Butajira Irrigation Project Summary

BUTAJ1RA IRRIGATION PROJECT
BRIEF PROJECT OtHLINE
By
C. J. N. DAVEY
Water Resources Development Digbrj<jr
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L-? *CONTENTS
PAGE
SUMMARY ------------------------------------------------------------------------------------ i
1.      Introduction-------------------------------------------------------------------------------------- 1
2.       Water Resources------------------------------------------------------------------------------- 1
3.       Soil Resources---------------------------------------------------------------------------------  2
4.        Crop Water Requirements-------------------------------------------------------------------  2
5.       Project Outline--------------------------------------------------------------------------------- 3
ANNEX A : Geological Investigation of Dam Site
ANNEX B : Preliminary Reconnaissance soil Investigations in Butajira Project area.BUTAJIRA IRRIGATION PROJECT
1.     Introduction
The proposed irrigation project is located approximately 25 km west of Ziway, on the main Ziaay-Butajira road, as shown in Figure 1. 
This project was identified in 198 5 and some dam site and reservoir
investigations were completed before the project was included as parr of
FAO Project ETH/82/008. The area to be irrigated was first identified on 
the available napping (1:250,000), which suggested that an area of nearly
500 ha would be available below the 1810 contour, as shown in Figure 2. However the more detailed topographical survey suggests that the gravity conrnandable area is substantially lower at approx irately 200 ha.
This study has investigated the available water and land resources for gravity irrigation development. It is intended that the
possibilities of pumping the available water to increase the irrigable area
would be considered later in the study.
2.       Water Resources
In the reconnaissance level investigations carried out during 1985 the proposed dam site on the Woja river was identified, a topographical survey of the reservoir area was produced, and the possible command area of the dam was estimated at over 3000 ha at the 1820 contour level. At this
level of 1820 the storage capacity of the reservoir would have been in 
g
excess of 500 x 10 m  . A capacity curve for the reservoir is shown as
Figure 3.
A more detailed investigation of the water resources has clearly indicated that the mean annual runoff from the catchment area is substantially lower than the initial figure. There are no river gauging 
stations on the Woja River but data are available for the Meki River. 
These indicate tliat the total runoff from the Meki catchment area is approximately 350 million cubic metres (2000 km ). The Woja catchment area of 435 km  is estimated to have a mean annual runoff of approximately 
c
3
75 x 10 m  . To store this volume of water the full supply level for the reservoir would be at the 1812.75 level.
3
2
7SUMMARY
This report gives a brief outline of the proposed irrigation development in the Butajira area. An evaluation of the v^ter resources indicates tnat there is sufficient water to irrigate in excess of 2500 ha. 
The soils surveys have been couple ted, indicating that the soils are suitable for irrigation, although some areas of unsuitable soils have to be excluded from the project area. The topography of the area indicates that only about 200 ha can be irrigated using a gravity system, and the balance of the project area would require a punped water supply.
f-2-
A possible dam site had been identified and investigated at the reconnaissance level study. An alternative dam site has been identified further along the Woja river, closes to the proposed irrigable area. A preliminary geological investigation of this site has been completed. This report (Annex A) indicates that further investigations including drilling 
will be necessary to determine the thickness of the weathered volcanic rock which is highly pemeable and will have to be removed during dam construction.
The
quality 
of
the
investigated. 
Samples
taken 
fro
m
water    in the     Woja
the Woja river has been briefly and Merechi rivers have indicated
that the water is suitable for irrigation.
3.        Soil Resources
A reconnaissance level soil investigation has been completed in the project area. A copy of this report is included as Annex B. This
report concluded that it wculd appear that the soils from the project area
are suitable for irrigation.
A more detailed feasibility level soil study has now been carried out in the project area, and this report is expected to be submitted soon. Discussions with the soil consultant have indicated that the soil
is more variable than anticipated, and it would be necessary to eliminate patches of less suitable or unsuitable soils from the proposed irrigable area.
• Crop Water Requirements
At present a variety of crops are grown on the proposed project area, including maize, sorghum, red peppers and wheat. In order to estimate the crop water requirements for the project a simplified cropping pattern of a
maize/wheat rotation has been used. Although the actual crops grown may still vary on the project it is not expected that the final water requirements
would vary considerably from this estimate.-3-
Ba inf all data from Koshe are available, indicating a mean annual precipitation of over 1000 inn. However data required for the
estimation of the crop water requirement including mean temperatures, relative humidity, wind speed and sunshine hours are only available for Ziway. These data were used, and the crop water requirement is estimated at approximately 1U50 mn for the wheat/cotton rotation. This estimate assumes an overall project irrigation efficiency of 40%.
5. Project Outline
In the initial reconnaissance level study the commandable area for this project was estimated at over 3000 ha. However the considerably 
reduced mean annual inflow and the topography of the area below the proposed dam site have now indicated that although these would be sufficient, water stored in the reservoir to irrigate approximately 2500 ha, this total area can 
only be irrigated by pumping from the reservoir. Inmediately below the main Ziway-Butajira road a snail area of approxijiately 200 ha gross has been identified below the 1812 contour which could be irrigated using gravity. 
This left bank area of approximately 100 ha has been surveyed, with 0.25 m contour intervals, and an outline design has been prepared. On the right
bank a similar area could be available, but no detailed survey data axe
available. No survey data are available for the remainder of the possible irrigable area.
It is proposed that two options would be considered for the proposed irrigation development. It is possible that a power line will be constructed to link Butajira with Ziway; in this event the development of the proposed irrigation project using relatively cheap electric power should be considered. The alternative development option would be to install a mini hydro station on rain punps to utilize part of the water stored to pump 
some water to the project area. It is estimated that an additional 500 ha could be brought under irrigation using these tecliniques.ANNEX A Geological Investigaion of Dam siteftrpcrt
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1    - Introduction
2     - Objective* of the Work
2.1.        Geolpglt description of t he <<mm
2.2.                  H
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2.1.          Geologic Description of the zone:
At both banks of Woja River there are outcrops which allow the observation and description of the different rock types. As it is stated in Dr. Kroupodorov’ s report, volcanic rock are developed. Throughout all the zone they are very cracked, fractured and weathered. In the right bank traces of faults can be observed, structural failure resulting from tectonic movements in the area.
The widely spread rocks in the zone are volcanic tuff and volcanic ashes, the latter lie almost horizontally over the former with variable thickness. In lesser amount occur
the aglomerates with boulders whose diameter ranges between 3-15cm Rhyolites and obidian, all them belonging to the
Trap series from the Pleistocene-Myocene.
2.2           Geologic Description of the Dam site:
Although the whole area is covered by top soil, the geology
of the dam site can be correlated by the presence of rocky outcrops. Which are found upto 150-200m approximately from the axis, up and downstream. A predominance of volcanic rocL deeply weathered, volcanic ashes and volcanic tuff can be observed.
.../3X
3
2.3. Determination of Che rock types in different parte of the
dam structure, as well as their physico-geologic fractures.
2.3.1. Embankment
For the investigation of this structure, together with the designers, six(6) test pits were allocated making up a cross-section profile in the direction of the proposed dam axis (Annex 1). The following lithologic sequence could be observed at the test pits:
Test Pit No. 1
0-0,40 Top Soil
0.40-2.30
Volcanic rock (volcanic ash) very alte
with seams ranging between l-3cm thlcki
and backfilled with sandy—clay material
traces of organic matter (roots)
2.30-2,60
stratified fine grained sand scone
Test Pit No. Il
0-0.30 Top Soil
0,30-1.00
Dark brown colored aandy-clay with trac
of organic matter.
.../4a
1.00 - 2.10                     Volcanic rocks (volcanic ..IO with rcsldc.l »lr- very altered, finegrained and seams ranging between l-4cm thickness backfilled with sandy-clay material and organic matter
2.10 - 3,00 Stratified fine-graned sandstone
3.00 -10,00 Volcanic rock (volcanic tuff) very altered. Test Pit No. Ill
0 - 0.40                         Top Soil
0.40       - 0.90             Dark brown colored sandy-clay with traces of organic matter.
0.90 - 2,20
Volcanic rock (volcanic ash) very altered,
fine grained with seams ranging between l-3ctn
backfilled with sandy clay material and organic
matter.
2,20 - 3,10
Stratified fine-grained sandstone
310 - 8.00
Volcanic rock (volcanic tuff) very altered
• • • • / 55
Test Pit No. IV
0 - 0.50
0.50 - 1.10
1,10 - 2,20
Top Soil
Black colored organogenic clay Dark brown colored sandy clay with content of organic matter (roots)
•>
Test Pic No. V
0     -      0.40
Top Soil
0.40 - 1.10
Black colored organogenic clay
1.10 - 2,00
Dark brown colored sandy clay with traces
of organic matter
2,00 -8,00
Very altered volcanic rocks (volcanic ashes)
Test Pit No. Vi
0- 0,40
Top Soil
0.40 - 1.40
Dark brown colored sandy clay with traces
of organic matter (roots)
1.40 -2,00
altered volcanic rock (volcanic ash)
As it can be observed from cross-section profile (Annex 2) the sandstone (sedimentary rock) occurs at the left bank
underlying the volcanic rock. This sandstone is
situated in the contact between the volcanic ashes and the volcanic tuff, which can be observed across the lithology of test pit No. 2.
.../ 66
From the geologic viewpoint it can be said that
the area  is complex  the rocks which are found in it are very  cracked and altered which increases considerably  the permeability of the rocks.
2.3.2                Spillway
At this structure three (3) test pits were allocated making up a longitudinal profile in the direction of the would be longitudinal spillway axis (Annex 3) the following lithologic requences were cut in the test pits.
Test Pit No. VII
0 -           0.50           Top Soil
0.50          - 1.00        Black colored organic clay 1.00         - 2.30       Altered volcanic rock (volcanic ash)
Test Pit No. V
0  - 0.40
0.40 - 1.10
1.10 - 2,00
2.00 - 8.00
Top Soil
Black colored organic clay
Dark      brown      colored      sandy      -clay      with      traces of organic matter.
Altered volcanic rock (volcanic ash)
/77
Test Pit No. VIII
0       -     0.40
Top Soil
0.40 - 1,20
Black colored organogenic clay
1.20 - 2,20
Dark brown colored sandy clay with
traces of organic matter.
2.20       - 2.40
Altered volcanic rock
Ac this structure as can be observed in the annex 3 appear
approximately 1 - 2mt of sandy-clay material.
The volcanic rock is situated after 2 mts depth and lfcs fractures
are similar to the left bank rock, with great alteration and cracking,
which Increases considerably the permeability of the rocks.
Allocation of construction materials
2.4.1.         Clay borrow area
With     the     objective     of     locating     the     borrow     area     in     the vicinity of the chosen axis , a test pit was dug
approximately     150m     from     the     dam     site     which     appears in Annex 3 and whose lithologic sequence is as follows:
0     -     0.40 Top Soil
0.40 - 1.20 Brown colored sandy clay 1.20 - 1.40 Altered volcanic rock.
. ../ H8
As it can be observed the thickness of clay material is of
0.80 m but a zone with a greater thickness can be created making
some excavations somehow far from the damsite 9 the material was
sampled in order to determine quality at the laboratory.
2.4.2.           Rip Rap Material
Due      to      the      great      alteration      of      the      rocks      found      at      area      its      utilization is not advisable . It is necessary to locate this material.
2.4.3.                 Sandy Material for Filter
Sand deposits which could be
in the vicinity of the area.
and Concrete
used for this purpose were not observed
It must be located.
Conclusions and Recommendations
Conclusions
Volcanic rocks are developed in the whole area with predominance at volcanic ashes and volcanic tuff
at the dam site area according to the outcrops
observed in the vicinity.
From the geologic viewpoint the area is complex the rocks occur very altered, fractured and cracked which increases considerably the rocks permeability .
.......... /99
3.1.3                According to the obtained results in test-pits No.II and III a weathered cross (volcanic rock ) with a thickness higher than 15m is expected, which considerably complicates the construction of the dam since the rocks are very altered and having a high permeability.
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3.1.4. Although the groundwater was not reached we can assume that
it is found between 10-12m depth according to the humidity of the rocks at the bottom (test pit No. Ill)
3.2.           Recommendations
3.2.1.                 Due to the great alteration of the volcanic rocks at the area, which increases considerably the permeability , the thickness
of weathered rock must be removed from the cut off trench during
construction.
The thickness of weathered crust (higher than 15m) must be determined as well as the variation of physical characteristics of the rocks. For this purpose drills must be made using drilling machines or some geophysical method.
The characteristics and homogeneity, as well in quality as in thickness of the borrow area must be determined with precision, for this it is necessary to increase the number of
test pits.
Eng. Serio BeANNEX B Preliminary Reconnaissance Soil investigation
In Bucajira Project area4)        about        7-10        additional        pit        observations        with        sampling        for textural and chemical analysis.
cc.      Mr.C.J.N.Davey
Water Resources               Development Engineer ET1I/82/008
Dr.I.Loerbroks
FAO Representative in -Ethiopia
Ms.D.von Werner
CPO Ethiopia, AGOE FAO Rome
Mr .J. II.Bruin
TSO AGLS FAO Rome
I
L

