Ethiopian Electric & Power Authority
Aleltu Hydroelectric Project Reconnaissance and Prefeasibility Studies
Volume 2 - Main Repo* t June 1984VOLUME 1 - EXECUTIVE SUMMARY VOLUME 2 - MAIN REPORT VOLUME 3 - APPENDIXES■
July 19, 1984
P6888.0   3.0   1 
.03.02 
.04.01
Ethiopian Electric Light
and Power Authority
P.O. Box 1233
Addis Ababa
Ethiopia
Attention:               Ato     Tamene     Wolde     Yohannes Manager, Planning and
Programing Department
Gentlemen:
Aleltu Hydroelectric Project Reconnaissance and Prefeasibility Study
We      submit      herewith      the      final      report      on      the      above      study      which comprises the
-    Executive Summary, Volume 1
-   Main Report, Volume 2
-    Appendixes, Volume 3.
We wish to draw your attention to the principal conclusion 
which indicates that the first stage development of Aleltu, 
300 MW, is technically and economically very attractive at
the prefeasibility study level. The ultimate development, 
based on a staged development of 12 subbasins, would be in 
the order of 1,020 MW.
We strongly recommend that EELPA proceed with a feasibility study of the Aleltu project to substantiate technical and 
economic feasibility. There are several tasks which should 
be pursued at this time prior to and in preparation for the recommended feasibility study; these are
- install additional streamflow and rainfall recording stations in the project area subbasins immediately
ensure that continuing  recordings the meteorological and  streamflow around the Aleltu project area
are     being     made     at     all     of recording stations in and
area.
•Hi•
1 \ I I IM11 I P
I i » |. . b | N h| H .1 I Il's k -iikhl J 
.’I 6WI
'
>i i .1 , , U HI t N't | »• • I •l'>
’• 1
\» Hi rjiEthiopian            Electric            Light
and Power Authority - 2
Details of these and  other recommendations
are contained in the report.
July 19, 1984 for ongoing work
We       take       this       opportunity 
to 
express
our
opportunity      to      work      with 
EELPA
staff
as
appreciation 
for
the
carrying out this challenging study.
a project team in
Yours very truly,
CJB:mam Encl
Executive Engineer
•’Naiionai
i iMin i)ACKNOWLEDGMENTS ACRES INTERNATIONAL LIMITED
wishes to express its appreciation to the Canadian International Development Agency and the Ethiopian Electric Light and Power Authority for their support in financing and for the many arrangements made in order to carry out this study. We also appreciated the significant contribution and the opportunity to work close with the staff of the Ethiopian Electric Light and Power Authority during this study. Thanks are also due to agencies of the Government of Ethiopia and others who contributed much of the basic information related to the Aleltu Hydroelectric Project.VOLUME  2  -  MAIN  REPORT TABLE OF CONTENTS
GLOSSARY OF ABBREVIATIONS list of tables
list of plates technical data
1 - INTRODUCTION ----------------------------------------------------------------
1.1       - General --------------------------------------------------------------
1.2       - Terms of Reference and Scope of Work
1.3       - Background ----------------------------------------------------------
1.4       - Aleltu Basin Description -------------------------------------------
1.4.1       - Location and Access ---------------------------------------------
1.4.2       - Topography --------------------------------------------------------
1.4.3       - Climate and Hydrology ------------------------------------------
1.4.4       - Geology ------------------------------------------------------------
1.4.5       - Study Execution ---------------------------------------------------
2 - RECONNAISSANCE STUDIES -----------------------------------------------------------
2.1       - Introduction ----------------------------------------------------------------------
2.2       - Work Program ------------------------------------------------------------------- 2.2.1       - General ------------------------------------------------------------------------- 2.2.2       - Field Investigation Program -------------------------------------------------
2.3       - Extent and Description of Development ------------------------------------- 2.3.1       - Extent of Development ------------------------------------------------------- 2.3.2       - Development Concept -------------------------------------------------------- 2.3.3       - Definitions --------------------------------------------------------------------- 2.3.4       - Description of Basin Development
Schemes ------------------------------------------------------------------
2.4       - Investigations and Project Layouts ------------------------------------------- 2.4.1       - Topographic Mapping, Aerial
Photographs and Surveys ---------------------------------------------- 2.4.2       - Hydrology and Reservoir Studies ------------------------------------------- 2.4.3       - Geological Assessment ------------------------------------------------------- 2.4.4       - Project Layouts ----------------------------------------------------------------
2.5       - Project Evaluation --------------------------------------------------------------- 2.5.1       - General ------------------------------------------------------------------------- 2.5.2       - Screening of Basin Development
Schemes ------------------------------------------------------------------ 2.5.3       - Screening of Jida Sites -------------------------------------------------------- 2.5.4       - Screening of Powerhouse Locations
Page 1-
1- 1- 1- 1- 1- 1- 1- 1- 1-
1
1
2
3
7
7
8
10 
11
12
_ 
°n the Basis of Penstock Length---------------------------------------
•5.5 - Description of Layout Optimization 
Mode L --------------------------------------------------------------------
2- 2- 2- 2- 2- 2- 2- 2— 2- 2- 2- 2- 2- 2- 2- 2- 2- 2- 2- 2-
1
1
2
2
3
5
5
6
9
12
46
46
47
54
59
67
67
68
70
73
2.5.6
Economic Parameters -------------------------------------------------------
2- 77
2- 78Table  o
f content5
2
,timization Analysis
757- Layout Op 
'            Base Schemes -------------------
7  5  8  - Layout optimization  Analysis -
Z                         independent and Diversion Sc
Schemes
Cost of individual Schemes - 2 5 9 - Energy Cost uf Ir.di”’ 1 Schp
c.mmiarv and Conclusions of
2.6
2.6.
2.6.
2.6.
- Summary ana
Reconnaissance Study
— •-
______________
. reneral Results------------------------ -
I Long-Range Project Development------------------------ 3 - Definition of the First Stage of
Development --------------------------------------------
3 - prefeasibility studies- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 3.1    - introduction ----------------------------
3.2    - Work program.
2
~ 81
2~ 94 2~l0i
2-102
2-102
2-104
2-105
3- 3-
3-
1
1
2
3.2.1   - Office Studies ------------------------------------------------------------- 3 2 2 - Field Investigations--------------------------------------------------------- 3*3’ - Concept of the First Stage Development--------------------------------- 3.4    - Topographic Maps and Surveys----------------------------- -------------- 3.4.1   - Available Mapping-------------------------------------------------------- 3.4.2   - Field Surveys -------------------------------------------------------------- 3.5    - Hydrology and Reservoir Hydraulic
Studies ------------------------------------------------------------------- 3.6    - Geology and Seismicity---------------------------------------------------- 3.6.1   - Scope of Work ------------------------------------------------------------- 3.6.2   - Field Investigations - Geological---------------------------------------- 3.6.3   - Test Pits and Auger Holes------------------------------------------------ 3.6.4   - Laboratory Testing-------------------------------------------------------- 3.6.5   - General Geology and Seismology--------------------------------------- 3.7    - Socioeconomical and Environmental
Assessment -------------------------------------------------------------- ” Description of the Present Setting---------------------------------------
t't’i ” Evaluation of Project Impacts-----------------------------------------------
• .3 - Cost of Mitigating Measures----------------------------------------------- ^.8 - Project Layout Alternatives-------------------------------------------------- j.o.l - General--------------------------------------------------------------------- '— 3-8.2 - Layout Studies --------------------------------------------------------------
3.9      -      Construction      Planning      and      Cost
.                Estimate ----------------------------------------------------------------- J,i  - Power and Energy studies, and Economic
3 H - o -------- -------------------------------------------------------------------------- Results of Prefeasibility Studies-----------------------------------------
3-
3-
3-
3-
3-
3-
3-
3-
3-
3-
3-
3-
3-
3-
3-
3-
3-
3-
3-
3-
3-
3-
3-
2
2
3
4
4
5
6
11
11
12
13
13
14
16
17
19
23
24
24
26
40
41
41
develSS of Recommended FIRST STAGE
J-VELOPMENT OF THE ALELTU PROJECT---------- ~ General
{’H - Topography --------------------------------------------- 4*2 _~ 4.2 :nfrastructure ---------------------------------------------
4,2      _ as^n Dev&lopment ------------------------------------
c
4.3    -
La y°ut Components------------------- THiku Development -------------------------------
4-
4-
4-
4-
4-
4-
4"
1
1
1
2
3
3
4Table of Contents - 3
Page
4.3.1       -    Tiliku          Aleltu     Reservoir -----------------------------------------------  4-    4
4.3.2       -    Tiliku          Aleltu     Main Dam----------------------------------------------  4-    7
4.3.3       -     Tiliku          Aleltu     Cofferdam and Diversion ----------------------------  4-  13
4.3.4       -     Tiliku          Aleltu     Spillway------------------------------------------------- 4-  14
4.3.5       -     Interconnection          Tunnel---------------------------------------------------  4-  16
4.3.6       -     Low-Level Outlet----------------------------------------------------------------- 4-  17
4.4       - Jida Development----------------------------------------------------------------- 4- 18
4.4.1       -     Jida Reservoir---------------------------------------------------------------------  4-  18
4.4.2       -     Jida Main Dam--------------------------------------------------------------------  4-  19
4.4.3       -     Jida---------------------------------------------------------------------------------  4-  21
4.4.4       -     Jida Spillway----------------------------------------------------------------------  4-  22
4.5       - Power Conduits - Tiliku Aleltu
to Powerhouse-------------------------------------------------------------------  4-  24
4.5.1       -      Geotechnical Considerations--------------------------------------------------  4-  24
4.5.2       -      Power Conduit Structures------------------------------------------------------  4-  26
4.6       - Powerhouse-----------------------------------------------------------------------------  4-  30
4.6.1       -      Geotechnical Considerations--------------------------------------------------  4-  31
4.6.2       -      Powerhouse Equipment Arrangement ---------------------------------------  4-  33
4.7        -  Aleltu Switchyard--------------------------------------------------------------------  4-  39
4.8        -  Transmission--------------------------------------------------------------------------  4-  39
4.9        -  Construction Power------------------------------------------------------------------  4-  40
4.10     -   Addis Ababa Switchyard------------------------------------------------------------ 4-  40
5 - CONSTRUCTION PLANNING -------------------------------------------------------------  5-             1
5.1        -   Contracts------------------------------------------------------------------------------  5-    1
5.2        -  Access and Transport---------------------------------------------------------------- 5-    2
5.3        -   Construction             Camps and Site Facilities --------------------------------  5-    2
5.4        -  Construction             Schedule-----------------------------------------------------  5-    4
/
6 - COST ESTIMATE------------------------------------------------------------------------------  6-
6.1       - Basis of Cost Estimate-----------------------------------------------------------  6- 6.1.1       -    General Approach---------------------------------------------------------------  6- 6.1.2       -    Code of Accounts---------------------------------------------------------------  6_ 6.1.3       -    Estimating Procedures----------------------------------------------------------  6-
6.1.4       -    Unit Costs------------------------------------------------------------------------  g_ 6.2       - Cost Estimate-------------------------------------------------------------------------  §_
0 •J - Effect
~ PROJECT POWER 7.1       - System 7-2       - Aleltu 7.2.1       - plant
of Changes in Cost Estimate---------------------------------------  6-
AND ENERGY CAPABILITY IN THE ICS — 
7-
Characteristics ------------------------------------------------------  7_
Project Planning Requirements -----------------------------------  7-
Capacity Factor -----------------------------------------------------  7_
Aleltu Project Generation Capability -----------------------------------------  7-
Timing and Plant Additions ---------------------------------------------------  7_
1
1
1
2
2
5
6
8
1
1
5
5
6
9Table of Contents - 4
8   economic analysis
8.1
8.2
- General --------
-  Alternative
Developments for
8.3
8.3.1 - Di
8.3
8.3
8.3.
8.4
8.5
Comparison -------------------
-   Comparison with Gilgel Gibe Project
a. Differences Between Projects-----------------------------
.2 - Adjustment -------------------------------------------------
.3 - Method of Economic Analysis--------------------------- 4 - Results of Comparison------------------------------------
-   Comparison with Future Available
Grid Energy Supply Cost ----------------------------
-   Sensitivity of Results-------------------------------------
8.5     -   Conclusion ------------------------------------------------
9 - CONCLUSIONS AND RECOMMENDATIONS
9.1     - Conclusions-------------------------------------  “
9.2     - Recommendations -------------------------------------------------------- PLATESVOLUME 2 - MAIN REPORT
GLOSSARY OF ABBREVIATIONS
Acres                 Acres International Limited
ACSR                aluminum core, steel reinforced
ASTM               American Society for Testing and Materials B/C                    benefit/cost
CI DA               Canadian International Development Agency
d
day
DSL
dead storage level
EELPA
Ethiopian Electric Light and Power Authority
ENEL
Ente Nazionale Per L'Energia Electrica
ERESA
Eritrea Region Electrical Supply Agency
FAO
Food and Agriculture Organization
FI
flood index
FSCL
full surcharge condition level
FSL
full supply level
ICS
interconnected system
MAF
mean annual flood
Mm3
million cubic metres
MM
modified Mercalli
MSL
mean sea level
PF
plant factor
PI
plasticity index
PMF
probable maximum flood
PPS
Power Planning Study
PTD
power tunnel diameterGlossary of Abbreviations - 2
PVC              polyvinyl chloride
r/min             revolutions per minute scs           self-contained system
t
tonne
UNDP           United Nations Development Program USER           United States Bureau of ReclamationVOLUME 2 - MAIN REPORT LIST OF TABLES
Number          Title
1.1
ICS Plant Capability
2.1                    Basins and Drainage Areas
2.2                    Summary of Basin Schemes
2.3                   Monthly Mean Summaries of
Subbasins Inflow Sequences
2.4                     Reservoir-Regulation
Relationships for Model Runs
2.5                     Evaporation Losses
2.6                     Flood Flows of Subbasins
2.7                     Reservoir Flood Routings
2.8                     Summary of Incremental Cost of
Energy for Individual Schemes
2.9                     Power and Energy Capabilities of
Staged Ultimate Aleltu Development
2.10                   Reconnaissance          Level          Definition          of          the First Stage Aleltu Project Development
3.1                     Flood Flow Analysis
3.2                     Model Evaluation of Jida Schemes 6.1                     Stage I -
Prefeasibility Cost Estimate 7.1                     System Load Demands
7.2                     Aleltu Project Energy Capabilities
7.3                     Aleltu Project-System Supply Capability
8.1                      Comparison With Gilgel Gibe EstimateVOLUME 2 - MAIN REPORT
LIST OF PLATES
Number            Title
1-1                    Existing and Committed ICS
2-1                    Potential Development
of Aleltu Projects
2-2                    North-South Projection of
Gorge Profiles and
Powerhouse Locations
2-3                    Reconnaissance Layout Arrangements
2-4                    Volume-Elevation Curves
of Dam and Reservoirs
2-5                    Screening of Aleltu 
Development Sites
2-6                   Screening of Jida Basin Sites
2-7                   Schemes Evaluated in Model Study
2-8                   Ultimate Development of Aleltu Basin 3-1                    Hydrologic Data Base
3-2                   Geology of Tiliku Aleltu Dam Site 3-3                   Geology of Jida Dam Site
3-4                     Geological Section of the Zega Wedem Gorge
3-5
3-6
Potential Jima Impervious Borrow Area Seismicity of Ethiopia
3-7                     Environmental Definitions
4-1
Project Layout Plan
4-2                     Dams-Layout Arrangement
4-3 Tiliku Aleltu Dam and Spillway
4-4
Properties of Impervious MaterialsList of Plates - 2
Number       Title
4-5             Tiliku Aleltu and Jida Structures 4-6             Jida Dam and Spillway
4-7             Power Conveyance Structures 4-8            Powerhouse Arrangement
4-9            Single Line Diagram 5-1            Engineering and
Construction ScheduleTECHNICAL DATA
A - ULTIMATE PROJECT
Drainage area (km2)
Number of subbasins
Average annual runoff (m /s) Generating gross head (m)
Economic firm energy potential (GW«h/yr) Installed capacity at 40% plant factor (MW)
3
4 230
12
49
1 0 70 3 592 1 023
B - FIRST STAGE TILIKU
ALELTU AND JIDA
Stage IA
Tiliku
Aleltu
Stage
Jida
IB
Combined First
Stage
Drainage Basins Drainage area (km2)
458
749
1 20 7
Annual average inflow
(m3/s)
5.6
8.2
13.8
Available mean power
flow (m3/S)
5.55
7.75
13.3
Degree of regulation (%)
60
100
100
Firm power flow (m3/s)
3.30
7.75
13.3
Reservoirs
Elevation
- Full supply level
(FSL) (m)
2 598.5
2 598.5
2 598.5
~ Dead storage Level
(DSL) (m)
2 570 .0
2 570 .0
2 570.0Stage
Tiliku
Aleltu
IA
Stage
Jida
IB
Combing^ First
Area
- at FSL (kn>2)
 at DSL (km2)
Volume, at FSL (lO^mS) Live storage (10 m3)
Flood handling
6
6.60
0.50
92
90
50.0
13.0
1 050
990
5 6.6
13.5
1 142
1 080
-   1:10 000-yr inflow
232 
329
561
(m3/s)
-   spillway type
uncon
t
uncon
—
rolled
trolled
weir
emer gency
weir
-   spillway width (m)
40
30
-
-   crest level (m)
2 598.5
2 600.0
—
-   spillway discharge
191
0
—
(m3/s)
-   flood surcharge
2 600.5
2 600.0
2 600.0
elevation (m)
3  - Dams
Type
earth
and 
rock
fill
earth
and 
rock
fill
Crest level (m)
2 60 2.0
2 602.0
-
Maximum height (m)
54
65
-
Crest length (m)
560
1 950
-
Embankment volume
(106m3)
1.0
7.1
8.1
-■ - Interconnection Tunnel
Type
unlinediii
Stage IA
Tiliku
Combined
Aleltu
Stage IB
jida
First
Stage
Length (km)
Diameter (m)
-
-
2.0
6.0
5  - Low-Level Outlet
Type
Length (km)
Diameter (m)
6     - Power Tunnel
Intake gate
unlined
0.8
5.0
3.5x3.5 m
roller
Tunnel
- type
-    length (km)
-   diameter (m)
Surge chamber
-   diameter (m)
-   height (m)
Valve         chamber         (valve type)
7     - Penstock
Type
Length (km) Diameter (m)
gate
concrete- lined
horse shoe
16.3
3.2
20 .0 110.0 spherical
steel surface
2.1
2.4
!      fa
-Stage IA
Tiliku 
Aleltu 
Stage IB
Jida
Combine^
First
Powerhouse
General
surface
surface
-   type
- number of units
surface
3
3
6
-  rated flow (m3/s)
3x6.33
3x6.33
38.0
-  dependable capacity
150
150
300
-   firm energy (GW-h/yr)
(MW)
u/ 1
260
6 20
1 040
-   average energy
450
630
1 060
(GW-h/yr)
Head
-  maximum gross (m)
-  minimum gross (m)
-   head loss at rated
flow (m)
-  head loss at mean
weighted flow (m)
-  minimum net at rated
1 0 70
1 0 40
140
31 .0
90 0
flow (m)
Turbines
-   type
-   number of units
-   turbine setting
elevation (m)
-  rated flow (m3/S)
-  rated net head (m)
- revolutions per
minute
rated unit
(MW)
capacity
pelton
6
1  530.0
6.33
900
750
51
Generators
~ number
- rated unit capacity
(MVA)
7
voltage (kv)
6
55.6
13.8V
Stage IA
Tiliku
Aleltu
Stage
Jida
IB
Combined First
Stage
9  - Transformers
Number
-
-
6
Type
oil cir culation forced air
cooled
Capacity (MVA)
-
-
56.0
Low voltage, high 
voltage (kV)
—
—
13.8/230
10 - Transmission
Number of lines
-
-
2
Voltage (kV)
-
-
2 30
Length (km)
—
—
88.01 - INTRODUCTION
1.1 - General
Electric power supply in Ethiopia is administered by the Ethiopian Electric Light and Power Authority (EELPA). The Aleltu project was identified in the 1982 Power Planning Study (PPS) * as one of a number of projects with potential
for economic hydroelectric development. The main project development is about 80 km north of Addis Ababa. Plate 1—1 shows the project location.
In the PPS, the Aleltu project was studied at prereconnaissance level. EELPA, as part of its overall generation planning, has decided to advance this project to prefeasibility level to determine
- more accurately                    its technical and economic merits
if it should be considered (in competition with other potential power projects) for meeting the nation’s
increasing load demand
- if it should proceed to a full feasibility level study.
In early 1983, discussions were conducted, and on June 29, 1983, an agreement was reached between the Canadian International Development Agency (CIDA) and EELPA for joint financing of the reconnaissance and prefeasibi1itv study of the Aleltu project. Acres International Limited (Acres) was engaged to perform the study jointly with professional and technical staff of EELPA.
*l982r Planning seudV» Acres International Limited, October1,2   - Terms of Reference
and Scope of Work_
The   Terms   ot   Reference   and   General   Scope   of   Work   as   embodied in the Agreement are summarized as follows.
(a)    Collect and review all available data pertaining to the Aleltu Basin complex, including hydrological, geo technical, topographical, environmental and infrastruc tural information.
