Omdel Dam

The Central Namib Water Scheme supplies Swakopmund, Walvis Bay, Rössing, Arandis as well as a number of smaller towns. It draws its water from underground sources in the Kuiseb River and the Omaruru River delta (OMDEL). The recharge of these sources is from intermittent flooding of the rivers. Abstraction from the OMDEL is currently 8.5 Mm3 per year but the estimated recharge before the dam was built was only 3.5 Mm3 per year.
In 1984, the Department of Water Affairs started to investigate ways to enhance the recharging of the OMDEL. It was already known that a thin layer of clay on top of the river bed gravel would limit the water penetration. The Omaruru River, like all westward flowing rivers, carries a high silt load. This silt seals the river bed so that most of the water flows over the top and out to sea. It was therefore proposed that the flood waters be contained for a time allowing the silt to settle out. The clean water could then be released into the infiltration areas, thus ensuring that most of the flood water would reach the underground aquifers.
A suitable site was selected and it was decided to use local materials for the construction of the wall. This was to reduce the costs and it also had the additional benefit of blending in with the surroundings. The wall consists of a clay core with sandy shoulders. The base of the wall stands on variable unconsolidated alluvium which allows a large amount of seepage. Many of the design features of the wall limit this seepage to prevent destabilisation of the foundation. The wall also has a complex set of drains to prevent through-seepage from destroying the clay core, when the dam is full. The drains also prevent internal water pressures (during rapid draw down) from collapsing the shoulders. The sandy shoulders are protected from wave and wind erosion by rock covering.
When the dam fills with water it is allowed to stand for 9 weeks to allow the silt to settle out. The clean surface water is then skimmed off by an abstraction tower and flows through a pipe under the wall to two infiltration ponds. The dam should be emptied between rainy seasons so that each time there is a flood, it can catch as much water as possible.
This dam allows the maximum amount of water to reach the underground aquifers and so making the maximum use of the periodic floods. It increases the recharge of the aquifer from 3.5 Mm3 per year to over 6 Mm3 per year. The total cost of this project was N$ 49 500 000.

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