/.      7
e,      •
I
X
y
i
BILATE RIVER BASIN DEVELOPMENT STUDY
PRELIMINARY REPORT
7
?/
VDLU /IE 5
C
D. Jovanovic                civ. eng.
Teclegiorgis Goilom civ. eng.
National Water Resources Commission
Executive Organ
!
Addis Ababa
July 19745
SA-72.I-S
BILATE RIVER BASIN
DEVELOPMENT STUDY
PRELIMINARY REPORT
D Jovanovic civ. eng
Teclegiorgis Goitom civ eng
Notionol Water Resource? Commission
Executive Organ
.1
Addis Ababo
July 1974».cknowl cdgment
The Bilate River Basin Development Preliminary Report
was prepared by the NVT
RC/EO from 1972 - 1974. The <7RC
f
provided all the necessary manpower and financial means for field survey, helicopter flight investigation and other expenses for the collection of all pertinent teennieal data
as well as for the office work. The efforts of the NWRC en ineers, surveyors, draftsmen, hydrometeorologists and other tecnnicians who assisted in the prep-u’ation of this report is greatly acknowledged. Special tnanxs are due to 
 .to Tsehay Woldai, who made the eolo. ical investigation of the project sites and prepared tne eology chapter of this report. The authors also wisn to express their sincere appreciation to the Imp-rial Ethiopian xr Force for their cooperation and willingness in making their military helicopter with its crew available for the ir.vesti - ation 
of project sites.
D. Jovanovic
Chief Engineer
’.ddis  baba July 26, 19741..BLE OF CONTENTS
TABLE OF CONTENTS
1. INTRODUCTION
2.      BJ SI ; D CRI ?ION
2.1.     Location
2.2.    The Upper Dilate Sub-Basin 2.3.     The yid Dilate Sub-Basin 2.4.     The Lower Dilate Sub-Basin 2.5>. kj € neral
3.     HYDROMETEOROLOGY
3.1.     Meteorology 
3.2.     Hydrology 
4.     GEOLOGY
4.1-      General Basin Geology 
Pase
4
5
6
7
&
8
8
13
4.1.1.       Boyo Swamp Dam Site Geology                  15
4.1.2.       Alaba Colito Dam Site              Geology      16
4.1.3.       Ropi Dam Site Geology 5.      TECHNICAL SOI UTI^NS
5.1.     General
5.2.     Water Acquirement 5.3.     Boyo Swamp Dari Site
17
20
20
20
21
5.3.1.       The nam
22
5.3.2.       The Spillway Design Flood 
23
5.3.3.       Power Plant
2-4
5.4.     Alaba Colito Diversion Dam
and pc er Plant
5.5.     Ropi Dam and Power Plant
5.5.1      Spillway Design        Flood 5.5.2      Po’- -r plant
(I)
24
25
26
27Page
5.6.     Ropi W.-ter Falls Hydro Power
Plant
27
5.7.     Electricity Demand 
28
5.8.     Quality of Voter            for irrigation 
28
5.9.     Sedimentation 
30
6. COST ; TIMATE ? ECC OMIC STUDY
6.1.      Method   of calculation
6.1.1.   Project capital costs
6.1.2.   Annual costs
31
31
31
6.2.     Capital cost estimate
32
6.2.1.   Boyo Swamp Dam
32
6.2.2.          Boyo jwamp Hydro Power Plant
33
6.2.3.   Alaba Colito “                     ”            ”
33
6.2.4.   Ropi Dam
34
6.2.5.   Ropi Hydro Power Plant
34
.2.6.   Roni ater Falls Hydro
Power Plant
35
6.3.       Annual  Cost Estimate
35
6.3.1.   Boyo Swamp Dam
35
6.3.2.          Boyo wamp Hydro Power Plant
36
6.3.3.   . .laoa Colito Hydro Power
Plant
37
6.3.4.   Ropi Ram
38
6.3.5.   Ropi Hydro Power Plant
39
6.3. .   Ropi Water Falls Hydro
Power Plant
40
6.4.     Cost of Stored ater and Produced
Electric Energy
41
- ■ 4 e 1 <    oyo Sw- p Storage
n
41
6.4.2.         3dvo Swamp Hydro Power plant
41
 .4.3.      lata Colito Hydro Power
plant
42
. +.4.     ori ’torage
42
6 • 4. ,   Ropi Hydro Tower Riant
42
6.4. .             ■topi   ter " alls Hydro Rower
Fl ant
^3
(II)6.5.       Proposed Solation 
6.5.1.       Vater Regulation 
6.5.2,       Electricity 
7. CONCLUSIONS - ND R^CO TC’ CATIONS
r
pa e
45
45
46
48
7.1.
 Vater Regulation 
48
7.2.
Electricity 
49
7.3.
Recommendations
50
(III)LIST OF DRAWINGS
1.Map of Ethiopia
✓            2. Bilate River Drainage Basin, 1:500,000
<3. Bilate River layout and longitudinal profile from Boyo Swamp to Ropi Water Falls
✓ 4. Hydrometeorological Stations of Southern .lift Valley Basin - 5. Bilate River flow at Alaba Colito, mass curve
6.            Geological map of the Lake District
*7. Boyo Swamp Reservoir Layout *
8.      Boyo Swamp Jam
9.      Alaba Colito Water Falls Dam and H.E.P.P. 10.      Ropi Dam Layout
11.      Ropi Dam and H.E.P.P.
12.      Ropi Jam Spillway
13.      Geological Cross section at Ropi Dam axis 14.      Ropi Water Falls Diversion Dam
 •’15. Recommended Schedule of work execution1.     INTRODUCTION
The Bilate liver Basin Study was initiated through tne order
of His Imperial Majesty. The Emneror appointed Ministerial Committee to study the problem of availability of water for irrigation in connection with th? complaint raised by the Bilate /agricultural Estate (BAE), that there is no sufficient water for irrigating the 53 gashas of land belonging to them, during the low flow period, and particularly after new concessione-rs upstream the B.^.E. were about to start irrig ating their farms.
The B.A.E. signed an agreement with the Ministry of Land Reform to lease 53 gashas of land in the Lower Bilate Valley. This agreement granted the rights to B.A.E. to use water
from Dilate River to irrigate the 53 gashas.
During the first few years of operation, it was observed 
that the Bilate River flow between the months of November
and April was too lew to satisfy the irrigation requirements
of the whole 53 gashas of land.
On March, 1971, His Imperial Majesty visited Bilate and the
B.A.E. brought tne water shortage problem to the attention 
of the Emperor. Then, the Emperor promised that a storage reservoir will be built by the government in order to solve
this problem and he appointed a Ministerial Committee to study this case. ./hen this situation was well known, several
new private concessions ard one government owned farm belong ing to the Tobacco Monopoly were established upstream the
B.A.E., thus worsening the irrigation water shortage
problem. Besides, two ctner farms w- rc- established downstream B.A.E.
The Ministerial Committee appointed a technical committee
to collect ana study the available data and recommend furt  er steps to be taken. Thus, National Water Resources Commission Executive Organ (LWRC/EO) started Held investigation works
□nd studies cl the Dilate River Basin with the budget provided for the Rift Vall.y. Shortly after the .thio-inn2
Government signed an agreement with the British Government that the British experts together with the HWRC/EO undertake the land resources study of the whole Ethiopian Rift Valley Lakes z.one. The Bilate River Basin was also included in this study. The British team concentrated mostly on land 
resources survey aspect and did not take detailed considera
tion of the water shortage problem in the Lower Bilate,
because NV. RC/EC was conducting a more detailed engineering study of this area.
The main purnose of this study was tn identify all possille
dam and reservoir sites in order to control and store
sufficient water to satisfy the critical needs of irrigation 
in the Lower Bilete sub-basin. However, the objectives of
this river basin plan should not only concentrate in one
use, but should also consider other needs and uses cf water
in -rder to have the optimum utilisation of water resources, consistent with sound engineering and economic principles. Thus, parallel with this investigation, all possible d?.m
sites for small diversion dams as well as sites for hydro power plants were also identified.
Four dan sites were identified along the main stream. Two dams will impound water to nrovide storage reservoirs, while the other two dams are diversion dams to provide small regulating reservoirs and diversions for rower purposes.
The first storage project identified is the dam on the
Bilate River about 800 m. downstream the Boyo Swamp outlet, with the Boyo Swamp acting as a reservoir. The stored 
water will to mainly used for irrigation, but could also be
used for rower production. The second storage facility will
be provided by?dam on the main stream located at about 5 kms. upstream from the waterfalls near Roni Village. Water stored in this reservoir could be used for irrigation as well as
for power. The two iivc-rsion dams identified are also both situated ( ,n th main river, ue is urstream the waterfalls
near \laha 3. lite Town, while- tne waterfalls r.~ar Ropi Village.
lies uro  -com of the
J
• 4 •- ?> -
It was impossible to determine the exact dam heights of the two storage projects, due to the fact that the following parameters were not precisely determined
-   long period of flow iecoru in the river
-   the amount of silt carried by the river
-   total water requirements for irrigation, because
presently there does not exist detailed land classifi
cation and cropping pattern studies in the Lower Bilate. From the study and investigation conducted by F WRC/EO, the i Boyc Swamp Dam seems to be the most feasible structure that / can satisfy the present acute irrigation water demand both economically and technically. If it is found that the
storage is insufficient or that siltation rate is higher
than anticipa ed, the reservoir water level can be raised without incurring a significant expense. Ropi Storage Dam
is f •und to be too expensive and difficult to construct
due to unfavourable geological conditions, hence the construction of this dam is neither technicallynor economi cally feasible.in the early stage of development.
The hydroelectric power potential of Bilate River as
indicated in this repul L could be very useful to the
concerned agencies in making comparative studies of electri city supply for rural areas and towns in this region.
From the three proposed hydropower plants about 71 X 10 kwn of electric energy could be produced annually.
Thus, several towns such as Hossana, Wollamo Soddo, Arba flinch, Yirgalem, Wondo and many rural settlements and farms could be supplied with electricity from hydrepovzer plants
in the Bilate River Basin.
6
»• /- Lt -
2. BASIN DESCRIPTION
2.1     Location: The Dilate River Easin is one of the major catchments in the Ethiopian Rift Valley Lakes Zone.
In the upstream watershed, it has two major tributaries, namely: the Guaer River which originates from the mountains near ’Iossana Town ana the .eira River which rises from the highlands in the northern part of the Dilate drainage basin. The Guder River flows eastwards into Doyo Swamp. The V/eira River flows southwards to join the Guder River at the outlet of theBoyo fwamp. Downstream from the confluence, the river takes the name Dilate. The Dilate River flows southwards and flov.s into LaKe ! argarita (Abaya).
