Borkena Irrigation Project Summary

bopjoxa irrigation mum
F.RTTT FROIECT OtHTJ’E
C. J. H. DAVEY
WATER RESO'JRCES DEVELORSJ.T E.‘SHEEP PROJECT EIH/82/008
FAO/WRDA FF.OJECT ETH/62/OC6 /JTIL 1966
ADDIS A2AEABORED LA IRRIGATION PROJECT
BRUT PROJECT OUTLINE
contents
page
GENERAL-------------------------------------------------------------------------------------------------------------------------------- 1
WATER RESOURCES----------------------------------------------------------------------------- 1
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CROP WATER REQUIRD-E1TS---------------------------------------------------------------  2
DESIGN DETAILS--------------------------------------------------------------------------------  3
U.l          Project Area----------------------------------------------------------------  3
M.2       Main Canal-----------------------------------------------------------------  4
U.3         Secondary Canals----------------------------------------------------------------------- 5
U.U        Drainage---------------------------------------------------------------------------------  5
U.5        Flood Protection------------------------------------------------------------------------  5
U.6        Roads and access tracks---------------------------------------------------------------  6
5. SUMMARY OF KEY POINTS----------------------------------------------------------  eB0RKD1A IRRIGATION PROJECT
GENERAL
The proposed Borkena irrigation project is located in the jwer Borkena Valley or Chaff a Plain approximately 330 km north of Addis baba on the main road to Dessie, as shown in Figure 1. As a
esult of the recent drought period a decision was made (in 1984) to proceed umeaiately with the construction of three dams on the Borkena River. These dams are now in various phases of constructibn with the lowest dam due to be completed later this year. The design of the main canal and part of the distribution system has now conmenced to utilize the water being made available for irrigation development.
2. WATER RESOURCES
The Borkena river has a total catchment area of approximately U65 km’ at the site of the lowest dam on the river, Borkena I. The
annual runoff from this catchment has been estimated at over 100.0 x lO^m2 3, and with the construction of sufficient reservoir storage capacity,
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approximately 65 x 10 m  could be made available for irrigation purposes in the Borkena valley. At present three dams are under construction with a total estimated storage capacity of 20.1 x 10Sri3, as shown in Table 1.
3-2-
lblel:         Borkena Dams, Storage Capacity and Height
Dam
Height
m
Storage Capacity
lowin’
Borkena I
Borkena II
Borkena III
18
31
25
1.8
12.0
6.3
Total
20.1
3
Allowing for transmission losses between the reservoirs, and seepage and evaporation losses, it is estineted that completion of these three dams would provide approximately 15.0 x 10^m  of water for irrigation purposes.
Additional dam sites have been identified above the lower sites, and it is possible that sufficient dams could be constructed to harness the canplete water resources of the Borkena Pdver above the Chaff a Pl Ain.
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3. CROP WATER REQJIRg-g^S
From the meteorological stations at Harbu and Kcmbolcha it is estimated that the mean annual rainfall over a 30 year period exceeds
1,000m per annum. In order to detenrane the effective rainfall available for cropping requirements the 75% probability rainfall has bean estimated at about 900 mm, and the effective rain fa T  at 630 nrn.
1-3-
The crop water requirements for the project area have been lemined assuming that two crops are grown each year (i.e. 200% cropping tensity), and, for calculaticn purposes, it has been assumed that
ipplenentary irrigation would be required for a maize crop grown during the iin rain season, and full irrigation would be required for a wheat crop 
jown during the dry season. The selection of other crops or a mixture of
Tops is unlikely to result in a significant increase in the estimate of the
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crop water requirements.
The overall project efficiency has been estimated at and with the above assumptions the project crop water requirement has been
calculated as 1,350 nn, with a peak requirement of 300 urn during February.
M. DESIGN DETAILS
U *1 Project Area
The total area of irrigable land in the Borkena Valley exceeds 10,000 ha, but the land to be irrigated would be limit pH by the available water resources. Tne completion of the first three dams would provide a total storage capacity’ of 20.1 x 10 m’ and it is estimated that these would yield sufficient water for approximately 1,100 ha cf full and 
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supplementary irrigation. Construction of the additional upstream dams would possibly increase this area tc about u,000 ha.-4-
Approxinately 1,000 ha of ideally suitable land for irrigation lopnent is Available above the Dima River. It is expected that the
sial development based on the three dams presently under construction would extend beyond this natural boundary.
The irrigable area along the right bank of the Borkena River s been selected for initial development since the gradients are lower, and ve area is closest to the lowest Borkena dam; thus reducing the length of the ain canal. The total area to be irrigated is limited by the availability of
jater and it is intended that the irrigation development would all be concentrated along the right bank of the Borkena River.
4.2 Main Canal
With the exception of the section of the main canal imrrediately below the dam it is intended that the main canal would be unlined, and located at a non-erodible gradient of 1:2,000. A number of drop structures would be necessary to maintain this profile, and the largest would be a 5 metre drop within the initial lined section of the main canal.
The first stage of the development to utilt7.p water from Borkena I would require 4,600 metres of main canal to a control reservoir to 
serve approximately 100 ha of irrigation development. To fully utilize the available water when all three Borkena dams are completed, the ^in canal would be extended an additional 2,800 m, to a total estimated length of 7,400 metres.3
The n»in canal is designed to pass a nexirum peak flw of
.6 m /s or 1.2 litres/sec/ha based on continuous water delivery in the main 
anal. Naturally this peak flow would be reduced at various stage along the
ain canal.
H.3 Secondary Canals
For the first 1,000 ha of development there are 6 secondary canals with a total length exceeding 10,200 metres. Tnese secondary canals vary in size depending on the areas they serve. The irrigation system proposed for Borkena is based on continuous flow in the main canal (2U hours per day during the peak demand period) and continuous flow in the secondary 
canals (10 hours per day). Thus water would always be available in the secondary canals to be diverted into the tertiary canals where rotational irrigation of the areas would be practised.
U.H Drainage
Tne brief soil survey carried out by the FAO Soil Expert has indicated that there already exists a relatively high water table in the
proposed irrigable area. Thus it will be necessary’ to incorporate sone deep open drains even during the initial development, and it nay be necessary to 
install sub-surface drains to provide adequate control of the water table.
U.S Flood Protection
Tne completion cf the three dams should reduce the floor -ng 
presently experienced in tr<e Borkena plains area, but sort flcxoc bur.d protectionL also be included in the design. In addition a bund is included in the ign to protect the main canal and to divert the surface flood waters into : Dima River.
4.6 Roads and Access Tracks
Since the project area is on the right bank of the Borkena river a permanent river crossing will need to be provided, possibly at the location of the existing causeway. No provision has been made for crossing the spillways of the dams.
Access tracks will be incorporated in the design of the irrigable area
5.       SUMMARY OF KEY POINTS
1.    Pean Annual Runoff from Borkena River
2.    Potential water available for irrigation from Borkena River
3.    Reservoir storage capacity of Borkena Dams
*4. Estimated irrigation water available from Borkena reservoirs
5.    Total potential irrigable area in Borkena Valley
6.    Mean Annual Rainfall
7.    Assumed project irrigation efficiency
8.    Estimated crop water requirement (raize, wheat)
9.    Irrigable area from 3 Borkena reservoirs
100.0 x 106m’
6 5.0 x 106m3 20.1 x 106m?
15.0 x 106m3 10,000 ha+
1,017 mn
1,350 mn. 1,100 ha-7-
10.     Estimated maxinum potential irrigable area
11.     Main canal maxinum capacity
12.     Length of main canal for stage 1: 100 ha
13.    Length of main canal for state 2: 1,000 ha
14.    Length of secondary canal for stage 1: 100 ha
15. Length of secondary canal for stage 2: 1,000 ha
4,000 ha 3.5 m’/s
4,600 m
7,400 m
1,550 m
10,200 m•Oarbeno ft
LEGEND Towns
All WiOthor Rood Mom Cotchfmnt I Subcoichmint Di.
Laks
Swomp
Dty Shiom/Wosh
F A-O/W' R.D A PROJECT ETH/82/OO8
WATER RESOURCES DEVELOPMENT AUTHORITY
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BORKENA IRRIGATION PROJECT
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Summery

