Birete Dam Project Hydrological Study Summary

rn
.. ..
kJO- 2/2 £(cj
f 1 <1 d + ft fl MH > M’t fr f « X H. f T « 
» «• 1 1               -
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 UST OF FIGURES
1  INTRODUCTION
e
2 . WATER REQUIREMENT
3.       WATER AVAILABILITY
a)    Catchment area
b)    Hydrometeorological data
c)    Runoff
d)    Mass curve
4.       HEIGHT OF DAM
5• DESIGN FLOOD
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.       ACKNO'wXEjDGzMLNT 9.       references- 3
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 hydrographBIRETE 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.
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 ,
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.
2- 5
i) Rainfall data
Only four years rainfall
, fnil d~ta is available from Derba
station. Annual figures are reproduced in table
Table 1- J
knnual
1             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 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 f6w discharge measurements were taken cn 3irete river near Inchini which are reproduced in table 3,
Discharge of Birete river
Date of measurement
near Inchini
28   March
1979
12  May
II
24 Litres/sec
81 ”
’»
26      11
I!
19”
"
16  Dec
11
11”
’•
17     h
It
23      "
”-5-
i) Rainfall data
. ^<11 Only four years rainfall d^
station. Annual figures are reproduced in tabl
is available from Derba
Table i- i
Year
knnual naimuix 
------------------------
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
mm
Actual Annual Evaporation E          =3 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. 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 Birete river near Inchini
Discharge of Birete river
Date of measurement
28  March     1979 12   May          II
26     it
If
16  Dec           II
17     11
II
near Inchini
24  Litres/       sec
81      11                          <1
19     11                         11
11      11                          11
23      11                         11LEGEND
9KC njlHK A UH fl
main VOLUMES OF  WORKS
OCHOBH'jIE OBbEMbl PA60T
-
E  °
z 5"?
Desc ri ption
HaMMeHotaHwe
Description
HdMwe-iOBaHHe
Unit of meas.
E
H3 'C
Ouai : •. Kop .
1
Over-Flovw dam
BOAoeimHaa nnoiuHa
1
Exca vation
Bbiepixa
_
M*
105 C-
2
Earth dam >
3eMD&Ha5) njioiMHa
3     Abutment....................................
i/CTOH
2
Backfill and f OSpaTHaa sacbin
ill
Ka h Hdcbinb
MJ
17 C- '
A 1 Pier
1 6blK
3
—
Gravel with pebble
rpaavwHO - raneHHKKOBbiw rp^HT
M>
K5•
5
' Energy dissipator
1 TacvATenb SHeprww
4
6 i Bridge
Moct
Riprap
Haopocxa ha hGc«
>pTnpOBaHHoro jumhj
i   M1
8 ■’
b
Slide gate t (bulkhead)
SdTBOp njlOCKUW CKOnbSflUWI (DeMO
H
TH
5 blu)
Filter (sand , q ravel I OfipaTHbivi cpwnbTp (necoK, rpajUH)
’mj
3
8
Radial gate
SaTBop cerMeHTHbiw
t
6
c        Operating          gear
MexanwiM noAteMHbiv,
Concrete and reinforced concrete BeTOH h xceneAooeTOH
\
7
Q Travelling gantry crane
Kpaw K.OAPOBOH
Reinforcement ApManpa
m
g.
1 Gantry crane rail track ¥                  rijpH KOAPOBOrO NpSHi
8
M etalwork
M eTannoKOHCTp^«
Ruh
m
i•
. 9 1 Site -for storage ot qates
| HnotnaA^ AA* xpaneHM saTBOpofc
9
Mechanical         ec MexammecKoe
uiprnent
coop^AosaHHe
in
1
50
•a 1S
n
20
ung
f-30
t - 30
FIG.6J4.RFGULATING         RESERVOIR.         DAM         SITE.         CONTROLLED         SPILLWAY         (Alternative         4)
Pvic6ApEn)WW)mAfl emkoctb. rnAPo^EJi. boaocbpoc c satbopamh
•■>)BIRETE DAM PROJECT HYDROLOGICAL STUDY
1 4 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 the water requircme of the proposed cement factory. Birete river is a tributory of
Sodoble river which in turn drains into Mugor 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
l                      O-20’E
site is approximately at 9°-29 N and 38
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 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 (O9°-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.BIRETE DAM PROJECT HYDROLOGICAL STUDY
1 4 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 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 CGment factory. The location of the dam site is approximately at 9O-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 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’Nt
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 yoars rain1311 Annual figures are reproduce
Onfall data is available from Derba
station.
d in table 1.
Year
1975
1976
1977
1978
Annual Rainfall in mra
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          = 0.8 x E
act                            pan
rnn
