6 8 Case River Engineering Approach Wadi Labka

6. 8 Case - ‘River Engineering Approach’ Wadi Labka – Gabion/ Soil Diversion Spurs

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

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

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

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 the cost of civil engineering headworks (incl. breaching bund, link canal and culvert, estimated at USD 7, 000 (or USD 1800/ha) for the same system

River engineering versus civil engineering • This ‘river engineering’ approach taken is low cost • It is also more appropriate than the civil engineering: – 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 wate 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