Birete Dam Project Hydrological Study Summary

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PROVISIONAL MILITARY GOVERNMENT OF SOCIALIST ETHIOPIA
» n a r* a <n i s
ETHIOPIAN WATER RESOURCESABSTRACT
This report covers the water availability study for Birete Dara Project and includes the derivation of an inflow design flood hydrography Flood routing has been 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 use in the case
of other basins, where enough data is not available, could only be with caution.1
ABSTRACT
This report covers the water availability study for Birote Dam Project and includes the derivation of an inflow design flood hydrograph. Flood routing has been 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 use in the case
of other basins, where enough data is not available, could only be with caution.2
TABLE OF CONTENTS
ABSTRACT
table OF CONTENTS
LIST OF FIGURES
1.       INTRODUCTION
2.       WATER REQUIREMENT
3.       WATER AVAILABILITY
a)    Catchment area
b)    Hydrometeorological data
c)    Runoff
d)    Mass curve
4.       HEIGHT OF DAM 5   DESIGN FLOOD
e
a)    Emperical methods
b)    Peak equations for a triangular hydrograph concept•
c)    Unit graph derivation
d)    Derivation of inflow design hydrograph from the unit hydrograph.
6.       FLOOD ROUTING
7.       CONCLUSIONS
8 • AC KN OULEDGzMLNT 9• REFERENCESe
list of figures
1  Reservoir capacity and area curve
2.       Mass curve of Inflow and Outflow-Draught 94 litres/sec.
3.       Mass curve of Inflow and Outflow-Draught 68 litres/sec.
4.       Lay out and dam axis cross-section
.5. Dimensionless graph
6.       Lag relation curve
7.       Birete River Basin
8.      Uni t hydrograph
9.       Inflow design flood hydrograph 10.       Spillway discharge elevation curve 11.       Inflow and Outflow hydrograph- 4 -
BIRETE DAM PROJECT HYDROLOGICAL STUDY
1 4 INTRODUCTION
The Birete river basin is located in Shoa administrative region about 106 km. North West of Addis Ababa. It is planned to conserve tho surface runoff of the basin to meet the water requircme of the proposed cement factory. Birete river is a tributory of Sodoble river which in turn drains into Mugc-r river. A site for
a fill-dam has been selected on Birete river which is about 10 km. from the proposed cement factory. The location of the dam site is approximately at 9°-29 N and 38°-2O'E
,
2.      WATER REQUIREMENT
According to the information received the present water
demand of the cement factory is 2,000 m^/day or 23.15 litres/sec. 
No projected demand is indicated, however the provision for increase
need in future is contemplated. To meet the water requirement
the following water availability study is conducted.
3t WATER AVAILABILITY
a)    Catchment area
Upstream      of      the      Bereti      Dam      site,      the      catchment      area      of the       drainage       basin       which       would       contribute       to       the       surface       runoff is measured as 37.85 km2.
b)    Hydrometeorological data
No hydrometeorological station exists’ in the catchment.
The nearest rainfall stations are located at Derba (09°-26’N, 38°-39’E) and Inchini (O9°-19'N, 38°-23'E), There is one
evaporation station at Hine in the catchment of Awash Basin which is close to Birete.- 6
i) Rainfall data
Only four years rainfall d_tw- is station. Annual figures are reproduced in
available          from          Derba table 1•
Table 1- / Year
knnuai xxaimuAx uv 
Arnim 1 Rainfall in mm
.--------------------
1975
1976
1977
1978
944.2
890.8
1,143.8
1,007.8
ii) Evaporation data
Three years annual evaporation data from Hine station is available as given in table 2.
Table 2 - Evaporation data for Hine Pan Annual Evaporation
mm
Year
Actual Annual Evaporation
E . s 0.8 x E 
act                             pan
mm
1976
1977
1978
1 ,624,2
1,525.2
1,499.1
1,299.4
1,220.2
1,199.3
The above values have been adjusted for the missing data
for 1977-78. Pan coefficient, K, for working out the actual
evaporation from the reservoir area is taken equal to 0,8.
c)           Runoff
A few discharge measurements were taken on Eirote river near Inchini which are reproduced in table 3.
Date of measurement
28  March     1979 12  May          II 26     11          It 16  Dec           II
Discharge of Birete river
near Inchini
24  Litres/       □ ec
81      11                      11
19     n                        11
17
it            11
11 1!
23 II
11
11- 5 -
i) Rainfall data
Only four years rainlaxx
4 r.fall data is available from Derba
station. Annual figures are reproduced in table
Table i- i
Year
knnual naimuxx u.u                         —------------------------- Annual Rainfall in mn
1975
1976
1977
1978
944.2
890.8
1,143.8
1,007.8
ii) Evaporation data
Three years annual evaporation data from Hine station is available as given in table 2.
Table 2 - Evaporation data for Hine
Year
Pan Annual Evaporation
mm
Actual Annual Evaporation
E . s 0.8 x E 
act                             pan
mm
1976
1977
1978
1,624.2
1,525.2
1,499.1
1,299.4
1,220.2
1,199.3
The above values have been adjusted for the missing data
for 1977-78* Po-n coefficient, K, for working out the actual
evaporation from the reservoir area is taken equal to 0.8.
c) Runoff
A few discharge measurements were taken on Birote river near Inchini which are reproduced in table 3.
Date of measurement
28   March     1979
12   May          II
Discharge of Birete river near Inchini
24 Litres/sec
81      II
II
26
11
II
19
II
It
16
Dec
II
11
II
II
17
11
II
23
II
IIlegend
3KC njl H K A UH fl
MAIN VOLUMES of works OCHOBHbl E OEfbEMbl PABOT
c       ? OC’
£§
Z S'?
—
Desc ription
HauMenoBaBue
Description HdHMeiOBaHue
1
Overflow dam
BOAOC-nuBHaa nnoiHHa
1
Exca vation BbieMKa
Unit me;
E^'
H3
m*
2
Earth dam                               > 3eMn«Ha5i nnoTnwa
3
Abutment
i/CTOI4
2
Backfill       and       F ill
OSpaTHaa 3acbin Ka h Hacbinb
A
Pier
BblK
3
Gravel with pebble
rpa8vwH0 - raneHHUKOBbiw rp^HT
5     Energy dissipator
TacHTenb aHepruu
4
6     Bridge
Riprap
flaopocKa ns hSco pTWpOBSHHOrO lUM
Filter (sand , gravel)
O6paTHbiH qpnnbrp (necox, rpa&nvt)
Concrete and reinforced concrete BeTOH h xceneiooeTOH
Reinforcement ApMarjjpd
M etalwork
MeTa/inoKOHCTp^K HUH
12
Moct
Slide gate ( (bulkhead)
SdTBOp njlOCKUM CKOnbiflUlUM (, PCMOHTHblu) Radial gate
SaTBop cerneHTHbiw_______________ Operating gear
MexanviBM noAbeMHM______________ Travelling gantry crane
Kpaw kosboboh____________________ Gantry crane rail track
n^TH NQanoBoro xpaaa_____________ Site    tor    storage    of    gates finomaAKa Ana xpareHus 3aTBQ?oe
5
6
7
8
9
Mechanical         ecj MexaHvmecKoe
Uiprnent coopyAo&dHue
M3
H?M1
MJ K5
m
rn
m
Seal M a cuj
, L 500 pao     .
50
•0        15        20
ling              t--3o t ;3O
FIG.6/LRFGULATING     RESERVOIR,     dam     SITE.     CONTROLLED     SPILLV/AT     (Altern pV!C6APEnnHPSWmAfl EMKOGTb. rHAPO^EJl . BOAOCBPOC C 3ATBOPAMH (Baf- 4 -
BIRETE DAM PROJECT HYDROLOGICAL STUDY
1 * INTRODUCTION