The Pil ite River Basin covers an area of 7,463 sq.xms. in the provinces of Shoa and Sidamo. ~he Basin extends between latitudes 06° 44’ 00" and 08° 22’ 00” north and between longitudes 37° 44’ 00” and 38° 30’ 00" ea:t.
2.2     General
The altitude of the Easin ranges between 3,600 m. and 1,285 t. above sea level. The soil cover in the catch ment is mainly sandy or loamy sand about 2-3 metc-rs in depth, which is probably a weatnered product of tuff and ignimbrite. The existing soils, bed rocks,
vegetation and land use have greatly influenced the drainage pattern of the Dilate River Basin. There exists tremendous ram and wind erosion in the whole catchment which is mainly due to the existar.ee of the erodable top soil, lack of vegetation cover  <nd dot land use.  henw mal gullies made by run-off erosion are exhibited in tue tributary streams an.’ banks of the main riv:r. Wind erosion plays an active role or 1 ■ luring the dr/ season. Most of the Dilate River I ?d (frr- Poyo w «:rp ■..■tlct up to the  ater ’’nils rear Ropi ’ nla;?) is f- (i. ni..ibri t*=> reck). Thus, J;er~
• • • •- 5 -
is no erosion in the river ted, while the river banks
are heavily scoured by run-off.
The shape of Bilate watershed is long and narrow, so flow in the tributary channels reaches the main stream
at different rimes, thus distributing the total run-off
over a longer span cf tine. The slope of tne watershed is r.ot. very steep, which gives more opportunity lor infiltration and detention storage.
There are three crater lakes in the lower portion of
the watershed which are the results of volcanic activi ties in the region. For simplicity and in order to 
avoid misunderstanding, the Bilate River Basin is divided into three sub-basins as proposed by the British team,
as follows:-
T
2.5 The Upper Dilate Sub-Basin
Comprises the area of the catchment upstream from the Boyo Swamp outlet. This part of ths Basin (excluding the swamp area) is not covered in this report except that it was inspected by helicopter. The two main tributaries of Dilate, the Jeira and Guder ^ivers lie
in this sub-basin and each of them carries nearly the same amount of flow. The slopes of these  wo tributar ies are very steep and tne topographical features are unfavourable for sites of big storage reservoirs.
In this sub-basin, there is no large area which is suitable for agriculture and the existing land is
situated high loove the river bed making it inconvenient for gravity irrigation or economical pumping.
2.       The i^id-Bilate Cub-Basin
This sub-basin stretches from Boyo Swamp outlet up to Bl at-nt ~na. -. oerdirn to the British survey, tn< lane suitatl for im. at± n is about
sit :atc . or. t • ■ -st  ■. iha of Bilate River north and 
ba. mostly
soutn 
lito _v.-n. Dur to unfavourable
••••/- 6 -
topographical conditions, gravity irrigation will be costly and must be combined with pumping. Another difficulty is that this area is intensively settled 
by srrall land owners with long established pattern of whom 70S are tenants. Thus, the problems to be encountered when preparing irrigation layout for such projects are numerous, namely: ’’land consolidation and consequent litigation, boundary adjustments, elimination of grazing, removal of houses, radical changes in field sizes anu shapes as well as in cropping patterns and imposition on tne traditionally independent farmers of
ar. unnaturally rigid discipline".
In this sub-basin, the section of the river basin 
between the water faxls near Ropi Village and Blatentena was not recommended for irrigation in the British report. This area is inaccessible and qaite bushy, however, wher. conducting helicopter investigation, it seemed that some of the land could be irrigated. Investigation of this
area was discontinued due to security reasons.
2.5 The lower Bilate Suc-3asin
This sub-basin extends from Blatentena up to Lake Margarita (Abaya). According to the British report, the estimated irrigable area in this sub-basin is
7,780 ha. However, as could be seen from the table below, the estimated irrigable land in this report is slightly more than that in tne British report.
/- 7 -
Area of Irrigable Land ir me Lower Dilate
1 ■
Name of
Concessioner
or Owner
[- n
Total
Area
(ha)
Area
culti vated 
(ha
Area
Irri
gated
fna)
Estimated
Irr. Area
by 
British 
(ha)
Estimated Irr. Area
ty 
NW7C/E0 
(ha)
Fanayotis
Colonel 3olonion Gen. Kebedc Worku
| Tobacco Monopoly ; Oilate Agri-
cultural Estate
Abaya Agri-
cultural .Estate
Ministry of
Interior
Unoccupied Land
80
600
840
1400
2120
1000
1000
2000
30
140
240
20
1300
450
-
40
-
20
1300
450
-
-
-
-
SO
300
400
1000
2000
1000
1000
2000
80
500
700
1200
2000
1000
1000
2000
i
T otais:
9060
2230
1810
7780
8480
Under rhe existin  circumstances, it is very difficult to jive an absolutely correct figure of the irri able area, since seme pert of the land is very inaccessible and quite bushy, esp-.cially the ar sanear Lake Abaya, The figures estj .aie by fv^C/EC1 i., the above table ar? considered as minimum.  ftcr all the pr-cer.tly forested land is cl ared, it is estimated V at up to 
10,000 ha. of land ccud be irri ted in the Lower Bllaie sub-basl.-. This fij was a basis for calc.l -
of wat i r*- dir nts in the Lower Pilate sub-bat- 8 -
3.    HYDROMETEOROLOGY
The existing hydrometeorological data m the Bilate
River Basin is very poor.
3.1      Meteorology
From the Table on page 9, it can be seen that
there are only 7 meteorological stations in the basin. However, for correlation purposes
additional neighbouring " stations outside the Bilate catchment were considered. The annual rainfall in the basin ranges from about 120C mm. in the highlands to about 600 mm. in the lowlands near Lake Abaya. However, due to meagre and incomplete rainfall data, it was impossible to determine the total annual rainfall of the basin.
Temperature, evaporation and other meteorological parameters are given in the annexes. Basically, mean temperature and evaporation increase south ward of the b^sin, while rainfall decreases.
3.2      Hydrology
There are only two hydrological stations in the basin. One is at Alaba Colito bridge which has an automatic water level recorder and a cableway where flow measurements are conducted. The other station at Blatentena has only staff gage. There are three years of flow data at Alaba Colito Station which is a very short period of time for the study. But,
the British team estimated on the base of long term rainfall records within the catchment that these three years belong tc the long-term mean for both monthly distribution and total annual rainfall. Therefore, it is felt that c .Iculation
• • • rSUMMARY OF METEOROLOGICAL GAUGING STATIONS
9
I
T
DESCRIPTION OF DISTANCE OF STATION
STATION                  FROM NEARBY TOWN
T
I
1
ST
N8
i
NAME OF STATION
DATE OF
INSTALLATION
Mar
I, 1953
OPERATING
ORGANIZATION
C.A A
NWRC
i
I
Feb
5, 1972
DURATION OF
RECORD
(Years)
16
(1953-1968)
2
(1972-1973)
I
GEOGRAPHICAL
LOCATION
I REMARKS
1
(a) HOSSAINA
(b) HOSSAINA
TYPE OF
STATION
(Class)
3 rd
3 rd
Elevation
ASI
(m)
2290
07* 32 
37*50 
07*32 
37* 51 
N
E
N
E
LOCATION
At   the
S.I.M
At ths Kembota
Mission
OR VILLAGE
j246Kms. from Addis Ababo
235Kms from Addis Abobo
Synod
2
| (a) ALABA COLITO
3 rd
Jan        1966
7
(1966-1972)
C A.A
.350
07*22’
38*06’
N
E
At
S.
I
’ 3l9Kms. from Addis Ababa .
j _ _ ----------
(b) ALABA COLITO -A"
gnd
Mar 6, 1973
I
N.W R C.
07*22 
38*06 
N
E
  3!5Kmt.from Addis Ababa j
(1973)
1_ ----------------------------
I
CAA
06*41 
38*01 
N
E
At the Bilate
BiLATE AGR ESTATES
3 rd
Mar 1971
(1972)
Agr. Estate
; 50Kms from Soddo
4
SHONE
5 BlLATEN TENA tA"
Fob 7, 1973
1
Jon. 29,1973
6
f
ABONSA
7         SAKI 8         ABELA
- • T
I
■A- -A
*
*
*
4th
4th
3 rd
3 rd
3 rd
3 rd
3 rd
41h
Evap   Pan   8
Raingauge
4th
4th
3 rd
Fob 1,1965
I
9
(a) WOLLAMO SODO
(b) WOLLAMO SODO
Doc 6,1969
10         YIRBA
II         WEST AWASSA 12        AJE
13        WULBAREGH
*
4
*
*
I
(1973)
I
(1     9   973)
(1965-1973)
9
(1965-1973)
3
(1971-1973)
14
(1954-1972)
3
(1970—1972)
I
(1973)
I
(1973)
I
(1972)
2
(1972-1973)
6
(1966-1971)
NWRC
07*20’
37*57 
NWRC
CAA
CAA
CA    A
CA    A
N.W   R  C
NWRC
N  W R   C.
NWRC
N.W R C
A.A R
i
N
E
N
E
25Kms. from Aioba Cohtc J
  68Kms. from Awassa
>
At the
Police Station
At the
Police Station
2000
I
1500
1540
2060
2050
1
07*08’
37* 59 
06*53 
38*00 
06*42 
37*56 
06*52 
3‘,O44’
06*56 
38*17 
07*10’
38*26 
07*15’
38*21 
07*42 
38*12 
07*03 
38*29*
N
E
N
E
N
E
N
E
N
E
N
E
N
E
N
E
N
E
N
E
In the
Adventist Mission
, 30Kmi from Alaba Collto
t
L..
In the
Adventist Mission
At    the
N CD
At the
S.I.M
I
30 Kms.from Aioba Collto
— j
390Kim from Addis Ababa
■
At
Police
At
i- Police
i
the
Station
  390Km» from
Addis Ababa
the
Station
I
AOKms from Awassa
J
I
May 30,1973
Feb 17, 1972
Fob. 3,1972
Jan 1,1966
At Eshstu s
Farm
At the
Police Station
Furfuru River
Bridge
At    the
AAR
:i8Xme from
33KiM f 0"»
I
5TKIM.
AwGSSO
Shoshomono
i
At
6utaj.ro
*
*
On t‘ e way
H>ss<y’na
14
(a) AWASSA
<760
gffK** Add!* Abdt^
••
(b) AWASSA
*
gnd
Jan 1,1972
2
(1972-1973)
i
AAR
2 tTKiM f^
1
07*03 
38*29 
N
E
At the
. Add-s Abeba
AAR
Or the wav
Alaba Cchto
LEGEND
LEGEND
■*                  Stations outside Bilate Basin
N.W.R C.
AAR
C. A A .
N C D.
Civil Aviation Administration
National Community Development
National Water Resources Commission
Awassa Agricultural Research
Sudan interior Mission10
of storages and power productivity based on these three years records are sufficient for this study.
Dilate is a perennial river. The two manor
tributarie-s upstream of Boyo Swamp (.tfeira and Cuder) river are also perennial. Downstream of Blatentena, there are numerous hot spr- lgs hich contribute a significant quantity of the Dilate
River flow, especially during the dry season.
During the rainy season there are many dry creeks which add a considerable amount cf flew to the main stream. Bas.i on the three years data, the minimum flow at Alaba Colitc was about 135 lit./sec. in April, 1973- Particularly prolonged low flow occured in the dry season of 1972-1973. The
return period of this dry periods was estimated 
by the British team to be once in 30 years.
the
As shown inATable on page 11, long-term annual flow of Dilate River at Alaba Colito is 351.20 X 10^
cu.m. The run-off at various dam sites was obtained by drainage area orcDort on. Due to surface and sub-surface geology, vegetation cover, end land use conditions described previously,
