ
th* prcsancD of fth under* liii C a it la asaumad that th, trap
■rficlsncy may ZvdUC, to UCjt and 20# «£ the ■□diaobC my paBl
downstream.
-- ,-u <i!
J■'
I Jtif7.1.1. ■ UR • “
-
-t f- *• ’tx ■-15 *
Therefore, yearly sediment volume, S^  eroded from
f
the catchment B«d retained in the reservoir,
3 
5     « A X h X 10 X 0-S ra
1
y 
‘
a
Where A = catchment am in «
h ■ B«diaont halghc eroded
9
Volurn* of the doposited sediment in
A x h x ttP i D.a
5        =                5
----------------
the reservoir,
■ 37*85 i l i lo^ z 37,830 n^/ywr Lt is proposed: to provide adiad BtorUfja v-oLumo of the
rcsarvoir vhich gives a rDajgnabU life bafor? the sediment
/
ChCroapbeB upon the llv« storage, Encroachment on Live stor&flv is Slow because progroexlvoly tho Bodiment passing downstream incrittivs and ultimately c, balanco ia reached.
Considering a dead storage = 3 x 1O^
Life of the dead Storage
« 7 * 
« 26 yoarn,
37,650
'*Jnlr
...
|7
t.
b CB F-- rc
M
fT
E-
i■
- .- t6 -
Table 9 - Reservoir surface on??, rciovnnt to each south Live storefio, s 2 x 10^ rn^ | Bond Storage, Dj =
Draught L> = 9^ litros/soc.
1K
I
1 1
1
■
Accumulated Vol. of Inflow
X,
Ace. Vol, of
Inflow + live
Storage
VJ + L
«? *
Acr. vol. of
Draught
D
m3
Available
Storage
V *L *D -D i       3^3
flr
Reservoir Surface Arc*
2
0
1
■
12
3
5
6
r
h
1
■»
<95,900
225,1+00
238,790
252,180
'289,940
355,870
433,360
905,490
1,727,660
l*, CB«+, 4d0
6,094,530
6,987,730
2,195,960
2,225,400
2,238,790
2,252,160
2,289+940
2,355,870
2,433,360
2,905,490
3,727,630
6,004,400
8,094,530
8,987,730
251,77c
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
2,729,980
2,Ugi,600
2,253,220
2,063,580
1,877,740
l,711,500
1,931,940
2,510,400
3,000,000
3,coo,OOO
3+000,000
592  OOO
570,000
538,000
506,000
4 80,DOO
452+000
424,000
460,000
542,000
600,000
600,000
600,000
9 is 6198? 7JO
f
TIlc. Qccunmlatgd volume of inflow (runoff) ostismtQd in table According to Mr* C^.Jhmoi^ not* attached
as Appendix A, the runoff baaed on thu minimum flow per Km ” of
-
Kujtr river c.t Chancho for tho driest yiar '971 works out
= O.339993S X 106 x 37.□$
B 12,860,760 a3
The r “■aff calculated.  in table 9  ia  Uss and la  cdoptsd fop
further study. 
~h.
I- 17 *
Table 10 - Net accufilulatetl volume of Inflow + liv& Btor<ig9
(continued from ts.ble    9)
Roservair Surface Area
2
m
Monthly
act. mp,
mm
Monthly
Vol, of Evap,
ITV3
Acc. Vol,
of Evap«
nP
Net Ac c. Vol.
qf Inflow +
Live storage
(Col, 3 of table 9 col* 5 of table 1O'
2
3
4
5
6
592,000
570 O0Q
t
538,000
506,000
1(80,000
452,000
<(24,000
460,000
542,00O
600,ooo
600,000
600,000
103
WO
110
134
122
130
1U1
96
86
82
1 IO
85
60,9*10
57,000
59,180
67,800
58,560
58,760
59,780
44,160
46,610
49,200
66,000
51,000
60,980
117,980
177,160
244,960
303,520
362,280
422,060
466,220
512,890
562,030
628,030
679,030
2,135,000
2,107,420
2,061,630
2,007,220
1,986,420
1,993.590
2,071,300
2,439,270
3,214,850
5,522,370
7,466,500
8,308,700
Mais curve is aboun in
““ curve ahd the tables, 9 and 10 that at the end of April
a> It would bfl sefin
(!,0U,3OQ - 1,721,7&o)
289,530 cubic metres of live storage
1- left in the reservoir which would provide enough cushion for ‘ very dry year or would add to the life of the project.1-Ifl-
I»■
■i
il ii
*m19 -
Tab io 11 - ilctserVQir surfaco 
MV* 
L . 1.5 * 1OS «?l
ral^vont to isch month^
•*'«•*» ■> n  = 1 x 10
3 ?
3
D «= 63 litr»s/sce»
Accumulated
Vol. of Inflow
Ace. Vol. of Inflow ♦ Live
storage
Act. Vol» of
Draught
D
Available
Stortigo
*-D -D
H
Roaervoir Surface Area
2
ra
3
i ,695,980
1,725,1*00
1,738,790
1,752,180
1,789,940
1,855,870
1,933,360
2,405,490
3,277,680
5,584,400
7,594,530
8,487,730
€
195.980
225.^00
238.790
252.180
289.9^0
355.870