(b)    Undertake an initial assessment of the Aleltu Basin at reconnaissance level with due recognition of multistage development aspects and considering the ultimate hydroelectric potential of this basin.
(c)    Select, as part of the reconnaissance study, the first stage of development for Aleltu.
(a) Define, at prefeasibility level, the first stage of the development, incorporating an economic and multi disciplinary technical evaluation of the proposed project.
(e) Prepare a report of the studies, including recommen- oations and the Terms of Reference for a full feasibi lity study, if appropriate.
i fi Conduct a training program for EELPA staff during the various stages of the study, involving various study aspects.
These conditions have been fulfilled in the performance of the reconnaissance and prefeasibility studies presented herein.1-3
1.3  - Background
Electrical power supply in Ethiopia is provided by two 
separate systems, the Eritrean Region Electrical Supply
Agency (ERESA) system in the Asmara region and the Main 
system in the central part of the country. Each system
consists of both an Interconnected system (ICS) and a Self-
contained system (SCS). The SCS mainly comprises local
diesel generating systems. EELPA has recently interconnected 
some of these SCS stations in the Bahar Dar/Gondar area to 
develop a separate Regional ICS. The extent of the ICS
systems is shown on Plate 1-1. The Aleltu project would be
interconnected with the Main ICS.
The Main ICS accounts for 84% of present power supply in 
Ethiopia and incorporates generation, transmission and
distribution facilities. The annual load factor is presently 
58% which is indicative of a modest industrial load, in addi
tion to the commercial, lighting and domestic demands. There
is no distinct seasonal pattern in either monthly energy 
generation or peak power requirements.
The installed capacity of the Main ICS is presently 213.7 MW. 
Of this, 97% is provided by four hydroelectric power plants, 
namely, Koka, Awash II, Awash III and Finchaa. The two 
diesel generating stations in the ICS at Alemaya and Dire
Dawa have already exceeded their life expectancy and are to 
be retired on commissioning of the Malka Wakana power plant
in 1987. Table 1.1(A) summarizes the power capabilities of
these existing plants.
The        Amarti        diversion        project        is        presently        under        construction and the raising of the Finchaa reservoir full supply level
(FSL) by 2 m is part of the construction program. TheTABLE 1.1
ICS PLANT CAPABILITY
Station
PRESENT ICS PLANT
Hydro
Koka
Awash II
Capacity (MW]----------- - Installed     Dependable
Energy  (GW  h/yr) Average Firm
Plant Factor*
capability
43.2 34.5
32.0 32.0
118 118
169 169
0.39
0.60
I
r
'i r
Awash III
32.0
32.0
173
173
Finchaa
100.0
100.0
565 
540
0.62
0.62**
I
Subtotal
207.2
198.5
1 025
1 000
0.57
Thermal (Diesel)
Alemaya
2.0
2.0
10
10
0.57
Dire Dawa
4.5
3.5
18
18
0.59
Subtotal
6.5
5.5
28
28
0.58
TOTAL A
213.7                 204.0                     1 053            1 028            0.57
(B) DEVELOPMENT OF ICS TO 1957
1984       Total A
1985       Amarti
1987 Malka Wakana
Alemaya***
Dire Dawa***
TOTAL B
213.7
150.0
(2.0)
(4.5)
359.2
204.0
150.0
(2.0)
(3.5)
348.5
1 053
1 028
175
560
200
560
(10)
(18)
(10)
(18)
1 760
1 760
0.57
0.43 (0.57) (0.59)
0.58
• Based on dependable
•• Pinchaa plant factor increases diversion project in 1985.
•••Retired.
capacity and        firm energy.
to 0.84 with increased energy from Amarti
I1-5
increased reservoir volume will augment the firm energy yield 
of the Finchaa power plant. In addition, Finchaa operation 
will allow for firming up of secondary energy generated by 
the Malka Wakana power plant (about 80 GW>h/yr). The Malka Wakana plant will have an installed and dependable capacity 
of 150 MW with an average annual energy generation of
560 GW-h. The ICS capability after 1987 will, therefore, 
be as shown in Table 1.1(B).
The growing economy and electrification programs will demand increasing supply of power and energy. The PPS shows that
there will be a need for additional generation, depending on 
the load forecast assumed, between 1989 and 1993. This load 
growth will arise from
- progressive electrification of the existing industrial
sector
- electrification of existing industrial boilers
- new industrial developments
- continuing regional and village electrification
increasing use of electrical energy for domestic cooking
- substitution strategies to reduce reliance on imported hydrocarbon fuels.
For the 1990s and into the next century, the optimum devel opment program for the ICS will be primarily hydro. With respect to other sources of generation, it is recognized in the 1982 PPS report that the role of geothermal generation could be economically attractive, when combined withincremental hydro capacity. Implementation of a geothermal plant is being considered in EELPAs generation planning.
The economic development of hydro generation arises from the unique topographic features which exist in the Ethiopian high plateau area. This plateau contains a number of large
drainage basins and is incised with deep ravines which permit the development of high head hydroelectric power schemes, a large number of potential developments mainly located in the Omo and Blue Nile basins have been identified. The most promising developments were found to be the Aleltu and Chemoga developments in the Blue Nile Basin and the Halele Werabesa development in the Omo Basin. In the 1982 PPS, it was recommended to advance these projects to full feasibility status as early as possible. In comparing the three recom mended developments, it was found that the Aleltu project appeared to be the most attractive from a number of viewpoints
-   large generating head, about 1,100 m
-  potential generating capability up to 800 MW
-  development can be in stages
favorable location close to Addis Ababa and adjacent to the main road to Bahar Dar
-   low investment cost.
For practical consideration of project management, the size of new major generating plants should meet the ICS load growth for a period of at least 5 years. Considering load growth in the 1990s, this would correspond to an installed capacity of 180 MW or more. Aleltu satisfies this condition.1-7
1.4   - Aleltu Basin Description
1.4.1 -             Location and Access
Location
The Aleltu project is located on the plateau area of Central Ethiopia. The plateau is at a relatively 
high elevation—much of it above 2,000 m and incor porates the large drainage basins of the Omo and Blue Nile rivers. The Blue Nile River starts at Lake Tana and flows initially in an easterly direction. It
then gradually turns south, toward Addis Ababa, and continues to turn in a westerly direction toward 
Sudan. About 130 km northwest of Addis Ababa, the Giamma River joins the Blue Nile River from a south easterly direction. This river has a number of tributaries of its own, all of which have cut deeply below the plateau area. The most southerly tributary 
is the Zega Wedem River, which is important in developing the Aleltu project.
The Aleltu project comprises parts of the Zega Wedem, Muger, Bersina and Giamma river basins and has a total drainage area of about 4,400 km2. The
project lies between longitude 38°45'E and 39°30'E, and 9°15'N and 9°45'N latitude. It is bounded approximately by Addis Ababa to the south, Fiche to the northwest and Debre Berhan to the east. The longest east-to-west and south-to-north dimensions of the project area are 105 and 90 km, respectively.1-8
Access
The Aleltu project area is readily accessible because
of its proximity to Addis Ababa. Two main highways cut through the project area. Highway 3 stretches
from Addis Ababa in a northerly direction to Fiche and on to Bahar Dar, while Highway 1 runs from Addis Ababa in a northeasterly direction to Debre Berhan and on to Asmara.
In the central part of the project area, an 
all-weather gravel road provides for access. From Muke Turi, this road runs in a northeasterly direc tion for about 22 km, halfway into the main project area, before it turns and heads north to Lemi. From Debre Berhan on Highway 1, a gravel road runs in a north-westerly direction along the eastern limit of the project area to the Bersina Gorge. Access to isolated locations ir. the plateau part of the project area is generally overland by four-wheel drive vehicles.
There is no roac
access into the possible powerhouse
locations in the Zega Weceir. Goj roac extends partly down tne q Deore Libanos. Foot trails lowest areas m the gorge.
1.4.2 - Topography
The        prominent        features plateau      area      which      will Zega     wecem     ana     Bersina neac car. 5e deveiopea.
-urge. Access by paved gorge from Highway 3 to 
extend from there to the
hola the
gorges m
r feservoirs
whicn the
are the high 
• and tne
hydraulic1-9
Plateau
The general elevation of the high plateau ranges between elevation 2,500 and 2,700 m near the dam locations and up to 3,000 m at the upper end of the drainage areas. The plateau is characterized by broad river valleys separated by flat highlands. Valley side slopes are reasonably gentle. The plateau generally falls in elevation from south to north and from east to west, i.e., as the rivers flow to the gorge. Land drainage is therefore generally to the northwest.
Both the valleys and highlands consist of agricul tural cultivated areas and grasslands. Some low- lying valleys have extensive "water-laden areas" in the wet season.* Tree growth everywhere is sparse. Small villages are generally located on the higher lands.
For most of their length on the plateau, rivers meander in overburden. As slopes steepen toward escarpments, rock is increasingly exposed in river beds, and eventually channels are cut in rock. The rivers have gentle bed slopes and they generally 
lose 300 to 500 m head throughout their length before finally dropping into the gorges.
*These areas are not swamps overlain with water but are more like bogs where the soil is laden with water in the wet season. Topographic maps of Ethiopia refer to these areas
as "land subject to inundation."1-11
beginning of July and lasts until the beginning of October. About 75% of the total annual rainfall and 
90% of the total annual runoff occurs during the rainy season.
Although the entire plateau receives heavy rains, the western part generally receives more rainfall during 
the wet season than does the eastern part. The mean annual rainfall is about 1,400 mm/yr in the western drainage basins of the project area, falling to
1,050 mm/yr in the eastern drainage basins.
In January, when the rain belt lies south of the
equator, the entire country is within the subtropical
high pressure region and very dry conditions prevail.
The climate in the Aleltu project area is temperate. Annual mean maximum temperatures are about 25 to 30°C in the Zega Wedem Gorge and about 5 to 10°C lower on the plateau. Annual mean minimum temperatures in the gorge and on the plateau are about 5°C. There is
little variation in mean temperatures throughout the
year.
1.4.4   - Geology
The tectonic setting and the regional geological and seismological aspects of the Aleltu Basin are described in detail in Appendix C. The formation and character
of the plateau is derived from over 14 geological units consisting of shales, sandstones, limestone, lacust- rines, agglomerates, pyroclastics, breccia and basalts. Predominant among these units, and largely determining the form of the landscape, are the upper basalt flows.1-12
These plateau basalts are mostly covered with a thin weathered zone of dark silty clay soil. Otherwise, these extensive basalt flows have resisted erosion to create the broad plateau and cliff face of the gorge.
With the Aleltu Basin being very close to the seismi- cally active Ethiopian rift system, geological activity is still very pronounced in the region. Earthquakes up to magnitude 6 on the Richter scale can be anticipated in the Aleltu Basin.
A number of major faults occur in the project area. A major east-west trending fault exists with down dis placement to the north and running almost parallel to and 0.5 to 2.5 km south of the Zega Wedem Gorge. Another series cf faults paralleling the main Ethiopian rift system are observed at the eastern end of the
basin near Debre Berhan.
1.4.5     - Study Execution
The study was executed in two phases—reconnaissance and prefeasibility—during the period from July 1983 to issue cf the final report in June 1984. Field investi gations were conducted in Doth phases of the study to develop site-specific data for the multidisciplinary office studies ana project designs.
Because of the complexity of the project and the extent of the studies in hydrology anc geology, the two study phases overlapped in their execution. The study work of the reconnaissance phase extended from July to 
Decker m . At the
3
teglnnlr,9 of ,JctGL^
reconnaissance of all basins had not
conceptual development of the flrst stayfe thg
n aa not been completed, 1-13
project was sufficiently defined to be able to start
the field investigations for the prefeasibility study. 
Thus, the prefeasibility stage of the study started in October 1983 and continued to February 1984 after which report preparation was undertaken.
The reconnaissance and prefeasibility studies are described in Sections 2 and 3 of this report, respec tively. Section 4 describes the recommended first stage development of the Aleltu project. The construc tion planning requirements and cost estimate for the first stage are contained in Sections 5 and 6, respec tively. The power and energy capabilities of the Aleltu project, as it would connect into the ICS, are presented in Section 7. The economic evaluation of the project is reported in Section 8.2       RECONNAISSASCE STLDIE>
2.1  - Introduction
The objectives of the reconna l s nance
-   identify all possible development alternative*
-   determine the concept of development and economic capability of the project
-   define the optimum first stage of development.
<
To       achieve       these       objectives,       the       reconnaissance       studion       wore conducted in several stages
-           identification        of        development        alternatives        from        existing topographic maps
-   initial site visit to assess field conditions of alterna
tive developments
-   multidisciplinary studies of alternatives and evaluation of development sequences
- site verification of project
layouts
- final analysis of alternatives and determination of overall concept of development.
Because of the complexity of the Aleltu project, with numerous alternative layouts to be studied, a mathematical computer "layout optimization model" was developed. This model regulates combined reservoir operations, optimizes the2-2
size of the project structures and provides project costing 
and economic evaluation for any complexity of development of the project. A detailed description of the model is
contained in Appendix A.
A description of the reconnaissance studies is contained in the following sections starting with the work program in Section 2.2. Section 2.3 describes the various development schemes. The multidisciplinary studies relating to topo graphy, hydrology, geology, layout and costing aspects are contained in Sections 2.4. Section 2.5, Project Evaluation, contains the results of the layout optimization model runs. The results are discussed in Section 2.6.
2.2   - Work Procrarr
2.2.1 - General
The reconnaissance studies were carried out through a coorcinetec program of office studies and field inves- t. at.ons. The work program, started at the beginning 
z
of July 1963.
' ! ' . <:e StuClfeS
<       • • 
1'
. net
»( I ui t uTct efetaL.. bl.eC j6 1 ng afiC ccl.fi. phOtQyrdpht.
-o/outfi of project
T ne existing topographic
MuClac ./ hy<3rGiog/f
ycciogy, c.
cet aihol ,n^ were ..ary .» r.
1 he 1
. i.!. i n.a i .
'-al I . c 
(        -ate
engineering and pre ll ml-
flvc
• flI -c —I . 1,^    1     ’ t M Jy bl ajCC.
! 1 ‘JU f ,  c            f ! . ■ c b 1 u 0 1 c b J j I ai\<
f
cj f i .41* 2-3
(described below), input parameters were *evel ,« 
for the layout optimization computer model.
Preliminary model runs were then carried cut.
On the basis of the preliminary model runs, a site verification program was undertaken, including topo graphic surveys to more accurately define the layout of structures. Further hydrologic and civil engi neering studies were also undertaken. The results of these studies then formed input for the final model runs which were carried out in December 1984 at the end of the reconnaissance study phase.
2.2.2     - Field Investigation Program
Initial Field Program
To ensure that the conceptual developments of the Aleltu project were feasible, an initial site inves tigation was made during July and August 1983. To cover the extensive area of the project, helicopter flights were made into the gorge and to the main damsites. Ground level investigations were also carried out. Topographic and geologic assessments were made to identify, in particular, the extensive basalt flows of the plateau and major geological units in the gorge.
From the preliminary field inspection, the potential feasibility of all sites was established. (Confirma tion of technical feasibility was to be substantiated by the ongoing studies.)
Topographic survey work was initiated to support the reconnaissance and prefeasibility studies.Social and environmental studies of the regiOn (as
discussed in Appendix D) were carried out early £n
the study period, August 1983 , to ensure that tech
nical aspects of the project would not impose
unacceptable social or environmental impacts on the
community.
Data collection programs were initiated for the study 
activities in hydrology, geology and for the socio-
environmental assessment.
Verification Field Program
Durinc October 1983, a second reconnaissance level
field investigation program was carried out. These
investigations were conducted to verify field infor
mation for the final reconnaissance layouts. During 
this period, prefeasibility level investigations, as
reported in Section 3, were carried out simultan
eously. The field investigations of the reconnais
sance study are reported below.
The topographic surveys required  to  establish  levels and  provide cross sections for dams and  other struc tures was substantial. This work was required
Sect.or. 2.4.1, only one third of the project area is2-5
To expedite the field program, helicopter flights
were again made into the gorge and to all recon naissance damsites. Walkover visits to main damsites were also undertaken. More specific investigations
for foundation conditions and power conduit alignment were carried out on this second field program.
The October 1983 field visits also included a further data collection program. This program was particu larly related to infrastructural, hydrological and environmental data.
The results of the field investigations are incor porated in Section 2.4 covering the multidisciplinary studies.
2.3 - Extent and Description 
of Development
2.3.1 - Extent of Development
Fourteen subbasins with potential for power development were identified on the high plateau. Plate 2-1 shows
the drainage basins and potential ultimate development of the Aleltu project. The rivers of the 14 subbasins presently discharge into four larger river basins. These are
Zega Wedem, in the northcentral part of the project area
Bersina, northeast of the Zega Wedem
-   Giamma to the east
-   Muger to the west.These     major     basins, 
together with the subbasins and shown in Table 2.1.
their
drainage areas, are
The estimated mean annual runoff (which is derived in later sections) from this total drainage area i  about
s
51 m3/s. With an average gross generating head of about 1,100 m and an overall project efficiency of about 80%, the potential firm energy yield of the development is about 3,800 GW«h/yr. At a system load factor of 40%,* this corresponds to an installed capa city of about 1 080 MW. As discussed in later sections of the reconnaissance study, the economic firm energy yield of the project area is estimated to be about
3,600 GW.h/yr with 1,020 MW of installed capacity at 40$ plant factor.
The development schemes within these subbasins and their contribution to the generating potential of the ultimate development are described in the following sections.
2.3.2    - Development Concept
A.s indicated in Section 1.4.2/ Topography/ the major features that create the Aleltu project are a combina
tion f rorr head pert
of reservoirs (river valleys) located upstream
the Zega Wedem ana Bersina gorges and the high that can be developed in these gorges. The maj
of this head is from the edge of the plateau to 
the bottom of the gorge. Plate 2-2 shows sections along the Zega Wedem and bersina gorges. in both gorges, the potential head increases to the west goi y away froff. most basin developments.
.e A*c J u pF ' eC t ! .• t Lr . ant to be
i c j u l! ©mt is,
*111 be a round 
.a coffipletea. over installed
after Year 2000 when A 40% plant factor allows
- meet system2-7
TABLE 2.1
basins and drainage areas
Major
Basin
Subbasin
No. *
River
Drainage
Area* * 
( km2)
Zega Wedem
1
Gur
g * * *
2
Weserbi
168
3
Agat
Tinishu Aleltu
86
4
77
5
Tiliku Aleltu
458
6
J ida
272' 4-
749
7
Robi-Gomoro
8
Gomoro
225
9
Robi
437
Bersina
10
Jinjer
238
' Muger
11
Girar
312
12
Duber
172
13
Sibilu
518
Giamma
14
Chacha
590
TOTAL
4 395
’ The subbasins are numbered from west to east and north to south, starting with the Gur, which is the most northwesterly basin.
** Drainage areas correspond to basin area at main damsites each river.
***Excluding the Weserbi tributary.
t Drainage area is downstream from Robi and Gomoro sites.
on2-8
the land slopes toward the gorge and
On the plateau, 
* i head dccra the potential
for optimizing Proje
g  downstream. Thus, development, a number of condi-
..kirh are not an exist which are
tions
compatible. These are
for the
- for
o area, damsites at the edge of maximum. drainage area,
gorge are preferred 
maximum head, da-sites upstream are preferred
-   also,   for   maximum   head,   powerhouse   locations   in   the lower sections of the gorge are preferred
- for minimum cost of power conduits, damsites at the edge of the gorge and powerhouse locations in the
upper sections of the gorge are preferred.
From a study of the topographic maps, it was found that many of the valleys are close to one another and have approximately the same elevations. They are the Agat, Tinishu Aleltu, Tiliku Aleltu, Jida, Gomoro and Robi. 
It was, therefore, considered that the reservoirs in 
these basins could probably be interconnected with low cost uncontrolled channels or tunnels and operated as
