Butajira Irrigation Project Summary

BUTAJ1RA IRRIGATION PROJECT
BRIEF PROJECT OtHLINE
By
C. J. N. DAVEY
Water Resources Development Digbrj<jr
[AC M-TW-’ L 'H".2/CJ8
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i.•
*m RCvtrrCONTENTS
PAGE
1. 
Introduction------------------------------------------------------------------------------------  1
2. 
Water Resources-------------------------------------------------------------------------------  1
3. 
Soil Resources---------------------------------------------------------------------------------  2
4.        Crop Water Requirements-------------------------------------------------------------------  2
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 part of
FAO Project EIH/82/008. The area to be irrigated was first identified on 
the available napping (1:250,000), vhich 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 conuendable
area is substantially lower at approximately 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 that the total runoff from the Meki catchment area is approximately 350 million cubic metres (2000 km  ). The Woja catchment area of U35 km  is estimated to have a mean annual runoff of approximately 
63
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
?SUMMARY
This report gives a brief outline of the proposed irrigation development in the Butajira area. An evaluation of the vster resources indicates tnar there is sufficient water to irrigate in excess of 2500 ha. 
The soils surveys have been completed, 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 wculd 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 
Grill be necessary to determine the thickness of the weathered volcanic rock which is highly permeable and will have to be removed during dam construction.
The quality of the water in the Woja river has been briefly investigated. Samples taken from the Woja and Merechi rivers have indicated that the water is suitable for irrigation.
3.        Soil Kesouxces
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 would 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.
4  • 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-
Ra inf all data from Koshe are available, indicating a mean annua] precipitation of over 1000 irm- However data required for rhe 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 1450 nrn 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, lnmediately below the m^in Ziway-Butajira road a small area of approximately 200 ha gross has been identified below the 1812 contour which could be irrigated using gravity. 
This left bank area of approximately 100 Iia has been surveyed, with 0.25 m contour intervals, and an outline design has been prepared. On the right
bank a similar aiea 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 site
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2.2.
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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 rock deeply weathered, volcanic ashes and volcanic tuff can be observed.
.. ./33
2.3.      Determination of the rock types in different parts 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  alter!
with seams ranging  between l-3cm thicknl and backfilled with sandy-clay  material’ traces of organic matter (roota)
2.30-2,60
Test Pit No. II
0-0.30 Top Soil
0,30-1.00
stratified fine grained sand stone
Dark brown colored sandy-clay  with trac
of organic matter.
.../41.00 - 2,10                     Volcanic rocks (volcanic ash) with residual air.
very altered, finegrained and seams ranging
between         l-4cm         thickness         backfilled         with         aandy-clay material and organic matter
2.10 - 3,00 Stratified flne-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-3cm
backfilled with sandy clay material and organic
matter.
2,20 - 3,10
Stratified fine-grained sandstone
310 - 8.00
Volcanic rock (volcanic tuff) very altered5
Test Pit No.
0 - 0.50
IV
0.50
1,10
1.10
- 2,20
Top Soil
Black colored organogenic clay
Dark brown
content of
colored sandy clay with
organic matter (roots)
9
Test Pit 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
0.40 - 1.40
1.40 -2,00
Top Soil
Dark      brown      colored      sandy      clay      with      traces of       organic       matter       (roots)
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 Che 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 Che 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
0.40 - 1,20
1.20 - 2,20
Top Soil
Black colored organogenic clay
Dark     brown     colored     sandy     clay     with traces of organic matter.
2.20       - 2.40                          Altered volcanic rock
At 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 ips fractures are similar to the left bank rock, with great alteration and cracking, which Increases considerably the permeability of the rocks.
2.4.            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.8
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 , 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      it9      utilization is not advisable . It is necessary to locate this material.
2.4.3.                 Sandy Material for Filter
and Concrete
Sand deposits which could                  be used for this purpose were not observed
in the vicinity of the area.
It must be located.
Conclusions and Recommendations
3.1.
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 .9
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.
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)
i
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.ANNembankment cross-section profile a a'
Osts IG- 10-6 6
Sampled pointSPILLWAY PROFILE BB‘
"S'
2 5m
-n .—
0,10 I,20q 2 00y
ANN EA
B'
0 v
F <- -r Q.40 q
-•
• *
Le gen d
BORROW AREA TOST PIT
YV
VV
YV
V
V   VV
v   1/ V    v   v
VV VVy
Vv vvV
VV vVV
VV
/ 1/ u
L 20 Y
■ . •  12.2 0 y
-     7 v 2,AO
e,oo
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Sampled Dam :
ScaleiVert
point V/oj a
i: ioo
Horiz : indicatedANNEX B Preliminary Reconnaissance Soil investigation
In Bucajlra Project areaAddis Ababa, 29 October 1986 TECHNICAL MEMORANDUM
To:                   Mssrs. Mirza Javed and S.Paris Soil survey consultants ETH/82/008
From:              E.De Pauw
Soil scientist ETH/82/008 Through:        Ato         Telahun         Eshetu
NPC ETH/82/008
,
Subject:          Prel            iminary            results            reconnaissance            soil            investigations jn Butajira project area
This memo summarizes the fin<