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. Pen 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 Sirota river near Inchini which are reproduced in table 3.
Table 3 - Discharge of Birete river near Inchini
Discharge of Birete river
Date of measurement
28  March    1979 12  May          II
near Inchini
24  Litres/       36C
26     11
II
16  Dec           11
17     11
II
81      11
19     11
11      11
23 it
it
it
it
11i)           Rainfall data
Only four years rain^ station. Annual figures are P
1LLD1W l — Year
1975
1976
1977
1978
ii)          Evaporation data
available from Derba
-infall data is
reproduced in table 1
Rainfall at Derba----------------- -------------------
Annual Rainfall in mm__________________ _
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
Year
Pan Annual Evaporation
mm
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. P^n 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 3irete river near Inchini which are reproduced in table 3.
Table 3 - Discharge of Date of measurement
- B——ne    irete rairve incr nenianir Inchini Discharge of Birete river
near Inchini
28    March    1979 12 May          "
26 »»              »
16 Dec           ”
17”                 "
24 Litres/sec
81  " 11
19" it
11" 11
23  " 1!As earlier within the catchment so a not possible. To
rainfall at Derba station Birete catchment
- 6 -
mentioned there is no rainfall station direct rainfall runoff corelation is
have an Based on
idea of the water availability, the is considered to be the same as for
this assumption an attempt has
been made to e
stimate runoff based on
i the method developed by published in 19^4,
the
U.S. Soil Conservation Service 1
Complete daily rainfall data for Derba is available four years from 1975 to 1978. However it is available fron 
for
to December for 197^ and from January to October for 1979 May
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 for every month, it would be representative storm for the direct runoff value could bG
To estimate the runoff* coefficient important to first arrive at the relevant month against which the read from the curve. For this
purpose the daily rainfall data has been analysed,
month the first ten values of daily rainfall storms
For every starting
from        the        maximum        value        in        descending        order        have        been        considered 
as
partial
series. 
The
value
s
are
listed 
Averages
of
storms
for
each
mont
h 
and 
in        tables        4        and        5. storms       for       a       return 
period 
of
2       years
hav
e
bee
n 
calc
ulated. 
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 of curves developed by U.S. Soil Conservation
Birete river catchment above the dam site has been considered to fall within the following characteristics.
i) Hydrological Soil Group
these for given Service
of cl
of clay and is likely to have
! iUCe thS t<>P SO±1 ln the CCtchment mainly consists
±nf “
considered to fall i  Group D. 
n
» t isAs earlier
 6 -
h there is no rainfall station mentioned there is
direct
rainfall
runoff
corelntion 
is
within  the catchment so  a not possible.  To  have an rainfall at Derba station
idea of the water availability, the is considered to be the same as for
Birete catchment,
Based on this assumption an attempt has
been       made       to       estimate       runoff       based       on 
• a the method developed by
the for
May
U.S. Soil Conservation Service published in 196U.
Complete daily rainfall data for Derba is available four years from 1975 to 1978. However it is available from to December for 197 * and from January to October for 1979
2
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 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 n the set of curves developed by U.S. Soil Conservation Service.
Birete river catchment above the dam site has been considered to fall within the following characteristics.
i) Hydrological Soil Group
-
e»"«iaor.a to fall
— 
in Group
Motion rate, it 1„7
ii)      Land Use and Treatment Class
Land use or cover is taken as Small Grair. , one the treatment or practice is classified as Good for hydrologic
condition for infilteration.
iii)        Hydrologic Soil-Cover Complex
Storm rainfall related to antecedent moisture condition, AMC II, has been considered.
For the above characteristics curve number, Cb‘ 85 has been adopted to read the direct runoff corresponding to the monthly representative storms arrived at in tables 4 and 5j and 
the runoff coefficients have been calculated.
Table 4 - Maximum daily rainfall storms,
return period storms and runoff coefficients
averages,
2 years
JAN
5 year
FEB
5 year
data
75-79
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