2*33.360
905.2*90
1,727.600
182,13°
358,390
54o,5ZO
722,650
887,160
1,069,290
*,245,540
1,427,670
1,603,930
1,786,060
1,966,190
2,144,450
2 500,000
2,367,010
2,190,270
2,029,530
1,902,780
1,786,580
1.687.820
1.977.820
2,500,000
540,000
524,000
500,000
472,000
456,000
440,000
4,  OOO
4 C0li IiOO
ft
2,500,OOO
6,094,530
6,987,730
2,500,OOO
2,500,GOO
464,000
540,000
540,000
540,000
540,000E
b
1Table 13 - Net sccLLmulatcd volume of Inflow + Live storage (continued from table 11 )
Reservoir
Surface
Area
2 
EB
Monthly
Act.. Evap,
mm
Monthly
Vol of
Evaporation
ACC* Vol. of Evaporation
Net Ace, Vol
of Inflow + Live
a tornjo
(col, 3
11
- col, 5 cf tab, t-2.
m3
2 /
3
4
5
6
54c,vOO 524,COO
500,000
472,000
456,000
440,000
420,000
464,000
540,000
540,000
540,000
540,000
10J
100
110
134
122
130
141
96
8$
02
1 10
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
108,020
163,020
226,270
281,900
339,100
398,320
442,86c
489,300
533,580
592,980
638,880
1 ,64o,j60
1,617,380
1,575,770
1,525,910
1,506,040
1,516,770
1,535,040
1,962,630
2,738,380
5,050,620
7,001,550
7,848,850
I 85
Maas curve is shown in Fig* 3# It would bo observed free the mass curve and tba table* 11 and 12 that at the end of April (.11-535p- l 2^5i5^) ■ 289,500 cubic metres of live
#
starage is left Ln the reservoir which would provide a cushion for
a very dry year cf add to the life of the project.
The present demand of tho ccmVnt factory is 23*15 litres per s&c* The accumulated draught by tho end of April dt
that rate starting from October would bop
= *3.15 * 36oo i 24 i “  = 4z4,034 m
G
3
The net Ace, Inflow frctu tables ID and 11 e 433,360 - 398,320 Storage reduirad ■» 424,034 - 35,040 ■» 388,994
■ 35j^O »3- —--jj- 2
•- 22 -
for      a      very      dry ba rwwmry*
year ,        a minimum
U* HEIGHT OF DAM.
Tlia dead storage for a reasonable life may be w 1.0 x IO6 m3s For the same dead storage, the three cases of
total storage ore listed in table 1J. Reservoir elevation .igiins
tha total storage? is read from the Cftpioity curve, Fig* 1* 
Cross-scction oi river at the dam axi.i, Fig* 
competent bod rock may be at elevation TG7 * 5 (with reference to tn assumed local bench nark).
shows that the
Table 13 - Alternative heights
of din
Draught
lit/see
Total
Storage
<?
Reservoir
El ovation
Bed   rock 
El* at doepest
point
Maximum
Height of dam including  3  m freeboard
r»
94
6S
23.15
1
3*0x10
2,5x10°£
1.5x10
125
124.2
122
107.5
1    107.5
1     107*5
20,3
19.7
17.5
The difference in.  the hei er live storage and  for 2*0  x  IC^trP choreas tho draught (withdrawal) la
ght or the damL for 0*5 x lO m^
6
of live storage in 3 raetrusf four times* For econgmi&al
utilization of the nvaiiable water and to provide for projected 
it is x-cottuntrnded to ftdopt=i2O.5 Bdt!r£ high d™ with o total Btarcffe of 3.0 x 1O^\
5. DESIGN FLOOD
a) Ltnpnrical iitithoda ;
i) Mr, C.A.Hunoz in his note, Appendix A, haa computed the flood peak by the two empirical methods, th* values are given below.
by liddos method, flood peak =
111*2 ih^/b&c.
By modified Hyes'3 usthod, flood poak o tOlj.6 tb /B*c.
323-
LAY0UP AWMV AM'S CROSS- SCG’ MM
I
IrlT 1- 2i|
v-tc-dtnff to the Blue Kile Basin report, Appendix III, 
■T-ps iri - i»6  peak flood value for once in ICQ years frequency
t
.' ''i
- curve — 8J ni^/set*
Lt , krzr.oz lias reccmmended to provide a peak value for
■R
' t.’ ’ j ‘ '3 r/sec,
1A}
r
? on basin charaetvriatics and rainfall pattern of
*_• 
"l
* for the Ribb and Guimnarnj flood control, the
-J
fr-ii":La has boon davtlopet! by Hr. M»Shx.m#
Q n 0,0076 H ’  C A ”
1  5           0  71
X3T K
where It = Maximum 100 year one day rainfall (ma)
C ■ Basin characteristic coefficient (in