there is a big run-off erosion within the catchment. During high-flow, there is a lot of sediment trans port in the river, particularly in suspension. The sediment samples were taken at Ztlaba Colitc Bridge and were ar.alised in the I-  RC/DO laboratory. Since the large portion of tne sediment consists of colloidal particles in suspension, which require
a very long time to settle, it is reasonable tc assume that a greater portion of these suspended solids will not have the chance settle in the
r< s-rv^irs. As regards to the chemical quality of rhe water, several electrical conductivity testsMean and Total Monthly and Annual Flows of
Dilate River at Al ata Colito
V. ar
7
I
Jon.
Feb.
Mar.
apr.
May
Jun.
Jul.
Aug.
iCD.
I
Oct. • Nov.
I
DeC •
Total
1G^ cu.m,
Avjragj fl gw
cu.m,/sc?.
0. >8
0.46
^971
1.41
1.65
•■♦
14.33
24.a6
34.30
91.85
»3.78
20 68
5 • 61
1.14
12.96
Total tl?w
10^cu,n..
•.55
1.11
3.78
4.26
14.53
66.86
113.48
55.38
1h.55
3.86
408.56
1972
z.vor  ,ff flow
cu.m./se3.
0.71
3.20
7.63
37.15 o.77
14.12
35.96
34.81
14.11
2.19
0.58
10.<>8
Total flow
10& cu.m.
1.91
7.99
7.60
?9.O2
’0.43
12.37
37.83
96.51
90.22
37.79
5.68
1.54
340.69
Average flow
cu.m./sec■
0.49
0.28
0.20
0.21
2.11
1.80
7.40
24.09
44.77
24.57
5.46
1.05
9.41
1973
Total flaw
10° cu.m.
1.31
0.67
0.54
0.54
5.65
4.67
19.83
64.51
116.03
65.81
14.16
2.82
296.54
L-------
Average Annual Flow for 1971-1973*. 351.20 X 1o‘? cu.m or
11.2 cu.m./sec- i* -
wct£ performed by the British team and Tobacco Monopoly. There was some discrepancy in the results of me two org.r.xoaU.^s, however, tabl.s cf both results are presented in this report. It
is not very important for our purpose which of these two tables is more accurate because if any 
t
of the proposed storages is built, concentration 
cf minerals during 1c* flow periods will be much lower than at present, since mineral concentration will be diluted by stored water- Therefore, after the storage project is built wate^ could be considered as convenient for irrigation through out the year.13 -
4. o e c i o g y
4. 1 General Lasir. lo
In many areas the relief of the basin consists of several small narrow ridges separated by narrow valleys, because the main river and the tributary streams in the drainage basin have caused severe terrain crcsicn.
The origin of the Bilate River, the main river
within the catchment is from the Gurage Mountain land region and is mainly from the eastern and southern part of the Mugo Mountain which is about 3,600 m. in altitude. The- two major tributaries
of the Bilate River a~e the Guder River which 
rises near Hossana and flows east into the Boyo Swamp and the   eira Liver, which rises in the
north and flows south to join the Guder River at the outlet of the Boyo Swamp. From Boyo Swamp, The Bilate Riv^-r flows southward into the Oolite Plain, with steep gradient ever an ignimbrite bed, with a number of minor waterfalls carrying high erosion of top soil.
Along the bank of the river, the rocks are very 
well exposed and almost fiesh except along the ridges where most of these are highly weathered and altered. Along the cultivated areas, it is very 
hard to find fresh outcrops.
In the Boyo Swamp, the soil is dark brewn in colour, and medium to fine grained in texture.
It is strongly mottled with weath red volcanic ash, numice and ignimbrite.  The soil of Cclit. Plain is more or l.ss similar to th-   sc l in Royc Swamp rut much li, nt.: in texture and xS nor * -r less sandy !■ v..14 -
The whole bilate Basin is situated in the main Ethiopian Rift Valley, an area to a great extent
net geologically wdl kfuwn. The Rift Valley area has been affected by volcanism probably since Pliocene, and the mosx dominant rocks consist of ignimbrite, interbedded tuff and oumice. According to D. Paola s report, the Ethiopian Rift Valley is
one of the most important geological structure in East Africa and is marked by a NNE-SSW normal faults, with rift in rif- structures, assymc-try 
and open tensional fissures forming its most important tec conic features. A number of steep faults produce a total difference of altitude of
more than 1,000 m. between the top of the plateau and the floor of the rift valley. A number of
normal faults of a TJE-SW trend occur along the Boyo Swamp and south of Alaba Colito Town. The widespread ignimbrite units may possibly explain some important volcano-tectonic collapses which produce a large regional calderaa employed by fissure erruptions. However, according to D. Paola’s
report, fissure erruptions occured alternately in 
□any periods of the geological history of the Ethio pian ift Valley, with th. outcoming of individual volcnnr.es. T nli)fc those volcanoes standing .on the plateau which rc larger r. ^i. nr, these rising from th. rift fl r r t’ r ycur.g nnu s-oll, shoeing 
i considerable fumar li c vity .t irosent. C’cven 
2.:k s rr-v ly: Zwai, Lan / no, Shalln, Abintn, Awassa, ..boyo   tv’ Chamo   io ■ r* of t ct nic r vrlcm -
tectonic origin occur in t i? Ethiopian Rift Valley floor (slwwn in drawi.nr r.u-b r----).
g . a » tri t ic;i Las f * t cn d^re on the existing ignimbrite outcrops of the area.
T
••>•c/15 -
Potential water regulation and power project sites
in the Bilate River Basin were selected and the geoljgical d^«Cx-pticn based ?n surface geological investigation and aerial photography of these sites
arc hereby discussed below:-
4.1.1 Boyo Swamp Par. Site Geology
Bc.yo Swamp is situated 26 kms. North-west of Colitc Town. A dam which will raise water level for about 5 m. has been proposed on the main stream about 800 m. downstream from we swamp outlet.
Bedrock consists mainly of very hard and massive ignimbrite. This i? a sound 
rock heavy and very hard to break even under the direct action of a hammer.
The reservoir area is marked by a NNE-SSW, NE-SW normal faults running parallel at almost 800-900 m. from the proposed dam site. Leakage of water from the reservoir
site is not nighly probable as the bedrock is less permeable. As a result,
careful detailed geological study of the area sup?  ■’minted by core drilling might well suggest as tc the exact nature cf the reservoir area.
Construction Materials
Impervious constructjon materials namely of silty clay, are available on the
slopes and ridges ne^r the proposed dam sit1 . Riprap and rock-fill can be
obtained Irm the interval? ey ignimbrite upstream or downstream from the proposed djm. site. This can aiso be crushed for
co.-icr tc aggregates ?r used f :r masrnory
1
.......... /- 16 -
1 <2 Alaba Colitc Dam Site Geology
It is situated on the Bilate River and is about 2 kuis. North- Zest of Colito Town. The site was surveyed and investigated 
as possible dam site for hydro-power production.
There exists a vertical waterfall, a few meters downstream from the proposed dam axis, where the water cascades about 12 m. from the top to the bottom of tnc silicic volcanic rock, mainly ignimbrite. A number of small rapids also prevail downstream.
Outcrops near the dam site mainly consist of fresh, hard and massive recent ignimbrite rock (probably Pliocene). It is bluishgrey to dark grey in colour and 
can not be broken easily with a hammer. Several columnar joints prevail within the axis of the dam, with rather remarkable horizontal and vertical fractures that
may nem oomc giculing.
Going upstream into the proposed reservoir site one can come across a green coloured ignimbrite which appears white to grey when weathered and altered. When fresh, the ignimbrite outcrop is extremely hard and very massive. It is a very good building materia] and seems very tight
and heavier than the one examined along the fall.
• • • • /17 -
No major or minor fault was observed along the dam axis cr along the reservoir, There will be no difficulty regarding foundation of dam 10-20 m. height.
nstruction Mat-rials
The site is geologically suited for either
a concrete mascnary or earth fill dam as relatively shallow or no excavation would be required to obtain a cutoff and foundation into the fresh ignimbrite rocks. Bedrock material in the vicinity of the dam site, namely ignimbrite is suitable for riprap, rock fill and masonary and for crushing of aggregates.
Impervious fill material is readily available from the silty clay overburden in the area of the dam and in the reservoir.
.Sand is also available along the main river itself and occurs at about 100-200 m. from the dam site.
^.1.3 Ropi Dam Site Geology
Ropi Dam site is situated on the main Bilate River and is almost 16 kms. south of Celite Town. The profile along the
dam axis is more cr less U tc 7 shaped rising to over 1700 m. a.s.l. on either side of the river bank. Outcrops mainly; consisting of compact tuff, pumicicus
tuff, ignimbrite and conglomerate are v_ry well exposed.
In th. lower portion cf the narrow gorge
"t th? d m sit
a
1  , the bedrock material is18 -
rather fresh and consists mainly: of a very hard, massive ignimbrite interbedded with 
a very compact tuli. Host of the fractures observe                 ar to be tight. The ignimbrite rock dees r.ot break quite easily and is a
very gjod building m
 • The inter
bedded tuff is rather hard but not tc much as the ignimbrite observed. At a still higher elevation on the river bank, at the dam site, ignimbrite outcrop occurs with joints sligntly open and may probably need some grouting. It is not clear as to 
weather this ignimbrite at the top is continuous downward but it is reasonable to assume that the top and bottom are of the same composition and are of the Same deposition though there exists no visible outcron which justifies the case. Core drilling, if performed at a later stage
of study might well give an explanation to this problem.
Above           ig-—10- _ v. - -it-rr p lies a rather compact tuff which is slightly porous ana nard «ut unlike the former
it breaks quite easily wten it becccee wet. It is grey in colour and very light in weight and the tendency to breakage increases wnen weathered.
big ly norous 3 nJ permeable nunicious tuff is also observed lying on tep of the cermet tuff below. It is highly weathered, soft and can easily be scratched with finger nail whon wet. A sample rf this
nor .us - id ncrmv-ablc numicious tuff when
• • • • /- 19 -
submerged in water will easily absorb water and disintegrate. This bed which lies almost horizontally at tne same elevation or. tne left and right bank of the river