MARCH 5 year
APRIL
5 year
MAY
6 year
JUNE
6 year
1
2
3
4
5
6
7
8
9
10
»c.
lc*
Runoff
data
75-79
20.0
20.0
6.6
5.7
^•7
4.6
1.6
1.0
0.4
0.4
6.50
7.46
5.55
0.00
Coeff
data
75-79
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
e
data
75-79
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
O.O5
data
7- -79
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
data
74-79
50.1
29.3
27.0
22.2
22.0
21.7
1' •. 9 
1°.O
1  i.O
17.5
2*-. 77 ‘*.63
2    .55
0.20Table 5 - Maximum daily
return period storms and runo
JULY
6 years
data
74-79
AUG. 6 years
data
7^-79
SEPT.
6 year:
data
7^-7
.-1 /-
1
1
2
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
IO.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.
30
2
2/ "
w
2 1.7
21.5
18.6
18.5
18.3
23.53
5.31
22.86
0.18
. - -
r 30.
17
1
3;
c.5
8.3
20.80
14.53
18.96
0.12
I
J
1.3
0.6
8.09
7.00 7.20 0.03
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 establish rater availability. Other parameters to carry out the study are the base flow, evaporation from the reservoir, infiltration and seepage through thG dam. Regrading base flow, tabl^ 3 rovoals that the minimum flow recorded is 11 litres/sec. It has been learnt that a ’V notch’ weir 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- 8 -
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,
• ' yo ’’.rs
data
r»
» 9EC. 
1 
j5 years
j j 74-7 8
1
2
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
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
2.61
0.00
*
I
J
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 establish vater 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  weir 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
*
Pan Evaporation
Month
E
mm
January February March April
May
June
July August
Septembe
October November December
? 106
168 153 163 176 120 108 102 137 129
125 138
Actual Evaporation E 
act = 0,8 x pan
mm
13^ 122 130 141
96 86 82 110
85 103 100 110
For the estimation of thG monthly runoff, monthly rainfall
data is
analysed in table 7. Rainfall data for the months of
January 
and for
to April, November and December is available for 5 years
years. 
the months from May to October it is available for 6 
Average rfSnfall is calculated for each month. Total
annual
rainfall for the four years for which complete data is
available and the total of average monthly rainfall is shown in 
the
the last column of totals. Out of the four years for which complete
last     column     of
data         is         availablet annual rainfall of
year 1976 has been the driest with a total 890.8 mm, which is
8 ^0,8 x 100 = 87.78% lOlUi-8- 10
of the total of average monthly rainfall# To reflect a dry year within the scope of data available  the monthly average rainfall
f
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 has 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 years 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.Table 7 - Monthly rainfall at Derba
1        Year
1 1974
I 1975
I 1976
I 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
jFeb.
1
3 1.9
22.8
U8.9
70.9
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
I 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
IOO7.8
82.7
16.5
14.5
23.4
4.7
4.1
1014.8
890.8- 12
Table 8 shows the computations of monthly volume of Inflow which is,
V + r
Where
±
V  = monthly volume of inflow
V = monthly volume of runoff
p
= Runoff coeff. x Catchipent Area x monthly rainfall depth
V x monthly volume of base flow calculated at b
rate of 5 litres/sec.
Table 8 - Monthly Volume of Inflow
the
nth
Runoff
Coeff.
Monthly
Rainfall Depth
mm
Monthly        Vol. of Runoff
V
m3
Monthly         Vol. of Base flow
Monthly Vol.
of Inflow
V. = V +V
i         «r b
mJ
1
2
3
4
5
6
it.
>v.
IC.
Ln.
sb.
irch pril
ay
une ifl.y
ug. Sept.
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,^90
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 reservoir level in the particular month is estimated with the help of reservoir
stage-volume and reservoir
stage-area curvot( 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. starts from October. L — 
computations initiate
for
the
plotting computations
to the particular month i full at the end of September, Therefore the mass curve study 
For the reservoir full stage two studies ----- --------
have been conducted,
is 2 million cubic metres with one
one for the condition when the live storage
million cubic metres as dead 
storage
and a constant draught (withdrawal) of 9^ litres/sec f
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. Tables 9 
and 10 show the computation for the first case and tables 1 1 