this caac- ■ 0*7}   J?
A = Basin area (km*’)
Acc
ording
 to ths Bluu
Nil® Basin Report th
a maximum
■1
■/Gar o:ig
 day ra
infall a 134
ram
Zionfore Q
racx
»      0,007
6      x      12U1*5
x      G.7      x      37,CS ' ’
07
u 96,9 □"’/sec,
b) ?cid'g. •qiiatlan, for _a, triangular hydrograph concept• 
Uoint; the general peak equations for a t ri umjular
‘-“drotj-apa,
0.305
(Rafi      Dvoign          of          Sauli          Dao?
by USSR)
Whore a Timo of concentration - travel time or water
from moat distant point to dam Bit* in Hours, f = Lvri^th of atraani in tulles
H = Elevation difference between th% upper point thei d'lti site? of the fi*rc?^u iri
The Length of the stroom u 11.91 and tho 8lov„tlon ttfroronoc between tho upper point end the don  ito of tho 
3
T“' i. (231,63. 
lira lonjtli ana Novation «Uf«rmcl is ,.,ppliad b
8I metre3_ (Thl, lnrlJraitt<m^0*305
T =
c
11 ,9(11.91 123.62
x 0.62137)J
x 3.2SC8 J
a 2* 59 hours                day = 2.6 hours
T a - + O.6 T p           *5
c
Where D ■. Rainfall excels period which is T = Timo frop start cf rise to peak 
P
T ■ 4 + (o,6 X 2-6) = 3,5fi hours
p 2
T = 2*6? T ■ 2.6? x 3>56 = 9f5 hours
taken al hours rate *
b 
p
T, = Time base of hydro graph. o
0.749 Aft
J
% ° T
P
Whore q a the peak disthargO in m /.ioe p
A = Drainage area in m
Q a Depth runoff in metres one day rainfall of 12**
3
50?t of the max* lOO year
ram i s taken as runoff*
T = Time from start of rise to peak rate in seconds, _ 0,749 x 37.85 x 1G  x 0.06?
6
p
3.56 * 3600 >37.2      nr/sec
c } Uni tfiraph dori yation.
To develop unit hydrograph for Birute unga^ed area p the dimensionless graph, Fig* 5 find lag relation curve Fig, 6 developud for Blue Nile Basin are used, Curves are taken from Blun Nile Basin lhjport, Appendix III - Hydrology prepared by USER.
The parameters used are:
Lag * time from centre of period of excess rain to centre of volume of runoff in seconds,
L ■ Length of longest vfitcrcourso from dam site to watershed divide in metros*■* .-BL.UE NILE fl A SIN. L AG_ £U_R VE
HG. CFIG. flIl -28
C v Controls of bnain, Flfi^ 7
L°         -         distance         from         dao         blU         to         th*         intersection         porpendiculoj on
from C to the stream ali^nnont in metres
S Q overall slop* in matros por flictre of longest watercourse from the dm si to*
L ~ H.91 kn ■ 11910 metros
L x± 5300 raatruB-found from norial phogacraphs sc- to it 50,000
Elovitlari
at           watershed           divide           point
Elevation at the d.-ci site
Dlffcironcij Of elevation
=         231.65 " 107.83
■ 123.82
S
_ 
1    <1  ■82   =   0.0103963 
1 1910 L&s ■ (3.0976}
3.0970
11910
/oTot
= 6 79O seconds or 1*89 hours 2 hours
t
Taking         unit         rainfall         duration         tino         as         one         fourth         of Ing time*
Unit rainfall duratian  t> * ^hour
t
Luk + ^7 =■ 2»2J hours
Volunio of 1 inch runoff « A x C
whoro Ae drainage arua in sq   miles
+
C» 26*B9-un£t conversion facter
Ar«a = 37 85km
#
Z
0  1U.61 sq. miles (1 aq.roile ■ 2.59 kc/)
Volwno of 1 inch runoff » lU,6l x 26.89
■ 393 second-foot-day Using dlmoneionloss graph, Fig. 5  thu orixnates of
t
unit hydrograph aro computed in Table 1A»1--29-
FIG- JO
Table 12* - Ordinal?v of unit hjnlrotfraph
Ticfr
Lrs»
> of
L=C ♦ j
*'00 . . .
" "■ = 44,^
2.15
0 rd 1 n - t c
(Froa hf, J )
Unit *;
c?>
Ord. jr --■*■
2.25
Unit s
aVltt,
t c 1.
35.>14
1
2
3
2*
5
0.5
1-0
1.5
2.0
3,5
3.a
3.5
4.0
4.5
r 5.0
5.5
**o
1 6.5
7.0
7.5
8,0
9.0
22*2
44.4
66.7
8B,$
nt j
133.3
155.6
177.B
200
222,2
244 Ji
266,7
2B8.9
311.9
333.3
355.6
4 Go
3
12
1?
20.8
16*2
12.3
7.0
4.7
3.1
2.13
1.53
0.97
0.65
0.45
0.31
0.21
0.)
525
ZO15