an integrated network with a common FSL. The FSL would be controlled by available reservoir volumes and regulating capabilities.
From the interconnected reservoirs, a power tunnel (°r canal) and penstock could convey power flow to a common powerhouse located in the Zega Wedem or Bersina gorges. Alternatively, more than one powerhouse could be consi-
i ne in the Zega Wedem Gorge for development of
basins.
basins and one in the Bersina Gorge for eastern 2-9
Basins distant from the gorges and rivers not suitable
for interconnection with other reservoirs could be
developed as diversion projects.
2.3.3     - Definitions
(a) Base Schemes and
Diversion Schemes
To discuss the development of the basins of the Aleltu project, they have been separated into 
three categories
-    independent schemes
-   base schemes
-   diversion schemes.
An independent scheme is a basin in which the reservoir operates independently of any other. 
Single reservoir power developments are indepen dent schemes.
A base scheme is a basin in which the reservoir operates in conjunction with neighboring reser
voirs as an integrated multi reservoir network 
having a common FSL. The power conveyance would be taken off one of the base schemes.
A diversion scheme is a river basin where the site is remote from the gorge and, therefore, better developed in conjunction with a base or indepen dent scheme; or because of other adverse features (which will be discussed hereinafter) would clearly not be economic as a base or independent scheme but has the potential for diversion. The feasibility of a diversion scheme depends on the economic viability of one or more other schemes.T he 14 basins were initially classified as below. However, as the study progressed, it determined that each of the independent schemes would be better developed as diversion schemes.
Independent Schemes
- Gur
Base Schemes
3 - Agat
Aversion
Sc h eine g
H Girar
- Weserbi
4 - Tinishu Aleltu
12 — Duber
- Robi-Gomoro
5 - Tiliku Aleltu
13 - Sibilu
- Jinjer
6 - Jida
14 - Chacha
8 - Gomoro 9 - Robi
(b)      Dam Types
In describing the alternative layouts, several
types of dams are referred to. These are as
follows.
Storage Dam - A dam creating a live storage volume equal to or greater than 50% of the mean annual runoff.
Diversion Dam - a dam built to divert part of the natural inflows to another basin via a rela tively large channel or tunnel. Reservoir storage, although small, would be sufficient to catch and divert peak flows.2-11
- overflow Dam - A low dam (of low cost) to divert most of the inflow into a conveyance tunnel or channel or to create a head pond for an intake structure. Flood peaks would overflow the
crest.
(c) Coding System
A coding system was devised to identify the alternatives. Basins are referred to by number
(1, 2, 3, etc); dam locations in each basin are referred to by lowercase letters (a, b, c, etc);
and powerhouse locations are referred to by capital letters (A, B, C, etc). Location of the
intake of the power conveyance system is indicated by underlining the relevant basin.
Total project development "plans" are referred to by basin numbers and powerhouse locations. Thus, Plan J345A is development of the Agat, Tinishu Aleltu and Tiliku Aleltu schemes with the intake
at the Agat reservoir and the powerhouse at Location A.
Within each basin development, "schemes" are referred to by basin number, dam location and alternative layout scheme number. Thus, Scheme la-2 is located on the Gur with a dam at Site a and is the second alternative layout.
Alternative "plans" and "schemes" are described hereinafter.2-12
2.3.4
- Description of Basin Development Schemes
At the reconnaissance level, all practical sites an(j development schemes have been identified as reported herein. A tabulation of all schemes identified is given in Table 2.2. This table, included at the end of Section 2, has been designed as a pullout, for easy reference.
Many  of the identified  schemes can, minary  assessment,  be determined  to comparison  with  other alternatives, are indicated at the end of each of
tions. Schemes identified as unattractive are not carried forward to the Project Evaluation, Section 2.5.
by a simple preli-
be unattractive in 
Such assessments
the basin descrip
7 |      1 - Gur Basin Schemes
The Gur River Basin is located close to the Zega
Wedem Gorge, south and southeast of Fiche. On the plateau       the Gur River has          two main branches—the werabesa which flows from the west and the Weserbi
from the south. These rivers flow into the Gur about 3 to 4 km upstream from the edge of the plateau. From the edge of the plateau, the Gur continues down the gorge another 4.5 km where it joins the Zega Wedem River.
The total drainage area is about 261 km , with the
2
Weserbi comprising about 168 km .
2
The Weserbi itself has several attractive development schemes,  independent of the Gur.  These are discussed after the Gur Basin schemes.2-13
(a)       Gur Site la
The most downstream site on the Gur is 2 km upstream from the gorge face at a location where the main valley narrows appreciably. The river bed elevation is 2,535 m and a dam up to 40 m high* could be constructed for a reservoir FSL up to 2,560 m. This site is 40 to 60 m lower in elevation than the major sites on the Weserbi, Agat and other rivers to the east. Two develop ment schemes can be considered.
(b)       Site la - Scheme 1
With a storage dam at Site la, the Gur could be developed as an independent scheme. The power flow could be conducted via a power tunnel
and/or a surface penstock directly to a power house in the Zega Wedem Gorge. The powerhouse could be located at Site A or further downstream near the confluence of the Gur and Zega Wedem rivers.
The Gur reservoir at Site la would not be capable of regulating the total basin flow. Upstream regulation could be provided on the Weserbi tributary, releasing flow to the Gur. The Girar could also be conveyed to the Gur
This is the potential dam height. Actual dam heights will be determined by the layout optimization model discussed here inatter.2-14
development by pumping to the Weserbi. This scheme, including the Weserbi and Girar, would provide a reasonable development of about 130 MW, at 40% plant factor.*
However, preliminary calculations indicate that the loss of head at Site la for the Weserbi and Girar water anc the longer penstock, reaching into the aoroe, make Scheme la-1 uneconomical. This scheme was not evaluated further, as diver sion Scheme la-2 is more appropriate for the Aleltu project development.
(c)       Site la - Scheme 2
The Gur could also be developed as a diversion scheme. With a storage dam at Site la, regu lated flow could oe diverted to the Agat scheme, 
which has an FSL at about 2,600 m. Because of the lower reservoir level of the Gur (below elevation 2,560 m), a small pumping station would be required to bring regulated flow over a distance of about 3 kit to a point just east of Highway 3 (see Enlargement A, Plate 2-1). From there, it could be discharged into an open canal anc conveyec to the Agat reservoir.
‘Plant factors of 40% have been usee for all of the recon naissance developments. This is based on present ICS capacity neecs anc future general strategies to overinstall new plants, as ciscussed in Serrinr -7 
energy Capability in the " feCtlOn 7' ^^t Pr  ower and
«
s2-15
If the Agat schemes prove to be uneconomical, 
then the Gur could still be developed by pumping 
and discharging into a series of canals and 
pipes to connect with the development of the
eastern basins of Tinishu Aleltu and/or Tiliku 
Aleltu. The arrangement of canals and pipes
would depend on the development of the other
basins.
As indicated in the following section, the
Weserbi would be better developed independently 
of the Gur. Scheme la-2 would therefore be a
small contribution to the Aleltu development and 
also depends on the development of the Agat or
other rivers to the east. It would not be a
part of the first stage development and was, 
therefore, not evaluated further in the recon
naissance studies. It should, however, be con
sidered in future studies in conjunction with 
the Weserbi.
2 - Weserbi Basin Scheme
The Weserbi River is about 30  km long.  Over most of its length,  it flows northeast,  parallel to  and  just west of Highway  3.  The upper reaches of the Weserbi River are very  flat,  resulting  in  large "inundation" areas. in the middle reaches, three damsites exist.
(a)      Site 2a
The most downstream site is about 5 km upstream from the confluence with the Gur. Topography is favorable for construction of a Low dam up to 20-m height. The riverbed would be at 2,620 m.K large shallow reservoir of high regulation 
potential would be created. The drainage area
would be about 168 km2. This site offers a
number of alternative development schemes.
With development of the Weserbi at Site 2a, 
water from the Girar can be diverted into the
Weserbi reservoir adding 182 km2 of drainage
area.
Topographic details of the Weserbi at Site 2a, 
as well as other major damsites on other rivers, 
are shown on Plate 2-3.
(b)    Site 2a - Scheme 1
The Weserbi could  be developed  as an  independent scheme with  a storage dam at Site 2a.  Regulated flow could be conveyed by a reasonably straight
9-km canal, at approximate surface elevation of 2,620 m, to a head pond at the edge of the plateau above Debre Libanos. The head pond would have an FSL of about 2,620 m. A power tunnel and/or penstock could conduct the power flow to a powerhouse at Site A (see Enlargement A, Plate 2-1).
Preliminary analysis indicated this scheme was too small to be developed as the first stage of the Aleltu project, it was not evaluated 
further in the reconnaissance studies but should be reviewed when the Weserbi is studied at
prefeasibility level together with the Girar.2-17
(c)        Site 2a - Scheme 2
With       a       storage       dam       at      Site       2a,       the       Weserbi      could alternatively be developed as a diversion
scheme. The storage levels in the Weserbi
reservoir are much higher than the reservoirs to the east (i.e., the Agat, Tinishu Aleltu, Tiliku Aleltu, etc). Therefore, the Weserbi could not operate interactively with these reservoirs but
could be considered as a diversion scheme.
Regulated flow could be conveyed via a tunnel
and an open channel to the Agat reservoir at a point 2 km upstream from the damsite.
If the Agat schemes prove to be uneconomical, then the Weserbi with the damsite at Site 2a, could still be developed as a diversion scheme. The Weserbi reservoir, being 20 m higher than 
the Agat, Tinishu Aleltu and Tiliku Aleltu schemes, can be readily diverted and connected with the conveyance structure of these schemes
for power development at a powerhouse in the Zega Wedem Gorge. As indicated above, regulated flow could be conveyed via a tunnel to the Agat side of the divide. At the outlet of the short diversion tunnel, a canal could be routed 
parallel to Highway 3 in a northerly direction, between the 2,620- and 2,600-m contours close to the edge of the plateau (see Enlargement A, Plate 2-1). A steel surface conduit (inverted syphon) crossing the Agat Gorge at this location would convey power flow directly or via a short open channel section to connect into development2-18
schemes of the Tinishu Aleltu and/or Tiliku
Aleltu.
This scheme is evaluated further
(d) Site 2a - Scheme 3
Weserbi Site 2a could also be developed for
upstream regulation, releasing flow to Gur
Site 12a - Scheme 1.
(However,     as     indicated     in     the     discussion     of     the Gur     Basin,     this     scheme     would     be     less     attractive and was not considered further.)
(e) Sites 2b anc 2c
Other camsites exist at Sites 2b and 2c, upstream from Site 2a. However, at these sites considerable drainage area is lost from the Tere and Wedanjo tributaries, and longer conveyance structures would be required to deliver power flow across the divide to the Agat. Also, with development at these sites, water from the Girar could not be diverted into the Weserbi
reservoir.
(Although not appearing attractive in the reconnaissance study, these sites should be investigated further in prefeasibility studies
of the Weserbi.)
i - Agat Easin Schemes
The Agat River Basin is located just east of Highway 3. It is only about 20 km long and has a2-19
2
small drainage area of about 86 km . The river
flows north and into the gorge to join the Zega Wedem River about 4 km from the edge of the plateau.
Two damsites were identified as having potential for power development; one at the edge of the plateau, 
the second on an upstream branch of the river.
(a) Site 3a - Scheme 1
A storage dam constructed in the Agat River at
the edge of the plateau would command the largest possible drainage area. This site would be located nearest (4.5-km horizontal distance)
to the most downstream powerhouse location
(i.e., powerhouse Location A). It would, there fore, be the most suitable location for the head pond and power intake of the conveyance works of the base schemes (Agat to Jida). At this site, 
the riverbed is around elevation 2,515 m. Topo graphy would permit a reservoir FSL up to
2,600 m with a dam height up to 85 m. However, the topography (i.e., a broad valley) of the
Agat Basin near the plateau edge is not favorable for dam construction.
(Although not a favorable damsite, this scheme is evaluated in the layout optimization model because of its head pond location.)
(b) Site 3a - Scheme 2
If upstream storage could be provided (at
Site 3b and in adjoining basins), a low overflow dam could be constructed to create a head pondfoca power intake at the same site, lns
a  storage dan. The head pond would ope^,/ independently. Operating slightly above Ped level, over 50 m of head would be lost bec' this scheme. This would not be attractive Z interconnected operation with the reservol °C
rs
(Preli'
ninary
be
 uneconomic as
ana lysis indicated this scheme individual scheme and it
“t evaluated further.)
( C) site 3b - Scheme 1
An alternative to the storage development at Site 3a would be to construct a storage dam in the upstream reaches of the Agat River, at Location 3b. A smaller storage dam, 25 m in height or less, could be provided to lower the dan cost relative to Site 3a. However, the drainage area would also be considerably smaller. A long conveyance tunnel or canal would be required to bring the regulated power flow to the edge of the plateau. Therefore, diversion to the Tinishu Aleltu would probably be a better arrangement if a storage reservoir is constructed in the Tinishu Aleltu Basin.
(Preliminary analysis indicated this scheme to he uneconomic and it was not evaluated further.)
Scheme 2
At tfle same site
^°w Cost overfj b' ^nstead a storage dam
to was
' >er!nit unreg j
U at
COu^d be constructed to to be diverted via 32-21
large open Aleltu reservoir.
channel into the adjacent Tinishu
The Tinishu Aleltu and other
reservoirs to the eas
t would then provide the
regulation         for         the         Agat         inflows, would overflow the dam and spill
(Because of the low cost of this scheme, it is considered further in the project evaluation.)
4  - Tinishu Aleltu
Basin Schemes
The Tinishu Aleltu flows nearly parallel to and is located approximately 10 km east of the Agat River. Like the Agat, the basin is only about 20 km long and has a small drainage area. Also similar to the Agat, two damsites were considered to have potential for power development; one site at the edge of the plateau and a second site on an upstream branch of the river. The schemes of development are also similar to those on the Agat.
(&) Site 4a - Scheme 1
High floods downstream.
At the edge of the
plateau,            the Tinishu Aleltu
Valley       is       very       broad       and       unsuitable       for       a       major damsite.        The        most        downstream        location        for        a storage dam on the Tinishu Aleltu is about
1.5 km from the edge of the plateau. At this point, a drainage area of about 77 km^ is available. The riverbed is around elevation
2,525 m, slightly higher than Site 3a (at the
gorge) of the Agat.  Maximum ground the valley  sides are about the same permitting  a reservoir FSL up  to  el 2,600 m with a dam height of up to
elevation on as the Agat, 
evation 75 m.2-22
The Tinishu Aleltu storage reservoir could be interconnected as a base scheme with the Agat storage reservoir at Site 3a. The power tunnel intake could then be located at the Agat or the Tinishu reservoirs. From the Tinishu Aleltu reservoir, a short 1.5-km tunnel and penstock could be considered to connect to a powerhouse at Sites A, B, C or D.
Similar to the lower Agat damsite, the broad valley of the Tinishu Aleltu at Site 4a is not favorable for dam construction.
(Preliminary analysis indicated this scheme to be unattractive. However, to confirm prelimi nary findings, this scheme was evaluated further in the layout optimization model.)
(b)      Site 4a - Scheme 2
Also, similar to the Agat, an overflow dam and head pond could be constructed at Site 4a, or nearer to the gorge face, if upstream storage could be provided economically.
(However, because of the significant loss of head, this scheme in preliminary analysis was determined to be unattractive and not evaluated further.)
(  Site 4b - Scheme 1
c
In the upper reach of the Tinishu Aleltu, a storage dam could be constructed. The oir could provide the upstream regulation2-23
for the Site 4a - Scheme 2 development and/or could be connected to the Agat and/or the Tiliku Aleltu base scheme. In the latter case, there
is a considerable loss of drainage area.
(As in the case of the Agat, this upstream
storage scheme was found to be unattractive by preliminary analysis and was not considered further.)
(d) Site 4b - Scheme 2
Similar to the Agat, an alternative diversion scheme for utilizing the Tinishu Aleltu flows would be construction of a low cost overflow dam at Location 2b and regulation of these flows by other reservoirs. The disadvantage of a much reduced drainage basin would be partly offset by the fact that the Tinishu Aleltu flows could discharge almost directly into the interconnec tion tunnel (by adopting a drop inlet at the dam and a downpipe to the tunnel) between the Agat and Tiliku Aleltu developments. If the Agat scheme is not economic, then the Tinishu Aleltu flows could be discharged directly into the
power tunnel leading from the Tiliku Aleltu reservoir to the Zega Wedem Gorge.
(This scheme was evaluated in the layout optimization model.)
5 - Tiliku Aleltu
Basin Schemes
The Tiliku Aleltu is about 4.5 km east of the Tinishu Aleltu. It flows north and essentially parallel to2-24
the       Agat      and       Tinishu       Aleltu       rivers.       However,       it       is         a much larger and longer river basin, as shown on
Plate 2-1. Three damsites were initially identified, 
one at the edge of the plateau, one in the lower
reach of the river and a third in the middle reach of
the river just downstream from inundated areas.
(a)      Site 5a - Scheme 1
For 5 km upstream from the gorge, the topography is fairly flat in the area of the Tiliku Aleltu 
River. No suitable storage damsites exist. However, if upstream storage is provided (at
Sites 5b or 5c), an overflow dam could be con structed close to the gorge to create a head 
pond for a power intake. The head pond would operate independently. A power tunnel or
penstock could convey power flow to a powerhouse at Sites A to D in the Zega Wedem Gorge.
Riverbed elevation of the Tiliku Aleltu at Location 5a is about elevation 2,515 m. About 70 m of head is lost relative to upstream
sites.
Although this scheme at Site 5a suffers from reduced head due to riverbed elevation, it bene fits from a short conveyance system. However, the value of the lost head is more significant.
(Preliminary analysis indicated this scheme to be unattractive for further evaluation.)2-25
(b)        Site 5b - Scheme 1
About 7 km upstream from the edge of the Zega
Wedem Gorge, the Tiliku Aleltu River runs over a
distance of about 2 km through a fairly narrow
valley. The local topography in this stretch of
the river is most favorable for dam construction 
with the best site appearing to be 0.5-km
upstream from the Muke Turi-Lemi Road. The
drainage area of the Tiliku Aleltu River at this
point is 458 km2 and this scheme can therefore
be a major contributor to the development of the
Aleltu project.
Dam height would be limited by the right bank 
topography, permitting a reservoir FSL up to 
about elevation 2,600 m. This would create a
narrow reservoir, about 10 km long with a
surface area which is small compared to its
drainage basin. Riverbed elevation is around
2,555 m at the selected damsite and the Tiliku 
Aleltu dam would be up to 45 m high (at
elevation 2,600 m).
Because of the distance to the Zega Wedem Gorge, 
it would be best to interconnect this reservoir
with the Tinishu Aleltu as a base scheme and
convey       power       flow       via       the       Tinishu       Aleltu       or       Agat
reservoirs
to 
the            powerhouse.            Alternatively, 
if
the Tinishu Aleltu and Agat schemes are not economic, a power intake could be provided at
the Tiliku Aleltu. Alternative powerhou^' v
:
-u v 
Locations A to E could be considered.(This        scheme        with        alternative        powerhouse        l
c
Oa
tions        is        evaluated        in        the        layout        °PtimizatlOn model.)
( c)     site 5c - Scheme______ 1
Preliminary calculations indicated that even with the maximum possible dam height at Site 5b the total flow of the Tiliku Aleltu could not be regulated. Therefore, an upstream damsite was considered.
Topographic maps indicated possible sites for construction of a low storage dam, for the purpose of impounding upstream flow and provid ing additional flow regulation in the Tiliku 
Aleltu Basin.
However, the mapping available in this area (east of longitude 39°) is only at 1 :250 000 
scale. Field visits revealed that
the area was generally flat and no obvious attractive site could be identified
being flat, any reservoir created would have a very large surface area to storage volume
r atio with attendant high evaporation losses. In the course of this study it was determined that
3n uPstream site development was not requif t0  establish feasibility of the Tiliku Aleltu
Basin2-27
- if evaluated, it will probably be uneconomic
(as lower cost storage can be provided in the Jida reservoir)
- detailed topographic mapping at a scale of about 1:2,000 will be required; the topo graphic survey and evaluation of this scheme could be carried out at the feasibility study stage.