and       12       for       the       second.       The       basis       for       adopting       tho       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 conservative side, but in the absence of data it has been adopted Further it has been assumed that the unit weight of the sediment trapped in the reservoir and deposited would be 80% of the unit weight of the loose soil including organic matter eroded from the catchment.
Regarding trap storage and the drainage
ef ^iciGncy           of area ratio i.e, 2 x 1Q  ,
6
the           reservoir,           the           live 37.85X1O  “ °«°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. xn
the presence of an undersluice it is assumed that the trap efficiency may reduce to 80% and 20% of the sediment may pass downstream.t15
Therefore, yearly sediment volume, S^, eroded from the catchment and retained in the reservoir,
S = A x h x 10J x 0.8 m 7 9  2 
3 
3
Where A = catchment area in m
h = sediment height eroded in mm.
g
Volume of the deposited sediment in the reservoir,
3
o
S « 
 A x h x 10^ x 0.8 
s
------------- 0?8
S 37.85 X 1 X 10  = 37,850 m^/year
- _J
3
It is proposed to provide a dead Btorago volume of the reservoir which gives a reasonable life before the sediment
encroaches upon the live storage. Encroachment on live storage is slow because progressively the sediment passing downstream increases and ultimately a balance is reached.
Considering a dead storage
=1 
x 10^ m^
Life of the dead storage
=
10  c 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
g
63
g
Draught D = 94 litres/sec.
/.CC. Vol. Of Inflow + live
Storage
V4 + L 
m3 3
Aoc. vol. of
Draught
D
m3
Available
Storage V +L +D -D
1     S n3
nr
I
Accumulated 
Vol. of Inflow
Reservoir
Surface Area
2
m
F"'
2
3
4
5
6
ct.
PV.
bc.
jin.
eb.
arch
pril
'nne
niy
)
pe.
lept.
195,980
225,400
238,790
252,180
289,940
355,870
433,360
905,490
I 1,727,680
|      4,084,400
! 6,094,530
1     6,987,730
2,195,980
2,225,400
2,238,790
2,252,180
2,289,940
2,355,870
2,433,360
2,905,490
3,727,680
6,084,400
8,094,530
8,987,730
251,770
495,420
747,190
998,960
1,226,360
1,478,130
1,721,780
1,973,550
2,217,200
2,468,970
2,720,740
2,964,390
2,944,210
2,729,980
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
452,000
424,000
460,000
542,000
600,000
600,000
600,000
9  is
The accumulated volume of inflow (runoff) estimated in table
6,987,730 m . According to Mr. C.A.Munoz's note attached
3
as Appendix A  the runoff based on the minimum flow per Km^ of
t
Huger river at Chancho for the driest year 1971 works out
■ 0.3399938 x 106 x 37.85
= 12,868,760 m3
The r i.off calculated in table 9 is less and is adopted for further study.17
Table 10 - Net accumulated volume of Inflow + live storage (continued from table 9)
——r-
!
1j
•
Reservoir
th 1     Surface Area
Monthly
act. evap.
mm
Monthly
Vol. of Evap. m3
Acc. Vol. of Evap.
i
f
1
n m
2
3
4
5
u
B
th
■11
|
1
592,000
570,000
538,000
506,000
480,000
452,000
424,000
p
y
E
460,000
542,000
600.000
600,000
600,000
103 100 110 134 122 130 141
96 86 82 110
85
60,980
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 9- col. 5 of table 10)
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
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.I
I
1
t
s
r
s
mass
o*
-i
Ftcw ‘° cvi^uw
9
«
J
,r M
&
{
s
r
3
4
3
i- 19 -
Table 11 - Reservoir surface area relevant to each month^ Live storage, L s 1.5 x 10^ nP; Dead storage, D  = 1 x 10 m ;
s
g
D = 68 litres/sec*
r
 -
th
Accumulated Vol. of Inflow
vi
m3
Acc. Vol. of Inflow + Live
storage
V i + LS
-
m3
Acc. Vol. of
Draught
D
m3
Available
Storage V.+L +D -D
i s s
m3
Reservoir
Surface Area
2 
m
L. ■
2
3
4
5
6
1
«
b 
l*
•
>
•ch 
■ >
•il
r
le
Ly
k
k
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 ,987,73O
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,COO
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
• r* t A t f*
p-<- 20 -
Table 12 - Net accumulated volume of Inflow + live storage (continued from table 11 )
a
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.12)
tn3
2
3
4
5
6
1
•
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.OCO
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 (1,535,040 - 1,245,54c) = 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 
The net Acc. Inflow from tables 10 and 11 = 433,360 - 398,32c
=  424,034 m3
= 35,040 m3
Storage required = 424,034 - 35,040 = 388,994 m31
1
9Considering a margin
live storage of 500,000 m 3 may
for      a      very      dry      year, be necessary.
a minimum
4, HEIGHT OF DAM.
The dead storage for a reasonable life may be = 1.0 x 105 6 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
n?
Reservoir
Elevation
Bed rock