bed may nc-ed extensive grouting. Conglo merate is seen exposed on the top of this pumicious tuff. It is more or less porous, slightly nerd and contains very course grained sand, intermingled with tuff, rhyolite and ignimbrite rocks. It seems
to be quite easily erodable under the- action of water.
Excessive leakage from the reservoir is not anticipated, but the permeability of the bedrock materials and the position of the ground water table should be explored before the site is found suitable for a storage reservoir. Foundation drilling is required to obtain the final design data. Construction Materials
Impervious fill material were not observed at the dam site. As a result silty clay 
would probably be hauled from an area of about 1-2 kms. upstream of the dam site to be used in the body of th.- dam. There are few small deposits ^f cemtnted gravel in the reservoir basin, but the quantity is toe small to m-..ct the total requirements of pervious embankment material or for natural aggr- gates. Instead, there exists hard and massive ignimbrite outcrops at about 2 kms. upstream from the dam site which could be used as a quarry. This can K used as a
rockfill, riprap, masonry
crushing for aggregates.
stone and fcr20
L
T
5. TEC ,JTC.-..L SCL 7Z \;S
5.1 Genera]
This    study    concentrated    only    on    the    Mid    and    Lower Bilate sub-basin due to the following reasons:-
-   The majority of irrigable lands are situated in 
the Lower Bilate Sub-Basin.
-   The possibility of building storage dams is in 
the Mid Bilate Sub-Basin.
- If storage dams are to be built in the Unner Bilate
Sub-Basin, they must be built on both tributaries
(Weira and Guder) in order to store sufficient
water. Besides, since the gradients of these two 
tributaries are very steep, high and costly earns
will be required to store the same amount of water
as in the main stream.
In     the     Mid     Bilate     Sub-Basin,     four     dam     sites     were identified along the Bilate  diver.
5.2 Water Requirement
As
In the table on 
. t. i c- r c will be about
10,000 ha. of irrigable land in Lower Bilate Suo-Besin.
A
With crop intensity of 130%. which is considered as a conservative figure, the total irrigable land is
13,000 ha., per year. The anticipated major crop is cotton and the estimated net water requirement for this crcn is 800 mm. from May to January. Adding 30% for field application and conveyance losses, the total water requirement is 1,100 mm.
Th * total rainfall in L >wer Bilate for the period May to January is about 400-500 mm. The monthly rainfall distribution is irregular, and since rainfall is not reliable, it was not considered in the estimation ?f water r.^ouir- ment.
• r ■» o21
T‘>     irrigate     all     the     irrigable     land     in     the     Lower     Bilatc, the annual water requirement is.
^,090  150 a i
100
or average annual flow of 4.50 cu.m./sec.
This amount cf water is available in Bilate River, but
its timely distribution is such that without storage reservoirs an intensive agriculture based on irrigation could not be achieved.
5.5 Boyo Swamp Dam Site
Bcyo Swamp situated about 26 kms. North-West of Alaba Colito is a flat plain with a total area of about
120 sq. kms. This plain is flooded every year, making some Portion of the nlain swampy. The area of the swamp varies between 5.5 sq. kms. (permanent open water surface) to 50 sq. kms. during the high flows.
Downstream from the Boyo Swamp outlet, the Bilate River flows through a valley with rocky river bed and banks. There are several good dam sites in this stretch of the river. However, one dam site 800 m. downstream from the swamp outlet was selected.
The drainage area at Bcyo Swamp dam site is 1,805 sq. kms. while at Alaba Colito Bridge is 2,620 sq. kms. The average annual flow at Alaba Colito Bridge for three years period (1971-1975) is 1^.2 cu.m./sec. Total flow regulation for these three years could be obtained by a storage volume of 226 X 10^ cu.m. Hence, the average flow nt Boy? Swamp d m site is:-
••••/22
Since the flow regime at Boyo Swamp Outlet is the same as at Alaoa Coliro, rhe required storage for total flow regulation at Boyo Swamp for the period oj these three years is.-
6
6
1,305 x 226 x 10  = 156 x 10
cu.m.
2620
For normal water level elevation 1895-80 m. in Boyo Swamp reservoir, the corresponding total storage is--
184 x 10 cu.m. Out of this total storage, 24 x 10°
cu.m, is allocated for siltation.
Based on field observation and information gathered from the natives, the Boyo Swamp is flooded from July to February reaching its highest elevation of about
1891.5 m. a.s.l. towards the end of August, and minimum elevation of about 1889.4 m. a.s.l. towards March,
Based on the existing evaporation data, the annual evaporation in Boyo reservoir is estimated to 1,800 mm.
By deducting annual rainfall of 900 i.im. the net
evaooraiion less is 900 mm./year, which has to be
applied on the difference between areas created by rhe dam (63 sq. kms.) and that of natural swamp^(_41 sq. kms.). The total annual evaporation loss is 0.9 (63-41) x 10^ =
= 20 x 10^ cu.m. Therefore, the total net annual flow is
(2^2 - 20) x 10  = 222 x 10^ cu.m or 7.0 cu.m./sec., wnic:
is more- than sufficient to satisfy the irrigation demand 
in the Lower Bilate.
f.3.1. Th. Dam
The Boyo .Swamp Dam is a concrete diversion weir wit’.- three redial gates (3.50m x 17.00m.).
The normal water level is 5.25 m. above the existing low water level in the river. This
6
• • • • /- 23
dam should be built in such a way that it
could be raised in the future. For this
reason the radial gates will be of bascular
type so that later on only the mobile parts should oc adaea.
?. 3.2. The Spillway design _Fiood
The three years i.eriod of flow measurement was not sufficient to determine the maximum flood by any statistical method. The maximum recorded flood at Alaba Colito during the three years record was 54 cu.m./sec. Using Creagers data for unusual flood discharges
for drainage area of 1,305 so. kns., the maximum possible flood discharge is more than 2,j00 cu.m./sec. Flood wave of this magnitude could give a total amount of flow of 300-400 x 10 cu.m., which is equal to the total annual flow at Boyo, therefore,.such flood is considered as improbable and it is rejected. This figure looks also excessive
for spillway design, especially when taking 
the big rniti ating effect of the reservoir
into consideration, which can impound additional storage of about 68 x 10^ cu.m, for cne meter increase in water level elevation in the reservoir a.ove normal water level. Thus, for il.W.L. elevation 1,893.80 m. the assumed spillway design flood is 600 m^/sec., for 51 m. length of spillway. However, if the water level reaches elevation of 1,895.5 m. the corresponding spillwa;  discharge is ab ut
1,3^0 cu.m./sec. In the feasibility study,
*he alternative with smaller spillway crest length and smaller discharge capacity should be Investigated as a cheaper solution but making- 24 -
some part of the land above 1,895*00 
be
flooded once in 10 - 20 years.
5.3.3 Power Plant
Since the slope of th- river for about 9 kms.
downstream cf Boyo Swamp is quite steep,
(7.5 /oo)^it is possiole to Droduce Hydro
electric power, by conducting the water throu .
an open channel cut on the right bank of the
river. Thus, two inlet slide gates (3.7 m.
X 2.4 a.) will be provided on the right
abutment of the diversion weir.
The capacity of the headrace canal is 14 cu.m,
sec. which is about twice the average net
annual flow. Two vertical Francis turbines,
each with capacity of 7 cu.m./sec. will be
installed with both of them working only 
during peak electric demand periods.
At the downstream end of the canal, there
will be a forebay with a gate leading through 
the 1.5 m. penstock to the power riant. The
power plant will have a net head of 51.5 m.
Hence, the installed capacity of this power
plant is 2 X 2,900 kw. = 5,800 kw. with an 
annual production of about 18.9 X 10^ kwh.
5.4 
Alaba Colito Diversion Dam and Power Plant
The dam will be concrete or masonary dam about 18.5 m.
high, with ogee type spillway. This dam is located 
just upstream of the water falls near Alaba Colito on 
hard rock nvc-r bed.
wat?r will be conducted through 2.0 m. diameter
reinforced concrete oipe, with inl^t gates at its
upstream end, leading water through the steel penstock 
of 1.5 m. j diameter to the power plant.
• • • • /- 25 -
The net head of this newer plant is about 30 m. with two vertical shaft Francis turbines for a discharge
of 10.5 nP/sec. per turbine. The total installed 
nower is 2 X 2,500 = 5,000 kw, Annual production of electricity will be about 16.2 X 10^ kwh.
• 5 Ropi Dan and Power Plant
This dam is the second alternative solution to store water for irrigating the land in the- Lower Bilatc
Valley. It is situated in a valley near Ropi Village about 5 kms. upstream from Roni Water Falls. The topography of the dam site is quite suitable for building rock cr earth-fill dam. However, geological conditions of the site are net promising and will
create many problems and expenses during construction in the- foundation of the dam and other pertinent structures.
The drainage area at Ropi dam site is 3,060 sq. kms.
/ /
The average annual flow at the
5,060 X i1.2 2620 
cr total annual flow 413 X 10
dam site is:
= 13.1 cu.m./sec. cu.m. v
Assuming net evaporation of 1,200 mm./year, the total annual evaporation loss is 1.2 X 7 X 10 
= 8.4 X 106 cu.m.
Therefore, the total net annual flew is 413 - 3.4 = 404.6 X 10 cu.m, or 12.8 cu.m./sec. The required storage for full regulation is: ,
»
3,060 X 226 X IO
6
------------------------- = 265 X 10 cu.m.
„ _~6
2062
Unfortunately, the topography of the dam d^es not all? to achieve the above mentioned storage. The maximum possi’d? height of the f> rescen dam will allow only
176 X 10^ "cu.m, with M.W.L. at 1,703 m.s.s.l., and- 26 -
2 m. free board. Allowing 23 X 10^ cu.m, for siltaticn, the net storage is: 176 - 23=153X10^ cu.m.
In this study the foreseen iam is rockfill type with 
7
maximum height of 63 m., having an upstream face made
of reinforced concrete or asphalt concrete impermeable membrane, supported bv dry rubble cushion. A re
inforced concrete drainage gallery will be constructed alcng the upstream periphery of the dam, which will
serve tc drain percolating water and to execute