Summery

Project Code: ZH1/6.2/C08

Author: C. J. N. Davey, Water Resources Development Engineer

Date: November 1989

Location: Approximately 25 km west of Ziway, Ethiopia

1. Introduction

The Butajira Irrigation Project was identified in 1985 as part of FAO Project ETH/82/008. Initial estimates suggested 500 ha of irrigable land, but detailed surveys reduced this to approximately 200 ha for gravity irrigation.

2. Key Findings

Water Resources

Mean annual runoff from Woja River catchment (435 km²): ~75 million m³
Reservoir capacity at 1812.75 level would store this volume
Initial estimate of 3000 ha command area reduced due to lower water availability
Water quality suitable for irrigation

Soil Resources

Reconnaissance and feasibility studies completed
Soils generally suitable but variable, requiring exclusion of some patches

Crop Water Requirements

Estimated at ~1500 mm for wheat/cotton rotation
Assumed irrigation efficiency: 40%

3. Project Outline

The project proposes two development options:

Gravity irrigation for ~200 ha below 1812 contour
Pumped irrigation for additional areas (up to 2500 ha total):
- Option A: Electric pumps if power line to Butajira is constructed
- Option B: Mini-hydro station or rain pumps

4. Dam Site Investigation

Geological Findings (Annex A)

Highly weathered volcanic rocks (ash and tuff) with high permeability
Weathered crust thickness >15m complicates construction
Need to remove weathered rock from cutoff trench
Further drilling investigations recommended

Construction Materials

Limited clay material found (0.8m thickness)
No suitable rip rap or sand deposits identified nearby

5. Conclusions

The project has sufficient water resources for irrigating 2500+ ha, but only 200 ha can be served by gravity. Pumping would be required for the remaining area. The dam site presents geological challenges that require further investigation before construction.

Annexes

Annex A: Geological Investigation of Dam Site (Pages 8-18)

Annex B: Preliminary Reconnaissance Soil Investigations (Pages 20-22)