Birete Dam Project Hydrological Study Summary

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IVO -
MINISTRY OF   MINES, ENERGY AND WATER RESOURCES
ETHIOPIAN WATER RESOURCES AUTHORITY
LAND AND WATER STUDIES AGENCY
UNITED NATION DEVELOPMENT   PROGRAMME DTCD PROJECT ETH/75/OO5
by
NAFIS AHMAD, UN ENGINEERING ADVISER GIRMA BEKELE, ENGINEER, LWSA
BANTEYEHUN HAILE, ENGINEER, LWSA
APRIL 1980 ADDIS ABABAi
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iABSTRACT
This report covers the water availability study for Bircte Dam Project and includes tho derivation of an inflow design flood hydrograph. Flood routing has been carried out to fix the spillway capacity. The study is basod 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 bo with caution.!
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ITABLE 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 curvo
k. 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,      ACKNOWLEDGEMENT
9.      REFERENCESLIST OF FIGURES
1.      Reservoir capacity and area curve
2.       Mass curve of InfLow and Outflow-Draught 9U litres/sec.
3.       Mass curve of Inflow and Outflow-Draught 68 litres/sec.
h. Lay out and dam axis cross-section Dimensionless graph
6.       Lag relation curve
7.       Birete River Basin
8.       Unit hydrograph
9.       Inflow design flood hydrograph 10.     Spillway discharge elevation curve 1 1 « Inflow and Outflow hydrographBIRETE DAM PROJECT HYDROLOGICAL STUDY
1 . INTRODUCTION
The Birete river basin is located in Shoa administrative region about 1 06 kin. North West of Addis Ababa, It is planned to conserve the surface runoff of the basin to meet the water requirer. 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 cn 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
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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 th© provision for i'nereas 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 kin .
b)    Hydrometeorological data
o
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
T
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evaporation station at Hine in the catchment of Awash Basin which is close to Birete.i)           Rainfall date
Only four years rainfall data is available from Derba station. Annual figures are reproduced in table 1•
laoxe 1- Year
/kin-l._______________________'---I. _           ■ -
V-
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           = 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 fGW discharge measurements were taken on Birete rivei near Inchini which are reproduced in table 3.
Table 3 - Discharge of Birote river near Inchini
Date of measurement
Discharge of Birete river
near Inchini
28      March      1979 12 May             "
26 "                   ’•
16 Dec              "
17"                    "
24 Litres/3ec
81 " "
19" "
11" "
23  " "I
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As earlier mentioned there is no rainfall station within the catchment so a direct rainfall runoff corelation is
not possible. To have an idea of the water availability, the
rainfall at Derba station is considered to be the same as for
Bircte catchment. Based on this assumption an attempt has
b^en made to estimate runoff based on the method developed by the U.S. Soil Conservation Service published in 1964*
Complete daily rainfall data for Derba is available for four years from 1975 to 197^. However it is available from
May to December for 197^ 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 tho corresponding rainfall storm. To estimate the runoff coefficient
for every month, it would be important to first arrive it the representative storm for the relevant month against which the
direct runoff valuo could be read from the curve. For this
purpose the daily rainfall data has been analysed. For every 
nonth 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 h 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 the following characteristics.
i) Hydrological Soil Group
Since the top soil in the catchment mainly consists of clay and is likely to have slow infilteration rate, it is considered to fall in Group D,7
ii)       Land Use and Treatment Class
Land use or cover is taken as Small Grain, and 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, CN 85 has been adopted to read the direct runoff corresponding to the monthly representative storms arrived at in tables 4 and 5, and 
the runoff coefficients have been calculated.