The Birete river basin is located in Shoa administrative region about 106 km. North West of Addis Ababa. It is planned to conserve the surface runoff of the basin to meet the water requircme of the proposed cement factory. Birete river is a tributory of Sodoble river which in turn drains into Mugcr river. A site for
a fill-dam has been selected on Birete river which is about 10 km. from the proposed cement factory. The location of the dam site is approximately at 9°-29 N and 38°-20’E
,
2.    WATER REQUIREMENT
According to the information received the present water demand of the cement factory is 2,000 nr/day or 23.15 litres/sec. 
No projected demand is indicated, however the provision for increase need in future is contemplated. To meet the water requirement
the following water availability study is conducted.
3.   WATER AVAILABILITY
a)    Catchment area
Upstream of the Bereti Dara site, the catchment area of the drainage basin which would contribute to the surface runoff
2
is measured as 37*85 km .
b)    Hydrometeorological data
No hydrometeorological station exists' in tho catchment.
The nearest rainfall stations are located at Derba (09°-26’N, 
J8°-39'E) and Inchini (O9°-19'N, 38°-23’E). There is one evaporation station at Hine in the catchment of Awr.sh Basin which is close to Birete.- 4 -
BIRETE DAM PROJECT HYDROLOGICAL STUDY
1 A INTRODUCTION
The Birete river basin is located in Shoa administrative region about 106 km. North Vest of Addis Ababa. It is planned to conserve the surface runoff of the basin to meet th© water requircme of the proposed cement factory. Birete river is a tributory of Sodoble river which in turn drains into Mugcr river. A site for
a fill-dam has been selected on Birete river which i9 about 10 km. from the proposed CGment factory. The location of the dam
site is approximately at 9°-29 N and 38°-20 E
f
,
2. WATER REQUIREMENT
According to the information received the present water demand of the cement factory is 2,000 m /day or 23.15 litres/sec. 
No projected demand is indicated, however the provision for increase need in future is contemplated. To meet the water requirement
the following water availability study is conducted.
3. WATER AVAILABILITY
a)    Catchment area
Upstream of the Bereti Dam site, the catchment area of the drainage basin which would contribute to the surface runoff
is measured as 37.85 km .
2
b)    Hydrometeorological data
No hydrometeorological station exists’ in the catchment.
The nearest rainfall stations are located at Derba (09°-26’N, 
38°-39'E) and Inchini (O9°-19'N, 38°-23'E). There is one evaporation station at Hine in the catchment of Awash Basin which is close to Birete.- 5 -
i) Rainfall data
Only four yours rnlnfoll data is available from Derta station. Annual figures are reproduced in table
lablei- >
innuax 
Year
----- -----------—
Annual Rainfall in mm
1975
1976
1977
1978
944.2
890.8
1,143.8
1,007.8
ii) Evaporation data
Three years annual evaporation data from Hine station is available as given in table 2.
Table 2 - Evaporation data for Hine
Year
Pan Annual Evaporation 
ram
Actual Annual Evaporation
E . = 0.8 x E 
act                            pan
mn
1976
1977
1978
1,624.2
1,525.2
1,499.1
1,299.4
1 ,220.2
1,199.3
The above values have been adjusted for the missing data
for 1977-78. Pc-n coefficient, K, for working out the actual
evaporation from the reservoir area is taken equal to 0,8,
c)           Runoff
A f6w discharge measurements were taken on Birete river near Inchini which are reproduced in table 3,
Table 3 - Discharge o Date of measurement
28  March    1979 12  May          II
f Birete river near Inchini Discharge of Birete river
near Inchini
24  Litres/3ec
81      11                      11
26     11
II
16  Dec           11
19 11
11 11
it
ti
17     11
II
23     11                      1111 Rlinfa11 “
Only four years ra station. Annual figures are P
lOD-Le 1 — 1            Year
1975 1976 1977 1978
ii) Evaporation data
lnl .oll d=ti 1. available fro. D.rba
±n table
a1 Rainfall at Derbg---------------------------------------------
Annual Rainfall in mn____________________
944.2
890.8
1,143.8
1,007.8
Three        years        annual        evaporation        data        from        Hine        station is available as given in table 2.
Table 2 - Evaporation data for Hine
Pan Annual Evaporation mm
Year
Actual Annual Evaporation
E . = 0.8 x E 
act                             pan
mm
1976
1977
1978
1,624,2
1,525.2
1,499.1
1,299.4
1 ,220.2
1,199.3
The above values have been adjusted for the missing data
for 1977-78. Pan coefficient, K, for working out the actual
evaporation from the reservoir area is token equal to 0.8,
c)           Runoff
A few discharge measurements were taken on Birete river near Inchini which are reproduced in table 3.
Table 3 - Discharge of B-—yne    irete rarive mrc ne niniar Inchini
Date of measurement
Discharge of Birete river near Inchini
28    March    1979 12 May           '•
26 *'                "
16  Dec          "
17     »           ••
24 Litres/3©c
81  " 11
19  " "
11" <>
23  " nAs earlier men within the catchment so 
not possible. To rainfall at Derba station Birete catchment.
- 6 -
tioned there is no rainfall station direct rainfall runoff corelation 
a
have an
idea of the water availability, the is  considered to be the same as for
Based on this assumption an attempt has
been       made       to       estimate       runoff       based       on 
• n the method developed by
the
U.S. Soil Conservation Service published in 19^4.
Complete daily rainfall data for Derba is available
for four years from 1975 to 1978. However it is available from
May to December for 1974 and from January to October for 1979 
which has also been utilized for computing runoff.
Based on the characteristics like, hydrologic soil
group, land use and treatment class, and hydrologic soil cover
complex, the U.S. Soil'Conservation Service has developed various
curves for direct runoff generated by rainfall storm. Runoff
coefficient is calculated by dividing direct runoff by the
corresponding rainfall storm. To estimate the runoff coefficient
for       every       month,       it       would       be       important       to       first       arrive       at        the representative         storm         for         the         relevant         month         against         which         the direct       runoff       value       could       bG       read,       from       the       curve.       For       this purpose the daily rainfall data has been analysed. For every 
month the first ten values of daily rainfall storms starting 
from the maximum value in descending order have been considered as partial series. The values are listed in tables 4 and 5. 
Averages of storms for each month and storms for a return 
period of 2 years have been calculated. In the hight of these representative storms, the values of runoff coefficients for
each month have been calculated with the use of curve as given in the set of curves developed by U.S. Soil Conservation Service
Birete river catchment above the dam site has been 
considered to fall within
withan the following characteristics.