(This scheme was not evaluated further in the reconnaissance studies.)
6 - Jida Basin Schemes
The Jida River has the largest basin in the project
area with a drainage area of about 749 km2. There
fore, the regulation of this river is most important
to the development of the Aleltu project. Four
promising storage sites for dam construction, as well
as a pondage site at the edge of the plateau, were identified over a 35-km reach of the river.
(a) Site 6a - Scheme 1
Similar to other basins on the plateau, the
river valley is very broad near the gorge and not suitable for a major storage dam. However, with upstream storage, an overflow dam could be constructed close to the gorge. Riverbed eleva tion of the Jida at this location is around elevation 2,500 m. About 80 m of head is lost relative to upstream sites.2-28
(Preliminary analysis indicated this scheme to be uneconomic and it was not evaluated further.)
(b) Site 6b - Scheme 1
Going upstream from the face of the gorge, the breadth of the Jida Valley gradually narrows such that the most downstream location for a storage dam is about 3 km from the edge of the Zega Wedem Gorge. At this location, an FSL of elevation 2,560 m would be feasible and the reservoir would operate independently. The reservoir, however, would be too small to effect sufficient river regulation. Construction of a second dam in the upper reaches of the Jida River could provide the required regulation. Such sites exist at Sites 6c, 6d and 6e as described below.
(c)      Site 6c - Scheme 1
A storage dam could be constructed about 12 km upstream from the gorge and about 3 km upstream from the Muke Turi-Lemi Road as shown on Plate 2-1. In this area, a number of dam align ments can be identified. The riverbed elevation is about elevation 2,540 m. Topographic con tours of the valley sides are well above eleva
tion 2,600 m and the dam could be in excess of 60 m high.
The impounded reservoir could be interconnected and operate interactively with the TiliRu Aleltu2-29
and Gomoro, utilizing short conveyance
structures.
(d)        Site 6d - Scheme 1
About 23 km upstream from the gorge, the Jida
Basin narrows to form a natural damsite. The
riverbed is around elevation 2,570 m. A reser
voir at FSL up to 2,620 m could be built with a
storage dam height up to about 55 m. This site
could operate as one of three alternatives
-       as     a     single     dam     scheme     on     the     Jida     River, interconnected        with        the        Tiliku        Aleltu        and Gomoro
-   as an upstream storage reservoir releasing flows to Sites 4c or 4b downstream
-   as a diversion scheme to the Tiliku Aleltu and/or Gomoro (i.e., operation at different reservoir levels).
(e)     Site 6e - Scheme 1
A fourth storage damsite is located in the upper reaches of the river, about 35 km upstream from the gorge. The riverbed is at 2,580 m and a reservoir at FSL up to 2,620 m could be created with a dam height up to 40 m. This site could only operate as
- an upstream regulation scheme with releases to the downstream reservoirs2-JU
- a diversion scheme to the Tiliku Aleltu Basin.
The reservoir volume impounded by a combination of any of the alternative schemes could provide suffi cient storage capability to fully regulate the
inflow. Additional regulating potential could be made available, if required, to augment the flow regulation from other schemes, such as the Tiliku Aleltu.
with four damsites anc possibly combined schemes, the selection of dam locations for the Jida scheme requires further detailed study. The Jida alterna tives are reviewed ir. the project screening evalua- t .or, Section 2.5.4.
Kob; -Gomorc
Mt « ' Schemes
<
Wot . —gobofc . ver
carries       the    combined flow of
c i .<     Wot . anc Gqbgtc r         vers.
It is
only about 5 km
AJ
a nr
^«»OBOJ            At we
‘w     -,       V.J
conainc t .urge -** tne ara. *ye areas
n
c -^r’ec Cr.acna
C.4>( a
»                . i
•      . a t < i j           ■«<
* ““ 
re^.
* 1 w* I
« . : ct c > o :
flt;ori
ofcc
re,,-..!.,
dra      lnage of the
one Jinjer c
j: ‘
’i
N.
ftber-
nbtruct
i •         . s. ! .
r. Jc
cr
■ o!e "-e : nr low.
. tAn           & 1
1  i ■ ihu b !
• A-jiUp«tr«*n. hci*;» d*m»
(cn the Gomer0) and >;<• ^a
on the Robi-Goracr have tiee* tion with thoee upatroan at ra^e •
tw
( <'M>
COf!
• i i ’ >•
< h# •
t< t * •
♦•
* *♦
a i de t ed i r
(a) Site 7a
Schomo 1
V•|i•)
<X>n• ’ t vt •
I•
extremely broad and not f.ivotibl® ! t
tion of a large dam.
However, if upstream storage in provided,
Site 7a at the edge of the gorge could be tun
able for constructing an overflow dam, to cr«r»'e an independent head pond at an FSL of about
2,500 m. A power canal and penstock could convey power flow to a powerhouse at Sites E or F in the Zega Wedem Gorge.
Advantages of this scheme are the large drainage area, the short power conduits and low cost storage dams. Disadvantages are the loss of
head. In the gorge, head is lost by turbining 
the power flow at powerhouse Sites E or F com pared to more downstream Sites A or B. On the plateau, the runoff is stored in upstream reser voirs on the Gomoro and Robi where FSL is about 2,600 m. Thus, by releasing water downstream to Robi-Gomoro Site 7a, instead of interconnecting to the Jida and Tiliku Aleltu which also operate at FSL of 2,600 m, about 100 m of head is lost.
(Because      of      its      large      drainage      area,      this      scheme is evaluated further.)k.
lb) sitejbj^scwnej
Upstream, 3 km from the edge of the gorge, a
major storage dam could he constructed with a
reservoir FSL up to 2,560 m. The riverbed 
elevation is about 2,515 m making the dam height
up  to 45 m. The reservoir would operate
independently and upstream storage would still
be required on the Comoro and Robi rivers.
The advantage of this scheme (relative to 
Site “a - Scheme 1) is that the storage dam
would permit full regulation of the drainage
downstream from the storage dams on the Gomoro 
and Roti. This drainage area is about 504 km2 
(if Jinjer flows are also diverted into the
Robi-Gomoro, see Scheme 10) and large enough to 
oe ar. attractive development.
Similar to  Site "t - Scheme 1,  a power canal and penctocr.  cou.c convey  power flow to  a powerhouse
•t Sites £ or F ir. the Zega Wedem Gorge.
tht  preliminary analysis, this scheme
economic but not competitive with &*** scheme oeve.opments for first stage
Qe vt? .'vjilfc-r.t
7r
• *ge , therefore, not evaluated
►
I
1 r
i 7 r
f'-rtnci .  .
r
study, but should be considered 
future Of Je- •
*             of the ultimate project.
•• L®rr*at
c
:'»Ue nt
*or bite 7L would be ab °
• ti*t total kot)i-Gomoro2-3 3
-   developing Comoro ar. * !• : .              - *♦•»•♦• fa *r which fully ragulata and divert then live?
flows to the Tiliku Aleltj Jlda ■.■•»,.**
a powerhouse site in the Bernina Gt;*
-   developing Robi-Gomoro Site 7b to regulate t * flow from the Jinjer diversion scheme .and the drainage area downstream from th* proposed *a* locations at Sites 8a and 9a
-   conveying the power flow via a 7-km tunnel from Site 7b connecting into the Tiliku
Aleltu/Jida complex.
In the latter case, the Robi-Gomoro reservoir at Site 7c is lower than the Tiliku Aleltu and Jida reservoirs and, therefore, the reservoirs could not operate jointly. The Robi-Gomoro reservoir would be operated independently. Special operating provisions would need to be imple mented. These provisions could be
- to incorporate hydraulic control gates or
valves in the tunnels in order to be able to switch operation from one reservoir to another at varying operating heads
to pump from the interconnection tunnel into the power tunnel or into the Tiliku Aleltu reservoir.
(Development of Site 7b - Scheme 2 in prelimi nary analysis appeared to be attractive but would only be developed after Sites 8a and 9a on2-J<*
the Gomoro and Robi rivers. It was therefor not evaluated further.)
8 - Gomoro Basin Schemes
The Gomoro River flows northwest, generally parallel to the Jida and about 10 km to the east. Before reaching the Zega Wedem Gorge, the Gomoro joins the the Robi to form the Robi-Gomoro River. Upstream from the confluence, the Gomoro Valley has low relief for about 10 km. Farther upstream, there is greater relief and the topographic map indicates four poten tially attractive sites for storage dams.
(a) Site 8a - Scheme 1 
A  storage  dam  over  30-m  high could be
constructed    about   10   km
upstream from the con-
fluence.
At this location, topographic contours
are    at    about    elevation 
anc over
2,580 m at the riverbed
riverbec,
height of
2,600 m on the ridges. With the high 
this site appears to offer a low dam
about 20 r or less.
crest length is elevation 2,600
-arge if the FSL is
very wide,
m, and dam
The Gomoro reservoir
. ts drainage area of potent.c. for storage in 
hig.n. would
However,         the being     about     2     km volumes would be
oe large compared
dam at
to
248    r.rni and there is a
Hie n t e
. .e
for 100* flow
i°ca r. Meaeir. Or
of
aacition 
regulation of
this aamsite with
'• uufi,
r
nersina Gorge would * -lC be best developed
to require- the Gomoro. respect to the indicate that
aS2-35
- a base scheme interconnected with the Jlda Robi reservoirs
- a storage
reservoir providing
upstream
re ju 1 a
tion for a
downstream site on
the Robl-
Gomero
River (see
Scheme 1).
Site 7a - Scheme 1
and Site
7b -
(This       scheme optimization
(b) Sites 8b, 8c and 8d
was      evaluated      in      the      layout model.)
About 3 km upstream from Site 8a is another
damsite, 8b, with similar topography. This site
offers the same type of development as Site 8a.
Two additional sites have been identified about
another 4 km upstream. These sites, 8c and 8d, 
could also be developed similarly to Site 8a, 
but being farther upstream, interconnection 
tunnels for interactive reservoir operation with 
Jida and/or Robi would be longer and more
costly. In addition, drainage area would be
lost.
(These schemes, being similar to Site 8a -
Scheme 1, were not evaluated further.)
No        other        sites        appear        attractive        for        additional upstream flow regulation.9 - RQbi Basin Schemes
over most of its length, the Robi River flows northwest in the direction of the Bersina Gorge. About 6 km from the edge of the gorge, it turns and flows in a westerly direction, parallel to the
Bersina Gorge. It is joined by the Gomoro River to form the Robi-Gomoro River. Like the Gomoro Basin, the Robi has low relief for a distance of about 13 km upstream from the confluence. Within a few kilo meters farther upstream, the valley relief becomes more pronounced and two damsites appear attractive. In the area of the potential damsites, the Robi is
about 8 to 10 km northeast of the Gomoro River and about 33 km in a straight line from a powerhouse at Site A.
(a)    Site 9a - Scheme 1
At about 14 km upstream from the confluence with the Gomoro, the topography permits construction of a dam with a reservoir at FSL in excess of
2,610 m. The riverbed elevation at about
2,590 m is higher than the Gomoro, Jida and Tiliku Aleltu. Dam height could be 20 m or greater. The drainage area at this site is
about 417 km2.
Based on indications from the available
1:250 000-scale mapping, there may be a need to construct low saddle dikes on low-lying ridges. The reservoir volume appears to be sufficiently 
tge for overyear storage and additional regu lation could be made available, as required, 
the Chacha and Jinjer diversion schemes.2-37
As a result of its location relative to the
Bersina Gorge and nearby rivers, the Robi has several development possibilities.
-   The Robi reservoir could be interconnected and operate as a base scheme with the western basins of Gomoro, Jida, etc, by constructing a
5-km long interconnection tunnel to the Gomoro reservoir. Robi River flows would then pass through the Gomoro, Jida and other reservoirs and on to the main powerhouse in the Zega Wedem Gorge. This is a water transport dis tance of about 45 km. The Robi Basin being at higher elevation than the Gomoro (riverbed elevation 2,588 versus 2,577 m) could have control gates installed on the interconnection tunnel and operate part of the year as a stor
age diversion scheme and part of the year as a base scheme. This refinement was not eval uated in the present reconnaissance studies
but should be studied in further prefeasibi lity studies of the Robi.
-   Alternatively, from this same storage site, a power tunnel could be taken to a separate powerhouse at Locations G or H in the Bersina Gorge. Such a development could incorporate the flows from the Chacha, and possibly the Jinjer rivers. If highly economic, Gomoro, Jida and Tiliku Aleltu flows could be added in stages as base schemes. The generating head, however, would be about 25% less (at Site 6) than that available at powerhouse Location A. Transmission lines would be longer and the development would also have to carry the costZ-JO
of a separate powerhouse access if
Powerhouse   A   or   others   in   the   Zega   Wedern   Gorge were to be developed separately.
As a storage reservoir,  the Robi can  provide upstream regulation  for a downstream site on the Robi-Gomoro River.
(The above alternative developments are
evaluated in the layout optimization model.)
(b) Site 9b - Scheme 1
There is an alternative damsite, 9b, about
1.5 km upstream from Location 9a, which possibly 
could eliminate some saddle dikes. Topographic
conditions are otherwise similar to Site 9a. 
Selection between the two schemes should be made
at prefeasibility level studies.
(The sites being similar, evaluation of Site 9a -
Scheme 1 will apply to this basin. Therefore,
Site   9b   -   Scheme   1   is   not   evaluated   separately   in   the layout optimization model.)
No    promising    sites    for    additional    upstream    storage could be identified.
10 - Jinjer Basin Schemes
The only river discharging into the Bersina Gorge is the Jinjer River. The Jinjer River has a small drainage area, and power development in the Bersina
r1 r r q
r
I
Gorge    is    not    economically    attractive at a high elevation (riverbed is at
•     This     river approximately
is2-39
2,620 m) and could be diverted to a Robi scheme or
Robi-Gomoro scheme. Two dam locations were identi
fied .
(a) Site 10a
About 2 km upstream from the edge of the Bersina
Gorge, just along the local road, is a potential
damsite. However, the river valley is very 
broad and since the drainage area is relatively 
small, a storage dam would be uneconomical. A
low cost overflow dam to divert unregulated 
flows may be economic. Diversion into the Robi
would require construction of a 7-km long open 
canal. For diversion into the Robi-Gomoro, a
short open channel to the Donji River would be
required.
(b)      Site 10b
A     second     site,     1.5     km     farther     upstream,     allows     a shorter length dam but the runoff from the
Gomanya       tributary       cannot       be       utilized.       This       site would have to be developed similarly to
Site 10a, with an overflow dam.
(Preliminary analysis indicated development of large storage dams was not attractive. However, diversion of the Jinjer could be attractive.
Since the Jinjer schemes both depend on the Robi or the Robi-Gomoro schemes being built before hand, they were not studied further. These schemes should be studied together with future studies on the Robi and Robi-Gomoro.)11 - Girar Basin Scheme
The Girar River initially flows in a southeasterly 
direction toward the Weserbi drainage basin. it then 
turns south to join the Muger River about 25 km west
of Muke Turi. Hydroelectric development of the Girar
in the Muger Gorge is not practical. However, the
runoff from the Girar River could be diverted into 
the Weserbi 3asin for power development in the Zega
Wedem Gorge.
(a) Site Ila - Scheme 1
A small dam could be constructed at a location nearest to the Weserbi drainage basin, about
14 km west from Debre Tsige and just 2 km north from the local road to Fital. The drainage
basin at this point is about 172 km2. The
fully regulated power flow would have to be
pumped in an easterly direction from about
elevation 2,540 m to about elevation 2,660 m, 
over a distance of about 5 km (see Plate 2-1).
From there, it would be discharged into the Tere
River which is a tributary of the Weserbi
River.
This Girar scheme depends on the development of the Weserbi beforehand.
(Although the Girar scheme has been identified as being attractive in the long-term development
of the Aleltu project, it has not been evaluated in detail in this study since it will come after
the Weserbi.)
j2-41
12 - Duber Basin Schemes
The Duber drainage basin is located in the central southwest section of the Aleltu project area. A
number of streams flow into an inundated area from which the Tinishu Duber and Tiliku Duber rivers emerge. These rivers drain in a westerly direction 
into the Muger River. The drainage area at the lower
2
edge of the inundated area is about 312 km .
Hydroelectric development of the Duber in the Muger Gorge is not practical. However, as with the Girar, 
part of the Duber runoff could be diverted toward the Zega Wedem Gorge. Various alternative schemes can be identified to utilize the runoff from the Duber
Rivers and to divert regulated or unregulated flow
into the Tiliku Aleltu or Weserbi basins, as shown on Plate 2-1. These schemes are as follows.
(a) Site 12a - Scheme 1
Downstream from the confluence of the Tinishu Duber and Tiliku Duber, a single storage dam could be constructed. Flows could be diverted by open canal and tunnel to the Weserbi.
(Preliminary analysis indicated that upstream sites were more attractive and this scheme was not evaluated further.)
(b) Site 12b - Scheme 1
Upstream from the confluence storage dams (bl and b2) on both the Tiliku Duber and Tinishu Duber rivers could be constructed to create acommon reservoir at an FSL of about 2,620 m. Regulated power flow could be conveyed by tunnel or open canal system to the Weserbi or Tiliku Aleltu reservoirs. The length of diversion to the Tiliku Aleltu (12 km) is less than to the Weserbi (14 km). However, due to unfavorable topography, diversion to the Tiliku Aleltu would be by tunnel construction, while diversion to 
the Weserbi would consist of open channel and tunnel sections which would be lower in con struction cost. The disadvantage of this scheme is that the topography of the damsites is not favorable for large dam construction.
(Because of the possible influence on develop ment of the Tiliku Aleltu, preliminary investi gations were undertaken. It was found that this scheme was less attractive than development at Site 12c below.)
(c)     Site 12c - Scheme 1
Unregulated flows could be diverted to the Tiliku Aleltu Basin by the following construc tion.
— An overflow dam (c3) and saddle dike (c4) built on the Tinishu Duoer River would divert runoff via the inundation area into the Tiliku Duber River (see Enlargement B, Plate 2-1).
- Small diversion dams (cl, c2 and c3), provid ing for storage of flood peaks only, could be constructed in the Tiliku Duber River just downstream from the inundated area.2-43
An uncontrolled tunnel diversion would dis charge the partly regulated flow over a distance of about 12 km into the Tiliku Aleltu reservoir.
The advantage of this scheme is that the construction of large dams in the Duber Basin can be avoided, while inexpensive storage can be provided by reservoirs of the base scheme.
(This scheme was evaluated in detail.)
(d)      Site 12d - Scheme 1
A series of small overflow dams (dl to d4), in the upstream reaches of the Duber and a 25-km long cut and embankment canal system could be constructed to collect and discharge Duber run off into the Tiliku Aleltu River (see Enlarge ment B, Plate 2-1). The diversion canal is long and the topography flat. Therefore, the over flow dams would have to be low structures, and the canals would have little gradient and be constructed wider than normal designs.
(Because of the topography and hydraulic design conditions, extensive geological field investi gations and hydrology studies would be required to appropriately assess this scheme. Topo graphic mapping at about 2-m contour intervals would also be required. However, preliminary analysis indicates that this scheme is not sufficiently attractive to be considered until
later stages of development of the ultimate project.)^2 - sibilu Basin Scheine
A substantial drainage area of about 518 km2 could 
be added to the Aleltu project by diversion of the
Sibilu River into the Duber Basin. A dam up to 50 m high could be constructed northwest of Chancho. This
scheme would comprise about 20 km of open canal and tunnel diversions with a number of control dams, pumping stations and pipelines to convey power fiOw to the Tinishu Duber River. The Duber diversion scheme would have to be designed to accommodate the additional power flow. The Sibilu scheme depends on the Duber scheme being developed beforehand. The Sibilu is also a complicated and costly scheme and the project layout and economic evaluation will require considerable effort.
(This scheme has been identified but not evaluated in this reconnaissance study. It should be evaluated in conjunction with further studies of the Duber schemes.)
14 - Chacha Basin Scheme
ihe most promising diversion scheme of the Aleltu project is the Chacha River diversion. Its large drainage area is located at the eastern side of the Aleltu project and runoff is in a northerly direc tion, into the Giamma River system. About 10 km north of the village of Chacha, the river flows through   series of deep narrow valleys and crosses
a
the local road from Highway 1 to Mendida. At this point the drainage area is about 590 km2.2-45
A number of topographically favorable dam locations
can be identified in this area. Dams would be high, 
but small in volume and the resulting reservoirs
would provide a considerable degree of flow regula
tion. The potential for additional upstream storage
at the crossing of Highway 1 from Addis Ababa to 
Debre Berhan was investigated during early site
visits. This proved to be unattractive.
In the main dam area, the riverbed is at elevation
2,710 m, considerably higher than the maximum Robi
FSL of 2,600 m, so that diversion could be made into 
the upper reaches of the Robi River.
It is noted that a future small power installation 
could be considered at this site. However, to 
develop the full available head, long conveyance
structures to the rim of the Robi reservoir will be
required affecting the economic potential of such a
scheme. This option was not studied.
Diversion of partly or fully regulated flows would be
by means of a 6-km long tunnel as shown on Plate 2-1. 