El, at deepest
point
Maximum
Height of dam including           3 m freeboard 
m
94
68
23.15
3.Oxio6
2.5x10b
1.5x10°
125
124.2
I        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^ 6 3
of live storage and for 2.0 x 10 nr 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
□•) 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.
By modified Myer’s method, flood peak = 104.6 m /soc.
J22 -
Considering a margin for a very dry year, a minimum live storage of 500,000 m  may be necessary.
4, HEIGHT OF DAM.
The dead storage for a reasonable life may be =
5
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 1G7.5 (with reference to an assumed local bench mark).
Table 13 - Alternative heights of dam
Draught
lit/sec
Total
Storage
m3
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°
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^m3 of live storage and for 2.0 x 10 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
e
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 r I°od peak by the two emperical methods, the values are given below.
By Fiddes method, flood peak =
111.2 m^/sec.
By modified Myer’s method, flood peak = 104.6 nr/sec.- 23-
LAY OUT AND DAM AXIS CROSS-SECTDN
**— • • i .aru15           C
According to the Blue ffile Basin report, Appendix ITX, Figure TIT - ^6, peak flood valu« for ones in t-C Th
read from tho curve = 83 m^/sec.
Mr. Munoz has recommended to provide - flood =         118 m /sec.
li) Based on basin cha.racti.ris tics aod
Blue Nile Basin for the Ribb and Gaairi fixed atxxazra2^ following formula has been developed by Xr.
Q = 0.0076 H *  C A **’ max
where H = Hoiizuz 1
■LZjLll
I
C = nasin c
CTfcld
ziffi cd *mx
this case = 0.7)
A = Basin area Ikn:'")
2
b) Peak equations for a triangular hydrograph concept
Using 
hydrograph.
the goneral peak equations for a trxan^rular 0.385
(Ref:
Design       of       Small       Dams by USSR)
Whore
T c
T o
11.9 L3
H
Timo of concentration -
from most L     e Length of
distant point stream in miles
travel         time         of to dam site in
H
= Elevation difference between th© upper
The
and the dati site length of the stream
water Hours
point
dlff
©renao between tho upper point
of the stream in feet
is 11.91 km and tho elovation and the dam site of tho
’trecm is (231.65 °f stream 1
^ » Tsa.Tn "l
107.83) » 123.82 metres. (This information ength and elevation difference is supplied by
rT c
11.9(11.91 x 0.62137)
123.82 x 3.2808 2.59      hours day = 2.6 hours
T P
D
2
+
0.6 T 
c
Where D
= Rainfall excess period which is
T  = Timo froo start of rise to peak P
T     =        + (0.6 x 2.6) = 3.56 hours P
taken as 4 hours rate
Tb
2.67 T = 2.67 x 3.56 = 9.5 hours p
= Time base of hydrograph
Where q
the peak discharge in nr/sec
A  = Drainage area in m
2
Q
= Depth runoff in metres, one day rainfall of 124
T   = Time from start of rise P
50%      of      the      max.      100      year mm is taken as runoff.
to peak rate in seconds
q= P
 0.749 X 37.85 x 106 x 0.062
3.56 x 3600
= 137.2 nP/sec.
c) Unitgraph derivation.
To develop unit hydrograph for Birute ungaged area, 
the dimensionless graph, Fig. 5 and lag relation curve Fig. 6 
developed for Blue Nile Basin are used. Curves are taken from
fi lue 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.0.385
T c
11,9(11.91 x 0,62137)
123.82 x 3.2808 2.59 hours day = 2.6 hours
T =
§
P2
+ 0.6 T       c
Where D
= Rainfall excess period which is
T  = Timo from start of rise to peak P
T = — + (0.6 x 2.6) = 3.56 hours
P2
T = 2.67 T = 2.67 x 3.56 = 9.5 hours
taken as 4 hours rate
b 
p
= Time base of hydrograph
0.7^9 AQ
q p = T
HP
Where q  = the peak discharge in nr/sec
p
A = Drainage area in m2
Q = Depth runoff in metres, one day rainfall of 124
T s Time from start of rise
P
 0.749 x 37.85 x 106 x 0.062
50$      of      the      max.      100      year mm is taken as runoff.
to peak rate in seconds
q= P
3.56 x 3600
= 137.2 m^/seo,
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.BLUE NILE BASIN IDEAL HYDROGRAPH
>
• • ibm nrr »u now him.off
M ■ mi FBOMCIBTIBOF RAMIXCIM MRjOO to TIM NALF or BUN-OFF NAB OCCURIO
•• 
ONICN BAMFALL IXCIBBB INFILTRATION
’W***r-MCOMFtlT (1,1 CUBIC —FOOT MB IICONO) INTfBRATIO
•rtB MBIOO OF BTONB TO BIT TOTAL MLM OF BUNFACt BUN-wK
■ M                KO                 MB
DIMENSIONLESS GRAPH F,G 5Ft? €-23-
*
fig 7- 30 -
Table 14 — Ordinates of* unit hydrograph
I
——
Time
</o Of
D
Lag + 2
5 00- - 44 4
2.25
Ordinate
(From Fig. 5 )
Unit q
cfs
 .          ~ 393 Ord . x ---------—
2.25
Unit q
nr / sec .
col. 4
n
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
311.9
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
0.65