Tabic 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
74 -79
JUNE
6 year
data
74-79
1
2
3
4
5
6
7
8
9
10
X
s
X2
Runoff
Coefft
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
17.2
13.6
12.5
12.0
9.2
7.5
7.4
7.4
6.9
6.8
10.05
3.58
9.60
0.04
34.2
20.0
14.5
9.0
8.0
7.5
7.3
7.2
6.7
6.6
12.10
8.89
10.87
0.05
44.1
44.0
34.1
30.9
26.0
24.3
17.5
14.0
13.9
12.0
26.03
12.03
24.55
0.20
' 50.1
29.3
27.0
22.2
22.0
21.7
19.9
19.0
19.0
17.5
24.77
9.63
23.55
0.20I
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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 » 
years
data
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•_EC .
5 years
data
?4-7 8
1
2
3
4
5
6
7
8
9
10
X
s
X2
Runoff
Cooff.
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
11.0
10.9
4.1
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
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 to establish 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, tabla 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 don site to record the base flow, however no further data other than listed in table 3 is available so far. As such it has
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been assumed that 10 litres per second would be 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 197^ (table 2) is adopted. Monthly figures of evaporation are given in table 6
Table 6 - Monthly evaporation data for Hine
Month
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Pan Evaporation
mm
A c tua1 Eva poration
E                                  E act = 0.8 x pan
mrn
January February
March
April
May
June
July
August SeptcmbG October November December
? 106
168
153
163
176
120
108
102
137
129
125
138
134
122
130
141
96
86
82
1 10
85 
103
100
1 10
For the estimation of the monthly runoff, monthly rainfall data is analysed in table 7. Rainfall data foi' the months of
January to. April, November and December is available for 5 years and for the months from May to October it is available for 6 
y cars. Average rainfall 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 last column of totals. Out of the four years for which complete <--ta is available, year 1976 has been the driest with a total annual rainfall of 890,8 mm, which is
-20,8 x 100 = 87.78# 1.014 ;-8- ■■
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of the total of average monthly rainfall. To reflect a dry year
within the scope of data available, the monthly averagG rainfall
is reduced to the same 87«78^« Final figures of monthly rainfall
are shown in th© last line.
The above approach of arriving  at the representative monthly rainfall for a dry year has been adopted because th© 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.
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Tablo 7 - Monthly rainfall at Durba
Y oar ssaaeeaocirs: 197'*
1975
1976
1977
1978
1979
TOTAL
Average Rep, dry
year
Jan          . S3 S3 C:
Fob.
Mar.
0.7
6.2
6.8
0.4
52.3
66. 4
13.3
11.7
3 1.9
22.8
48.9
10.9
14.0
128.5
25.7
22.6
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
Juno
104.4
133.7
107.2
115.4
115.7
154.7
731.1
121.8
106.9
July
288.4
268.0
219.5
258.2
330.5
202.9
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
Oct.
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
Doc.
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
890.8I
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Table 8 shows the computations of monthly volume of inflow which is,
V = V + V
i                r o
Were V  = monthly volume of inflow
±
V     = monthly volume of runoff
= Runoff coeff. x Catchipent Area x monthly rainfall
depth
V     = monthly volume of base flow calculated at the
rate of 5 litres/sec. 
Table 8 - Monthly Volume of Inflow
\
Dnth
Runoff
Coeff.
Monthly
Rainfall Depth
mm
Monthly Vol.
of Runoff
V
m3
Monthly           Vol. of Base flow
m
Monthly Vol.
of Inflow
V = V +V_
i          3  b
r
1
2
3
4
5
6
kt.
lov.
)>c.
Jan.
rob. March April
Hsy
•J uno July
Sopt.
0.12
0.03 i
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
77,490
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 curvee( capacity and area
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curves). Reference is made to Fig. Fig. 1 are attached as Appendix B, To of the reservoir surface area relevant
it is assumed that the reservoir
which is the end of rainy season
computations
initiate
for               plotting
computations
the
to the particular month
is full at the end of September,
Thereforo the mass curve study
starts from October