grouting curtain. During construction, water will be diverted through the 2.5 m. diameter C4inforced 
concrete cut and cover conduit, built on the left bank 
cf the valley. The construction area of the dam site
will be kept dry by diverting the river through a
diversion pine and building upstream and downstream coffer dams. The upstream coffer dam will be later incorporated in the body of the dam as an upstream toe.
5.5.1  Spillway Design Flood
Regarding the spillway design flood, every thing what was said for Boyo Swamp is also applicable- here. By raising the reservoir water level for two meters eoove N.W.L.,
rhe reservoir can impound additional 25 X 10 cu.m, and the corresponding spillway capacity will be 1,400 cu.m./sec. Therefore, the spillway capacity at M.W.L. and spillway crest at 1,699.5 m. is 700 cu.m./sec. The crest length will bo 51 m. The spillway will be located on left nd of the dam embankment.
From tne snillway wat r will flow over the chut-, intc the steelin  pool which will be constructed in th  river bed. It will be an
• type srillwnv with 3 radial gates (17.CM 3.5 m.) which will kc-^p the at - !■ v* t ion 1 ,703 m.
c
.
• • •»,- 27 -
5.5.2       Fewer PlF;nr
The dam being 63 m. high, a hydroelectric power plant could be constructed downstream of the dam using the stored water in rhe reservoir. The race conduit will be
2.5   m. diameter, and 140 m. long reinforced concrete cut and cover pipe. The penstock will be a steel pipe 2 ci. diameter and 65 m. long, rxt the upstream end of the penstock 
t ere will be an automatic valve chamber. Since the headrace conduit is short, it will not be necessary to have a surge tank.
The net head in the power plant is 46 m. and installed capacity is 2 X 4,750 = 9,500 kw. Annual electricity prod ct.cn is about
31.0 X -IO6 = kwh.
5.6  \ ~ 7.1 .. ?• r-.r r-11.7    _• r 1
The foreseen 9 m. high diversion dam will be located on the rocky river bed Just upstream of the Small Rcpi Water Falls. The main storage will be provided 
either by the Roni reservoir or by the Boyo Swamp reservoir. If Boyo Swamp is selected as a main storage reservoir, a small dam should be built in this reach of the river, in  rder tc provide at least
d.ily regulation. This could be achieved easily 
since there are several good dam sites for small dams and with rocky riv-r bed on this stretch of the river.
The headrace c nduit will be an open channel with the forebay  t its dcwr.strtnm .nd, followed by penstock 
n
leading f the cower plant located on the right bank f th* riv r.
Tn- ■ -t l> i f the ^ow.r riant is nb ut 5^ m. Th. ir.-talled :ai -itv is 2 X 5,500 = 11,000 kw. wnual - r d;?ti r is ab^ut X 10  kwh.28 -
5.7     El?ctricity Demand
The Dossible hydroelectric potential of the Mid Bilatt. Sub-Basin is about 102 X 10 kwh/year, However, since Rcpi power plant and dam are net recommended for construction in the near future, due to the low benefit cost ratio about 71 X 10  kwh/year can be produced 
c
cheaply from the other three power plants only.
The produced electricity if transmitted within the radius of about 100 kms. from the power plants, the following towns and rural areas could be supplied with electricity:
Hossana Wclamo Sodo Arba Minch Alaba Collto Yirgalc-m iVondo
- 10,000 Population
-   15,000
9,000
5,000
18,000
7,000
•i
it
•i
ii
Total:              - §?xQQQ
The Lower Bilati Valley Farms including Tobacco Monopoly Factory, Farms around Arba Mincn, WAP’. Farms and many others which will be developed in the near future.
thp pst.imated lectric demand for all these towns end
itopi Vater Falls Hydro Power Plant alone could satisfy-A present demand and if n- cessary Hoyo Swamp Hydro Power Plant could b: built as * next one.
9.8      Quality of Water l^r Irrigation
Electrical c^r.< activity of Bilato i-.iver water samples was conducted by T .baccn p’?n poly and the Eritish Tear
at various 1 ces in tl
■<!-*■ Sub-Basin. The
^c-sults .r 7- wn in the +wo tables on the next page
••••- 29 -
Electrical Conductivity of Dilate River in 3C X 106 at 25°C
(performed by Tobacco Monrooly)
Date
Unstream of Panayotis
Farm
Panayctis Mineral
Spring
Blaten-
tena
Bridge
Tobacco Monopoly 
Farm
Nev.       16, 1973
Dec.         5, 1973
Dec.       26, 1973
Jan.       12, 1974
March 12,1974
May 18, 19^4
150
-
400
430
-
240
900
-
930
885
-
900
185
-
500
530
4J0
210
194
270
530
570
460
340
i
Electrical conductivity cf the Dilate River and tributaries at selected sites, March,
1973 (oerf rmed by British Team)
Site
Electrical
Conductivity fT!h? > r cm)
Water
Temperature
CO
Wc-ira Tributary
nlaba Oolite
Pilate Tena
Tobacco Monopoly Dilate Agricultural Estate Bisare Tributary
Abaya Agricultural Estate
170
200
900
960
1000
1100
2100
23
24
29
29
32
31
32
Fr-m then t.nbl.s, it is quit-.. clear th^t during lew flew mineral concentration is high vnd it injurious and uno?tirf-»ct ry f-r irrir + i n uses xc-rt frr salt tolr rant plants. ’’ w v rf the rirh erne ntr tirn < f
/30 -
minerals during the dry season can be reduced by building storage dams, in which case tne water will
be very good for irrigation all around the year.
5.9      3-di-  -nr           , n
Me regular sediment samples were made but several samples were taken from the main stream at Alaba Colito bridge during high and low flow periods. Laboratory analysis of these samples showed that the amount of suspended solids in saturated and unconso lidated state is between 2-6% of the volume cf flow. However, the amounts cf dry and consolidated sediments is 1-3 /oo of the volume of flow.
Based on this figure, and taking into account bed-load sediment which is not measured the annual sediment volume in Boyo reservoir will be 300,000 cu.m, and in Ropi Dam, 600,000 cu.m, which means tnat dead storage of the Boyo reservoir will be completely filled with sediment in 70 years, while Rcpi reservoir will be
filled in 40 years.
However, as the major portion of the sediment consists of suspended colloidal particles which need several months for settling, it is expected that a
large pertion of the suspended colloidal particles
will be discharged when water is released for irrigation and power purposes. Besides, the greater portion of the sediment will be discharged over the spillways during high flews. For this reason the
above mentioned life of the two reservoirs will be lentnr thou that estimated above. The more precise life of th.- reservrirs should be estimated by conductin’ accurate sediment me. .a sun me nt before and after
c nr*ruction f th-  dams.
• • • • /51 -
• ?-°-ss t i  nate and Econo- ic dy
6.1 Jethod ol_ "ale ill at ion
6.1.1        Projects "a-ital Costs
Capital cost was calculated for storage dams anu nydro-electric power plants separately. The capital costs of hydropower plants includes all pertinent structures as well as diversion da:.is access roads, living centers, air
t
conditions, step-up transformers, switchyards etc.
6.1.2         tn.mal C osts
Annual cost consists of
-   depreciation and obscolescence
-   Co.t of money
-   ”>xes and insurances
-   Operation and maintenance
epreciation and Obscolescer.ce is calculated by "Sinking-Fund Method’’ proposed by W. Creager and I. Justin. In this mete cd, each year a sum will be paid into a r serve fund such *hat at
the end of t :e useful life of an item the reserve will be sufficient in size to replace it or its equivalent in usefulness. Useful life of structures ani equiments used in this renort
is as proposed by the above mentioned authors except for those structures whose estimated useful life was indefinite. In this study the
useful life of such structures was taken to be
:n .ears, which ives a conservative estimate
Tor t e annual cost -.
-   depreciation and C-bscclc-sconce values are based at - ’ annual interest rate.32 -
- Cost of money was calculated at 5.5^ of the
total capital cost.
- Taxes and Insurances were  estimated at 0.5/ of *he total capital cost.
- Operation and Maintenance       was calculated as follows;
- for storage dams at  0.5/$ of the total capital cost
- for hydroelectric power plants at
2 Eth# per installed                 KW.
Since water stored in storage reservoirs will serve
for irrigation and power purposes, of storage dams was distributed as 20/ for power
80^ for irrigation
6.2 Capital Cost Estimate
6 2.x        3oyo Swamp Dam
the annual cos: follows:-
Item
--
~th.$
Access Road and Living Center
300,000
Dam
310,000
Fridge
195,000
Qadial Gates
570,000
Mobile Crane & Accessories
150,000
Grouting
200,000
Land expropriation
300.000
Sum:
2,025,000
Investigation 1%
20,250
Contingencies 5^
101,250,
Sut>ervisirn & Consultation 8/
162,000
Total Capital Cost:
2x208^00-
I33 -
6.2.2        Bcyo Swamp Hydro Power Flant
Item
access      load     and     Living     Center Headrace conduit
Pc ns toer.
Power house
Turbines with Accessories
Eth.
340.000 
1,100,000
100,^00
700,900
450,300 
Generators
with 
Accessories
Air Condition
Transformers Switchyard
Sum:
Contingencies 5/
430,000
60,000
150,000
400,000 
3,730,000
136,500
Supervision       and       Consultation       8£ Total Capital Cost;
6.2.3        Alaba Colito Hydro Power Flant
Item
Access Road and Living Center Dam
Intake
Headrace conduit
F r.stock
power House
Turbines with Accessories Generators with Accessories
ir Condition
Transform rs
Switchyard
ur.;: Contingencies Y
uu.rvision and Consultation 9/ Total Capital Cost:
293,400 ^214^200
Eth. t
100,000 
2,500,000
40,000
400,000
100,000
500,000
400,000
330,000
50,000
140,000
360,000 
4,9 0,000
243,500
397,600
2x=1^iqo- 34 -
6.2.4 R pi Dam
I rem
Sth. 3
Access Road and Livi
ng Center
450,000
Water Diversion
2,710,000
Dam
14,480,000
Spillway
1,661,500
Grouting
750,000
Land Expropriation
___ 25,006
Sum:
20,076,500
Investigation 1%
200,765
Auscultation 
5$
100,363
Contingencies 5%
Supervision and Consultation 5%
1, 003,825
1.003.825
Total Capital Cost:                       ^i2«2x228 6.2.5 Reni Hydr.? nc>wcr Fl^.t
Item
Eth. 3
Access Road and Living Center
250,000
Intake
200,000
Headrace conduit
1,100,000
Penstock
120,000
Gates and Valves
700,000
Pcwar House
1,700,000
Turbines with Accessories
800,000
Generators with Accessories
730,000
Air Condition
100,006
Transformers
250,000
Switch yard
660,000
Contingencies 5a
Sum:                     6,6.30,000 331,500
Supervision ar.d Consultation 8%
T- tr.] C -rit-»l Oft:                       7XZ
= - = = = if:= =
530,400
91.900
• • • •6.2.6