i) Hydrological Soil Group
consists > it is- 6 -
As earlier mentioned there is no
• 
rainfall station
direct rainfall runoff corelatio
n is
within th© catchment so a not possible. To
rainfall at Derba station i
have an
idea of the water availability, 
the
is considered to be the same a
s for
Birete catchment,
Based on this assumption an attempt has
been made to estimate runoff based on
the
U.S. Soil Conservation Service published in 1964.
for May
Complete
four years from 1975 to 1978 to
i the method developed by
daily rainfall data for Derba is available However it is available from
December for 1974 and from January to October for 1979
which has also been utilized for computing runoff.
Based on the characteristics like, hydrologic soil group, land use and treatment class, and hydrologic soil cover complex, the U.S« Soil'Conservation Service has developed various curves for direct runoff generated by rainfall storm. Runoff coefficient is calculated by dividing direct runoff by the corresponding rainfall storm, To estimate the runoff coefficient
for every month, it would be important to first arrive at the representative storm for the relevant month against which the
direct runoff value could bG read from the
purpose the daily rainfall data has been analysed• For every 
month the first ten values of daily rainfall storms starting 
from the maximum value in descending order have been considered as partial series. The values are listed in tables 4 and 5. 
Averages of storms for each month and storms for a return period of 2 years have been calculated. In the hight of representative storms, the values of runoff coefficients each month have been calculated with the use of curve as In the set or curves developed by U.S. Soil Conservation
Birete river catchment above the dam considered to fall within the following cha
1) Hydrological Soil Group
m
 For this
these for given Service 9
site has been aracteristics.
or clay and Is iik91y considered to Tall i„ Group D. 
i^t i™1"17 C°n9ists iteration rate, it Is7
il) Land Use and Treatment Class Land      use      or      cover
treatment condition
ill)
or       practice       is       cl< for infilteration
Hydrologic Soil-Cover Complex Storm rainfall related to
antecedent moisture condition,
AMC II, has been considered. has been
85 to tho
monthly 
representative
storms
For the above characteristics curve number, Ch adopted to read the direct runoff corresponding 
arrived at in tables 4- and
5, and
the
runoff coefficients have been calculated.
Table 4 — Maximum daily rainfall storms, averages,
2 years
return period storms and runoff coefficients
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
7  -79
:
JUNE
6 year
data
74-79
l.j A
1
2
3
4
5
6
7
8
9
10
X
s
Y
X2
Runoff
20.0
20.0
6.6
5.7
4.7
4.6
1 .6
1 .0
0.4
0.4
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
9.58
7.23
8.66
0.03
Coeff
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
8.89
1^.87
0.05
e
44.1
44.0
34.1
30.9
26.0
24.3
17.5
14,0
13.9
12.0
26.08
12.03
24.55
0.20
50.1
29.3
27.0
22.2
22.0
2 1.7
1 ‘.9
1 ° . O
1 1.0
V.5
2--. 77
''.63
2 J. 55
0.20
J8 -
Table 5 - Maximum daily rainfall storms, averages, 2 years
return period storms and runoff coefficients
JULY
6 years
data
74-79
AUG.
6 years
data
74-79
SEPT.
6 years
data
74-79
OCT.
6 years
data
74-79
| NOV,
1 •'‘EC.
• ' yo ars        j5                years
data
» data
r»   1
)     74-7 0
1
i2
3
4
5
6
7
8
9
10
X
s
X2
Runoff
Coeff*
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
37.0
31.9
30.4
29.1
29.1
28.9
28.0
27.7
24.3
33.34
12.27
31.78
0.26
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
45.4
30.0
17.6
18.0
14.2
1 1 .0
9.4
8.5
8.3
20.80
14.53
18.96
0.12
I
23.0
12*0
12.0
1 1 .0
10.9
4.:
4.0
2.0
1.3
0.6
8.09
7.00
7.20
0.03
__________
9.7
3.6
2.8
1 .0
1.0
0.2
-
-
-
-
3.05
3.50
2.6l
0.00
To calculate the runoff, rainfall data for the year 1976, which happens to be the driest year in the four years
record, has been adopted*
d)           Mass curve
Mass curve study is carried out tc establisli water availability* Other parameters to carry out the study are the
base flow, evaporation from the reservoir, infiltration and seepage through the dam. Regrading base flow, table 3 reveals that the minimum flow recorded is 11 litres/sec* It has been learnt that a ’V notch  wuir is contemplated downstream of the dam site to record the base flow, however no further data other than listed in table 3 is available so far. As such it has
1- 9 -
been assumed that 10 litres per second would ba available even in the driest month. It has further been assumed that out of
10 litres per second base flow, 5 litres per second would be lost due to infilteration and seepage through the dam. For evaporation from the reservoir, data for the year 1976 (table 2) is adopted. Monthly figures of evaporation are given in table 6.
Table 6 - Monthly evaporation data for Hine
Month
»
Pan Evaporation
mm
Actual Evaporation E 
act = 0,8 x pan mm
134 122 130 141
96 86 82 110
85 103 100 110
E
January February March April
May
June
July
Augus t Septembe
October November December
168 153 163 176 120 108 102 137 106 129 125 138
data is January and     for years.
For the estimation of thG monthly runoff, monthly rainfall analysed in table 7. Rainfall data for the months of
to April, November and December is available for 5 years the months from May to October it is available for 6 Average rdfnfall is calculated for each month 
Total
annual rainfall for the four years for which r
available and the total of average monthly rainfall
the last column  of data is available, annual rainfall of
complete data is  is shown in 
totals. Out of the four yeOut ar of s f tcrhe w fhiour ch c yeomarspl feorte which year 1976 has been the driest with a total
890.8 mm, which is
8 ^0-- X 100 = 87.78% 1014»810
of the total of average monthly rainfall. To reflect a dry year
within the scope of data available, the monthly average rainfall
is reduced to the same 87.78%* Final figures of monthly rainfall are shown in the last line.
The above approach of arriving at the representative monthly rainfall for a dry year ha9 been adopted because the pattern of monthly rainfall is not regular. Perusal of the
monthly rainfall data would show that there is a large variation in the rainfall for the same month in various yaars and this variation is much more in the dry months. As such the figures of monthly rainfall adopted in further study are considered to give a better picture in which large variations within the scanty data are absorbed to a certain extent.T          W               f -Mt
Table 7 - Monthly rainfall at Derba
1  Year
1974
1975
1976
1977
1978
1979
TOTAL
Average
Rep. dry
year
Jan.
0.7
6.2
6.8
0.4
52.3
66.4
13.3
11.7
peb.
I
3 1.9
22.8
48.9
10.9
14.0
128.5
25.7
22.6
Mar.
25.4
43.4
74.6
30.6
23.7
197.7
39.5
34.7
Apr.
76.3
51.0
19.9
17.7
29.5
194.4
38.9
34.1
May
165.0
38.3
14.3
62.6
0.7
132.9
413.8
69.0
60.6
June
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
1567.5
261.2
229.3
Aug.
168.1
202.3
285.7
199.7
298.8
232.8
1387.4 231 .2
202.9
Sept.
128.7
164.2