3320
3635
2150
<225
S20
5mQ
370
265
170
in
79
54
37
id
n.
59.32
?M .O1
>02.93
ISO* till
PO.BS
n.09
2 3.22
i 5.29
IO.J48
7.50
4.Bl
>.?3
3.24
1.53
1.05
0.51
Tho unit hydrografih 1* plotted    Fi^ru fl.
in1-3I-I33
Derivation of inflow design hydrograph. from the unit hydrograph Since there is no racordins raln^ugB wi thin th* catchment
crca,. the 1 day 300 year probable precipitation of 124 ra"n- la
u$&d for the design storm. The storm rain is arranged in a
□are or leas syinotrical timo distribution Fi-gn 9r with the greatest inlanslty in the middl* of ths storm to give the worst possible condition for flooding* The rainfall excess are assumed in the
Following naojiiiar*
1st   hour     flat             tc%   of rainfall
2nd     91
ii                                 91
3rd      r           m          75%      ■ I
4th      H
■i          60$       H
IB
ii
IB
Th*       unit       hydrographs       of       -J-       hr,       duration       are       durlvod
for rainfall excess of 1cira  1 razi6nm  12mm  22 5imDt
p
#
fd
Tmra,
and 2 j 4 nun a, Ujiing the principle of unit hydra graph superposition the design inflow flood hydrograph is obtained both graphically (Fig, 9) and analytically in table 15.
The unit hydrograph is computed for 1 inch rainfall
Miich in inotric units is for 25*4 nun or 1 
The rainfall excess t from ana day 100 yg&r probable
= 25*4ntm*
precipitation         is         distributed         into         -J         hour         unit         rainfall         durmtiona find is term ad by 1
Therefore
(
Ordinate for oaelk * Hour unit = QrdlnQt* °f
firs ph * *
4 
£r■ as a ii
- 33-■ij. *'?
*u
p
'. r ?
* 3*-
'r.
- ’ *■ .
TablM       1J — Computation of inflow flood hydrograph
Et>
■ rva.
Ordinate for
1
*2-'
*2-r
1^=6
ijn12
i 3-22,5
lj-2.4
Q
Total
J"3 
n /stc
}
f
>
5
r
OiQ
0.5s
2. 3 *
)
1
)
3
i
k
*
J
3.70
<(.O5
3.15
2.39
1.36
0.92
c.6o
0.41
0.30
o.i?
0.12
0.06
0.06
0.04
0,03
0.0
0.58
2,34
3,70
4.05
3,15
2.39
1.36
0.92
0.60
o.4i
0.30
0.19
0.12
0,08
0,06
0.04
0,02
0.0
3,48
14.04
22,20
24,30
18.90
14,34
0.1(5
5,52
3.60
2,46
t.&o
1,14
0.72
0,40
0,3*
0.24
0.12
0,0
6.96
28.08
44,40
40.60
37.BO 28,60
16,32
11(04
7.20
4,92
3,6
2,28
1,44
0.96
0,72
0,40
0.24
0.0
T3,O5
52,65
83.25
91,52
70,07
53,77
30,6
20.7
13,50
9,22
6.75
4.27
2,7
1,8
U35
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
1.0
1.2
0.9
0.6
0,3
0.0
1.74
7.02
11.1
12,15
9.45
7.17
4,06
2.76
1,00
1 ,23
0,90
0.57
0.36
0,24
0,18
0,12
0.06
0.0
1.39
5.62
8,88
9.72
7.56
5.74
3.26
2.20
1,44
O.98 ’
0*72
0,45
0.29
0.19
0,14
0.096
0,040
0.0
0.58
2.92
9.52
28.75
70.53
135.59
191.34
209.45
106.70
144.90
100.97
65.98
44.15
29.18
20.04
13.46
8.B1
5-8?
3.96
2.57
1.42
0*56
0.156
0.048
Fbf inflow dcairijn flood hydrograph is platted in f±g  9. Ths
B
=CinpUtations show that the peak of the inflow flood is
^hich is the maxinjum computed ooopared to all the ta&ihods so It l«i t
wcpted for routing It through the reservoir to fix the spillway *J»cityB
■■A"'-W f_    A 3 0 £ £!, £ VO T jq? j  _c   U R y_f=^
— 5 6
’"h „ ■ •
it’ *w a.I
...fl
- 35
j flood routing
For flood routine inflow flood hydrograph with a pwftk
df 209*^5 ro^/soc baa bom computed. The outflow hydrograph has
t  he computed for which a spillway width is assumed limiting
the head above the crest to 1,5 metres so that the total bright
fl f the dusi above the full conservation level Is ressonablo, For
itanofflic reasons ths crest is provided in the shape of a joraieircle keeping a straight wiath of SO metres. So. for a diameter of
2D metres, the semicircle length would be 31 .i* metres, For
computing the rating carve of the spillway, the length of the
ereit is taken as 31 ictros. The discharge over the spillway