Note               As the capital for both Alaba Clllo and Ropi Water Faile Hydro Electric Power Plante are very coiner votive, It is ejected thot attar detailed investigation* and studies
the cost of produced electricity will be cheaper than shown in this fable
4^- 45 -
6.5 Proposed Solution
6.5.1. AV? ter Regulation
From the table on the previous cage it could be
seen that the cost of water stored by Boyo Swamp Dam is 10 times cheaper than that stored by Ropi
bou-/- ~r
Dam. W»>,000 ha. of Boyo Swamp land will be flooded when Bcyo Swamn Dam Is built. This land is already marshy and is used for cattle grazing only few months yearly, while a negligible
portion of it is used for agriculture. Once the
lake is created, when the water retreats land 
along the rim of the reservoir could also be used for cattle grazing as well as for agriculture,
as it is the case with Koka Lake shores. The s il which is presently saline, will improve its
quality by the existance of fresh silt which will
be brought to the reservoir during the flood flows. Since Boyo Swamp storage reservoir is far away from the irrigable land in Lower Bilace Valley, it
is anticipated that some water will be wasted du<_ to the reservoir operation- Ir order to mirimizv 
this wastage, daily or weekly regulation dam at Ropi Water Falls er another convenient place should be built. ^nether very attractive advantage to 
build Boyo Swamn Dam is the long life of its reservoir. Even when the reservoir becomes silt therequired net storage could be regained cheaply by very small increase in the dam height.
Due to all the above mentioned reasons, it is recommended to regulate Bilate River by construc ting Boyo Swamp St.rate Dam.
• • • • /46
. r.2           -l?ctrcity
Hosana, Sodo, Yirgalcm, aw?ssa, Alaba Cclito and other villages and farms in this region are
getting electricity from diesel generators.
i bacco her. :;v ly, uses diesel oil for the
irrigation pumps, Tobacco factory, as well as
for lighting th< factory and living centers.
This diesel energy costs at lorst h—5 Zth.(Z/kwh.
^EdLFx/S figure before the increase of oil price).
The cost f hydroelectric energy produced from
oilate River including the costs of transmission lines ~nd step-down transformers will be much cheaper than the cost of diesel energy. However, Ropi Hydroelectric Fewer Plant is not recommended for construction since it requires very costly 
dam.
Rcpi hater Falls Hydr Power Plant is recommended for immediate implementation since it is the cheapest one (0.012 Eth.^/kwh). The next cheap- st is the Boyo Swamp Hydro electric Power Plant
(0.020 Etn.^/kwh), which is recommended t~ be constructed when th. c no ur.ption of cl<-c tricity approaches the production nf Roni ’Vater Falls Hydr • Power Plant. The third one is the /.la ba
C lito Hy-’ro Power Plant (0.026 Eth.g/kwh), which should be built only after more detailed 
inv-srigati~r.s nJ studies snow that it could b clrwr than c loul^ted in this study.
In this study, th cost of pr~duc^d energy fr m the nt  Vu three .-.ydro electric pc’Ar slants is bos< d on present   rices. Tao cost cf produced 
nrrry tr m tl* xiotirf hvdr^ power ^l~nts in Et-.V • L  , t-- J n t  rices d< . Lop t. ir
e r. t
i n is s t J 1 ws: —
••••U7 -
Koka