97.2
146.7
139.3
207.2
883.3
147.2
129.2
Oc t.
12.8
3.4
12.0
183.8
27.8
35.0
274.8
45.8
40.2
Nov.
0.0
0.0
31.3
24.3
27. 1
Dec.
0.0
0.0
0.2
8.9
14.3
Total
944.2
890.8
1143.8
1007.8
82.7
16.5
14.5
23.4
4.7
4.1
1014.8
890.8- 12
shows the computations of monthly volume of
Inflow which
Table 8 is,
v i       =     Vr
+V b
Where
Vi
V
r
Vb
monthly volume of inflow
monthly volume of runoff
Runoff coeff. x Catchipent Area x monthly rainfall depth
monthly volume of base flow calculated at the rate of 5 litres/sec.
Table 8 - Monthly Volume of Inflow
Runoff
Coeff.
Monthly
Rainfall Depth
mm
Monthly         Vol. of Runoff
V
Monthly         Vol. of Base flow
nthth
Monthly Vol.
of Inflow
V. = V +V
i         3  b
r
mJ
1
2
3
4
5
6
t.
.V.
r-
in.
»b.
1
irch
pril
ay
line
uly
ug.
frpt.
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
14.5
4.1
11.7
22.6
34.7
34.1
60,6
106.9
229.3
202.9
129.2
182,590
16,460
-
-
25,660
52.540
64,530
458,740
809,230
2,343,330
1,996,740
880,240
13,390
12,960
13,390
13,390
12,100
13,390
12,960
13,390
12,960
13,390
13,390
12,960
195,980
29,420
13,390
13,390
37,760
65,930
77.U9O
472,130
822,190
2,356,720
2,010,130
893,200
In table 8, evaporation losses from the reservoir have not been accounted for. For calculation of evaporation losses, 
the reservoir surface area relevant to the resarvoir level in 
the particular month is estimated with the help of reservoir stage-volume and reservoir stage-area curvo«( capacity and area13
curves). Reference is made to Fig. 1* Fig. 1 are attached as Appendix B. To of the reservoir surface area relevant it is assumed that the reservoir is which is the end of rainy season.
computations for plotting initiate the computations to the particular month 
full at the end of September, Therefore the mass curve study
starts from October. 1
have been conducted, one
is 2 million cubic metres with one
For the reservoir full stage two studies
for the condition when the live storage million cubic metres as dead 
storage
and a constant draught (withdrawal) of 9^ litres/sec;
and       the       other       for       the       condition       when       the       live       storage       is       1.5 
million
cubic metres with, the same dead storage of* one million
cubic metres and a constant draught of 68 litres/sec, 
and 10 show the computation for the first case and tables 1 1 
and 12 for the second. The basis for adopting the dead storage
as one million cubic metre is explained below.
i) Sediment inflow
There is no sediment flow data available. No sediment sampling has been done. In view of the topographic features, vegetation cover, rainfall intensity and other characteristics
of the catchment, it is assumed that 1 mm depth of soil over the catchment area would flow as sediment annually. In view of the vegetation cover in the catchment it may be an assumption on the
Tables 9
conservative side,
but in the absence of data it has been adopted 
Further       it       has       been       assumed       that       the       unit       weight  »**<* va uu.e  of   sedithement
trapped in the reservoir
weight of the loose soil
the catchment.
Regarding trap storage and the drainage
and deposited would be 80% of the unit including organic matter eroded from
G ^iciency           of area ratio i.e. 2x10  ,
6
the           reservoir,           the           live 37.85x10  “ °«°53, 
6
gives a value of 86% as read from the graph given In th. Blue Nile Basin, Hydrology report, prepared by U.S.B.R. In 
the presence of an undersluice it is assumed that the trap efficiency may reduce to 80% and 20% of the sediment may pass downstream. 
H15
Therefore, yearly sediment volume, S^, eroded from
the catchment and retained in the reservoir,
S = A x h 
y
Where A 3
a
x 103 x 0.8 m3
catchment area in m
2
h 
s
sediment height eroded
Volume of the
deposited sediment in
in mm.
the reservoir,
'yr’
S
yr
A x h x 103 x 0.8 
s
0.8
m3
S 37.85 x 1 X 10  = 37,850 m /year
3
3
It is reservoir which
encroaches upon is slow because
proposed     to     provide     a     defcd     storago     volume     of     the
gives a reasonable life before the the live storage. Encroachment on progressively the sediment passing
sediment
live            storage downstream
•t. '
increases and ultimately a balance is
reached.
Considering
a dead storage
m3
Life of the
dead storage
= 1 X 106
1 x 106
= 26 years.
37,850
*
«- 16 -
Table 9 - Reservoir surface area relevant to each month^ Live storage, L = 2 x 10  m | Dead Storage, D  = 1 x 10 m J
63
s
g
Draught D = 94 litres/sec.
Accumulated
Vol. of Inflow
ACC.  Vol.  Of
Inflow + live
Storage
■m
V V ♦ L 
i3
m3 3
Aoc. vol. of
Draught
D
m3
Available
Storage V +L +D -D
i      3 o3
nr
I
Reservoir
Surface Area
2
m
!2
3
4
5
6
(           195,980                  2,195,980 225,400             |    2,225,400
|  238,790 2,238,790 252,180 2,252,180 289,940 2,289,940
1             355,870                  2,355,870 L             433,360                     2,433,360
905,490             :   2,905,490
I 1,727,680
|  4,084,400
I 6,094,530
3,727,680
I 6,084,400
8,094,530
• 1      6,987,730                      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
i 2,964,390
2,944,210
2,729,980
2,491,600
2,253,220
2,063,580
1,877,740
1,711,580
1,931,940 2,510,480
3,000,000
3,000,000
3,000,000
592,000
570,000
538,000
506,000
480,000
rI
452,000
424,000
460,000
542,000
600,000
600,000
600,000
3
2
The accumulated volume of inflow (runoff) estimated in table 9 is 6,987,730 m . According to Mr. C.A.Munoz's not. attached
as Appendix A, the runoff based on the minimum flow per Km  of Muger river at Chancho for the driest year 1971 works out
= 0.3399938 x 106 x 37.85
= 12,868,760 m3
The r -..off calculated in table 9 is less and is adopted for further study.
I17 -
Table 10 - Net accumulated volume of Inflow + live storage (continued from table 9)
---------- r—
I
J
• Reservoir
th ! I
I
Surface Area n
m
t-
Monthly
act. evap.
mm
Monthly
Vol. of Evap. m3
Acc.        Vol. of Evap.
3 m
9- 10)
2
3
4
5
552$OOO
570,000
538,000
506,000
480,000
h           452,000
1            424,000
460,000
542,000
y
600,000
(• 600,000
E 600,000
103 100 110 134 122 130 141
96 86 82
110 85
60,990
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
512,830
562,030
628,030
679,030
Net Acc. Vol. of Inflow +
Live storage (Col. 3 of table col. 5 of table
m3
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
8,308,700
F
Mass curve is shown in Fig. 2. It would be seen from the
"ass curve and the tables, 9 and 10 that at the end of April (2.011.300 - 1,721,780) = 289,520 cubic metres of live storage
is left in the reservoir which would provide enough cushion for a vory dry year or would add to the life of the project.« «□> • u * •
Fie-18-- 19 -
Table 11 - Reservoir surface area relevant to each month^
Live storage, L = 1.5 x 10^
9
; Dead storage, Dg = 1 x 10
D = 68 litres/sec*
th
Accumulated Vol. of Inflow
vi
m3
Acc. Vol. of Inflow + Live
storage
V i * LS
3
m
Acc. Vol. of
Draught
D
m3
Available
Storage V,+L +D -D
i s s
m3
Reservoir
Surface Area
2 
m
2
3
4
5
6
*
I
•
C.
•ch
ie Ly
L
U