Project Overview

The Borkena Irrigation Project is located in the Borkena Valley (Chaffa Plain) approximately 330 km north of Addis Ababa. Initiated in 1984 following a drought period, the project involves the construction of three dams on the Borkena River to support irrigation development in the valley.

Water Resources

The Borkena River has a catchment area of 465 km² at the Borkena I dam site, with mean annual runoff estimated at 100.0 × 10⁶ m³. Three dams are under construction with the following specifications:

Dam	Height (m)	Storage Capacity (10⁶ m³)
Borkena I	18	1.8
Borkena II	31	12.0
Borkena III	25	6.3
Total		20.1

These dams are expected to provide approximately 15.0 × 10⁶ m³ of water for irrigation after accounting for losses.

Crop Water Requirements

The mean annual rainfall is 1,017 mm, with effective rainfall estimated at 630 mm. The project assumes two crops per year (200% cropping intensity) with:

Supplementary irrigation for maize during the main rain season
Full irrigation for wheat during the dry season

The estimated crop water requirement is 1,350 mm annually, with a peak requirement of 300 mm in February. Project irrigation efficiency is estimated at 40%.

Design Details

Project Area

The Borkena Valley contains over 10,000 ha of irrigable land, but initial development will be limited to:

1,100 ha using water from the three dams
Potential expansion to 4,000 ha with additional upstream dams

Main Canal

Mostly unlined with a gradient of 1:2,000
Maximum capacity: 6 m³/s (peak flow)
Stage 1 (100 ha): 4,600 m length
Stage 2 (1,000 ha): 7,400 m total length

Secondary Canals

6 secondary canals totaling 10,200 m for 1,000 ha development
Continuous flow system (24 hours in main canal, 10 hours in secondary canals)

Other Infrastructure

Drainage: Deep open drains and possible sub-surface drains due to high water table
Flood protection: Bunds to divert flood waters into Dirma River
Roads: Permanent river crossing and access tracks

Key Project Statistics

Mean Annual Runoff: 100.0 × 10⁶ m³
Potential irrigation water from river: 65.0 × 10⁶ m³
Reservoir storage capacity: 20.1 × 10⁶ m³
Available irrigation water from reservoirs: 15.0 × 10⁶ m³
Total potential irrigable area: 10,000 ha
Initial irrigable area (3 dams): 1,100 ha
Potential maximum irrigable area: 4,000 ha