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Understanding Dredging

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Dredging for Drinking Water in Africa

2021-07-02 Lauren Grieco
dredging, drinking water, africa, reservoirs, silt

In flat landscapes across Africa, reservoirs and dams are a common solution to provide water for drinking and agricultural purposes to nearby communities. With dams blocking the flow of water comes the natural process of siltation, and without maintenance in the form of dredging, the capacity of the reservoirs – and the water they contain – is gradually reduced.

Hundreds of reservoirs constructed across the flat landscape of Burkina Faso store water for drinking water, industrial and agricultural activities as well as fishing. Nine large water reservoirs saw siltation caused by land erosion, increased demand for water as well as dams which were poorly maintained. A consortium is developing sustainable reservoir rehabilitation plans for the Nakambé and Gourma water authorities. Witteveen+Bos is leading Develop to Build (D2B), a programme executed by Rijksdienst voor Ondernemend Nederland or the Department of International Development in the Netherlands. Through sustainable land and sediment management measures, water resources will be preserved over the long term, and aim to generate income for the local communities.

In the KwaZulu-Natal province in eastern South Africa, another project was recently executed by Dredging Africa. The project’s goal was to increase the storage capacity for fresh water provided to nearby communities including the eMondlo area south of the town of Vryheid. The South African contractor was tasked with removing more than 35 years of silt accumulation in a reservoir created by the Mvunyane Dam.

The storage capacity behind the dam had been diminished over decades through siltation. The reservoir and dam is part of the eMondlo Water Supply Scheme which includes a pumping station and water purification plan. The water intake at the abstraction tower was also obstructed, requiring dredging over an area of approximately 3000 square metres. To increase storage capacity, the reservoir needed to be deepened by 20 metres and required the removal of a layer of alluvial gravel as well as silt.

Back in 1984, tropical storm Domoina brought heavy rainfall which expanded rivers and led to sediment flows that deposited in the reservoir. Dubbed the Consolidated Domoina Layer, the top, consolidated layer consists of alluvial gravel and requires the power of a Cutter Suction Dredger (CSD) to break through the coarse material. Supplied by Dutch vessel builder Damen Shipyards Group, the CSD250 has a normal max dredging depth of six metres but the ladder was extended to reach a depth of nine metres. The hydraulically powered cutter then broke through and pumped out material at a rate of 1,000 cubic metres per hour. With the Consolidated Domoina Layer removed, the DOP150, a submersible dredge pump, removed up to 20 metres or the fine sediments and silt which were revealed, successfully increasing water storage capacity. Since the dam will always lead to siltation, the CSD will be tasked with maintenance work in the future to maintain the desired level.