Additional regulation, if necessary, could also be
provided by the Robi reservoir.
(This       scheme       was       evaluated       in       the       layout       optimization model.)
The above economically promising schemes were consi dered in the multidisciplinary studies described in the following sections.2-46
2.4 - Investigations and
Project Layouts
2.4.1 - Topographic Mapping. Aerial
photographs and Surv y-------------
The entire project area is^ vrea is covered by 1 :250 000-scale
mapping wit•. u c    h 5(Jn m-m c conuuui       ontour intervals• Mapping at
1 : 50 000-scale with 20-m contour intervals is available over that part of the project area east of longitude
39°00' (see Plate 2-1). This covers about one third of
the project area. Aerial photographs are available
over an area covering the Zega Wedem Gorge and a 5-km wide strip of the plateau adjacent to the gorge.
The Ethiopian Mapping Agency is preparing 1 :50 000 topographic mapping to cover the project area east of longitude 39°00*. None of this mapping was available for the current study but would be required to confirm
or extend studies associated with the ultimate develop ment of the Aleltu Basin.
The mapping was scheduled to be completed in September 1984 and this was the basis of the original work 
program for the Aleltu studies. The unavailability of
a full set of 1:50 000 topographic mapping imposed an additional work load on the EELPA land surveyors and reduced the extent of reconnaissance work that could be performed on the Gomoro and other eastern basins.
Topographic field surveys, carried out to augment the topographic maps, comprised an open traverse from the THiku Aleltu to the Jinjer Basin and cross section
surveys of the reconnaissance dams2-47
Survey monuments were established near the selected 
damsites at each river. Vertical control was trans
ferred to these monuments from an existing geodetic
benchmark near Debre Libanos.
The field surveys are not tied to the Universal
Mercator Grid in the absence of horizontal control
stations in the study area.
Dam Profiles
During the site verification program, topographic
profiles were established at the proposed Tiliku
Aleltu, Tinishu Aleltu, Jida, Gomoro, Robi and Jinjer
dam locations, based on a 40-km long line survey 
carried out from the Tiliku Aleltu damsite to the
Jinjer damsite. Plate 2-3 shows the modified pro
files, resulting from the survey information (dotted 
lines).
It can be observed from Plate 2-3 that the surveyed 
profiles of the Jida, Gomoro and Robi damsites are
significantly different from the profiles derived 
from the 1:250 000-scale mapping. At the Robi site
particularly, the riverbed elevation was found to be
about 35 m higher. Adjustments such as this are to 
be expected with 1:250 000-scale mapping, underlining 
the importance of the site verification surveys.
2.4.2 - Hydrology and
Reservoir Studies
(a) General
Hydrologic      studies      of      basins      in      the      Aleltu      project area were undertaken to assess- the available flows for hydroelectric
generation
-   the required reservoir volumes (and dam heights) at varying degrees of flow regulation for separate and integrated reservoir operation
-   the flood flows which will govern the design of diversion works and spillways.
The hydrologic studies are reported in detail in Appendix B and are summarized in the following subsections, covering
-   determination of monthly inflow sequences
-   reservoir regulation analyses
-   evaporation studies
-   flood flow studies and reservoir routing.
(b) Determination of
Monthly Inflow Sequences
To derive an appropriate data base for generation of subbasin stream flows, the recorded flow data of the following four hydrometric gauging stations were analyzed
-   Muger River at Chancho
-   Chacha River at Chacha
-   Beressa River at Debre Berhan
-   Awash River at Melka Konture.
The available records of these stations are about 20 years of which some years are incomplete or missing, simple statistical functions and2-49
regression equations were used to fill in the missing data. The stations were then compared to select a most representative data set from which inflow sequences for the Aleltu subbasins could be derived.
It was found that the Muger and Chacha stations would be most suitable for this purpose. Both stations are within the project area with the Chacha located at the east end and the Muger at the west end. Both stations have drainage basins comparable in size with the areas of the Aleltu subbasins. However, historical correlations of
the flow data were found to be poor.
A more extensive analysis was, therefore, required to determine representative inflow data. For this purpose, a rainfall data analysis was undertaken. Again, it was found that reliable historic rain
fall correlations did not exist in the Aleltu project area. It was, therefore, apparent that deterministic methods could not be employed to derive the required inflow sequences.
Rainfall isohyets were drawn to determine the variation of the rainfall over the project area
and probabilistic methods were used to establish the rainfall-runoff relationships in the basin. A Markov single step sequential simulation method was then used to generate monthly flow series based on the records of the Chacha gauging sta tion. The resulting inflow sequences each contain 20 years of monthly runoff data and were used for the reservoir regulation studies. Monthly mean summaries of the flow data are contained in Table 2.3.table 2.3
MONTHLY MEAN SUMMARIES OF
SUBBASIN INFLOW SEQUENCES
Moan Discharge* (m^/s)
SI to          SubbasIn
2a            Wosorbl
3a            Agat
4b Tlnlshu Aleltu
5b T111ku Aleltu
6c             J 1 da
8a            Gomoro
9a              Robl
10a           J 1n Jer
12c           Duber
14a           Chacha
Jan           Fob          Mar           Apr           May           Juno        July           Aug           Sept          Oct          Nov          Dec          Yr
0.06          0.06          0.11          0.09          0.12          0.10             7.10         16.70              3.12          0.16          0.10          0.06          2.3
0.04           0.03          0.054        0.05          0.06          0.40             4.30             0.30             1.71         0.09          0.06            0.03          1.3
0.01          0.01           0.05          0.03          0.03          0.02             1.40             3.40            0.71           0.04          0.02          0.01          0.5
0.17           0.22          0.29          0.24           0.29          0.27           15.70           40.00             0.67           0.32          0.27          0.17           5.6
0.23          0.21           0.32           0.38           0.40          2. 10            31.50         50.40           10.95          0.50           0.42            0.25         8.1 0.07           0.07           0.12           0.12           0.13          0.41               0.00          16.20             3.40           0.10            0.13           0.10         2.4 0.12           0.13           0.26          0.24            0.20         1.77             16.50          26.30             5.35            0.35           0.20           0.12         4.3 0.07           0.11           0.11           0.10            0.13         0.11               9.30           13.10            3.37            0.19           0.13           0.13         2.2 0. 13          0.11           0.18           0.18            0.20         0.83             13.70           25.60            6.12            0.30           0.23           0.13          4.0 0.21           0.25           0.29           0.31             0.33           0.40            20.30           41.50            9.60            0.48           0.38           0.20          6.2
-502-51
(c)      Reservoir Regulation Studies
For each subbasin, reservoir storage volumes were
calculated for various degrees of regulation 
utilizing a simple reservoir balance model. These
calculations permitted the definition of the
relationship between reservoir storage and power
flow.      The      results      of      these      analyses      are      shown      in Table 2.4.
(d)      Evaporation Studies
Evaporation        losses        were        calculated        to        determine their effect on power flow.
Free surface evaporation and natural evapotrans piration in the Aleltu Plateau area were deter mined as 1,100 and 675 mm/yr, respectively. The net reservoir loss was thus 425 mm/yr. The
results of these calculations are shown in Table 2.5. An average loss of about 3.5% of available inflow was found for all subbasins and less than 4% for the main base schemes at a mean reservoir operating level of 2,590 m. Being low and within the expected error of the hydrologic analysis, evaporation losses were not included in the reservoir balance model. (Evaporation losses are accounted for in the prefeasibility studies.)
(e)      Flood Flow Studies
and Reservoir Routings
To size construction diversion and flood handling facilities, estimates of flood peak magnitudes and shapes of inflow hydrographs were derived. Daily peak flow recordings of a large number of stations2-52
TABLE 2.4
RESERVOIR-REGULATION RELATIONSHIPS FOR MODEL RUN—
Site        Subbasin
Drainage
Area _
M
Long-Term ean
Annual Flo*—
Reservoir Volume Required (Mn^)
for Degree of        Reau 1 at 1on (J)
50 60_ 70 80 90 100
(km^)
(rn^/s)
28.5
42.2
72.6
107.7
143.1
2.30
182.0
2a
Weserbl
168
5.6
6.8
9.1
11.7
0.47
15.3
20.4
4b
Tlnlshu
Aleltu
37
66.3
106.8
149.2
202.9
297.7
5.57
401.1
5b
Tlllku
Aleltu
458
6c
Jlda
749
8.16
148.2
193.2
238.2
372.5
525.5
707.2
35.9
49.3
73.4
129.8
198.0
8a
Gomoro
225
2.41
27.5
9a
Robl
437
4.29
49.8
73.4
97.1
132.9
197.1
279.5
238
2.24
40.3
52.6
65.9
97.0
133.0
189.8
10a
Jlnjer
12c
Ouber
312
3.96
53.3
75.3
100.1
134.6
169.0
216.5
14
Chache
590
6.19
74.1
109.8
145.6
193.7
249.0
304.33-53
TABLE 2.5 evaporation losses
Site         Subbasin
Drainage Area
(km )
2
2a           Weserbi                         168
Mean Reservoir Level
(m) 2 620
Reservoir Area
(km2) 10.0
Net Evaporation Loss     as     Percent of Runoff
5.8
3a
Agat
92
2 590
8.0
8.6
4b
Tinishu Aleltu
37
2 590
0.5
1.4
5b Tiliku Aleltu 458
6c Jida 749
8a Gomoro 225
9a Robi 437
2 590
2 590
2 590
2 590
4.0
36.0
16.0
3.0
1.0      ) Base
) schemes
6.0      ) average
) evapora- 9.0      ) tion
) equals 1.0      ) 3.9%
10a
Jinjer
238
2 640
7.0
3.7
12c
Duber
312
2 610
1.5
0.5
14a         Chacha                          590 Basin Average Evaporation
2 640
12.0
2.7
3.4located in the Omo and Blue Nile basins were COn_ sidered. These stations were tested for regiOnal homogeneity. Frequency analyses were then carried out and, from these, regional average relation ships between flood flows and the mean annual flood were derived. The mean flood was calculated as a function of drainage area and these relation ships then permitted calculation of the
1:10 000-yr flood peaks for the Aleltu project subbasins.
A representative nondimensional shape of the flood inflow hydrograph was determined by averaging the four largest flood hydrographs recorded at the Chacha station as a function of their mean annual flood and by relating the time basis to drainage area. Flood hydrographs with a 1:10 000-yr recur rence interval for each of the stations were then calculated from this basic hydrograph. Table 2.6 shows the results of the flood flow calculations.
To assess the spillway requirements for the main subbasins of the reconnaissance study, simple reservoir flood routings were carried out for various spillway widths and reservoir surcharge levels. The results of these analyses are shown in Table 2.7 and are generally valid for reservoir FSLs of about elevation 2,580 m (base schemes).
2 '4 '3 ‘ 2^2icaj_Assessment
Regional and local in the Alelt-,, 
geological conditions were assessed
Aleitu Project area for structures (at
ror the P  P
ur
ose  of designing
and          cost          estimatin          n31SSance              level),          preparing          layouts iiy e2-55
TABLE 2.6
FLOOD FLOWS OF SUBBASINS
3
Drainage
Area
Flood
Flow (m /s)
Site
Subbasin
MAF
Q 20
°10 000
2
( km )
2a
Weserbi
168
35.4
56.6
109.6
3a
Agat
92
22.8
36.5
70.6
4b
Tinishu
Aleltu
37
11.7
18.8
36.3
5b
Tiliku
Aleltu
458
73.5
117.7
228.0
6c
Jida
749
105.4
168.6
326.6
8a
Gomoro
225
43.8
70.1
135.7
9a
Robi
437
71.1
113.7
220.4
10a
Jinjer
238
45.6
73.0
141.4
12c
Duber
312
55.6
88.9
172.3
14a
Chacha
590
88.5
141.6
274.4
Note:        MAF indicates mean annual flood.TABLE 2.7
RESERVOIR FLOOD ROUTINGS
Site
Subbasin
Inflow
Outflow
Reservoir
Surcharge
Spillway Width
3
(m /s)
3
(m /s)
(m)
(m)
2a
Weserbi
-
-
-
-
4b
Tinishu
Aleltu
36
36
-
—
5b
Tiliku 
Aleltu
228
205
2.0
33
6c
Jida
327
167
2.0
27
8a
Gomoro
136
-
0.7
-
9a
Robi
220
-
1.6
-
10a
Jinjer
141
43
2.0
30
12c
Duber
172
65
2.0
10
14a
Chacha
274
185
2.0
302-57
General geological information was obtained from the 1975 geological map of Ethiopia, supplemented by Acres/ EELPA knowledge from previous site visits to other projects in Ethiopia. Seismicity data were abstracted from the detailed seismologic study carried out in 1979 by Pierre Gouin. Site-specific geological and geo technical data were obtained from the reconnaissance field investigations by EELPA and Acres specialists.
Regional tectonic discontinuity was not observed during the 3-dimensional survey. However, previous studies carried out by the United States Bureau of Reclamation (USBR) in 1964* show the existence of northwest to southeast fracture lines on the eastern extremity of
the project area.
A 3-dimensional geological traverse over the proposed project area was carried out during August 1983. Damsites were visited and it was found that foundation conditions comprise a series of thick plateau basalt flows, likely interbedded by lacustrine sediments in the lower flows. The upper basalt flows are flows predominantly characterized by columnar joints, which are mostly closed. The plateau basalts are mostly covered with a thin (0.5 to 2.0 m) layer of dark clay.
The dam abutments are, in general, gentle slopes in the order of 5H:1V and covered with residual soils and talus.
*USBR, Land and Water Resources of the Blue Nile Basin, 
Ethiopia, 1964 . 
'Based  on  the bedrock 
the damsites,  a side
maximum heights less
overburden conditions assessed 
slope of 2.5H:1V for dams with 
than 25 m and 3H:1V for heights
excess of 25 m were
considered appropriate at the
reconnaissance           level of study.
The geology of the Zega Wedem Gorge near the confluence of the Agat River is largely covered by talus. Rock outcrops in the Agat River indicate that the underlying bedrock is likely sandstone or limestone. The side
slopes of the gorge show that the thick basalt flows rest on fine- to medium-grained massive sandstone.
The proposed penstock route from the plateau to the powerhouse Location A traverses the various plateau volcanic flows, interbedded lacustrine sediments and the underlying pinkish upper sandstone formations.
The proposed powerhouse Location A is presumably located on the shale, gypsum, limestone and sandstone layers.
Geological and geotechnical conditions were considered to be acceptable at powerhouse Location A, but are likely more favorable at Locations 3 and C. However, considering the additional power generating potential
of Site A, it was decides that the Initially preferred layout arrangement at Location A would be retained Construction materials available in
are limited to large quantities nf , 
... 
, 
.. 
laDle ln the plateau area entities of columnar basalt and
small guantitles of residual clayey soil ,  ■
r
y  soil for impervious
material, obtained from inundation areas in the drain age basins. The basalt can be quarried f '
,      . 
and riprap, and crushed for filters,
for rock fill transitions and2-59
concrete aggregate. Fine-grained river sand is available in the Jima Gorge. Access is difficult, however, and hauling distances are long (about 70 km). This sand would not be economically exploitable for the Aleltu project. Sand would therefore have to be crush ed from basalt or hauled from a qarry on the plateau, whichever is more economical.
The results of the geotechnical site verification program confirmed that similar assumptions initially made regarding general geology, foundation conditions, type of dam and availability of construction materials were adequate for purpose of the reconnaissance study. No modifications to the design of the dams and power flow conveyance structures were therefore introduced.
2.4.4 - Project Layouts
Conceptual designs and layouts of the alternative developments were prepared for the comparative evalua tion in the reconnaissance studies. The work included
identification of main project cost components and determination of hydraulic and structural requirements
layout of project components within topographic constraints to suit the ultimate project development
establishment of relationships between project layout and project operation requirements, i.e., dam volume/ reservoir FSL, reservoir volume/elevation, etc.Conceptual layouts, as shown on Plate 2-2, were developed utilizing the available 1:50 000 and 
1:250 000-scale topographic mapping and additional survey work reported in Section 2.4.1.
The layout work was limited by the need to use the
1:250 000 mapping for two thirds of the study area. 
This mapping provided 50-m contour intervals on the plateau, thus requiring broad assumptions of topography for reservoir volume determinations and, in preliminary layout studies, until field surveys could be completed. However, with the mapping supported by field topo graphic surveys, the locations of dams, spillways and interconnection tunnels were fixed for each scheme of
the reconnaissance studies.
Project Components
The selection of the structures, in conjunction with conceptual layouts, was based on experience and engineering judgment.
(a)  Infrastructure
Access to the plateau is not a problem, but the extremely difficult descent to the powerhouse sites could be difficult and expensive, 
involving over 15 km of road to descend the
1,000 m. In the final model runs, these costs, which were common to most of the schemes, were included on the basis of a fixed cost of about
$10 million.
The costs related to erection and operation of tne townsite and construction camps were not2-61
considered in the reconnaissance studies. These costs are common to all developments and are normally expressed as a fixed percentage of
total costs. For the comparative analysis of
the model runs, such costs would not affect the results.
(b)       Dams
Major storage dams selected were earth-core
rock-fill which are appropriate for the gener
ally broad river valley topography and available construction materials. A crest width of 6 m
and upstream and downstream slopes of 3H:1V were selected.
Since detailed information on construction materials was not known in the initial phase of the reconnaissance studies, it was not possible to estimate unit prices for specific materials. Therefore, based on experience, it was estimated that an average unit price of $9/m3 for high dams 025 m) and $ll/m3 for the lower dams would be appropriate. The higher cost for the smaller dams reflects the premium cost of open ing up the borrow pits and restricted placing areas on the smaller dam structures.
(c)       Spillways
Spillway construction does not constitute a major cost component. Considering the small inflows and relatively large reservoir volumes of the Aleltu projects, the quantities of spill
are low and uncontrolled rock cut spillways were2-62
selected. It was assumed that the rock excavated from the spillways would be utill2e(j as fill material for dam construction and spill way costs were incorporated as part of the dam costs.
Spillway dimensions were selected based on routing the 1:10 000-yr flood inflows through the reservoir, as discussed in Section 2.4 2
(d)       Reservoir interconnection
Tunnels and Channels
The integrated operation of the base scheme reservoirs will require a network of water conveyance structures. Two alternative means of interconnection were considered
-   interconnection tunnels or channels
-   collector channel.
(i) Interconnection
Tunnels and Channels
In establishing the most economic type of conveyance structure for each scheme, it
was realized that the reservoirs Aleltu project will be subjected large water level fluctuations, due to the particular hydrologic
of the to very This is cycle in 
r
r
—
Ethiopia which p
provides reservoir inflows
over a wet period of 3
over a dry period of 9 
these conditions, 
require cuts,
open
up to 40
months and drawdown months. Under channels would 
m deep.
f r
f2-63
Interconnection tunnels would be pres surized. Because of favorable geological subsurface conditions, tunnels were assumed to be unlined.
The selection of tunnels or channels for reservoir interconnection was based on cost comparisons. For most schemes, tunnels were found to be more economical.
Tunnel diameter was a variable in the layout optimization model and was derived by assuming minimum operating heads to convey power flow or unregulated flow between reservoirs.
(ii) Collector Channel
Instead of interconnection tunnels or channels between reservoirs, an alterna tive concept considered was a collector channel. This consists of control struc
tures which discharge power flow into a collector channel at each reservoir, 
routed below the dams. After collecting the power flow from each reservoir, a power canal (or tunnel) would convey the power flow to the edge of the gorge, from where a penstock would convey the water to the powerhouse.
The invert level of the collector channel system would have to be set below dead storage level (DSL). Energy dissipation2-64
r^auired downstream from the control structures on each reservoir. The operation and control of the system wouid >? more complicated and costly than a tunnel system. The collector channel concept was therefore not further consi dered .
r T*—    1
an-d or Power Canal
□pen canals or tunnels can be utilized to convey power flow across the plateau. A preliminary assessment was undertaken to determine the rela tive cost and economic factors in selection of
t~e oeto.od of conveyance.
•or the t_nnel concept, the subsurface excava tion and concrete lining are the high cost items, oemg aoo_t 50% higher than straight canal costs. However, open canals have addi tional costs and detractions.
can=_s car. require oridges and river -ng str_ct>res.
€ *  T--re stilling basins down-
re
E  =--~- control structures and have "* “e£i lcss in the stilling basins. 
■”e tetrain and deplete
..ss greater environmental ’-he cost of the iand.
Z ' «r a