For the reservoir full stage two studies
have been conducted, one for the condition when the live storage
is 2 million cubic metres with one million cubic metres as dead storage and a constant draught (withdrawal) of 9^+ litres/secj
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 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 ram 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 boen assumed that the unit weight of the sediment
trapped in the reservoir
voight 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
efficiency of the reservoir, the live area ratio i.e. 2 x 10  ,
6
'37.85x1 o6 " °’°53.
gives a value of 86% as read from the graph given in the 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
downstreami .)’*
I.
(X
.
K
.
..
_ ---------------------------------------------->----- --  : -• ,15
Therefore, yearly sediment volume, S^., eroded from th© catchment and retained in the reservoir,
S = A x h x 10"x 0.8 nr
y 3  2
3 
3
Where A = catchment area in m
h =
s
Volume of the
sediment height eroded   in mm.
deposited sediment in   the reservoir. S f           r     yr ’
S
yr
=
Axh x 10^ x 0.8
s
0.8
m^
It is proposed to provide a dead fltorago 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
= —-—^-2—   26 years.
s
37,850.1
h re: h
I pe L7
mtTable 9 — Reservoir surface area relovant to each month
Live storage, L = 2 x 10 m J Dead Storago, D = 1 x 10 m ;
S
Draught D = 9^ litrea/sec.
3
1 ♦
1 J
Accumulated Vol. of Inflow
V. 
1 _3 m
Acc. Vol, of
Inflow + live
Storage
V. ♦ L 
m3 S
Acc. vol. of
Draught
D
m3
Available
Storage
V +L +D -D 1      3 gS
Reservoir
Surface Area
2 
m
1 12
3
4
5
0
h
1
1 •
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
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,OOO,OOO
3,000,000
3,OOO,OOO
592,000
570,000
533,000
506,000
480,000
452,000
424,000
460,000
542,000
600,000
600,000
600,000
The accumulated volume of inflow (runoff) ostimatod in table 9 is 6,987,730 m . According to Mr, C.A.Munoz s not* attached 
t
as Appendix A, the runoff based on the minimum flow per Km  of
2
Huger river at Chancho for the driest year 1971 works out = O.3399938 x 106 x 37.35
= 12,868,760 m3
The r noff calculated in table 9 is less and is adopted for further study* 
—TM*~^**^■ ...
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Table 10 -   Net accumulated volume of
Inflow + live storage
(continued from table   9)
Reservoir
Surface Area
2 
in
Monthly
act. evap.
mm
Monthly
Vol. of Evap.
n?
Acc. Vol*
of Evap.
nP
Net Acc. Vol.
of Inflow +
Live storage
(Col. 3 of table
col. 5 Of table 10!
m3
2
3
4
5
6
592,000
570,000
538,080
506,000
4 80 ,  000
z
452,000
424,000
460,000
542,000
600,000
600,000
600,000
103
100
110
134
122
130
141
96
86
82
110
85
60,9^0
57,000
59,180
67,800
58,560
58,760
59,780
44,l6o
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
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 mss 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 c very dry year or would add to the life of the project.
7.___________________________________51J
SHXMOR
OUWJLATIVt
X aec MKYtm
t
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Table 11 - Reservoir surface area relevant to each raonth^ Live storage, L  «= 1.5 x 10  m ; Dead storage, D  =
g
63
g
1 x 10 tn |
D   68 litres/sec,
e
1
Accumulated
Vol. of Inflow
V
vi
Acc. Vol. of
Inflow + Live
storage
V + L
is
m3
Acc. Vol. of
Draught
D
3
m
Available
Storage
VJ +L +D -D iss
3
m
Reservoir
Surface Area
2
m
2
3
4
5
6
/
1
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f
B
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
1,789,9^0
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,OOO
2,367,010
2,198,270
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 ",ooo
464,000
540,000
540,000
540,000
540,000
t20 -
Table       12 - Net accumulated volume of Inflow + live storage (continued from table 11)
Reservoir
Surface
Area
2 
m
Monthly
Act. Evap. 
mm
Monthly
Vol of
Evaporation
3 
m
Acc.       Vol.       of Evaporation
m3
Net Acc. Vol
of Inflow + Live
storage
(col. 3 of table 11
- col. 5 of tab. 
m3
12]
2 /
3
4
5
6
540,000
524,000
500,000
472,000
456,000
440,000
420,000
464,000
540,000
540,000
540,000
540,000
103
100
110
134
122
130
141
96
86
82
1 10
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 (l,535fO4O - 1 245>5^0) = 289,500 cubic metres of live
t
storage is left in the reservoir which would provide a cushion for a very dry year or add to the life of the 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 = ^33,360 - 398,320 Storage required = 424,03^ - 35,^40 = 388,99^ n»