i.
?t.
Km.
195,980
225,400
238,790
252,180
289,940
355,870
433,360
905,490
1,727,680
4,084,400
6,094,530
6,987,730
1,695,980
1,725,400
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
182,130
358,390
540,520
722,650
887,160
1,069,290
1,245,540
1,427,670
1,603,930
1,786,060
1,968,190
2,144,450
2 500,000
2,367,010
2.193,270
2,029,530
1,902,780
1,786,580
1 ,687,820
1,977,820
2,500,000
2,500.000
2,500,000
| 2,500.000
540,000
524,000
500,000
472,000
456.000
440.000
4' ,000
464,000
540,000
540,000
540,000
540,000- 20 -
Table 12 - Net accumulated volume of Inflow + live storage (continued from table 11 )
tl
Reservoir Surface
Area
2 
m
Monthly Act. Evap. 
mm
Monthly
Vol of
Evaporation
m3
Acc, Vol. of Evaporation 
m3
Net Acc. Vol
of Inflow + Live
storage
(col. 3 of table 11
- col. 5 of tab.$2)
m3
2
3
4
5
6
i
540,000
524,000
500,000
472,000
456,000
440,000
420,000
464,000
540,000
540,000
540,000
54u,OOO
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
108,020
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
1,575,770
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
Mass curve is shown in Fig. 3. It would be observed from the mass curve and the tables 11 and 12 that at the end of
[April (1,535,0^0 - 1>245,5^0) = 289,500 cubic metres of live storage is left in the reservoir which would provide a cushion for a very dry year or add to the life of th6 project.
The present demand of the cement factory is 23.15 litres per sec. The accumulated draught by the end of April at that rate starting from October would be,
= 23.15 X 3600 x 24 X The net Acc. Inflow from tables 10 and 11 = 433,360 - 398,320
= 424,034 m3
= 35,040 m3
Storage required = 424,034 - 35,040 = 388,994 m3s
J
■*
________ t
*
i
x
i22 -
Considering a margin live storage of 500,000 may
4, HEIGHT OF DAM.
for      a      very      dry      year, be necessary.
a minimum
6
The dead storage for a reasonable life may be = 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 the competent bed rock may be at elevation 107.5 (with reference to an assumed local bench mark).
Table 13 - Alternative heights of dam
Draught
lit/sec
Total
Storage
w3
Reservoir
Elevation
Bed rock
El, at deepest
point
Maximum
Height of dam including  3  m freeboard
m
94
68
23.15
3.oxio6
2.5x1Ob
X
1.5x10°
125
124.2
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 10^m^ of live storage and for 2.0 x 10 of live storage is 3 metres, 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 m •
5. DESIGN FLOOD
a) Emperical methods:
6 3
i) Mr. C.A.Munoz in his note, Appendix A, has computed the flood peak by the two emperical methods, the values are given below.
By Fiddes method, flood peak =
111.2 m /sec.
q
By modified Myer’s method, flood peak = 104.6 m /soc.
J5
22 -
Considering a margin for a very dry year, a mi live storage of 500,000 m  may be necessary.
4, HEIGHT OF DAM.
The dead storage for a reasonable life may be =
1 0 x 106
e
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 the competent bed rock may be at elevation 107.5 (with reference to an assumed local bench mark).
Table 13 - Alternative heights of dam
Draught
lit/sec
Total
Storage
3 
m
Reservoir
Elevation
Bed rock
El* at deepest
point
Maximum
Height of dam including  3  m freeboard
m
94
68
23.15
3.0x106
2.5x10°
X
1.5x10
125
124.2
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^m^ of live storage and for 2.0 x 10 nr of live storage is 3 metres, whereas the 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^m^.
5. DESIGN FLOOD
a ) Lmperical methods:
i) Mr. C.A.Munoz in his note, Appendix A, has computed the flood peak by the two emperical methods, the values are given below.
By Fiddes method, flood peak =
By modified Myer’s method, flood peak = 104.6 nr/sec.
111.2 m^/sec.- 23-
LAY OUT AND DAM AXIS CROSS-SECTDN
5T4
.iim
4- 24 -
According to the Blue Rile Basin report, Appendix TTI, 
Figure TH -
peak flood value for once in 
f'r
Th
fr^uancT
read from the curve = 83 m3/sec.
Mr. Munoz has recommended to provide flood =         118 m /sec.
ii) Based on basin characteristics Blue Nil© Basin for the Ribb and
following formula has been developed by Xr.
Q = 0.0076 H *  C A **’
max
r
rd
15           C
max 
15
where H = Maxinum 1CC y^ar zme
C = Basin chzrzc’Lrzsric nz-eddaciaxil this case = 0.7)
A = Basin area ()
According to the Blue Nile Basin Rupert ttf maximum 100 year one day rainfall » 124 ma
Thorofore Q = O.OO76 x 124 *  x C.7 x 37.£5~’71
-
a 96.9 cr/sec.
b f Poak equations for a trian/cular hvdro^riph concept
Using
hydrograph.
the goneral peak equations for a trian^cular
0.385
T c
T o
11.9 L3 H
(Reft
Design       of       Small       Dams by USSR)
Where
Timo         of         concentration         -
travel
to dam site
time
a',..
from most L     e Length of
distant point stream in miles
H
= Elevation difference between tho upper and tho daci site of the stream in feet
of water in Hours
point
Tho
length of the stream is 11.91 km and the elovation 
diff
©renoo between tho upper point and the dam site of tho 
trocm is (231.65 - 107.83) o 123.82 metres. (This information stream length and elevation difference is supplied by
Dr< Tsoga)25
T c
T
P
11,9(11.91 x 0.62137)
123.82 x 3.2808 2.59      hours day = 2.6 hours
0.6 T 
c
Where D =
Rainfall excess period which
taken as 4 hours
is
T =
P = — + (0.6
Timo fro# start of rise to peak
rate
T
 x 2.6) = 3.56 hours
P
Tb
qp
2.67 T = 2.67 x 3.56 = 9.5 hours P
Tfc = Time base of hydrograph 0.749 AQ
T P
Where q     P
= the peak discharge in mJ/sec
A := Drainage area in m2
Q
= Depth runoff in metres, one day rainfall of 124
T   = Time from start of rise
P
q  _ 0.749 x 37.85 x 10  x 0.062
50%      of      the      max.      100      year mm is token as runoff.
to peak rate in seconds
6
P
3.56 x 3600
= 137.2 nP/sec.
c) Unitgraph derivation.
To develop unit hydrograph for Birete ungaged area, the dimensionless graph, Fig, 5 and lag relation curve Fig. 6 developed for Blue Nile Basin are used. Curves are taken from
Blue Nile Basin Report, 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 watercourse from dam site to watershed divide in metres.25
T c
11.9(11.91 x 0,62137) 123.82 x 3.2808
2.59 hours day = 2.6 hours
T 
P Where
D
2
+  0.6 T 
c
taken as 4 hours rate .
T P
Tb
%
D  = Rainfall excess period which T : = Timo from start of rise to peak
P
+ (0.6 x 2.6) = 3.56 hours
2.67 T = 2.67 x 3.56 = 9.5 hours P
= Time base of hydrograph 0.749 Aft
T P
Where q     P
the peak discharge in nr/sec
A =     Drainage area in m2
Q =    Depth runoff in metres, one day rainfall of 124 
T = P
Time from start of rise
0.749 X 37»85 x 106 X 0.062 3.56 x 3600
137.2 m^/seo,
5096     of     the     max.     100     year mm is taken as runoff.
to peak rat6 in seconds
c) Unitgraph derivation.
To develop unit hydrograph for Birete ungaged area, the dimensionless graph, Fig. 5 and lag relation curve Fig. 6 developed for Blue Nile Basin are used. Curves are taken from
Blue Nil© Basin Report, Appendix III - Hydrology prepared by USER.
The parameters used are:
Lag s time from centre of period of excess rain to centre of volume of runoff in seconds.