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
0.51
The unit hydrograph is plotted in Figure 8Time in hours!• M«C
33
FIS •4
the catchment
f 124 mm is
the
32
of inflow desi
Since there is no recording raingauge wi thin 1 day 100 year probable precipitation o
us©d . for
more ' or l099
intensity
condition
following
th e design storm
The storm rain is arranged in a
in
for
syraetrical time distribution Fig. 9j
the middle of the storm to give th©
flooding. The rainfall excess are
with the greatest
worst possible
assumed in the
manner•
1st    hour    net     10%  Of
2nd       ii
3rd       ii
4th        ii
ii 40% II
ii 75% 11
ii 6o% It
rainfall
it
It
II
The unit hydrographs
of
I
hr. duration
for
and
the
rainfall excess of 1mm, 1mm, 6bid, 12mm, 22.5mm, 1 2mm ,
are derived
15mm,
3mm 9
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 = Ornate of unit graph * ±
2434 -
Table 15 - Computation of inflow flood hydrograph
Ordinate for
^2=1
"2s1
i 2=12
i 2=22.5
±2=15
i 2=3
i 2=2.4
Q
Total
rn /sec
0.0
0.58
2.34
3.70
U.O5
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
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
q'.O
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
0.36
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
*,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
O.19
0.14
0.0
0,58
2.92
9.52
28.75
70.53
135.59
191.34
209.45
I86.7O
144.90
100.97
65.98
44.15
29.18
20.04
13.46
8.81
5.87
3.96
2.57
1 .42
O.56
The mflo
w design
flood
hydrogr
Tim®
nt^rval
D.O
3.5
1.0
1.5
, 02.°
2.5
1 0
u
L
5.0
5.5
.5
.0
r.5
.0
.5
>0
»5
|,o
1.5
2.0
—-■
L
vhich • 1003 Sh°W thQt thG PSak °f thS inflow flood is 209.U5m3/soc
~ph is plotted
i
.n Fig.
0.096
0.048
9. The
0.156
0.048
a
do  13 th° 'naxirauin co«nputed compared to all the method a so it ±3
c *pa ty routing it through the reservoir to fix the spillway
ci6.
of
to the
35
flood ROUTING
For flood routing inflow flood hydrograph with a peak ,.5 n /sec has been computed. The outflow hydrograph has
209.45 m
computed for which a spillway width is assumed Hmlting
be <— -
head above the crest to 1.5 metres
of the economi c keeping a 20 metres, computing crest is
is given
dam above the full conservation reasons the crest is provided
straight width of 20 metres, the semicircle length would
the rating
taken as 31
by formula,
curve of the
metres. The
= C B H1,5
where Q
B
so that the total height
level is reasonable. For
in the shape of a semicircle
So, for a diameter of
31.4 metres. For
spillway, the length of the discharge over the spillway
spillway discharge in nr/seo. coefficient of discharge which
is taken as 2
Crest length of spillway width, which is taken as 31 metres
i
values of
Table 16  shows the
head, H* Spillway
H          Head of water above crest in
metres.
values of discharge for various rating curve is plotted in Fig. 1036
■auwkm i3utae mmabmiv
SPIir.YAY ni$CHAi\GE ELEVATION CURVE-37 -
Table 16 - Spillway rating ourvo
computations
H
m
Q = C B H *
s
15
o
nr/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
145.70
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 routing computations
3Tm| Crest level = 125
Sec
Inflow at
time t
L        3/sec
m
Average rate
of inflo^ q^
for dt nr/sec
m3
Trial Reservoir Storage elevation
m
Out flow
at time T
nP/sec
4
5
6
7
0
1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800
1800
5      1800
0
1800
3
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
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
14.038 -
Tfl bie 17
_ Flood routing computations, B = 31mj Crest level a 125'
- 39 -
d rQ  ^  ^ computations, B = 31mj Crert level = 125
ur   11
Out flow
Incremental
Storage
ds
Total
Storage
nP
Reservoir
elevation end of d19 M
Remark
9
l±50
900
900
2250
7200
23400
61650
119250 117350 206100 219600 211950 191700 168300 141300 119250
97200
79200
65250
54900
47250
40950
32850
27450
10
11
12
13
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
0.01
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
0.00o
TlM£ ,H HOURS
FIG II„ Ef^N
CES
a)
p.nign Of Small Dams
by United States Bureau of Reclamation
Land and Water Resources of the Blue Appendix III - Hydrology by 
United States Bureau of Reclamation.
NilS Baaln» £thi  pi
O- 43
APPENDIX A
Mr. C.A. Munoz’s NoteTo
be used have
information
of the spillway and runoff values per square kilometer.
design o-
No hydrometeorological stations exist in the catchment
^ nd only published maps at a scale of 1:250,000 which are not
^equate; the tributary area has been determined by Dr. Tsega from
lvailable aerial photographs at scale 1:50,000 and has found to
have a value of 37.85 sq. knits.