= 424,034 m3
= 35,040 m3
q*I
r
IOMM-MTlVf KFLOw/OUTFLOWE
V
-•3
Considering a margin live storage of 500,000 m  may
b. HEIGHT OF DAM.
22 -
for       a       very       dry       year, be necessary#
a minimum
3
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 tho total storage is read from the capicity curve, Fig. 1 •
Cross-section of river at the dam axis, Fig. U, 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
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° Z*
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 1O m
63
of live storage and for 2.0 x 1O m  of live storage is 3 metres, 
63
63
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 1O m .
5. DESIGN FLOOD
a) Emperical methods:
i) Mr. C.A.Munoz in his note, Appendix A, has computed the flood peak by tho two emperical methods, the values are given below.
By Fiddes method, flood peak =
111.2 m3/sec.
By modified Myer’s method, flood peak = 104.6 m /sec.
3r- 23-rr r!i24
According to the Blue Nile Basin report. Appendix III,
- U6  peak flood value for once in 100 years frequency 
f
ths curve 83 m /sec,
J
o
X •_ o
Munoz has recommended to provide a peak value for
■»
' ra
i
. O J -
/sec
a
~.G
Q m:
? on basin characteristics and rainfall pattern of
for the Ribb and Gummara flood control, the
been developed by Mr. M.Shim.
= O.OO76 H1’5 C A0,71
where H = Maximum 100 year one day rainfall (mm) C = Basin characteristic coefficient (in 
this case = 0.7)
A = Basin area (km
2)
According to the Blue
Nile Basin Report the maximum
~0 year one day rainfall =
124
mm
Therefore Q     max
=      0.0076      X      1241’5          X      c.7      X      37.85°’71 = 96.9 m^/sec.
b)                equations for a triangular hydrograph concept
Using
L’/dro graph .
the
general peak equations for a triangular
.0.385
(Ref:
Design       of       Small       Dams by USBR)
Where
T 
c
= Timo of concentration -
travel          time          of
water
from most distant point
to dam site in
Hours
L
H
point
*o
The
length of the stream is 11.91 km and the elovati
on
cifferenoe between t
ho upper point and the dam site of
the
=troor. is (231 .65 - 107.83) = 123.82 metres. (This information of stream length and elevation difference is supplied by2,59
hours day = 2.6 hours
A?  \TICt1
T 
P           I-
0.6 T 
c
D        Rainfall excess period which Where
T = Tima froc» start of rise to peak P
T     = ^ +        (0.6 x 2.6) = 3.56 hours P
rate
Tb
= 2.67 T = 2.67 x 3.56 = 9.5 hours P
T. = Time base of hydrograph D
0.7^9 AQ Q =   m----------------
PT p
Where q     P
= the peak discharge in nr /
A =   Drainage area in m2
Q T s
P qp!
: Depth runoff in metres f
one day rainfall of 124 
; Time from start of rise
0.749 x 37.85 x 106 x 0.062 3.56 x 3600 
= 137.2 n?/sec,
50% of
the max. 100 year
mm is taken as runoff to peak rate in. seconds.
c) Unitgraph derivation.
To develop unit hydrograph for Birote ungaged area , tho 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 metros.iHYDROGRAPH
• • Cumq fait pu bacord hub*orf
.M > NOURI FROtt CANTAB OF RAIN AXCASB FARM TO T|M<
WMN HALF 0* RUM-QFF NAB OCCURCO
•* NOURI QVAR WHICM RAINFALL AMCKAOB INFILTRATION
■ OF • RAT— ■ ICONS MIT (I. A CUBIC'FOOT RAR 11 COHO) INTIBRATAO
OMAR PARlOO OF BTQRtf TO NAT TOTAL VACUA OF 1URFAC A RUN-OFF
DIMENSIONLESS GRAPH F»® 5FIG. 0BLUE NILE BASIN LAG CURVE
In mUlloni tach llirM In mtftrtAC = Centroid of basin, Fig» 7
L Q = distance from dam sito to the intersection perpendiculx ca
from C to the stream alignment in metres
S = overall slope in metres por metre of longest watercourse from the- daa site.
L = 11.91 km = 11910 metres
L = 5300 metres-found from aerial phogographs sc' ’.e ca
Is 50,000
Elevation at watershed divide point; Elevation at the dam site Difference of elevation
= 231.65
- 197.83
= 123.82
S =
bag
123.82
1 1910 = (3.0978) = 3.0978
= O.OIO3963
_/s
11910      x      530(5 ^0.0103963
0.38
= 6,790 seconds or 1.89 hours-5=^ 2 hours
Taking unit rainfall duration time as one fourth of
lag time.
Unit rainfall duration, D e -Jiiour
Lag + a 2.25 hours
Volume of 1 inch runoff e A x C
where Ac drainage urea in sq< miles
2
Ca 26.89-unit conversion factor Area = 37.85km2
= 14.61 sq. miles (l sq.mile = 2,59 kn )
Volume of 1 inch runoff a 1U.61 x 26.89
“ 393 second-foot-day Using dimensionless graph, Eig. 5, the ordinates of
unit hydrograph are computed in Table 1*4.c .i
jFIG-730
Table It* - Ordinates of unit hydrograph
% of
Tine
hrs •
T0
♦2
*00 _      4
U1<
Ordins to