L = Length of longest watercourse from dam site to watershed divide in metres.-2 6~
BLUE NILE BASIN IDEAL HYDROGRAPH
f
• • UM nrr ru ucom rim-off
Ml FROM CINTIR OF RAM II CI M MRXM TO TIM
HALF OF RON-OFF WAR OCCURIO
•• WAI OVIR VMltt RAMFALL IXCIIM INFILTRATION
WOAT-AICOM MIT (l,f CURC-FOOT MR IICQRO) INTCARATtD
M» MRIOO OF ATOAN TO OIT TOTAL MLU« OF AURFACI RUNorN
DIMENSIONLESS GRAPH F,G 5j i hi
r i
BLUE NILE BASIN LAG CURVE
»*
fw
i!
i
tk .
f
Fs'W
Ft; 6 LScale I « 50,000
FIG 7- 30
Table 14 — Ordinates of unit hydrograph
Time
hrs-
</> of
D
Lag + 2
*'0Q_ _ 44 4
2.25
Ordinate
(From Fig. 5 )
Unit q
cfs
„ 393
2.25
Unit q
rn /sec,
col. h
35.314
1
2
3
4
5
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
8.0
9.0
22.2
44.4
66.7
88.9
1*11.1
133.3
155.6
177.8
200
222.2
244.4
266.7
288.9
31 1.9
333.3
355.6
400
3
12
19
20.8
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
18
14.86
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
1.05
O.51
The unit hydrograph is plotted in Figure 8riQ. 8- 3332 -
4
ar© >
Since there xs no r«uuruxjig *■aAu ^u
6b
w
 ~ —
a
the
used       for more     or     - intensity 
condition following
1 day 100 year probable precipitation o the design storm. The storm rain is
less symetrical time distribution Fig*
9,
in
for
manner.
the middle of the storm to give the flooding* The rainfall excess are
1st hour net             10%  Of   rainfall
2nd      "           "     40%    II
11
3rd " 
4th " 
"     75%    11             11
"     60%    II
11
The unit hydrographs
of  -J- hr. duration
are derived
for
and 
the
rainfall excess of 1mm, 1mm, 6mm  12mm  22.5mm, 15mm  3mm,
t
f
f
2.4mm. Using the principle of unit hydrograph superposition
design inflow flood hydrograph is obtained both graphically
(Fig. 9) and analytically in table 15
The unit hydrograph is computed for 1 inch rainfall which in metric units iaT for 25*4 mm or i^ = 25.4mm.
The rainfall excess, from one day 100 year probable precipitation is distributed into -J- hour unit rainfall durations and is termed by i^
Therefore,
Ordinate for each ± hour unit = OgU-Qate of unit frraph
s- 33-34 -
Table 15 ~ Computation of inflow flood hydrograph
Ordinate for
” 2=1
■val     i
0).o
0>.58
2 .34
3 .70
4,.05
3..15
2<.39
1 <.36
0..92
0..60
0..41
0..30
0..19
0..12
0..08
0. , 06
0..04
0..02
± 2=1
i2’6
i 2o12
i 2=22.5
i 2=15
A 2 = 3
i 2=>2.4
Q
Total
nf/sec
.0
.5
.0
.5
.o
.5
o
5
c
5
0
5
0
5
0
5
0
5
0
5
5
0
5
0
0.0
0.58
2.34
3.70
4.05
3.15
2.39
1.36
0.92
0.60
0.41
0.30
0.19
0.12
0.08
0.06
0.04
0.02
oio
3.48
14.04
22.20
24.30
18.90
14.34
8.16
5.52
3.60
2.46
1.80
1.14
0.72
0.48
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
2.28
1.44
0.96
0.72
0.48
0.24
040
13.05
52.65
83.25
91.12
70.87
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
4.08
2.76
1.80
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
0.98
O.72
0.45
0.29
0.19
0.14
0.096
0.048
0.0
0,58
2.92
9.52
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
3
Th t;
dO3itorn flood hydrograph is plotted in Fig. 9. The vhich **iOnS Sh°W that the Peak of the inflow flood is 2C9 .45m /sec
ado Pted3 thG rnaXiraUn’ com
P  ®d compared to all the methods so it ±3
ut
ca
Pac
. 
r routinS it through the reservoir to fix the
HQCi y.
spillway
t- 35 -
6.
flood routing
of 209.45 
* y',r°*r”h «“» « P~*
to        be        computed        for        which        a        spinVQy        width0!-^0*        hydrOSTaph            has
the head above the crest to 1,5 metres
of the dam above the full
(
------------------------
economic reasons the crest is
keeping a straight width of 2n .
20 metres, the semicircle len^+K
computing the rating curve or m. 
conservation level is
----  is assumed limiting
so that the total height
__ -J reasonable. For
provided in the shape of
metres. So
’
crest     is     taken     as     31
is given by formula,
m ©tres
length would be H h
J  **
’ ’Pillvay, th. length or
a semicircle
So, for a diameter of
metres. For
ua’ the
The discharge over the spillway
Q9
t
spillway discharge
coefficient of
is taken as 2
B     = Crest length of
which is taken
H
e in m3/seo>
discharge which
spillway width,
as 31 metres
t
water above crest in
Table 16
values of head, H*
= Head of
metres.
shows the values of discharge for various Spillway rating curve is plotted in Fig. 10.3637 -
Table        16 - Spillway rating ourvo
computations
H
m
Q = C B H *
s
15
/sec.
0.2
0.4
0.6
0.8
1 .0
1.2
1 .4
1.5
1.6
1.8
2.0
5.54
15.68
28,81
44.36
62.00
81 .50
102.70
113.90
125.48
149.70
175.36
With the help of spillway rating curve fig. 10, reservoir capacity curve Fig. 1 , and the inflow flood hydrograph Fig. 9>flood routing computations are carried out by trial and error method as shown in tables 17 and 18. The crest level of the spillway for the full conservation level is taken as 125, earlier indicated in table 13.38 -
- Flood r
•outing computations, B = 31m, Crest level a 125
____
Second dt a
Inflow time t m-^/sec
at
Average rate of inflow qi for dt nr/sec
Inflow
Trial Reservoir Storage elevation
m
Out flow
at time T
nP/sec
4
5
6
7
1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800
1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800
0.58 2.92 9.52
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
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 294228 360711 356535 298440 221283 150255
99117
65997
44298
30150
20043
13212
8847
5877
3591
1782
644
184
125.01
125.01
125.01
125.08
125.20
125.46
125.85
126.22
126.45
126.57
126.58
126.5
126.38
126.25
126.09
125.98
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- 38 -
Flood routing computations
31m> Crest level = 125
Inflow at
time t
m^/sec
Average rate
of* inflow q.
for dt nr/sec
Inflow
Trial Reservoir Storage elevation
m
Out flow
at time T
nP/sec
1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800
1800
1800
0.58
2.92
9.52
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
4
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
364665
24.61
16.75
11.135
7.34
4.915
3.265
1.995
0.99
0.358
0.102
5
522
3150
11232
34443
89352 185508 294228 360711 356535 298440 221283 150255
99117
65997
44298
30150
20043
13212
8847
5877
3591
1782
644
184
6
125.01
125.01
125.01
125.08
125.20
125.46
125.85
126.22
126.45
126.57
126.58
126.5
126.38
126.25
126.09
125.98
125.86
125.75
125.65
125.59
125.52
125.45
125.39
125.35
7
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■K
- 39
h routing computations, H = 31m| Crert level = 125
T               _
^S
cra
oUC
8
'^0,25
o5
c
o,5
1.25
i. 0
4 ju
13.0
34.25
66.25
95.75
114.5
122
117-75
106,5
93.5
78,5
65.O
54,0
44,00
36,25
30,5
26,25
22,0
18.25
’5.25
.»‘" ,e
_____________ 1 Out f L°w
r?
__________________
Incremental
Storage
ds m-^
_____________
Total
Storage
m-’
__________________ Reservoir
elevation
end of dt, M
___________
Remark
' 9
10
1 1
12
13
^50
900
900
2250
7200
23400
61650
119250
117350
206100
219600
211950
191700
168300
141300
119250
97200
79200
65250
54900
47250
40950
32850
27450
72
2250
10332
32193
82152
162108
232578
241461
184185
92340
1683
61695
92583
102303
97002
89100
77157
65988
56403
49023
43659
39600
32206
27266