?g"|  / °"n faO/WRD A-team.
Sr
These findings are based on
reconnaissance undertaken in July 
2  days field obser^
description of 8 soil profile pits °"ly’
about 40 
WRDA-laboratory.
samples- have not yet been receive
observations and the
reCeived from the
1.
The
the
and
project area  covers about 700  ha
Weja  river south of its confluence
on the west bank of
with the Meki
bordered
parallel with and below the
and below the 1815 m contour,
in the west by the road between Butajira and
river
It is
Zway.
2.
project          lend          1,          generally          a          flat          or          .["»»>          f*“
2
b1
3.
with very uniform slope in the range
~ *- The area is fully cultivated wi
Proportion under grazing. The separation « **®" and grazing land is probably rotational, not perna
soils are most likely formed in
ignimbrites and are very homogeneous. Char
Presence of a lithological discontinuity ^°Ujf^ont?nuity cm (varying between 30-70 cm) Abov®. J^/^nsol idated
soils appear to have formed in
icamc ash, below in more compacted nimbrite. In all inspected profile P* 
soil horizons (sometimes a few horizons are
° ld be observed:
. bstantiai
cultivated
t i^i^thJ
depth of
or in
sequence
t present)
cuPinuph           horizonLAP J 
of thickness 10-15 cm, fairly
1)       indistinguishable in uncultivated land from:
2 )       A-horizon(Aj_, 
fairly thick with depth of 20 cm to max
60-70 cm (if the E-liorizon is absent).
The Ap- and A-horizon have similar colors (mainly  black to very  dark brown,  moist),  similar field textures (silty loam-silty  clay  loam) and similar structures (weak to moderate crumb) and consistence (usually  friable,  moist). The uverage pH is 6.5.
3)       E-horizon (E) characterized by lighter colours (especially
dry:         (light) grey brown), somewhat lighter field textures
(mainly  silty  loam),  mottling  (usually),  poorer structure (usually  s t rue t ure 1  ess ) and variable thickness (this horizon may  be absent but thickness usually  varies,  even within the profile,  from a  few cm to  10-15  cm,  with a  maximum of 20 cm). The average pH is 6.8.
4)
Discontinuity  (IIBt) formed by  very  dark (grey)brown to black silty  clays with fairly  uniform thickness (although the horizon may  be absent in a  few places),  strong structural development (medium to  coarse prisms breaking into  fine (sub)angular blocky  aggregates),  friable to  firm consistence (moist) and presence of broken to  continuous, Moderately thick clay cutans. The average pH is 7.3.
5)
5®£ond____discontinuity
------- CJIlBj       formed       by       (very)       dark       brown
22j
with     thickness
of 30-40             cm (varying from
a ubanguiar
° to co
Uy Clay
1  j loams
cm) ,
friable        concsonistseinstceen, ce,
fine to medium
^
ay
re sembling
---- «uii
 cutans;
na
blocky structure and
mixture of various
Parent ma t
Similar u
Varie gatp?Oclty
11 1 nR but
ial Ugnimbrite?) . The average pH is 8.3.
 Probably
broken,  moderately thick
brown               colors somewhat
due to weathering of
6)
re
fra «»ent  ’ ^°   —- ----------------------------------------  --
to above (lllBt)
soi 1
s t ructure, 
clay cut ans and
but more tendency  to
colors somewhat more
more weathering rock
8T
The
^ nera1 u
e
a)
average pH is also 8.3.
C Qracteristics are therefore:
b)         w*akly to
w ateri           1eachedPQ ° y structured loamy topsoil
c)           r*°Eging    an       .    i    u       v    i    a    1    horizon    with    evidence    of    temporary
L hi ? darh col ateral water movement
as Qc
well structured silty clay layer with
d)
*    tholoaj    _U.    few      to       15    cm    at    a    depth    of    30-50    cm,    acting
Str Uct°re bro •dlsco«tinuity
Ur
al deve^op ^ and more loamy layers with weaker
8In this arrangement of layers and th
of  water the soils show similarity with ??°ciate  movement
a
d
Ku lito plains, which have been rated “ 
° fp the Alaba
irrlg
ation         because         of         their         extreme         erodn         ?         for              «ravity
erosion hazard is, however, 
following reasons:
no(  expected
Pected in these soils for
The sa«*
a) there is no
river banks.
b)            the      dark
discont    inui    t    y
visible erosion in the area, not even in the
colored clay that forms
is porous and permeable and
the
lithological
barrier
absorb
indicated
transport
to root and water penetration,
irrigat ion
by
i n
It
a
does not appear a
should be able to
lower rate as is
the
the
water stagnation and
This will need to be
evaluated from the
These
appearance of
Kulito plains.
water, albeit r*at
signs of lateral
eluvial horizon,
infiltration tests.
the
non-saline.
(8.3) which
Na, particularly
instability. Further
following objectives:
From
Project
the first
area
The pH
could
f rom ab ou t
studies
clays are well structured and do not have the typical the easily dispersible clays of the Alaba
observations it appears that the soils of are suitable for irrigation. The soils are
in the subsoils is, however, fairly high point to substantial amounts of exchangeable
70        cm,        and        associated        structural are therefore aimed to achieve
. better eborecterizetlon
level
b)      an assessment o 
determine the application
and maPPing
a at feasibi^ity
. e
rates
waterlogging in the topsoi
c)         assess the erosion
hazard on
infiltration characteristics to
required to avoid
the basis of field
observations and lab data
MditXonal studies at f eas ibklA V~
nk_
triplir-‘About
5 infiltration tests to
----- ~ 
sequ-e ^ncceate. On some sites (having   ®. ! “ _
On some
th
excavated
dir*
3t  ---------- 1 after the tests to study the a r ibution of the infiltrated water.
!  of layers) tlaye  he mrsoi) sture proili®
rof  He is to be
dl t/b°Ut 24
PF ‘®
£ curve (2
rmination
ring core samples
of the bulk density
Profiles) in duplicate
3)
obSerr°utine
deU?ation
ler »»ned i-
auger
density
in each augerhole.
observations
of at least
be carried out in
representative
. and vertical
have to be taken for
(1 profile) and the
on all signi f icant
leading to overall
1/16 ha with pH and EC4) about
textural
7-10 additional pit observations with sampling for and chemical analysis.
cc.      Mr.C.J.N.Davey
Water Resources               D.velop.enl Engineer ETII/82/008
Dr.I.Loerbroks
FAO Representative in Ethiopi
Ms .D.von Werner
CPO Ethiopia, AGOE FAO Rome
Mr.J. II.Bruin
TSO AGLS FAO RomeMDote Due
TITLE
CALL NO.
ED1TI0N ------------ VOLUME-------------------------- CAcc. NO.----------A