When comparin ; the above figures of the cost of  ■roduced energy from the existing hydro power
plants with tl.at ci the proposed Bilate Hydro Power Plants it is obvious tnat the cost of energy from Pilate River is cheaper than that from Wash and nearly equal to that of Pinchaa. However, if the cost of energy from the existing power plants is adjusted to the present prices the cost of energy from Bilate River might be even cheaoer than that from Finchaa.L© -
7• Cc nc 1 usins sii. l bY
• t i ms
7.1 Water Regulation
Further irrigati:n development of Lower Bilate and eventually Mid Bilate Valley could be humpered only by the shortage of water. Climatic and soil conditions ar
se favourable that this development should be encourages since it could contribute a significant amount to the national economy. By gr .wing fluejcured Virginia tobacco cn the Tobacco Monopoly farm, about 8,5 million Eth. $ could be saved every year in foreign currency. Bilate agricultural Estate is already exporting vegetables and 
fr its to Europe bringing foreign currency to Ethiopia. Similar benefits cculd be obtained from other farms which will be growing cotton, harricot beans and other cash crons.
There is sufficient water in Bilate River to irrigate
farms in the Lower and Mid Bilate Valley. However, the unequal timely distribution cf water is such that without regulation very small portion jf the available land could be irrigated. Farms in the Lower and Mid Bilate Valley end around Arba Minch on Hare and Sile rivers as well as future farms on the Gallana River and others will reduce the inflow into Lakes Margeritta (Abaya), Chamo and Stefanie (Chew Bahr). It is true that some part of the Bilate water will be lost by evaporation and consumptive use Of irrigated plants, which will have influence on th regime f the ab’ve mentioned lak.s. However, this
bolds true not nly with the irrigation development in the Bilot V llc-y, but with all  ther developments within the basins of the above mentioned three lakes as well os -n any river basin. Very r "ugh calculation sh-ws that
,J uc- to th< irrigation tvcl nt of th. Mid and Low< r - il t. sub-basins ns ar ^ Si i in this study, the av r^ level of Lake Morrorittn will he reduced by 
car.-
maintain the same r- Auction in water level f Lake Chamc, very small or no regulation werk will be needed 
on the creek connecting both lakes. This reduction in 
water level is of less importance for Lake Stefanie,
because this Lake is not perennial. As found by the British Team, it is only since 1966 (in living memory)
that Lake Charao has overflowed consitently into the
Sagan, and there can be no guarantee for how long this condition will be maintained.
The- anticipated effect of implementing these developmental nlans *n the ecology of the region is beyond the scone f this study. The land resources study of the Rift Valley Lakes Zone financed by the British Government if continued, should give an answer to the ecological consequences which can result from the dev--lonment of this region. Since the ecological study is a very long-
term study due to the lack of basic data whose
collection can take several years, it is logical tc
conclude that development in Dilate River should not
wait until this long-term*study is ready, because it is
clear that this development is economically and technically feasible and since already a considerable investment is mode. All lot r developments could be implemented based on long-term study conclusions.
2 Electricity
Production r-f cheap electricity fr^m Bilnte River will facilitate rural development as well as the raising cf standards ?f living of the whole population in this
r -;ion. Tobacco Monopoly Factory will employ about 1,200 workers, and other farms will be one uroged to establish apr- —industry f r tneir own ^r*ducts with th<_- heir cf
ilo.blo cheap lectricity.- 50 -
-   Detailed studies of Bcyo Swamp storage dam and of Rcpi Vater Falls and Bcyo Swamn Hydroelectric Power Plants should be immc :i°tely continued.
-   After the mcessary consultant’s work is finished,
it is recommended to start construction of Boyo Swamp storage dam.
-   This dam c uld be completed before the rainy season of 1976, so that storting with September of 1976 
all existing and new farms c  uld have full use of water for irrigation.* It
HYDROMETEOROLOGICAL STATIONS
southern R,ft„Y.ALLEY basin
, b i-
*yI
.     • *GEOLOGICAL cross-St
AT ROP» DAM AXISWORK DESCRIPTION
19 7 5
Survey Work        I Detail Survey of Boyo Swamp Dam Silt
2 Additional Survey of Boyo Swamp area
3 Detail Survey of Ropi waler foil* HE P P. Site
4’
Investigation
Work
__.L G_ao,5>®*col mappingi of Boyo Swamp Reservoir B Dam S ts _2. Geological mapping of Ropi water falls Site
_ B. Core drilling for Boyo Swamp Dom Site
< Core drilling for Ropi water foils HE PP Site
5 investigation of
permeability for
6 Investigation of
soil B rocs for foundation 8
Boyo  Swamp  Dam,  Reservoir  B  H  E  P  P  Site construction materials at Boyo Swamp Doti
and Ropi water fall* HEPP Site*
Hydrometeoro
logical Work
I To continue flow measurement of Bilate River
2. To start Sediment flow measurement at Aiaba Coiita
3                   To_eetoblish river gauging Station at Boyo swamp outlet 4. To perform a series of flow measurements at Boyo swamp
5 To establish river gauging Station at Ropi water tali*
_6 To perform o series of flow measurements at Ropi wots- ta is 7   To establish meteorological Stations of different classes
8   To establish I*-1 class meteorological Station at Boyo swcmp
Design Work
I
Feasibility study of Boyo Swamp Dam
---- • j
t 1
1I
l
i__
—
Bidding and
Construction
2.  Feasibility study of Ropi water fobs H.E.P.P
3.  Detail design and contract documents tor Boyo Swamp Dom
4. Detail design and contract documents for Ropi water fails H t pp
I.   Tender and selection of contractor for Boyo Swamp Dam
2   Tendering and selection of contractor for Ropi water tain h E
&
Construction of Boyo Swamp Dam
4. Construction of Ropi water falls HEPP• .* •