3000072 3002322 3012654 3044847 3125999 3288107 3520685 3762146 3946331 4038671 4040354 3978659 3886075 3783772 3686770 3597670 3520513 3454525
3398122 3349099 3305440 3265840 3233634
3206368
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
23 99
125.86
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
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.01
0.00
0.01
0.01
0.00
0.00
0     0
•    • 0 —4042 -
kEFERENCES
9«
b)
Small Dams
by  United States Bureau of Reclainatlon
Land      and      Water      Resources      of      the      Blue      Mile         Sasin Appendix III - Hydrology by
United States Bureau of Reclamation.
Ethiopia- 43 -
APPENDIX A
Mr. C.A. Munoz’s Note- 44 -
November 5th, 1979
ro
pfO#
«
Mr. G. Dekker
project Manager
r A.Munoz
^drometeorologist
ETH/75/0O5
ETH/75/OO5
hg used 3S
Ref:        Hyd.rometeorologd.cal data
for Birete River,
For the design of a small dam on Birete River which will storage for the water supply of the cement factory, I
^ve b^en q
nonnested to provide the basic hydrometeorological
inform3 r 101
such as rainfall, evaporation, peak discharge for the
r the spillway and runoff values per square kilometer,
design 01
No hydrometeorological stations exist in the catchment ind only published maps at a scale of 1:250,000 which are not adequate; the tributary area has been determined by Dr. Tsega from ivailable aerial photographs at scale 1:50,000 and has found to
lave a value of 37*85 sq. krots.
The area was visited with Mr. N. Ahmed and Dr. Tsega m Friday November 2nd, 1979 nnd field data were collected for
•ho possible peak flood design. It appears that the catchment
las been properly located on the aerial photographs and the
-lculations have been based on the value of the area as computed Tsega.
Tho nearest evaporation station is located at Hine with
nnual values in mm of 1624.2, 
1525.2 and 1499.1 mm observed in
Operation pan class A, installed in December 1975; those values ^  spona to 1976, 1977 a d 1978 respectively. These values
re
n
n  adjusted for missing data in 1977 and 1978*
09°]j Observations of rainfall at Derba (09° 26*N) and Inchine
(
8°39, 
> Ej t
^ ° 3'E) could be used. (The longitude at Derba is
82- 45 -
Values of runoff have been
on  Huger River with a tributary area 
ftVaUablc
pi ver is a trib
utary of Huger. 
Sq * kmt3» Blreto
Enclosed herewith
xewitn please find th
information. 
ho  detailed available
C.C#
Dr. Tsega, E]mA.
t- 46 -
tation of peak flood for Birete River at possible dam sijte
----------
z
f Ba 5 v
on a tributary area of 37.85 sq. knits, calculated by
p r Tsega from available aerial photographs 1:50,000)
1  j psddeg Method
. H^ta: Length of stream 11*91 kmts.
Elevation at upper part:
” 
11  dam site:
Difference of elevation:
Slope of main stream;
(information supplied by
231.65
107.*3
123.82
123.82           = 0.0103963
11910
Or. Tsega)
Standard         contribution         area         coefficient         Cs         for         a         rolling         catchment slope and soil type with impeded drainage Cs = 0.5
Catchment wetness factor Cw for wet zone and perennial stream
Cw = 1.00
Land use factor Cl (intense cultivation) = 1.50
C = Cs x
A
Catchment
Volume of
33
Cw X Cl = 0.5 X 1.00 X 1.5c = 0.75 lag time K = 3.00 (cultivated land)
runoff RO = CA (P - Y) A x 1O3
= 0.75 (60-0) 37.85 x
= 1703.25 x 10  (m )
Average flow         0
0.93 RO              440
~  36OO x T
n
-D
’T
JO
Q =
103
440 
T
B
44o
B   Q
=
Base Time:
T
B
T + 2.3K + T.
P 
A
T
A
0.028 L
0.028 x 11.91
0.33348
Q* s* 
Q* x 0.0103963* 
0.1019622Q*
3.2706
Q*
tb
= 0.96 + 2.3 x 3 + T =7.86 + ?* 270fi228
A
q*- 47 -
By
trial and error:
, 
3.2706238
7.86 + 3.2706238
440
Q
Qv
Qv
Q*
Q
5O.O
2.659147 
1.229952
9.089952
8,80
48.4
2.637614 
1.239993
9.099993
9.0909
OX
48.35
2.636933 
1.2403135
9.100313
9.10031
2.3 Q = 2.3 x 48.35
= 111.20 n?/sec
Q Max
III-46
2)
From Blue Nile               Studies  Appendix III  Figure
y
y
Frequency
Once in x years
5
10
25
50
100
o
Peak value in nr /sec
for 37.85 sq . knits
40
49
64
75
83
3
3) By the modified Myer's method for a C coefficient of 17
Q = 17 x = 17 x 37.85^ = 104.6 m /sec.
From the above values and to satisfy the results of the Fiddes method it is advisable to design the spillway with
a capacity of 75 n?/sec with a freeboard of 28 m^/sec and an
o
emergency spillway of 15 m/sec. The total capacity for
evacuation of the peak flood would be:
Q = 75 + 28 + 15 = 118 m /sec.
Rainfall data-
The station installed at Derba (09° 26'N and 38°39'E) ^pril 1974 gives the following values:
3
Y ear
1975
1976
1977
1978
Annual Precipitation mra
944.2
890.8
1143.8
1007.8- 48 -
The station located nt Inchine (O9°19'N and 38°2J-E) indicates higher values due to higher altitude but the data
a vail able are Questionable. Details
o f the Land and Water Studies Agency
stations •
rvaporstion:
Observations have been made
are available in the office
of EWRA for those two
in  cn  evaporation 
catchment of the
close to Birete.
pan class A; however
at Hine
Hine is
since June
located on
22,        1975
the
Awash basin and not on th© Nile basin, but relatevily The adjusted values would be:
Year
Annual Evaporation in mm
1976
1977
1978
Vater Studies Agency of EURA.
Runoff values:
1624.2
1525.2
1499.1
Daily and monthly values ar© available in the Land and
No hydrological station is existing on Birete River, Continuous data of runoff are available for Muger River at Chancho for the period 1971-1976; the tributary area is 484 sq, kmts, The values of runoff are as follows:
Year         Runoff ra^ x 10^ Runoff per sq. kmt. in
x 10^
1971
1972
1973
1974
1975
19Z6
164.557
206.08
275.033
208.907
303.991
207.195
0.3399938
0.4257851
O.56825OO
0.431626
0.6280805
O.428O888
Average
227.627
0.47030449
APPENDIX B
Area and Volume Computation
for plotting Area and Capacity Curves50 -
Area & Volume computation
No Z
1 ra
1   I 2
A
2
m
3
I (Al) + (Al + 1
2
m2
4
906.6
-I Av    I      V
| 
m3
5
6
1   1 110                1360
!             906^6    I           906,6
2
111
7160
4260
4260
! 5166.6
3
112
113
1668O  I
11920
11920
1 7086
■   33680      I
25180
2518O
42266
I4
5
114
I 50600  1 
42140
42140
84406
6       115           68040
59320
59320      1         143726
7
8
I 116           87O8O                           77560                      77560               221286
117        1 18000
102540
138540 
102540
323826
9      1 18 I      159080 i
10      119         200680
11      120 1    256400
(           138540 |             462366
179880 ,       179880
228540 
!         228540
642246
870786
12      121            321880 1                 289140
13      122       392480
14 1 123 465440 1
357180 
428960 
289140              1159926
i      357180 i           1517106
i      428960              1946066
15
124            533200 1                 499320                 !     499320              2445386
16           125       603600
17      126       669400
18           127       740600
128 1    812200
568400 
j   568400             3013786
6365OO ' 6365OO 3650286
7O5OOO ; 705000 4355286
19
776400 
;   776400              5131686I
<
I