Summery

Project Overview

The Butajira Irrigation Project is located approximately 25 km west of Ziway, Ethiopia, along the main Ziway-Butajira road. Initially identified in 1985, the project was later included as part of FAO Project ETH/82/008.

Key Details:

Original estimated irrigable area: 500 ha (based on 1:250,000 mapping)
Revised gravity-commandable area: ~200 ha (after detailed survey)
Potential total irrigable area with pumping: ~2500 ha
Proposed dam site on the Woja River
Project prepared by C.J.N. Davey, Water Resources Development Engineer (1989)

Water Resources

Woja River catchment area: 435 km²
Mean annual runoff: ~75 million m³ (revised from initial higher estimates)
Reservoir full supply level: 1812.75 m contour
Initial reservoir capacity estimate at 1820 m: >500 million m³ (revised downward)
Water quality found suitable for irrigation

Soil Resources

Reconnaissance and feasibility-level soil investigations were conducted:

Soils formed in volcanic ashes covering ignimbrites
Generally suitable for irrigation but with some variability
Key characteristics:
- Weak to moderately structured loamy topsoil
- Bleached eluvial horizon showing signs of temporary waterlogging
- Dark colored, well-structured silty clay layer at 30-50 cm depth
- Non-saline with pH averaging 6.5 (topsoil) to 8.3 (subsoil)
Some areas of less suitable soils need to be excluded
Erosion hazard appears low based on field observations

Crop Water Requirements

Current crops: maize, sorghum, red peppers, wheat
Simplified rotation used for calculations: maize/wheat
Mean annual precipitation: >1000 mm (Koshe data)
Estimated crop water requirement: ~1500 mm for wheat/cotton rotation
Assumed irrigation efficiency: 40%

Project Outline

Two development options proposed:

Gravity irrigation for ~200 ha below 1812 m contour
Pumped irrigation for additional areas (possibly ~500 ha more):
- Option A: Electric pumps if power line to Butajira is constructed
- Option B: Mini-hydro station to power pumps

Geological Findings (Annex A)

Dam site geology dominated by volcanic rocks (tuff and ashes)
Rocks are highly weathered, fractured, and permeable
Weathered crust thickness >15m complicates dam construction
Key recommendations:
- Remove weathered rock from cutoff trench
- Conduct additional drilling to determine exact thickness
- Locate suitable construction materials (clay, rip rap, sand)

Note: The document includes detailed annexes with geological investigations and soil survey data that provide technical support for these findings.