The area was visited with Mr,   N. Ahmed and Dr. Tsega )D Friday November 2nd, 1979 and field data were collected for
dw possible peak flood design. It appears that the catchment
ns been properly located on the aerial photographs and the nlculations have been based on the value of the area as computed
Tsega.
Tho nearest evaporation station is located at Hine with
November 5th, 1979
ETH/75/OO5
.^““teorologlst ETH/75/OO5
Ref:        Hydrometeorological data
for Birete River,
For the design of a small dam on Birete River which will n S storage for the water supply of the cement factory, I
requested to provide the basic hydrometeorological -i such as rainfall, evaporation, peak discharge for the
^ual vaiUGS in mm of 1624.2, 
1525.2 and 1499.1 mm observed in
Operation pan class A, installed in December 1975; those values
^ resp°na to 1976, 1977 and 1978 respectively. These values
n  adjusted for missing data in 1977 and 1978.
°5°19« 0bsGrvations rainfall at Derba  (09° 26»n)
and Inchine
8 °39'E)
2 3*E) could be used
(The longitude at Derba isI
4 5 -
Values of runoff have .
Ki ver is a tributary of Huger. 
Enclosed herewith r^i information. 
c. c.
Dr. Tsega, EI/Ra.
„u .r River ,
g
please finrt + u
lth  „ trlbutary Q;sa” »»»«.*•<. r„a dQta avalJabl() Sq< kznts f Blreto
tho               detailed          avaiiableu
- 46 -
tation of peak flood for Birete River at possible dam site rotnp -----------------
(Based on a tributary area of 37.85 sq. knits, calculated by p r Tsega from available aerial photographs 1:50,000)
fjj-ldes Method
id data: Length of stream 11.91 kmts
Elevation at upper part:
” 
11  dam sites
Difference of elevation:
Slope of main stream:
(information supplied by
231.65
1 07 a *3
123.82
123.82           = 0.0103963
11910
Dr. 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
Cw x Cl = 0.5 x 1.00 x 1.50 = 0.75
Volume of
lag  time K = 3.00  (cultivated land)
runoff RO = CA (P - Y) A x 1O3
33
Average flow Q
Base Times
= 0.75 (60-0) 37.85 x
= 1703.25 x 10  (m )
0.93 RO 440
3600 xT  ’ T
Q =
fl         b
103
440 
T B  Q
440
=
B
T
B
T
T + 2.3K + T
P
0.028 L
A
0.028 x 11.91
0.33348
A
si
Q  x 0.0103963^
t
0.1019622Q^
3.2706
Q*
tb
= O.96 + 2.3 x 3 + T = 7
.86 + 3ul27°^238
A
1- 47 -
By
 trial and errors
, 
Q
3.2706238
Qv
7.86 + 3.2706238
Q*
440
Q
5O.O
2.659147 
1.229952
9.089952
8,80
48.4
2.637614 
1.239993
9.099993
9.0909
48.35
2.636933 
1.2403135
9.100313
9.10031
OX
Q Max
2.3 Q = 2.3 x 48.35
= 111 .20 mJ/sec
Once in x years
5
10
25
50
100
for 37.85 aq■ kmt3
40
49
64
75
83
3
3) By the modified Myer's method for a C coefficient of 17
Q=17 X A* = 17 X 37.85^ = 104.6 n /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 nP/sec with a freeboard of 28 m^/sec and an 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 3S°39'E) pril 1974 gives the following values:
3
Year
1975
1976
1977
1978
Annual Precipitation mm
944.2
890.8
1143.8
1007.8- 48 •
The station located nt Inchino (09°19’N and 38°23«E) indicates higher values due to higher altitude but the data
are available in the office
-
a vail able are Questionable. Details
pi  the Land and Water Studies Agency
of EVRA
for those
two
stations.
rva
oration:
Observations have been made
in an evaporation
catchment        of        the close to Birete,
pan class A; however
at Hine
Hine is
since June
located on
Awash basin and not on the Nile basin, The adjusted values would be:
22, 1975
the
but relatevily
Year
Annual Evaporation in mm
1976
1977
1978
Vater Studies Agency of EWRA.
Runoff values:
1624.2
1525.2
1499.1
Daily and monthly values are 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, knits. The values of runoff are as follows:
Year         Runoff ra^ x 10^
Runoff per sq. knit. in nr x 10
3               6
1971
1972
1973
1974
1975
19Z£
164.557
206.08
275.033
208.907
303.991
207.195
0.3399938
0.4257851
O.56825OO
0.431626
0.6280805
0.4280888
Average
227.627
0.47030449
appendix b
Area and Volume Computation
Tor plotting Area and Capacity CurvesNo
1 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
z
m
2 110 111 112
113 1 14
115 116 117 118 119 120 121 122 123 124 125 126 127 128
A
2
m
3 1360 7160
1668O 33680 50600 68040 87080 118000
159O8O 200680 256400 321880 392480 465440
5332OO
603600 669400 740600 812200
Area & Volume computation <Al) + (Ai + Q
2
2
m
k 906.6
4260
11920
25180
42140
59320
77560
102540
1385^0
179880
2285^0
289140
357180
428960
499320
568400
6365OO
705000
776400
5
906^ 6
4260
11920
25180
42140
59320
77560
1025^0
1385^0
179880
2285^0
289140
357180
428960
499320
568400
6365OO
7O5OOO
776400
4
V
n?
6
9O6 6 5166.6
17086 42266 84406
143726 221286 323826 462366 642246 870786
1159926 1517106 1946066 2445386 3013786 3650286 4355286 5131686