, 
C0n7eyance, canals generally tne contours of the land within2-65
limits, thus increasing the length (and cost) relative to tunnels.
Therefore, in laying out the project, the following criteria were adopted.
-   Where a canal wanders more than 30% off the straight line distance, power tunnels have been selected.
-   Where canals are straight or wander less than 30% off the straight line distance, canals or tunnels have been selected on the basis of total cost (including bridges, stilling 
basins, and value of energy losses) with consideration of the environmental impact.
The power tunnels which convey the water from the reservoirs to the face of the escarpment were assumed to lie within the basalt cap rock which forms the plateau. A circular-shaped tunnel was adopted for ease of calculation. The tunnels were initially assumed to be unlined and to require minimum roof support at a unit cost of $80/m3. During the later model runs, a concrete lining with a thickness of 1/15 of the diameter, was added at a unit cost of
$350/m3.
Surge Tank
A study of surge tank requirements indicated that the variation in cost between alternat schemes was not a significant factor m t'e layout consideration. Concrete-lir.ed sur~etanks v/ere studies as surge tank cost of S4
adopted in the reconnaissanc lump sum cost items.
and shaft were included
16 Cost r>c
ds  a f,-
f
million.
To compare
all  schemes on the same basis, a
single c_ 
gorge i.— 
cal in forma
"    '    3      t      for    po«er      flow    conveyance    in    the ’     C0nC6P        ired.     Because     of     lack     of     geolOgi. 
- —i  s requ   tion m          gorge during the recon-
wa
naissance t
stage or
rhe study, the selection of ;
power l
sh aft  and  tunnel,  at  depth,  was  not
consi
" to be appropriate. Instead, a steel enstock was adopted for all schemes.
surface Pen The penstocks 
various powerhouse loca-
,
,
* ’ were designed with increasrng thickness at increasing head. The routing of the penstocks
is a function of powerhouse location and is described in Section 2.5.4.
Penstock diameter, length and thickness were variables in the layout optimization model.
A conservative design criterion, in accordance current international penstock codes, was
Pted from which steel requirements and thick' were determined. High-strength steel is
equired t0 control plate thickness. The
$4 '500/t inpi
penstock steel
t WaS
cos
inciUdlng
ln 9. After receipt of quotations
assumed to be tlo n, supports and test'
turers, this c ’
’----------------: frorn manu^aC'
the final modei°St reduced to $3 000/t for
runs.
f2-67
From a layout perspective, both an underground or surface powerhouse could be considered. A surface powerhouse would be better suited to a staged development. Also, as in the case of the penstock, an underground powerhouse was not considered appropriate because of lack of geolo gical information. Therefore, a surface power house was adopted for the reconnaissance study.
Costs for the powerhouse in the model runs were derived on the basis of dollars per megawatt of plant capacity, using values obtained from the other studies. The rate used was $250,000/MW. These costs covered the civil works, mechanical and electrical equipment, and architectural housing of the plant.
Transmission
Transmission lines were routed from the power- house sites in the gorge to the Gafarsa sub station near Addis Ababa by the most direct route. Transmission costs were evaluated at
$ 100,000/km. These costs were entered in the model as lump sum costs.
2.5 - Project Evaluation
2.5.1 - General
As discussed in the previous sections, the Aleltu project is a complex development. The combination
W           JF'fTT2-66
of its 14 basins, each with alternative schemes of development, including dam locations, reservoir sizes, interconnection tunnels or open channel conveyance structures, power tunnel, penstock routes and power house locations, provides for a large number of development options.
A special approach was required to address these options. Initially, alternative development options
for the basins and the powerhouse location were screened to identify the most promising sequence of schemes. This is discussed in Sections 2.5.2 to 2.5.4. The screening analysis was followed by extensive use of the layout optimization model, which is discussed in Section 2.5.5 and subsequent sections.
2.5.2 - Screening of Basin
Development Schemes
Prior to performing the more detailed analysis with the layout optimization model, the significant parameters related to basin developments were reviewed.
The largest variable cost item in any development is
the dam volume. Energy production is a direct function of power flow and available head. If the latter is
assumed to be constant, costs and benefits can be expressed in terms of power flow (from the reservoir) only. From this relationship, two sets of curves have
been prepared, as shown on Plates 2-4 and 2-5.
The first set of curves, on Plate 2-4, shows the
ratio of Dam Volume/Power Flow against Power Flow for2-69
the major development in each basin. This ratio corresponds to the cost per kilowatt hour of the basin (dam-reservoir) part of the project. Thus, the lower the ratio, the more economic the development.
- The second set of curves, on Plate 2-5, shows the
ratio of Dam Volume/Live Storage against Live Storage for the major storage development in each basin. 
This ratio is equivalent to cost per cubic metre of storage for the dam-reservoir part of the project. 
The lower the ratio, the more economic it is to store water at a site.
It is noted that these screening curves only provide a comparative assessment of the dam-reservoir component. Other major factors, such as the length of power
conduits, are not considered here.
A number of observations can be made from these
graphs.
- The Agat, Tinishu Aleltu and Jinjer schemes have a
high cost storage and corresponding high unit energy cost compared to the other schemes. It appears that these schemes will be very costly as high dam reser voir regulation schemes and development should con centrate on low dam diversion of unregulated flows.
- Apart from the above three schemes, all other developments show lower, competitive costs.
- The Robi provides the lowest storage cost followed by the Gomoro.-   The Robi also provides the lowest unit cost for
energy (based on consideration of basin development
only)       and       should       be       considered       for       early       development along with the Gomoro and Chacha.
-   Less than full regulation is obtained for the Tiliku
Aleltu, Chacha and Jinjer schemes.
-         Additional         project         regulation         potential         is         available at     the     adjacent     basins     of     Jida,     Gomoro     and     Robi     base schemes.
- The Jida, although relatively high in unit energy 
cost compared to the Robi and other schemes, provides
the largest storage volume and power flow and 
therefore the potential of providing the largest
energy benefits. Geographically, the Jida is also 
adjacent to the Tiliku Aleltu and can provide the
additional regulation capacity needed for the Tiliku 
Aleltu development. The economics of the Jida Basin 
development, therefore, requires further analysis.
Potential       alternative       dam       locations       in       the       Jida       Basin are evaluated below.
2.5.3      - Screening of Jida Sites
As discussed in Section 2.3.4, the Jida Valley has five potential damsites. There are numerous combinations of development schemes considering alternative damsites, dam heights and interconnections. The river can be developed as a single reservoir or as a multireservoir development. The most practical schemes of develoment were selected and a set of curves prepared to assess their potential storage and energy generation. These schemes are shown on Plate 2-6 and include2-71
Scheme 6b-l - Lower and Middle Jida Dams Scheme 6c-l - Main Jida Dam
Scheme 6c-2 - Main Jida Dam (at elevation 2,570 m) and Middle Jida Dams
Scheme 6c-3 - Middle Jida Dam
Scheme 6d-l - Main Jida Dam (at elevation 2,590 m) and Upper Jida Dam
Summarizing from Section 2.3.4, these schemes are as follows.
Scheme 6b-l comprises storage dams at the lower and middle Sites 6b and 6d. At the lower site, the reser
voir FSL would be about elevation 2,560 m and the available live storage would be about 350 Mm3, This volume would be insufficient to provide full regulation of the inflows and, for this purpose, a dam at the Middle Jida (or Upper Jida) location would be required. This dam could be constructed up to about elevation
2,690 m. An interconnection tunnel from the lower site to the power tunnel (leading from the Tiliku Aleltu scheme) would be required. Because of the much lower FSL at the lower dam, this interconnection tunnel would require special provisions to control the different operating heads.
Scheme 6c-l comprises a large storage dam about 3 km upstream from the Muke Turi-Lemi Road. The reservoir would be integrated with the Tiliku Aleltu and operate at a common level. Because of the broad valley of the Jida Basin, high dams will be large in volume and costly. Therefore, upstream storage sites in series
with the main Jida reservoir were considered, as discussed in Schemes 6c-2 and 6c-3.2-72
Scheme 6c-2 incorporates the Main jiua nho Main jida dam and a second dam constructed at the middle Jida site. This scheme
will permit the main Jida dam to be built lower at FSL about 2,570 m
In Scheme 6c-3, the second storage dam is constructed at the upper Jida site. In this case, the FSL of the
main storage dam is assumed at elevation 2,590 m and the upper Jida reservoir would provide for additional regulation releasing water to the reservoir at
Site 4c.
In Scheme 6d-l, the construction of a single storage dam at the middle location is considered. Because of
its more upstream location, the interconnection tunnels from the middle Jida reservoir to the Gomoro and Tiliku Aleltu reservoir will be considerably longer than for
the main Jida scheme. This reservoir could be inte grated with the Tiliku Aleltu and Gomoro or operate independently, diverting flow to the Tiliku Aleltu.
The       results       of       an       initial       comparison       of       the
five
schemes are shown on Plate 2-6.
this plate that schemes with the lower reservoir and upper reservoir are not attractive. This is due to the dam volume required at the lower Site 6b being large
It can be seen from
while, volume
appear single
for the upper site 6e, the available storage
is small. The most attractive alternatives
to be the middle Jida dam, site 6d, either as a scheme or in conjunction with the main Jida dam.
The     main     Jida     dam     as     a     single     scheme     is,     <---
is, according to
this        evaluation,        less        promising        when        compared 
middle Jida dam.
- -i to the2-73
This evaluation, however, does not consider a number of factors which would benefit the project economics of the main dam alternative. These are
- about 10% less drainage area is available at the middle dam location
-   access and interconnection tunnels to Tiliku Aleltu and Gomoro reservoirs are longer from the middle location
-   this analysis considers only the Jida reservoirs. System operation requirements such as the availabi lity of additional regulation potential and optimum system operating reservoir FSL are not considered; both are less favorable for the middle dam location.
On the basis of these considerations, the main Jida scheme was selected for the reconnaissance studies. The location of damsite is further reviewed in the prefeasibility studies, after the concept of the ulti mate development was established. The results of these reviews are contained in Section 3.8.2.
2.5.4     - Screening of Powerhouse
Locations on the Basis
of Penstock Length
A large number of powerhouse locations were initially identified. These are Locations A to H, as shown on Plates 2-1 and 2-2. For developing the power project in the Zega Wedem Gorge, Locations A to F were selected, and for the alternative development in the Bersina Gorge, Locations G and H were considered.2-74
To obtain a perspective on the effect of penstock cost relative to the powerhouse locations, a preliminary assessment was carried out for powerhouse locations. In this asessment, the penstock length is the major variable governing the selection of the powerhouse location. To exclude any large variation of other parameters in the comparison, it was assumed that
- all power flows would be delivered to the penstock at the edge of the gorge directly above the powerhouse under consideration
-   all power flows would be delivered to the penstock at the same head (at elevation 2,580 m), i.e., not considering tunnel route or losses to the penstock
-   all associated project components, such as access, transmission and powerhouse, would be equal in cost.
The intent of this comparison was not to determine the actual cost of each penstock routing, but to discover
if the benefit of additional head obtained from a lower powerhouse location would be economic compared to the additional cost of a longer penstock. The penstock profiles were plotted and each penstock was designed and costed assuming a unit price of steel of $2.50/kg. 
A mean power flow of 8.0 m3/  was assumed.
s
Powerhouse Locations
in Zega Wedem Gorge
From a study of the topographic maps, the river gradient downstream from Site A flattens out and it
appears     that     the     gain     in     head     would     not     offset     the increased cost of longer conveyance works. To2-75
confirm this, a powerhouse Location A* at the confluence of the Gur and Zega Wedem was considered.
The basic data for the shown below.
seven powerhouse locations are
Available
Location
Penstock
Head
(m)
Length
(km)
Cost ($xl06)
A*
1 140
4.65
58.5
A
1 100
3.75
45.0
B
990
2.30
26.9
c
960
1.95
22.4
D
900
1.60
17.3
E
900
2.15
24.7
F
860
1.25
14.9
It can  be seen  that Location  E with  higher cost than Location  D and  no  additional head  is not attractive. This alternative was not further considered.
The remaining locations were compared on the basis of incremental cost of energy (when selecting a lower powerhouse location). This value was compared to the long-term marginal cost of ICS grid energy supply of 35 mills. The results are as shown below.2-76
Location
Annual Incre-
Head  mental Gain  Energy TmT" (GW-h)
Annual ized Cost Increase ($x!0b)
to D
40
23.4
0.30
to C
60
35.1
0.61
to B
30
17.5
0.54
to A
no
64.3
2.18
to A*
40
23.4
1.58
Incremental Cost of
Energy________ (mills/
kW-h) 12.8 17.5 31.0 33.9 67.4
It can be concluded that the incremental cost of
energy increases as the powerhouse moves downstream
and the optimum development considering the Aleltu 
project only would be Location F. Although increas
ing, the incremental cost from Locations F to A is
always less than the marginal cost of future ICS
energy        supply.        Also,        as        the        Aleltu        project        develops in the future and the flow to the powerhouse
increases, the benefits of Location A will increase compared to that of Location B to F. Furthermore, it is shown that locations lower than Location A (i.e., such as Location A*) are not economic. Location A was therefore considered to be most promising with regard to economic development in the ICS. However, to confinn this conclusion, Locations A, B and C were further evaluated in the layout optimization model.
Powerhouse Locations
in Bersina Gorge
A similar assessment was provided for powerhouse locations in the Bersina Gorge. Because of the2-77
topography at the edge of the gorge, there are fewer natural locations for penstock alignments.
Locations G, H and G* were investigated. In a simi lar analysis, as for the Zega Wedem Gorge, Site G was determined to be the most promising. The gorge widens appreciably downstream from Location G. From a study of the 1:250 000-scale topographic maps, it
was confirmed that the gain in head would not offset the increased cost of longer conveyance works. Therefore, Location G* was rejected, and Locations G and H were further evaluated in the layout optimiza tion model.
2.5.5 - Description of Layout
Optimization Model
The layout optimization model, developed for the Aleltu project, incorporates a hydraulic network strategy with reservoir operation and economic analysis. A detailed description of the model is contained in Appendix A, Layout Optimization Model.
This model evaluates any combination of schemes in the Aleltu project. By deterministic equations, individual project components are optimized as a function of regulated power flows. The model computes the costs of the various components and the total project cost. The energy generation was maximized by comparing the incre mental cost of energy production to the long-term average cost of grid energy supply to the ICS.
The model considers five variable project components which have a major influence on project economics. These are2-78
-    dams
-    interconnection tunnels
-    power tunnels
-    penstocks
-    powerhouses.
Fixed components, such as access and transmission, are included as lump sum items in the cost of the power house component.
The costs of townsite and construction camps, amounting to about 13% of total cost, were not included because these costs are equal for all schemes and would not affect comparisons between schemes.
2.5.6      - Economic Parameters
(a) Long-Term Grid
Energy Supply Cost
The main economic parameter used in the layout optimization model is the long-term average cost of future ICS grid energy supply.
This parameter was derived from the 1982 PPS report. Therein, the future cost of available* grid energy at the generating stations** was calculated from the average estimated investment
_________ L
’ Available energy is energy that can be supplied and may be
greater than the energy demand. 
r*
**Energy generation in this report is at the station 
L
generator terminals unless stated otherwise.2-79
cost* of future system expansion, including 
transmission to distribution centers. A value of
44 mills/kw.h was found, expressed in January
1982 United States dollars.
To obtain a comparative value of available grid 
energy cost for present conditions, the following adjustments were carried out.
-    Inflation adjustment based on an average annual inflation rate of 8%/yr over 2 years.
-    Increased costs to provide for additional infra
structure, socioeconomic costs and substation 
costs, amounting to about 30%.**
The corresponding 1984 investment cost of future
available grid energy supply resulting from these
adjustments is 67 mills/kW-h.
Since the principal emphasis of the reconnaissance
phase was to compare the alternative schemes (and 
not to establish capital cost estimates for any 
particular development), special consideration was
given to evaluating differential costs and bene
fits. Accordingly, common costs such as townsite
and construction camp and proportional costs
(which are straight percentages of the construc
tion cost) such as contingencies, etc, were
* The investment cost included contingencies, engineering and construction management, and interest during construc
tion, but excluded escalation.
**Aleltu Hydro Project Prefeasibility Study, Supplement 1. A comparison of cost estimates of the Aleltu pre feasibility study with the current Gilgel Gibe Feasibility Study and with project estimates in the 1982 PPS, indicates that PPS costs should be about 30% higher, to cover such items as increased infrastructure, socioeconomic costs and the substation at the load center.analysis at this stage, excluded from the . _.?ferred to in this study
resultant unit cost is re
Value of future available grid 
The
as the "1984 Base energy supply
This value was used 
ion^model and determined optimization —
as
in the layout follows.
1984     investment     cost     of     future available grid energy supply
Excluding    common    costs    and proportional costs
- townsite and construction
Mills/kW* h 67.0
camp at
-   contingencies at
-   engineering, management
and owners cost at
-   interest during
construction at
Subtotal
1984 Base Value of future
available grid energy supply
at the station generator
terminals
(67.0 - 31.7) is
The criterion for economic analysis reconnaissance study is, therefore incremental cost of energy supplied project schemes must be less than 35
13%
20%
12%
25%
31.7
in the
that the
by the Aleltu mills.2-81
(b) Annualized Costs
A discount rate of 12% over 50 years was used to 
calculate the annual cost of the developments
resulting in an annualization factor of 0.12042.
(c ) Benefit/Cost Ratios
For evaluating  alternative plans,  benefit/cost (B/C) ratios were used.  These were defined  as follows.
B/C Ratio
Long-term ICS grid energy supply cost (35 mills/kW.h) Average energy cost in mills per kilowatt hour
2.5.7     - Layout Optimization
Analysis - Base Schemes
The analysis was carried out in two parts
-   initial (September 1983) runs
-   final (December 1983) runs.
The first series of runs were carried out with the purpose of defining, at an early stage, the require ments of the October/November field investigation pro gram. These runs were, therefore, carried out using available data including the results of initial hydro logic analyses and initial assumptions of the cost of energy supply and project layout assumptions regarding unlined power tunnels, dam sections and elevations, etc.
These assumptions and analyses were refined after the results of the field investigation program becameavailable and detailed hydrologic analyses had been 
prepared. These refined input data were then used in 
the December 1983 runs.
Because of the comparative nature of the reconnaissance
schemes studies, the results of the September and 
December 1983 runs were used separately to evaluate
development plans. As the data were revised, the
earlier results were reviewed and rerun as necessary. 
The results in any one of the evaluation tables
presented in the following sections are always
comparable.
A total of about 80 development alternatives were
investigated. The purpose of these runs was
- to establish economic viability of the individual
base schemes and diversion schemes
- to determine                the most economic              sequence        of developing the Aleltu Basin
- to determine the most economic powerhouse location and routing of power conduits.
These issues are addressed in the following sections.
The basin schemes selected for evaluation in the layout optimization model are shown on Plate 2-7. In the
preliminary analysis, it was determined that the following base schemes would best operate as an2-83