Summery

1. Project Overview

The Birete Dam Project is located in the Shoa administrative region, approximately 106 km northwest of Addis Ababa, Ethiopia. The dam is planned to conserve surface runoff from the Birete River basin to meet the water requirements of a proposed cement factory.

Key Project Details:

Location: 9°29'N, 38°20'E
Catchment Area: 37.85 km²
Distance from cement factory: ~10 km
Current water demand: 2,000 m³/day (23.15 liters/sec)

2. Water Availability Study

Hydrometeorological Data

The study was conducted with limited data, using information from nearby stations:

Rainfall Data from Derba Station (mm)
Year	Annual Rainfall
1975	944.2
1976	890.8
1977	1,143.8
1978	1,007.8

Evaporation Data from Hine Station (mm)
Year	Pan Evaporation	Actual Evaporation (0.8 × Pan)
1976	1,624.2	1,299.4
1977	1,525.2	1,220.2
1978	1,499.1	1,199.3

Runoff Estimation

Runoff was estimated using the U.S. Soil Conservation Service method with curve number CN 85. The catchment was classified as:

Hydrological Soil Group: D (clay soil with slow infiltration)
Land Use: Small Grain with Good treatment
Hydrologic Condition: AMC II (average antecedent moisture condition)

3. Dam Specifications

Alternative Dam Height Scenarios
Draught (liters/sec)	Total Storage (m³)	Reservoir Elevation	Dam Height (including 3m freeboard)
94	3.0 × 10⁶	125	20.5 m
68	2.5 × 10⁶	124.2	19.7 m
23.15	1.5 × 10⁶	122	17.5 m

Recommended Design: 20.5 m high dam with total storage of 3.0 × 10⁶ m³ (including 1.0 × 10⁶ m³ dead storage)

4. Design Flood Analysis

Multiple methods were used to estimate design flood peaks:

Fiddes Method: 111.2 m³/sec
Modified Myers Method: 104.6 m³/sec
Blue Nile Basin Curve (100-year flood): 83 m³/sec
Triangular Hydrograph Method: 137.2 m³/sec
Unit Hydrograph Method: 209.45 m³/sec (adopted for routing)

5. Flood Routing and Spillway Design

The spillway was designed with:

Crest length: 31 meters (semicircular shape)
Discharge coefficient: 2
Maximum head above crest: 1.5 meters
Maximum discharge capacity: ~113.9 m³/sec at 1.5m head

6. Sedimentation Analysis

Estimated sediment deposition:

Assumed erosion rate: 1 mm/year over catchment
Trap efficiency: 80%
Annual sediment volume: 37,850 m³/year
Dead storage (1.0 × 10⁶ m³) expected life: ~26 years

7. Conclusions

The study recommends a 20.5 m high dam with 3.0 × 10⁶ m³ total storage capacity to meet current and projected water demands. The design accounts for limited hydrological data and includes conservative estimates for flood routing and sedimentation.

Note: The study emphasizes that the results are based on very scanty data and should be used with caution for other basins.