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6.11 Case: Wadi Labka – What to Do When Traditional System Becomes Unsustainable. Wadi Labka (En-River), Eritrea . Wadi Labka Gorge. Three main command areas. Flow is divided in the riverbed. Scan of Map. Riverbed downstream of gorge. Flow. Traditional method of diversion. - PowerPoint PPT Presentation
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6.11Case: 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 andthe earth bund will be increased
Use of geotextile will reducewash 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