integrated network at a common FSL*—Agat, Tinishu Aleltu, Tiliku Aleltu, Jida, Gomoro and Robi.
A full tabulation of the final model analyses is
contained in Appendix A, Tables A.1 and A.2.
(a) Agat and Tinishu
Aleltu Plans
From a consideration of maximizing head (by developing the downstream powerhouse Site A in the Zega Wedem) and minimizing the length of the power conduits, the Agat followed by Tinishu Aleltu appears to be the most promising development plan. The significant parameters of the model runs for
the Agat and Tinishu Aleltu plans are shown below.
Plans
( from
Table
A.l)**
3A
34 A
5A
45A
345A
Annual energy** 
(GW-h)
80
153
352
466
440
Average energy
79.7
71.8
24.9
34.0
39.3
cost
(mills/kw-h)
* Adopted DSL is at elevation 2,560 m for all runs in Table A. 1, Appendix A, and at elevation 2,570 m for all runs in Table A.2.
**For full details of model runs, see Appendix A, Table A.l.Plans
(from
Table
3A
34A
5A
A.l)*
45A
345A
B/C ratio
0.44
0.49
1.40
1.03
0.89
Incremental energy cost relative to plan in brackets
64.9
(5A)
(mills/kW*h)
If Plan 3A were developed for the Agat, with a storage dam at Site 3a, the firm annual energy yield would be about 80 GW*h. The average cost of energy would be about 80 mills/kW’h, which is far in excess of the long-term ICS grid energy supply cost. The benefit/cost ratio would be
0.44. Construction and interconnection of the Tinishu Aleltu storage at Site 4a would only slightly improve the economics of the Agat plan as shown by the results of Plan 34A.
The Tiliku Aleltu by itself (Plan 5A with a storage dam at Site 5b) would generate about
For full details of model runs, Table A.l.
see Appendix A,2-85
350 GW*h/yr firm energy at a cost of about
25 mills/kW-h. The benefit/cost ratio would be
1.40. This plan is therefore economically 
attractive.
To further assess the economic viability of the
Tinishu Aleltu, Plan 45A was evaluated. With 
this combination, the benefit/cost ratio is 1.03, 
but the incremental cost of energy for Tinishu 
Aleltu is about 65 mills/kW-h which is in excess
of the long-term ICS grid energy supply cost.
Therefore,  the Tinishu  Aleltu  as a storage scheme by  itself or integrated  with  the Tiliku  Aleltu  is economically not attractive.
A confirmation run, combining Agat, Tinishu Aleltu and Tiliku Aleltu, was carried out and it can be
seen from Plan 3_4 5A that the addition of the Agat scheme causes a further deterioration of economic benefit.
(b) Tiliku Aleltu
and Jida Plans
With the Agat and Tinishu Aleltu (as storage schemes) being unattractive, the focus comes on the Tiliku Aleltu and Jida schemes for first stage development to powerhouse Site A.
The significant parameters of the alternative project development plans for the Tiliku Aleltu and Jida are shown below.2-86
Plans
(from Table A.2)
5A
6A
56A
Annual energy 
(GW-h)
240
584
1 003
Average energy cost
34.6
30.9
25.6
(mills/kW-h)
B/C ratio
1.01 
1.13
1.37
Both the Tiliku Aleltu and Jida as single basin developments would be economic, with the Jida scheme more economic than the Tiliku Aleltu scheme. This is shown by Tiliku Aleltu, Plan _5A (with a storage dam at Site 5b) and Jida, Plan 6A (with a storage dam at Site 6c). It is noted that the reservoir volume at Tiliku Aleltu is small and regulation is limited* because the reservoir FSL on the Tiliku Aleltu is topographically limited to elevation 2,600 m. This becomes clear when both schemes are integrated and the Jida reservoir provides partial regulation for Tiliku Aleltu inflows. Plan ^6A indicates that the energy yield
*The maximum regulation which can be achieved is about 60% (see Table A.2). It is noted that the existing reservoirs of the ICS have an additional regulating poten tial of about 210 GW-n/yr (see Section 7). This would be sufficient to provide for full utilization of the Tiliku Aleltu inflows, if adequate installed capacity is provided. Generally, the available regulating potential of the ICS will be a benefit to the Aleltu project, however, this was not incorporated in the analysis with the layout optimiza tion model.2-87
for the combined schemes is greater than the sum
of the single schemes and the benefit/cost ratio 
of the combined scheme is higher than either of
the single schemes.
Tiliku Aleltu and Jida
With Gomoro and Robi
Schemes Interconnected
The Gomoro and Robi schemes (Plans 8_A and 9_A) were not investigated as individual schemes with the powerhouse in the Zega Wedem Gorge due to the unrealistically long power flow conduits. However, if constructed in sequence with the
Tiliku Aleltu and Jida schemes, the Robi and Gomoro storage schemes at Sites 8a and 9a are economically attractive, as shown below.
Plans (from Table A.2)
568A
^689A
_5 6A
Annual energy
1 181
1 498 
1 003
(GW-h)
Average energy cost
25.4
24.2 
25.6
(mills/kW« h)
B/C ratio
1.38
1.45 
1.37
It is noted that the benefit/cost ratios are
higher compared to the Tiliku Aleltu and Jida (Plan ^6A) due to decreasing average cost of energy, when the Gomoro and Robi schemes are added.The Robi and Gomoro as base schemes are consi dered later with the powerhouse in the Bersina Gorge.
Power Intake Location for
Tiliku Aleltu/Jida Development
For power development at powerhouse Sites A, B or C, the apparent best location for a power intake is at Tiliku Aleltu, being closest to all three powerhouse locations. This was confirmed by the layout optimization model runs of
Plans j>6C and 56C, with the power intake alter natively at Tiliku Aleltu and Jida. The results
are shown below where Plan _56C is slightly more economic than Plan 56C.
Annual         energy (GW-h)
Average energy cost (mills/kW-h)
B/C ratio
Powerhouse Locations
in the 2ega Wedem Gorge
Plans (from Table A.1) 56C               56C 999                994
19.8              20.2 1.76              1.73
A large number of powerhouse locations were initially identified. These are Locations A to2-89
F, as shown on Plate 2—1. For developing the power project in the Zega Wedem Gorge, Locations A, B and C were selected by the screening process. Plans 56C, _56B and j>6A show the economic comparison of these powerhouse locations when Tiliku Aleltu and Jida are constructed only.
Plans
(from Table A.1)
56C
56B
56A
Annual energy 
(GW-h)
999
1 016
1 122
Average energy cost
19.83
19.97
20.42
(mills/kW-h)
B/C ratio
1.51
1.50
1.47
Incremental energy cost relative to 
plan in brackets
27.1
(56C)
24.8 (56B)
(mills/kW« h)
Increased energy yield
and highe
r energy
costs
arise as the powerhouse is moved downstream. Benefit/cost ratios for the three powerhouse locations differ only slightly with Location C being most favorable. However, additional generation at Locations B and A is obtained at incremental costs of 27.1 mills/kw-h, going from Site C to B, and 24.8 mills/kW«h from Site B to A. As these costs are much less than2-88
The Robi and  Gomoro  as base schemes are consi dered  later with  the powerhouse in  the Bersina Gorge.
Power Intake Location for
Tiliku Aleltu/Jida Development
For power development at powerhouse Sites A, B or C, the apparent best location for a power intake is at Tiliku Aleltu, being closest to all three powerhouse locations. This was confirmed by the layout optimization model runs of
Plans ^6C and 56C, with the power intake alter natively at Tiliku Aleltu and Jida. The results
are shown below where Plan _56C is slightly more economic than Plan 56C.
Plans
(from Table A.1)
56C
56C
Annual energy
999
994
(GW-h)
Average energy cost
19.8
20.2
(mills/kW.h)
B/C ratio
1.73
Powerhouse Locations
in the 2ega Wedem Gorge
A large number of powerhouse locations were
initially identified. Thesp Arc r
mese are Locations a to2-89
F, as shown on Plate 2-1. For developing the power project in the Zega Wedem Gorge, Locations A, B and C were selected by the screening process. Plans 56C, 56B and 56A show the economic comparison of these powerhouse locations when Tiliku Aleltu and Jida are constructed only.
Plans
(from Table
A. 1)
56C
^6B
56A
Annual energy 
(GW.h)
999
1 016
1 122
Average energy cost
19.83
19.97
20.42
(mills/kW-h)
B/C ratio
1.51
1.50
1.47
Incremental energy cost relative to plan in brackets
27.1
(56C)
24.8
(56B)
(mills/kW« h)
Increased         energy         yield         and         higher         energy         costs arise as the powerhouse is moved downstream.
Benefit/cost ratios for the three powerhouse locations differ only slightly with Location C being most favorable. However, additional generation at Locations B and A is obtained at incremental costs of 27.1 mills/kW-h, going from Site C to B, and 24.8 mills/kW«h from Site B to A. As these costs are much less than2-90
the long-term ICS energy supply cost of
35       mills,       Site       A       provides       the       greatest       economic benefit. Site A also generates an additional
106 GW-h for the ICS compared to Site B; and 123 GW*h compared to Site C. Plans ^689A, 5689B and ^689C further confirm these findings as shown below.
Plans
(from Table A.1)
5689C
5689B
5689A
Annual energy (GW«h)
1 518
1 544
1 705
Average        energy        cost (mills/kW-h)
B/C ratio
Incremental energy cost relative to scheme in brackets (mills/kW-h)
21.09          21.17
21.40
1.42           1.42                  1.40
26.3 (5689C)
23.5 (5689B)
In these runs, the benefit/cost ratios for Locations C, B and A are nearly equal. The incremental energy costs however are even lower for Plan 5689A compared to Plan 5689B, confirm ing the preference for Site A.
The evaluations carried out above show that, from a benefit/cost point of view, there is little difference among the three locations.2-91
However, because of the higher head available, considerably more generation can be obtained at Location A at a cost which is only slightly higher (about 1.5%) than for the other locations
but at an incremental cost considerably lower
than the long-term ICS energy cost. It is
therefore concluded that Location A is the most economic powerhouse location. However, if geotechnical or topographical considerations are found to be unfavorable, any other location in 
the Zega Wedem Gorge, between Locations A and C, would be attractive for further investigation.
(c)       Gomoro and Robi Plans
As indicated in Section 2.5.2, the Robi has the
most economic scheme—considering development of the basin only. The Gomoro and Chacha are also economic schemes which could be developed with the Robi as a first stage development or as a later development separate from the Tiliku Aleltu/Jida, development.
Because of the distance from the Gomoro and Robi sites to powerhouse Location A, an alternative development in the Bersina Gorge was investigated. This plan comprises the construction of a storage dam and intake at the Robi reservoir, Site 9a, and 
a power conduit to Locations G or H in the Bersina Gorge. Various model runs were carried out to investigate this concept, the results of which are shown below.2-92
Plans
(from
Table A.1)
2G
689G
689H
Annual energy 
303
245
1 128
927
(GW-h)
Average        energy        cost
32.75
33.69
23.87
24.64
(mills/kW-h)
B/C ratio
0.94
0.89
1.26
1.22
Because of the higher head available, powerhouse Location G is more economic than Location H. This is shown by Plans 9G and 9.H for the Robi scheme only, and confirmed by Plans 689G and 689H, in which the Gomoro and Jida schemes were considered to augment the generation potential of Robi.
The Bersina Gorge development was then compared to that of the Zega Wedem Gorge to determine where it is better to turbine the waters of the Gomoro and Robi. In the Bersina Gorge, the waters would be utilized in Plan 89G. In the Zega Wedem, the same water would be used in Plan _5689A. Thus, the best scheme is determined by comparison between the average energy cost of Plan 89G against the incre mental cost of 89 in Plan 5689A.Annual energy 
(GW-h)
Average        energy        cost (mills/kW- h)
Plans (from Table A.2)
89G              5689A        56A 465               1 498          1 003 27.85            24.18          25.61
B/C ratio
Incremental energy cost relative to scheme in brackets
1. 26
1.45
1.37
-
21.3
(56A)
-
(mills/kw-h)
The waters of Gomoro and Robi are better turbined 
at powerhouse Site A at an average (incremental)
cost of 21.3 mills/kW-h compared to
27.9 mills/kw-h at Site G. From the same water, 
powerhouse Site A also produces an additional
31 GW-h/yr for the ICS.
These results are due to the fact that, at
Location  G,  the available head  is less,  access is longer and  more difficult,  and  transmission  lines to Addis Ababa are longer.
It is concluded that a single power development in the Zega Wedem Gorge utilizes the economic poten tial of the Aleltu project most beneficially.2-94
2.5.8 - Layout Optimization 
Analysis - Independent
and Diversion Schemes
Considering only the base schemes, the optimum develop ment is to turbine the Tiliku Aleltu/Jida/Gomoro/Robi power flows at powerhouse Site A.
Diversion schemes will not change the concept of the basic development. However, the diversion schemes can influence the sequence of development after Tiliku Aleltu, i.e., the Duber or Weserbi and Girar may be more economic than Gomoro or Robi. To assess the influence of the diversion schemes, a number of these have been evaluated with the layout optimization model. It is noted that the Agat, Tinishu Aleltu and Weserbi schemes, initially considered as potential base
schemes, have now become potential diversion schemes.
Tinishu Aleltu Overflow Dam
f
----------------------------------------------------------------------------------  L
"
With storage schemes
being uneconomic on the Tinishu 
-
Aleltu, a diversion scheme at Site 4b was considered.
This       consists       of      a       small      overflow       dam      diverting       unre gulated flow via a drop inlet and downpipe into the
power tunnel running from Tiliku Aleltu to powerhouse Site A as described in Section 2.3.4.
The performance of this scheme is determined by comparing Plan 4_56A with Plan _56A and Plan 45689A with Plan 5689A.
i L
L
r
-2-95
Plans (from Table A.2)
56 A
456A
5689A
45689A
Annual energy
(GW-h)
1 003
1 039
1 498
1 533
Average energy 
cost
(mills/kW-h)
25.61
26.10
24.18
24.56
B/C ratio
1.37
1.34
1.45
1.43
Incremental energy cost relative to scheme in brackets
40.2
(56A)
40.8
(5689A)
(mills/kW-h)
It can be seen that the average cost of energy increases and the benefit cost ratios decrease, when the Tinishu Aleltu scheme is added to the project development. This is due to the fact that the
available runoff from this diversion scheme is too small to generate sufficient energy to offset the
cost of the dam and downpipe works. Further, as shown by the incremental value of energy, the cost of the additional energy generated by the Tinishu Aleltu scheme is in excess of the ICS grid energy supply cost which indicates that this scheme is unattrac tive .
Agat
The Agat diversion scheme at Site lb is not practical unless a Tinishu Aleltu scheme is constructed.Therefore, the Agat diversion scheme is not evaluated further.
Weserbi (and Girar)
Because the Agat reservoir storage scheme is not economic, the Weserbi diversion scheme (with the steel pipe as an inverted siphon crossing the Agat) has to be considered instead of the more simple diversion to the Agat reservoir. This is Weserbi Site 2a - Scheme 2. The economic attractiveness of this Plan 256A is as follows.
Plans (from Table A. 2)
Annual        energy (GW-h)
Average energy cost (mills/kW«h)
B/C ratio
Average incremental cost relative to scheme in brackets (mills/kW.h)
56A
1 003
256A
1 171
25.61
25.1
1.37
1.39
22.12-97
calculated for Jida at 22.8 mills/kW«h and Gomoro 
at 24.0 mills/kW«h. With the costs so close, 
further studies at prefeasibility level would be
required to determine the timing of the Weserbi. It
is expected that Weserbi would continue to be more
economic than Gomoro and, therefore, precede Gomoro 
in the sequence of development of the Aleltu project. 
Jida, however, has several advantages over Weserbi as
follows, but not reflected in the analysis.
Jida is about 4.4 times the size of Weserbi and 
would better meet ICS load growth.
- With Jida being adjacent to Tiliku Aleltu, the camp 
and infrastructure would be more economical for
first stage construction.
- Jida is a conventional hydroelectric development, 
whereas the Weserbi requires an uncommon design of
long canal, inverted siphon and connection into the
surge chamber (or other section of the power con
veyance). These aspects will require further
design and operation studies before economic
assessment of Weserbi can be fully confirmed.
Therefore,       it       is       considered       that       Jida       should       precede Weserbi in development of the Aleltu project.
The Girar will also need to be included in further evaluation of the Weserbi since it has the potential
of improving the economics of the diversions from the west. However, with a medium-sized drainage area (172 km , about the same as the Weserbi) and the requirement for pumping, the Girar may not improve
22-98
. ■
the competitive position of t Jida and Gomoro.
Robi-Gomoro Plan
Fhp Weserbi relative to
The Robi-Gomoro River with an overflow dam at the edge of the Zega Wedem Gorge (Site 7a - Scheme 1), and upstream regulation on the Robi and Gomoro, has the potential of a low cost development as an inde pendent development separate from the Tiliku
Aleltu/Jida development. This Plan 7E has the power house at Site E and storage dams at Site 8a on the Gomoro and Site 9a on the Robi. It would be in competition with the Tiliku Aleltu/Jida powerhouse Site A development, Plan _5689A, for utilization of the Gomoro and Robi waters. As shown in the following tabulation, the Robi-Gomoro Plan 7E at
30.2 mills/kW«h is economic compared to the
long-term cost of grid energy. However, the upstream Robi and Gomoro waters can generate energy at a cost of 21.3 mills/kW-h at powerhouse Site A in the Zega Wedem Gorge, as shown by the incremental energy cost of Plan _5689A. Therefore, Plan 7E for development of the total Robi-Gomoro Basin is not attractive and the upstream Gomoro and Robi waters are better turbined at powerhouse Site A in the Zega Wedem Gorge.
This analysis also indicates indirectly that the
Robi-Gomoro scheme at site 7b has potential for economically developing the drainage areas below the Gomoro Site 8a and Robi site 9a, but the energy cost is likely to be in excess of 30 mills/kW-h.2-99
Plans
(from Table A. 2)
56A
J5689A
ZE
Annual energy
(GW-h)
1 003
1 498
385
Average energy cost
25.6
24.2
30.2
(mills/kW-h)
B/C ratio
1.37
1.45
1.16
Incremental energy 
cost relative to
-
21.3 (56A)
-
scheme in brackets
(mills/kW* h)
Duber Diversion Scheme
The Duber diversion Scheme 12c was evaluated as an addition to the system after Jida or after Robi. The results are shown by comparing Plan 56,12A with Plan 56A and Plan 5689,12A with Plan 5689A.2-100
plans (from Table A.2)--------------------------
56A
56,12A
5689A
_5689,12A
Annual          energy 
1 003 
1 300
1 498 
1 796
(GW-h)
Average energy
cost
(mills/kW-h)
B/C ratio
25.61 
26.47
24.18 
25.13
1.37 
1.32
1.45 
1.39
Incremental energy cost relative to scheme in brackets
29.4 
-
29.9
(56A)
(5689A)
(mills/kW*h)
It can be seen that the average cost of energy increases when the Duber diversion scheme is incor porated. However, the incremental cost of energy generated by the Duber scheme is below the future ICS grid energy supply cost indicating that this diver
sion scheme is economic for future development, after the Weserbi and the Chacha.
Chacha Diversion Scheme
The Chacha diversion scheme would discharge partly regulated flows in the upper reaches of the Robi River. The Robi reservoir, together with the other base scheme reservoirs, would provide regulation for the remaining flow. The chacha diversion comprises the second largest subbasin of the Aleltu system and provides a substantial contribution to the2-101
development potential of the Aleltu project. By 
comparing Plan ^689 ,14A with Plan 5689A, the Chacha
diversion scheme is very economic.
Plans (from Table A. 2)
5689A                   _5689,14A
Annual         energy 
(GW«h)
Average       energy       cost (mills/kW-h)
B/C ratio
Incremental energy cost relative to scheme in brackets (mills/kW* h)
1 498 
1 956
24.18 
23.82
1.45 
1.47
22.6 (5689A)
The energy of the Chacha Basin is added to the system at an incremental cost of 22.6 mills/kW«h which lowers the average energy cost and increases the benefit cost ratio of the system.
As can be seen from the comparison with the Duber, the Chacha is a much more attractive scheme.
2.5.9 - Energy Cost of
Individual Schemes
The economics of individual schemes, as additions to the Aleltu power development sequence, were calculated.2-102
The average costs of energy generated by the individual schemes were used as a base parameter to establish their economic attractiveness. The results of the calculations are shown in Table 2.8. As indicated in the previous Section 2.5.8, based on an ICS grid energy supply cost of 35 mills/kW*h, the Tinishu Aleltu upstream overflow weir is not economic. All other schemes are economic.
2.6 - Summary and Conclusions
of Reconnaissance Study
2.6.1 - General Results
The reconnaissance study, carried out to analyze the optimum utilization of hydropower resources of the Aleltu project area, identified 14 basin schemes, each with alternative concepts of development. A number of powerhouse locations were also identified with alterna tive conveyance concepts.
The reconnaissance study yields a clear definition of
the ultimate development concept of the Aleltu project. The Tiliku Aleltu, Jida, Gomoro and Robi schemes would be developed in an integrated manner; the reservoirs could be interconnected for operation at one reservoir level. All power flows would be turbined at powerhouse Site A. There are three diversion schemes which the reconnaissance study has evaluated to be economic; namely, the Weserbi, Duber and Chacha. Four other basins—the Gur, Girar, Jinjer and Sibilu—have been identified as diversion schemes but have not been evaluated. These schemes are expected to be developed 
at a later stage of the Aleltu project and do not2-103
TABLE 2.8
SUMMARY OF INCREMENTAL COST OF ENERGY FOR INDIVIDUAL SCHEMES
Basin
Number Basin
Scheme Definition
4                    Tinishu Aleltu                   Site 4b - Overflow Dam Diversion
Incremental
Cost      of      Energy (mills/kW.h)
40.9
5
Tiliku Aleltu
Site 5b - Base
20.8
6
Jida
Site 6c - Base
22.8
8
Gomoro
Site 8a - Base
24.0
9
Robi
Site 9a - Base
19.8
7
Robi-Gomoro
Site 7a - Base
30.2
(including upstream storage on Robi and 
Gomoro)
2
Weserbi
Site 2a - Diversion
22.1
12
Duber
Site 12c - Diversion
29.9
14
Chacha
Site 14a - Diversion
22.6