1, given by formula,
Q 5 C B H ’
whore Q = spillway diachorgo in »/«•€*
&
15
C = coefficient gf dlachnrsa which
is taken aa 2
B a CrMt Unffth of spillway width, which la taken as 31 ci&troa
H ■ Hoad of watiij* ctbovQ craat in
Tdotros.
Table l6 4ows tho values of discharge for various
• j
values of head, H* Spillway rating curve is plotted in Fig, IO,Table l6 — Spillway rating oum> Computation*
ih
Q=CEH*         |
15
3 
Q /flQCi
Jz
0.2
0*4
0,6
0*8
1*0
1.2
1*4
t.5
1*6
1*0
2.0
j                5*54
15.63
2d ei
r
44*36
62,00
01.50
102/70
113*90
125*43
149*70
175*36
With the help of epillvny rating cum fig. IO, rtiurvoir capacity curve Fig* 1, and the inflow flood hydrograph fig* 9jflood ro ling computations are carried out by trial and error method a> shown in tables 17 and IB* The crest level of tha spillway for the Tull conicrvatUii level is taken as 125, earlier indicated in tnblfi 13*r
■J
,c
r
■;
i
i
J
I- I.
I J*
r
■<
f- J8 -
Tabla I? - Flood routing computa ti on 5   13 = 31 l Croat Uval * 125
r
p
-a
dt
StfCOTlCi
Inflow at
tirat t
D^/sdC
Averarfo rate
of inflow q. 
for dt ni^/sie
Inflow
Trial Reservoir
Storage elevation
in
Out flow
fct tin# T
q3/*«e
2
3
4
5
6
7
)
i-
i
p
l
i
1
l
|
l
|
li
1
i
1
1
I
1
1
I
i
i
J* 
lBOO
I8CH)
1000
1800
ifioo
1000
1800
1800
1800
iaoo
i8oo
1800
1800
1800
1800
1800
1800
1800
1800
1800
1800
1800
1800
1800
|    1800
0,0
O.5B
2.92
9*52
28.75
7D.53
135-59
191-34
209.45
186,70
144,90
100,97
65*?S
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
4$. 6<U
103,06
163,46
200.395
19S.075
165.8
122.935
83.475
55.065
36,665
24,61
16,75
11.135
7,34
4.915
3.265
’.995
0.99
0.358
0.102
522
3150
11232
34443
89352 1855O8
294228 360711 356535 298440 221283 150255
99117
65997 44 298 JO15O
20043
13212
8B47
5077
3591
1782
644
184
125,01
125.01
125.01
125,08
125,20
125.46
125.85
126,22
126.45
126.57
126.5S
126.5
>26.38
126.25
126.09
123.98
125.86
123.73
125.65
125.59
125,52
125.45
125.39
123.35
0.5
0.5
0.5
2.0
6.0
20.0
48,5
84.0
107-3
121.5
122.3
113
100
87
70
60
48
to
32.5
28.5
24.0
20.0
16.5
l4,o0
■- 39 -
Tablo IB - Flood routing compu
£J = 31m| Crott level - 125
Averse
rite
tut flow
Out flow
w3
Incraretntai
St^ra^e
drt ei-
Total
Storgga
Ttoservoir
elevation
and of dtf M
Roni ark
fl
9
10
11
12
13
0.25
0.5
0.5
U25
4.0
n.o
34.25
66.25
95.75
114*5
122
117*75
106.5
93*3
76.5
65*0
j4.Q
44.00
36.25
JO. 5
26,25
22*0
IB.25
15*25
1*50
900
$00
2250
7200
231*00
61650
119250
117350
206100
219600
211950
191700
168300
161 JOO
119250
97200
79200
65250
5^900
1*72 50
40950
32850
27450
72
2250
10332
32193
B2152
16210B
23257B
241461
104185
92340
T683
61695
92583
1023^3
97002
89100
77157
65988
56403
49023
43659
39600
32206
27266
3000072
3002322
3012654
3O44B4?
3125999
32B81O7
3520685 3762146 3946331 403B671
4040354 3978659 38060?5 37B3772 3686770 3597670 3520513 3454525 3398122 3349099 3305440 3265840 3233634 32O6368
125.01
125*01
125*015
125*075
125*21
125.47
125.86
126,21
126,45
126.57
126.58
126.5
126.38
126.25
126*09
123*98
125,06
125.75
125,66
125.59
125.51
125.44
125*39
125*35
0,00
0*00
0,005
0.005
0,01
0,01
0.01
0,01
0.00
0.00
0,00
0,00
0,00
0.00
0,00
0*00
0,00
0,00
0.01
o oo
a
0,01
0*01
0,00
0,00
Mt ■- 4tO -
Flood routing computation3 show that th* nuixiffiun
the discharge through tho
J
reflorvolr level attained ia 126,58 
spillway for that level is 122*5 a /»ec, Head above the creat of spillway is (12^,56-125*00) > 1,58 metres which is very close to desired limit of head above crest of 1.50 metre), Therefor® the crest length of 31 metres is adopted.
The inflow and outflow hydrographs have been plotted on the some sheet. Fig. 11. Surcharge volume le shown as hatched and camoa to 95^ OO0 ta .
(
7, CONCLUSIONS