(Pros Fig. 5 )
2.25
Vn 1 t r
c
Ord. x 322,
2.25
Uni• r<
col.  4
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
t.o
6.5
7.0
7.5
8.0
9.0
22.2
kk.k
66.7
88.9
111.1
133.3
155.6
177.8
200
222.2
244.4
266.7
288.9
3H.9
333.3
355.6
400
3
12
19
20.8
16.2
12.3
7.0
4.7
3. i
2.13
1.52
0.97
O.65
0.45
0.31
0.21
0.1
525
2095
3320
3635
2830
2 1 50
1225
820
540
370
265
170
114
79
54
37
18
■ ■ — «
14.86
59.32
94.01
’02.93
80.14
60.88
34.69
23.22
15.29
to. 48
7.50
4.91
3.23
2.24
1.53
1.05
0.51
Tho unit hydrograph i» plotted tn Figure 8.*-31f32
d)t Derivation of inflow design hydrograph from the unit hydro graph.
Since there is no recording raingauge within the catchment
area, the 1 day 1 OO year probable precipitation of 124 mm is used for the design storm. The storm rain is arranged in a
□ore or less symetrical time distribution Fig. 9  with the greatest
f
intensity in the middle of the storm to give the worst possible condition for flooding. The rainfall excess are assumed in the following manner.
1 st   hour     net      10%  of rainfall
2nd      II          ii      40%      ti
ti
3rd
II
ii
75%      it
it
4th
II
H
60%      ii
ii
The unit hydrographs of -J- hr. duration are derived for rainfall excess of 1mm, 1mm, 6mm, 12mm, 22.5mm, 
and 2.4mm. Using the principle of unit hydrograph superposition the design inflow flood hydrograph is obtained both graphically (Fig. 9) and analytically in table 15.
The unit hydrograph is computed for 1 inch rainfall which in metric units is' for 25.4 mm or ±  = 25.4mm.
15mm, 3mm,
1
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 4 hour unit = °.rdinate of unit firaphV
f- 33-«
5
D
5
0Table 15 — Computation of inflow flood hydrograph
.me
■rval
Ordinate for
V1
i 2=6
i 2=12
i 2=22.5
±2=15
i 2=2.4
Q
Total
n 
nr / sec
1
>
)
>
)
5
)
i
)
5
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
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
0.0
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
0*0
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
A.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 '
0.72
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
Hie inflow design flood hydrograph is plotted in Fig, 9. The confutations show that the peak of the inflow flood is 209.45m /sec
*hich is the maximum computed compared to all the methods so it is
■
‘dopted. for routing it through the reservoir to fix the spillway opacity,
r--ItMVMl VLCMTIM
9£
4
FIG io
/
*
-•—
i
-«
ru- 35
flood routing
For flood routing Inflow flood hydrograph with a peak
of 209*45 nP I sec has been computed. The outflow hydrograph has
to be computed for which a spillway width is assumed limiting
the head above the creit to 1.5 metres so that tho total height
of the dam above the full conservation level is reasonable. For
conomic reasons the crest is provided in the shape of r. semicircle
keeping a straight width of 20 metres. So, for a diametor of
20 metres, the semicircle length would be 31.4 metres. For
computing the rating curve of the spillway, the length of the
crest is taken as 31 letres. The discharge over the spillway
is given by formula,
Q z C E H *
s
where Q = spillway discharge* in nr/aec.
s
C = coefficient of discharge which
is taken as 2
B = Crest length of spillway width, which is taken as 31 metros
15
3
I
H s Head of water above crest in
motros.
Table 16 #ows tho values of discharge for various values of head, H, Spillway rating curve is plotted in Fig. 10.rTable 16 - Spillway rating ottrve computations
ih
Q = C B H *
13
1 s
«
m
s
«□
nr/sec.
0.2
0.4
0.6
0.8
1 .0
1 .2
1.4
1.5
1.6
1.8
2.0
j                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 epillway rating curve fig. 10, reservoir capacity curve Fig* 1  and the inflow Flood hydrograph
f
Fig, 9>flood ro >ing computations are carried out by trial and orror method as shown in tables 17 and 18, The crest level of the spillway for tho full conservation level is taken as 1251 earlier indicated in table 13,I
,c
’!*
c
,c
c
1 >
r
,c
5
,C
•J r
•-
.£
I
■-- 38 -
Table 17 - Flood routing computations, B =x 31mj Crest level » 
125
dt
Second
■3
Inflow at
time t
/sec
Average rate
of inflow
for dt nr/sec
Inflow
Trial Reservoir Storage elevation
m
Out flow
at time T
□3/sec
2
3
4
5
6
7
)
)
1
i
i
I
•
1
j
1
)
I
i
I
i
•
i
)
j
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
0.0
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
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
1OO
87
70
60
48
40
32.5
28.5
24.0
20.0
16.5
14.0• • 39 -