Summery

Aleltu Hydroelectric Project Summary

Aleltu Hydroelectric Project - Reconnaissance and Prefeasibility Studies

Project Overview

The Aleltu Hydroelectric Project is located about 80 km north of Addis Ababa, Ethiopia. The study was conducted by Acres International Limited in collaboration with the Ethiopian Electric Light and Power Authority (EELPA) between 1983-1984.

Key Project Data

Parameter	Ultimate Project	First Stage (Tiliku Aleltu & Jida)
Drainage Area	4,230 km² (12 subbasins)	1,207 km² (combined)
Average Annual Runoff	49 m³/s	13.8 m³/s (combined)
Gross Head	1,070 m	1,070 m
Firm Energy Potential	3,592 GW·h/yr	1,040 GW·h/yr (combined)
Installed Capacity (40% plant factor)	1,023 MW	300 MW (combined)

Project Components

First Stage Development

The recommended first stage includes:

Tiliku Aleltu Development
- Earth-and-rock-fill dam (54m high)
- Reservoir with 92 Mm³ storage at FSL 2,598.5m
- 60% regulation of inflow (5.6 m³/s average)
Jida Development
- Earth-and-rock-fill dam (65m high)
- Reservoir with 1,050 Mm³ storage at same FSL
- 100% regulation of inflow (8.2 m³/s average)
Interconnection: 800m unlined tunnel between reservoirs
Power Conveyance:
- 16.3 km concrete-lined horseshoe tunnel (3.2m diameter)
- 2.1 km steel surface penstock (2.4m diameter)
Powerhouse:
- Surface powerhouse with 6 Pelton units (51 MW each)
- Total installed capacity: 300 MW
- Firm energy: 1,040 GW·h/yr

Key Findings

Reconnaissance Study Results

Identified 14 subbasins with hydroelectric potential
Recommended staged development approach
Tiliku Aleltu and Jida schemes selected as most economic first stage
Powerhouse Location A in Zega Wedem Gorge identified as optimal

Prefeasibility Study Results

Confirmed technical feasibility of first stage
Hydrologic analysis showed:
- Mean annual rainfall: 1,188 mm (Tiliku Aleltu), 1,103 mm (Jida)
- 1:10,000-year flood peaks: 232 m³/s (Tiliku Aleltu), 329 m³/s (Jida)
Geological conditions generally favorable for construction
Estimated construction period: Approximately 5 years

Economic Analysis

Compared against long-term ICS grid energy supply cost of 35 mills/kW·h
First stage average energy cost: 25.6 mills/kW·h
Benefit/Cost ratio: 1.37 for first stage
Incremental energy costs for future stages below grid supply cost

Recommendations

Proceed with feasibility study of first stage development
Install additional streamflow and rainfall recording stations
Ensure continuous recordings at existing meteorological stations
Obtain 1:50,000-scale mapping of entire project area

Conclusion: The first stage development of Aleltu (300 MW) is technically and economically very attractive at the prefeasibility study level. The ultimate development could reach 1,020 MW through staged development of 12 subbasins.