Summery

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 to meet water requirements for a proposed cement factory.

Key Details:

Location: 9°29'N, 38°20'E
Catchment Area: 37.85 km²
Current Water Demand: 2,000 m³/day (23.15 litres/sec)
Proposed Dam Height: 20.5 meters
Total Storage Capacity: 3.0 × 10⁶ m³

Hydrological Data

Rainfall Data (Derba Station)

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

Evaporation Data (Hine Station)

Year	Pan Evaporation (mm)	Actual Evaporation (mm)
1976	1,624.2	1,299.4
1977	1,525.2	1,220.2
1978	1,499.1	1,199.3

Key Findings

Water Availability

The study analyzed water availability using mass curve analysis with two scenarios:

Live storage of 2 million m³ with draught of 94 litres/sec
Live storage of 1.5 million m³ with draught of 68 litres/sec

The analysis showed sufficient water availability to meet current and projected demands.

Dam Height and Storage

Draught (litres/sec)	Total Storage (m³)	Reservoir Elevation	Dam Height (m)
94	3.0 × 10⁶	125	20.5
68	2.5 × 10⁶	124.2	19.7
23.15	1.5 × 10⁶	122	17.5

The recommended design is a 20.5m high dam with 3.0 × 10⁶ m³ total storage.

Design Flood Analysis

Multiple methods were used to estimate design flood peaks:

Fiddes method: 111.2 m³/sec
Modified Myer's method: 104.6 m³/sec
Blue Nile Basin curve: 83 m³/sec (100-year flood)
Triangular hydrograph concept: 137.2 m³/sec
Unit hydrograph method: 209.45 m³/sec (adopted for design)

Spillway Design

The spillway was designed with:

Crest length: 31 meters
Crest level: 125m
Maximum discharge capacity: 209.45 m³/sec

Conclusions

The hydrological study confirms the feasibility of the Birete Dam Project to meet the water requirements of the cement factory. The recommended design includes:

20.5m high dam with 3.0 × 10⁶ m³ total storage capacity
31m spillway capable of handling 209.45 m³/sec peak flow
Dead storage of 1.0 × 10⁶ m³ for sediment with estimated 26-year life

Note: The study acknowledges limitations due to scarce data and recommends caution when applying these findings to other basins.