6.11 Case: Wadi Labka – What to Do When Traditional System Becomes Unsustainable

Wadi Labka (En-River), Eritrea Wadi Labka Gorge

Flow is divided in the riverbed  Scan of Map Riverbed downstream of gorge Three main command areas Flow

Traditional method of diversion Combination of earthen bunds protected with acacia brushwood and natural vegetative bunds (tamarisk)

Vegetative bunds not always protected

Increasingly difficult to find acacia shrubs for the traditional bunds Haulage may take more than three days

What to do in this area?  The traditional system is becoming gradually non-sustainable.....  There is a gorge yet the river widens out considerably  There are command areas on either bank of the river

Discussion in groups...

In the end..  An ‘improved traditional system approach’ was followed - in this case focussing on river engineering rather than the development of a headworks>>>>>>

Construction of agims with bulldozer and gabions  Replacement of upper section of the traditional agims with soil bunds and gabions  Gabions at section where flood channel hits the diversion bund  Flood protection on left bank to prevent further movement of Wadi Labka to the left Agim = Traditional diversion bund

Construction of agims with bulldozer and gabions

Labor for gabion filling provided by farmers without charge

Wall of gabion baskets with earthen protection Another layer of gabions will be added and the earth bund will be increased Use of geotextile will reduce wash out of fine material at the bottom and prevent collapse

Scope of work  1200 meter gabion works  120 days of bulldozer use  Total costs USD 430,000  Total cost/ ha USD 110

This may be compared with a civil engineering approach that was considered earlier Headworks (incl. breaching bund) Single off-take Link canal and culvert, estimated at USD 7,000,000 (or USD 1800/ha) for the same system

River engineering versus headworks approach  This ‘river engineering’ approach taken is lower cost. The headworks option in fact was dropped earlier as being too costly  In this case the river engineering approach was also more appropriate than the headworks approach:  It follows the traditional system of diverting flood flows, based on splitting the flow in smaller proportions to make its management easier  It follows the existing water rights and does not interfere with it  Very large floods will wash out part of the structures but will also ensure that the floods remains in the river bed and does not enter the command area, causing erosion