Table 18 - Flood routing compilations, B = 31mj Crest level = 125
Average rate of out flow
Out flow
3
m
Incremental
Storage
ds
Total
Storage
Reservoir
elevation
end of dt, M
Remark
8
9
10
11
12
13
0.25
0.5
0.5
1.25
4.0
13.0
34.25
66.25
95.75
114.5
122
117.75
106.5
93.5
78.5
65.0
54.0
44.00
36.25
30.5
26.25
22.0
18.25
15.25
450
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
'.23,98
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
0.004-3
40
Flood routing computations show that the maximum reservoir level attained is 126.58 and the discharge through the spillway for that level is 122.5 m /sec. Head above the crest of spillway is (126.58-125.00) = 1.58 metres which is very close
to desired limit of head above crest of 1.50 metres. Therefore
the crest length of 31 metres is adopted.
The inflow and outflow hydrographs have been plotted on the same sheet, Fig. 11. Surcharge volume is shown as hatched
3
and comes to 954,000 m .
7. CONCLUSIONS
The foregoing study is based on very scanty data* Tt should be,as such, treated as applicable to this particular case
and not used as a general guide line, Similar approach for
other basins, where enough data is not available, should be used
with caution.
The water availability study shows that with a live storage of 2 million cubic metres a draught of 94 litres/seo
would be possible which has been recommended in view of economical conservation of water and catering for long term demand,
The spillway capacity is based on a 100 years probable flood with sufficient free board over the flood surcharge level.
The top level of t he dam has been recommended J metres higher than the spillway crest level. The free board would however be checked for wave height when the design of the dam is carried
out.
8. ACKNOWLEDGEMENT
Thanks are conveyed to Mr. Egbert H.J. Schroten for computing the X  values in tables 4 and $  and to Mr.
g
f
Carlos A.Munoz for reading through the report and agreeing to it in full.I
I
u■
ADISCHARQlL kN m’/MC
QCM'/SeC] ,
X
0 Qi I/O U 1.0 
2»        10       15      40      4.5       »X) M «£ 
<5       7.0 ff gg t5 g.g >,| jg g pg K0 |U |t 0 Tl« IN NOURS
FIG .11J
I
I
I
I 1
■
I-i
~ 42 -
9,
REFERENCES
a)      D *nign of Small Dams
by United States Bureau of Reclamation
b)      Land and Water Resources of the Blue Nile Basin, Ethiopia Appendix III - Hydrology by
United States Bureau of Reclamation.- M -
APPENDIX A Mr, C,A. Munoz’s NoteI
»*>
,1 i
j- 44 -
November 5^^*              1979
To              : Mr. G.Dekker
Project Manager ETH/75/005
From         : C.A.Munoz
Hydrometeorologist ETH/75/005
Ref:         Hydrometeorological data
for Direto River
For the design of a small dam on Birote River which will be used as storage for the water supply of the cement factory, I
have been requested to provide the basic hydrometeorological information such as rainfall, evaporation, peak discharge for the
design of the spillway and runoff values per square kilometer.
No hydrometeorological stations exist in the catchment and only published maps at a scale of 1:250,000 which are not
adequate; the tributary area has been determined by Dr. Tsegu from available aerial photographs at scale 1:50,000 and has found to
have a value of 37.85 SQ. kmts.
The area was visited with Mr. N. Ahmod and Dr. Tsoga on Friday November 2nd, 1979 and field data were collected for
the possible peak flood design. It appears that the catchment
has been properly located on the aerial photographs and the
calculations have been based on the value of tho aroa as computed
by Dr. Tsega.
Tho nearest evaporation station is located at Hine with annual values in mm of 1624.2, 1525.2 and 1499.1 nun observed in
an evaporation pan class A, installed in Docember 1975| those values correspond to 197^, 1977 and 1978 respectively. These values
have been adjusted for missing data in 1977 and 1978.
Observations of rainfall at Derba (09° 26»n) and Inchine (09°19>N) (38°23'E) could bo used. (Tho longitude at Derba is
38°39'E).
•. - .... ,v ..
A-r*'.'Values of runoff have b»en computed from data available on Muger River with a tributary area of 484 sq. kmts ; Bireto
River is a tributary of Muger.
Enclosed herewith please find the detailed available inf orrnation.
c.c.
Dr. Tsoga, EWRA•J
£E
■S
Computation of peak flood for flircte River at possible dim sHe (Based on a tributary area of 37*35  Q • krats. calculated by
Dr. Tsega from available aerial photographs 1:50 000)
f
1 ) Fiddes Method
Field data: Length of stream 11.91 knits
Elevation at upper part:
"             " dam site:
Difference of elevation:
231.65
107."3
123.82
Slope of main stream: 123.32
11910
0.0103963
(information supplied by
Dr. Tse$a)
Standard         contribution         area         coefficient         Cs         for         a         rolling         catchment slope and soil type with impeded drainage Cs = 0.5
Catchment       wetnoss       factor       Cw       for       wet       zone       and       perennial       stream
Cw = 1.00
Land use factor Cl (intonse cultivation) » 1.50
= Cs x Cw x Cl = 0.5 x 1.00 x 1.5C = 0.75
Catchment lag time K = 3.00 (cultivated land)
Volume of runoff RO = C^ (p • y) Ax 10^
»      0.75      (60-0)      37.85      x      103
= 1703.25 x 10  (m )
33
Average flow Q =
Base Time:
36OO x T
n
K°
a
bUo
tb
Q
M
^2
aT
B
-Tr B ’ —^0
T
 + 2.3K +
p
0.028 L
Q* s*
0.028 x 11.91 
0.333/48
Q* x O.OIQ3963*                  O.1O19622Q*
T  = O.96 ♦ 2.3 x 3 + T « 7.86 <• 1,2706238
q- 47 -
By trial and error:
3.2706238         ?.e6 <■ 3.2706238
440
Q
Q*
Qv
Q*
Q
50.0
2.659147
1.229952
9.089952
8.30
48.4
2.637614
1.239993
9.099993
9.0909
48.35
2.636933
1.2403135
9.100313
9.10031 
OK
Q Max =          2.3 Q =            2.3 x 48.35               = 111.20 m3/sec
2)       From Blue Mile
Studios, Appendix III, Figure
Frequency
Once in x yoars for 37-85 ag, krats
5 
10 
25 
50
100
IIX-46:
*1
Peak value in iaJ/anc
40
49
64
75
83
3)      By tho modified Myer's method for a C coefficient of 1?
Q = 17 x = 17   37.85* » 104.6 m /sec.
x
3
Prom the above values and to satisfy the results of the Fiddes mothod it is advisable to design the spillway with
a capacity of 75 m /sec with a frooboord of 28 m /soc and an
3
3
emergency spillway of 15 m /sec. Tho total capacity for
3
3
evacuation of tho peak flood would bos
Q = 75 + 28 + 15 => 118 m /sec.
Rainfall data:
The station installed nt Derba (09° 26'N mt! 38°39’E) in April 1974 gives the following values 1
Yoar
1975
1976
Annual Precipitation ran
944.?
890.8
11|«3.8
1007.3iI
ii I
1
I
•i
I
i
I
-jihife i )■ jif- 48
The station located at Inchine (O9°19'N and 38 23'E) indicates higher values duo to higher altitude but the data
available are questionable. Details are available in the office of tho Land and Water Studies Agency of EdRA for those two s tations.
Vvn.pora ti on :
Observations liavc been made at Hine since June 22  1975
f
in an evaporation pan class A; however Bine is located on the catchment of tho Awash basin and not on the Nile basin, but relatevily close to Eirote. The adjusted values would be;
Year
Annual Evaporation in mm
1976
1624.2
1977
1525.2
1978
1499.1
Daily and monthly values are available in the Land and Water Studies Agency of EURA.
Runoff values:
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
Ms. The values of runoff are as
Year         Runoff nr x 10 
follows:
Runoff        per sq. knit, in
x 10^
1971
164.557
0.3399938
1972
206.08
0.4257851
1973
275.033
O.56825OO
1974
208,907
0.431626
1975
303.991
0.6280805
1976
207.195
0.4280888
Average
227,627
0.470304I
I- 49 -
APPENDIX B
Area and Volume Computation for plotting Area and Capacity Curves50
Area & Volume computation
(Ai) + (Al + 1
No
z
m
A
2I
2
Av
V
m2
m
3
m nP
1
2
34
56
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
110
1 1 1 1 12
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
1360                          906.6
7160                            4260
1668O                          11920
33680                          25180
50600  i                   42140
68040                          59320
87O8O                          77560
118000 ,                   102540
159080                        138540
200680                        179880
256400                        228540
321880                        289140
392480                        357180
465440                        428960
533200                        499320
603600                        568400
669400                        636500
740600                        705000
812200                        776400
906^6                  906 4 6
4260                    5166..6
11920                   17086
25180                   42266
42140                   844o6
59320                143726
77560 ,          221286
102540                323826
138540                 462366
179880                 642246
228540                 870786
28914C              1159926
357180               1517106
428960               1946066
499320           2445386
568400           3013786
636500           3650286
705000           4355286
776400           5131686• r
II