The foregoing study in baaed on very scanty data* It ehould bd aa such, troated aa applicable to this particular case
t
and not used a» a general guide linei Similar approach for
other basins, whoru enough data ia not available, should bo used with caution.
The water availability study shows that with a live ptcragc of 2 million cubic metres a draught of 9^ litres/sec
would ba possible which has been recommended In view of economicnl eemaervntion of water and catering for’ long term denmnd*
The spillway capacity is based on a 10O yaars probable flood with sufficient frno board over the flood surcharge Level*
The tap level of t he dam has been recosunsnd^d 3 metres higher than the spillway crest level. The fr«o board would however be cheeked for wave height when the design of the dam is carried
out.
6. ACKNOWLEDGEMENT
Thanks are conveyed to Hr. Egbert K*J  Sohroten for
-
computing the X^ values in tables 4 and 5  and to Mr.
f
Carlas A*Kunax for reading through the report and agreeing to
it in full.I-
•i
——J< lP/MC
FIG III
I ’la)
of Snail Dnma
by United States Bureau of Reolamation
b)
Land and Water itosgurcofi of the Blue Nilo Biain, Ethiopia
Appendix HI - Hydrology by
United States Bureau cT Reclamation.-M
APPENDIX A Mr, C A. HwwEfj Note
t
.jr* ■ ■■fioVOmOer
1979
Ta             t Mr. G,Dekker
Project Manager ETH/75/O°5
From        : 0.A.Munoz
Hy drome-tvoro logi a t ETIt/7 5/^05
Reft Hydrometeorological data
______ far Dlrato _Riverx
For tho design or a small dam on ZJlrato River which will bo used as storaao for the water supply of the cement factory, I
have bven requested to provide the basic hydroaetaarolosical information such 3* rainfall, evaporation, peak discharge for the
design of the spillway and runoff values per squaw kilomoter.
No hydroiDO two re logical stations exist in the catcbmotit
□nd only published maps at a scale of It250,000 which aro not
adequate i the tributary area has boon dotorfained by Dr. Tsoga fro« available aorinl photographs nt scale 1*50,000 and has found to
havo a value of 37.85 sq. kmts.
The area was visited with Mr, M, Ahmed and Or, Tsoja on Friday November 2nd, 1979 and fivld data were collected far
the possible peak flood design, It appears that the catchment
has boon properly located on the aerial photographs and the
calculations have been based on tho value <jr the am »a computed
by Dr. Tsoga.
Tho nearest evaporation station is located at Mino with annual values in min of A62U.2, 1325.2 and 1^99.1 mm Observed in
nn evaporation pan class A, installed in December IJTJj thdso values correspond to 1976, 1977 and 1978 respectively. Those values
have boon adjusted for missing data in 1977 and
Observations of rainfall at Dorba (09° 26'N) and Inching (O9°19'K) (3B°2J‘E) could bo used. (Tho Longitude at Dorba is
38°39'E)*
■-™
“■ —r<ei
i--
1
r
I
II
I
IValues of runoff have taon computed from data available
on Huger Hivor with a tributary area of
Stiver is a tributary of Huger.
Encl03cd herewith pl erase find the detailed available
i n l'y rrna t i, qh .
c*c.
Dr. Tsoffa  EWftA
sq. kmta p Blreta
f- 46 -
C ofiijp u t □ t i o n of pea K f 1 d_ for Ji re te H 1 ver - t. po-* s 111 o d — (Br.?cd on e tributary arci of 37«®5 *A« Jccaes • calculated by
Dr. Tsvja from available aerial photflfifr.phs 1-jO OOO)
t
1) Fiddos Method
Field data: Length of stream 11.91 knits-
Elevation at upper parti
rt
" d<im mites
Differtncci of elevation:
Slope Of main striyfimi
{Information supplied by
231.65
i?7*Z2
125.32
123-82
119W
Dr* Tsetfft)
0.0103963
Standard         contribution         area         caeffioionc         C*         for         a         rolling         eatchaent slope end moil typo with impodwd drainage C51 = 0.5
