Managing Siltation in Algeria’s Reservoirs
October 1, 2007
By Canadian Consulting Engineer
In Algeria, water is essential for economic development, but its scarcity requires the use of dams for human use and irrigation. The Algerian agency for dams, L'Agence Nationale des Barrages et Transf...
In Algeria, water is essential for economic development, but its scarcity requires the use of dams for human use and irrigation. The Algerian agency for dams, L’Agence Nationale des Barrages et Transferts (ANBT), has an ambitious program to build 70 new dams by 2010.
However, soil erosion has helped to reduce the capacities of the existing dam reservoirs by 20% since their original construction. In 2003, the ANBT therefore decided to do a comprehensive study of the risks of soil erosion and siltation for 21 dams. Of these, five are existing dams and 16 are in the detailed planning stage.
TECSULT was commissioned to do the study and was also asked to develop watershed management plans to reduce hillside erosion and ultimately decrease siltation in the reservoirs.
At the call for tender stage, the watersheds had been divided into six groups with an invitation to tender for each group. Tecsult responded to all tenders and in its proposals offered a 20% rebate for the study of all 21 watersheds under a single lump sum contract. The strategy succeeded and Tecsult began the study in the fall of 2003. Originally, the client asked the firms only to locate slopes prone to sheet erosion. Tecsult suggested also locating areas prone to gully erosion and landslides, knowing that these processes contribute more than 90% of the sediment loads reaching Algerian reservoirs.
Tecsult also carried out a technical transfer program which included installing state-of-the-art GIS (Geographical Information System) equipments at the ANBT offices, and training local technicians and engineers in new technologies.
A large group of Algerian stakeholders were invited to discuss the conclusions of the study and the ensuing tasks.
Elevation models using satellite images
Tecsult engineers had to study an area of 23,800 square kilometres, and in a time span of 18 months. They had limited access to the field sites due to national security reasons, but the study was completed in the planned schedule.
The project involved the digitization of vast amounts of data such as the contour lines of nearly 50 large-scale topographical maps. The challenges were overcome by conducting research and development. For example, Tecsult developed algorithms for extracting a digital elevation model from satellite images. Tecsult also adapted pedotransfer functions developed in Australia that can predict difficult-to-measure soil properties such as texture from geological maps. And because no model exists for locating gullies, Tecsult created one for estimating the proneness of slopes to gully erosion.
Erosion control strategies to serve the rural population
Each watershed was mapped for different types of erosion, and the results were combined in a consolidated map that was used to develop an erosion management plan for each watershed.
With the help of a rural sociology researcher from the University of Algiers, Tecsult suggested erosion control techniques acceptable to the needs of the local rural populations, such as those that would provide additional income for the farmstead or family, that were low cost and simple to construct and maintain. An example is the erosion check-line using the typical “prickly pear” cactus (opuntia), a very successful technique used by Tunisian and Moroccan farmers.
Because the hillside protection measures will require many years before they reach maximum efficiency, Tecsult proposed special erosion control actions that will rapidly reduce silting in the reservoirs. These measures include trapping sediments that are presently in the river systems using strategically placed storage basins and check structures. Tecsult also recommended ways of protecting river and roadside banks from erosion.
Tecsult created 16 technical guidelines (specifications, equipment, labour, cost) in order to facilitate the implementation of all the control measures, including the required actions by farmers, contractors and local communities. Finally the studies included drafting “invitation to tender” documents for local contractors.
Costs would be less than solutions like dredging
Lastly, Tecsult studied the cost of implementing the plans for each watershed-with-dam. The cost studies took into account the special erosion control actions and sedimentation management, as well as public information programs. The costs were found to be significantly less than alternative solutions such as dredging (which would be required on a continuous basis and would have serious environmental impacts), raising the dam crests or ultimately having to decommission existing dams. Each management scheme was examined by comparing the total estimated gain of water in the reservoir over 50 years using the “with” and the “without” scenario. The conclusion is that this strategy will secure a water volume equivalent to the annual consumption of more than nine million people.CCE
Name of project: Development of a Management Strategy to Reduce Hillslope Erosion and Reservoir Siltation for Algerian Watersheds
Award winning firm prime consultant: TECSULT, Montreal (Andr Lauzon, Pierre Roy, P.Eng., Henri Tichoux, Guy Parent, Dr. Jacques Langlois, Martin Harvey, P.Eng., Francois Trudeau, Raphael Fauchre, Salah Rechoum, P.Eng., Abdellah Hayoun)
Owner: Agence Nationale des Barrages et Transferts (ANBT), Algeria
Other key players: Dr. Brahim Benmoussa (sociologist), Dr. Victor Galay, P.Eng. (hydraulics), Dr. Mourad Arabi (forestry), Dr. Benina Touaibia (hydraulics), BNEDER (GIS)