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ISMAR7 Abstract

Optimisation of water release from a dam to recharge a downstream unconfined alluvial aquifer

Diganta Sarma1, Yongxin Xu2 and Greg Christelis3

1Namib Hydrosearch CC, Box 11546, Windhoek, Namibia
2Department of Earth Sciences, University of Western Cape, Cape Town, South Africa
3Geohydrology Division, Department of Water Affairs and Forestry, Windhoek, Namibia

The Oanob Alluvial Aquifer (OA) located near Rehoboth Town south of Windhoek, Namibia was the main source of water supply to the town till 1990 (17.28Mm3 in 1988). A dam constructed on the Oanob River, upstream of the alluvial aquifer, currently supplies the town and the aquifer is largely unused. Historically, seasonal flow in the Oanob River recharged the aquifer. Recharge ceased since the completion of the dam except when water from the dam is either released or when water spillage occurs during years of exceptional rain.

The OA is made up of fluvial sediments laid down in an incised valley cut into impervious basement rocks. The aquifer is elongated extending 21.5 km towards the south-east and is in average 2.5 km wide. The fluvial sedimentation by episodic flow in the ephemeral river has resulted in poorly sorted, laterally and longitudinally heterogeneous sedimentary layers. The aquifer is unconfined but could be locally under semi-confined conditions. Elevated bedrock at 5.8kms from the northern end of the aquifer separates the OA into two compartments.

Historically the OA received recharge from seasonal flow on the ephemeral Oanob River. It was estimated that up to 33% of surface flow recharged the groundwater (DWA, 1989a). It was planned prior to the completion of the dam that ‘surplus’ water from the Oanob Dam would be released on a regular basis to sustain the vegetation downstream. Flow was recorded in one such release made in 1992 and in three high rainfall years that resulted in spillage from the dam. The infiltrated groundwater formed a mound underneath the active river channel and this dissipated gradually into the full lateral extent of the aquifer. Significant throughflow losses were noted. It is estimated that currently the aquifer is 24% saturated.

In order to reduce evaporation losses from the impounded water in the dam, the utilisation of the additional storage available in the aquifer, and to limit the negative affect of the dam on the downstream vegetation, artificial recharge to the aquifer via releases from the dam is being considered. This may also significantly reduce the costs of water treatment. It was noted from hydrographs that during the spillage events the large surface flows did not result in higher recharge volumes but water was lost through surface runoff. Therefore, to optimise the release volumes to obtain maximum infiltration, a groundwater flow model is being developed. The existing data on water releases and spillage, and groundwater levels is being used to calibrate the model and predict an optimal scenario. Underground dams at the downstream end of the compartments are planned to reduce throughflow loss and allow sufficient time for water to flow laterally and saturate the full lateral extent of the aquifer. The effects of such dams are also being simulated through the groundwater model.