Catchment       wetness       factor       Cw       for       wet       zone       and       pwronjiial       stream Cw ■ 1.00
Land use factor Cl (Xntonaa cultivation) ■ 1 ■ JO
CA ■ Cm x Cw x Cl e 0,5 x 1.00 x 1, JC =■ 0-75 Catchment lag lino K = 3.00 (cultivated lend)
Volume of runoff HO = C^ (P - Yj A x 1£p
-   0*75 (60-0) 37.8} x io3
-    1703.25 X 103
UtiO
(ta )
3
A^eraga flow Q . ^gJ 
Base Tines
.
t b            « .          H
4i*0
t b “ tp * 2*3K * \
T . 0.028 L
0.038 x
A  7-             4                     4 
Q&
<U x 0,0103963’
X
o.yniia
O.1O1962SQ^
2
T - 0.96 + 2,3 x 3 * 1\ « 7.86 + 3, 7Q6238J*7 -
By trial and error:
Q
3,27062.10
Q'
7.P6 * 3,2704238
Q*
-+ft0
Q
50.0
2.659147
1,229952
9.089952
8.80
48.ft
2,637614
1.239993
4S.35
2.636933
1,2403135
Q Kax =
2,3 <1        =   2,3 x 48.35
9.099993                  9.0909
9.100313                  9.10031 OK
T
t 1T T.20 ciJ/scc
2)       From Blue Nile StudiosB
Frtiguoney
Appendix
Peak
III, Figure 1X1*46
value in
pnco in x yeara
5
TO
35
50
100
for 37.85 sg,
40
49
6ft
75
83
3)      13y the ntocllliqij Myer's method lor a. 0 coefficient of 17
Q a 17 x A+ « 17 X 37,85+ . lQft.6
from tho nbovo vuiuca and to unitary thu result* of tho fiddas method it la advisable to daaign the spillway with
a capacity of 75 m /sec with a freeboard of 28 m^/aoc and an onorgoncy spillway of 15 m /aoc, The total capacity i’or evacuation of thn peak flood would bot
Q * 75 + 28 ♦ 15 - 113 pPMc.
3
3
in April
Th® aval ion iim tilled nt Durba (09° -6>N rnd >6*3$"£)
jjivos tho Following vnlucsi
Year
Annual Pt vclpLtr.Huti arc,
'975
1976
94 ft ,?
090,8
I
1977
1970
1143.8
1007*3■' ■■■ --1
™ 4S -
The station located at Iflcluno (Oy°l?'N and 36 2 J E) jndicoUs higher values duo to higher altitude but th« data
ftvailahlu ara qu^ationable. Detail* are available in the office of the Land find Water Studios Agency Of EVRA far those two station^-
y va porr, ti on:
Observations Jiavo bo an irami-L- at I lino
in       an       evaporation catchment of tho
pan
/■.wash
-5 however [fine io
b?r.in
and
not
on
tlio 
since June located on Nile      ba#ifli
22, 1975
the
but r&lfltovily
closo ta Eirete.
The adj
usted va
lues wo
uld hot
Yr* .ar
Jinn 11=1 Evaporation in mm
1976
1977
1973
1624.2
1525.2
1499-1
Lally and monthly values arc available in the Land and Vater Studies Agency of
Runoff valuos;
No hydrological station is existing on Birc-tc River.
Continuous data of runoff arc nvciltblc for Huger River at Cliancho for the period 1971**1976; tlio tributary area is
aq*
i. Tlla           values fif runoff
are a s        follows:
Year
Runoff
x 1 c/‘
Runpff
per He. knit, in
1971
164.5 57
&-339993&
1972
zo6,OE
0.4257851
1973
275-033
0.5682500
1974
208.907
0.431626
1975
303.991
0.6280805
157-6. ,
207.195
O.42BOBER
Average 227-627 
0,470304
■ — -■
IIAPPENDIX B
Arsa and Vol unto Computation for platting Area and Capacity Curvpj’■
- raArena & Vplurm? computation
| No
z
m
A            |(Ai} + (Al + 1 }
2
2
m2
m
> Av
m3
V
1
2
3
4
5
6
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
110
111
1 12
113
114
115
116
117
118
1 19 120
121
122
123
124
125
126
127
126
1360
7160
16680
33680
50600
68040
87080
118000 159080
200680 256400 321880 392400 46^440 533200 603600 669400 740600
812200
906.6
4260
11920
2 51 80
42140
59320
77560
102540
138540
179880
228540
289140
357100
42S96O
499320
568400
636500
705000
776400
906^6
4260
11920
25160
42140
593*0
77560
102540
138540
173880
228540
20914C 3571SO 420960
499320
568400
636500
705000
776400
906.6
5166-16
1 7086
42266
84406
143726
221286
323826
462366
642246
870786
1159926
1517106
1946066
2445386
3013786
3650286
4355206
5131686i I i
»
I
i