Summery

Project: Birete Dam Hydrological Study
Location: Shoa administrative region, Ethiopia (106 km NW of Addis Ababa)
Coordinates: 9°29'N, 38°20'E
Authors: NAPIS AHMAD, GIRMA BEKELE, BANTEYEHUN HALLE
Date: April 1980

1. Introduction

The Birete Dam Project was planned to conserve surface runoff to meet water requirements for a proposed cement factory. The dam site was selected on Birete River, a tributary of Sodoble River which drains into Muger River.

2. Key Findings

Water Requirements

Present demand: 2,000 m³/day (23.15 litres/sec) for cement factory
Future increased demand anticipated

Water Availability

Parameter	Value
Catchment Area	37.85 km²
Annual Rainfall (Derba station)	890.8 mm to 1,143.8 mm (1975-1978)
Annual Evaporation (Hine station)	1,499.1 mm to 1,624.2 mm (1976-1978)
Base Flow (minimum recorded)	11 litres/sec

Dam Specifications

Option	Height (m)	Total Storage (m³)	Live Storage (m³)	Dead Storage (m³)	Draught (litres/sec)
Recommended	20.5	3.0 × 10⁶	2.0 × 10⁶	1.0 × 10⁶	94
Alternative 1	19.7	2.5 × 10⁶	1.5 × 10⁶	1.0 × 10⁶	68
Minimum	17.5	1.5 × 10⁶	0.5 × 10⁶	1.0 × 10⁶	23.15

Design Flood

Multiple methods were used to estimate peak flood discharge:

Fiddes method: 111.2 m³/sec
Modified Myers's method: 104.6 m³/sec
Blue Nile Basin report (100-year flood): 83 m³/sec
Shim's formula: 96.9 m³/sec
Triangular hydrograph concept: 137.2 m³/sec
Unit hydrograph method: 209.45 m³/sec (adopted for design)

Spillway Design

Crest length: 31 meters (semicircular shape)
Maximum discharge: 122.5 m³/sec
Maximum reservoir level during flood: 126.58 m
Crest level: 125.00 m
Head above crest: 1.58 m
Surcharge volume: 954,000 m³

Sedimentation

Assumed annual sediment inflow: 1 mm depth over catchment
Estimated sediment volume: 37,850 m³/year
Dead storage life: 26 years
Trap efficiency: 80%

3. Conclusions

        The study recommends a 20.5m high dam with 3.0 million m³ total storage (2.0 million m³ live storage)
Design flood peak of 209.45 m³/sec was adopted for spillway design
A 31m semicircular spillway crest was designed to limit head to 1.5m
The study acknowledges limitations due to scarce data and recommends caution in applying methods to other basins
Mass curve analysis shows sufficient water availability for the cement factory's needs

               
            