Summery

Bilate River Basin Development Study Summary

Bilate River Basin Development Study - Summary

1. Introduction

The Bilate River Basin Study was initiated to address water shortages for irrigation in the Lower Bilate Valley, particularly for the Bilate Agricultural Estate (BAE). The study identified potential dam sites for water storage and hydropower generation along the Bilate River.

Key Objective: Identify dam and reservoir sites to control and store sufficient water for irrigation in the Lower Bilate sub-basin while considering other water uses.

2. Basin Description

The Bilate River Basin covers 7,463 sq.km in Ethiopias Shoa and Sidamo provinces, with elevations ranging from 1,285m to 3,600m. The basin is divided into three sub-basins:

Sub-Basin	Area	Characteristics
Upper Bilate	Upstream of Boyo Swamp	Steep gradients, unsuitable for large reservoirs
Mid Bilate	Boyo Swamp to Blatentena	4,500 ha potentially irrigable land
Lower Bilate	Blatentena to Lake Abaya	7,780-10,000 ha irrigable land

3. Hydrometeorology

Key findings about water resources:

Annual rainfall: 600mm (lowlands) to 1,200mm (highlands)
Average annual flow at Alaba Colito: 11.2 m³/s (351.2×10⁶ m³/year)
Minimum recorded flow: 135 l/s (April 1973)
Water quality issues during low flow periods (high mineral concentration)

4. Proposed Technical Solutions

4.1 Boyo Swamp Dam

Location: 800m downstream of Boyo Swamp outlet
Type: Concrete diversion weir with radial gates
Storage capacity: 184×10⁶ m³ (160×10⁶ m³ net after siltation)
Primary purpose: Irrigation water supply

4.2 Hydropower Plants

Plant	Capacity	Annual Production	Estimated Cost
Boyo Swamp	5,800 kW	18.9×10⁶ kWh	4.2 million Eth. $
Alaba Colito	5,000 kW	16.2×10⁶ kWh	5.0 million Eth. $
Ropi Water Falls	11,000 kW	36.0×10⁶ kWh	6.0 million Eth. $

5. Economic Analysis

Comparative costs of water storage and energy production:

Project	Water Storage Cost (Eth. $/m³)	Energy Cost (Eth. $/kWh)
Boyo Swamp	0.0116 (capital) 0.072 (annual)	0.020
Ropi Dam	0.116 (capital) 0.78 (annual)	0.026

Key Finding: Boyo Swamp storage is 10 times cheaper than Ropi Dam for water storage. Ropi Water Falls hydropower is the most economical for electricity generation.

6. Conclusions and Recommendations

Boyo Swamp Dam is the most feasible solution for irrigation water needs (technically and economically)
Ropi Water Falls Hydro Power Plant should be implemented first for electricity generation
Detailed studies should continue for Boyo Swamp dam and Ropi Water Falls power plant
Construction of Boyo Swamp dam could begin in 1975 for completion by 1976 rainy season
The development would support irrigation for 10,000 ha and provide electricity to regional towns