Summery

Project Overview

The Birete Dam Project was a hydrological study conducted in April 1980 to assess water availability for a proposed cement factory in Ethiopia. The study was prepared by Nafis Ahmad (UN Engineering Adviser), Girma Bekele, and Banteyehun Halle of the Land and Water Studies Agency.

Key Project Details:

Location: Birete River basin, 106 km NW of Addis Ababa (9°29'N, 38°20'E)
Catchment Area: 37.85 km²
Purpose: Water supply for cement factory (current demand: 23.15 l/s)
Dam Type: Fill-dam

Methodology

The study used limited available data from nearby stations (Derba for rainfall, Hino for evaporation) and applied various hydrological methods including:

U.S. Soil Conservation Service method for runoff estimation
Mass curve analysis for water availability
Multiple empirical methods for flood peak estimation
Unit hydrograph approach for design flood derivation
Flood routing to determine spillway capacity

Key Findings

Water Availability

Scenario	Live Storage (million m³)	Dead Storage (million m³)	Draught (l/s)	Reservoir Elevation (m)
Recommended	2.0	1.0	94	125
Alternative 1	1.5	1.0	68	124.2
Minimum	0.5	1.0	23.15	122

Design Flood

Multiple methods were used to estimate the 100-year flood peak:

Fiddes method: 111.2 m³/s
Modified Myers's method: 104.6 m³/s
Blue Nile Basin curve: 83 m³/s
Triangular hydrograph concept: 137.2 m³/s
Unit hydrograph approach: 209.45 m³/s (adopted for design)

Dam Specifications

Parameter	Value
Recommended Height	20.5 m (including 3 m freeboard)
Total Storage Capacity	3.0 million m³
Spillway Capacity	122.5 m³/s (31m crest length)
Maximum Reservoir Level	126.58 m
Sediment Life	26 years (based on 1 mm/year erosion assumption)

Conclusions

The study recommends:

A 20.5 m high dam with 3.0 million m³ total storage (2.0 million m³ live storage)
A semicircular spillway with 31m crest length capable of handling 209.45 m³/s design flood
Provision for future water demand increases (current factory demand is only 23.15 l/s)
1.0 million m³ dead storage for sediment accumulation over 26 years

Note: The authors caution that the study is based on very limited data and results should not be generalized to other basins.

Appendices

The report includes two appendices:

Appendix A: Mr. C.A. Munoz's note with additional hydrometeorological data and flood peak calculations
Appendix B: Area and volume computations for reservoir capacity curves