Aquifer response to artificial recharge, Omaruru Delta (Namibia)      
 

Framework

The OMDEL (Omaruru Delta) alluvial bed aquifer in Namibia constitutes one of the groundwater sources supplying water to coastal towns (e.g. Henties Bay, Swakopmund) and the Rössing Uranium Mine located in the so called Central Namib Area. This hyper-arid region is currently solely dependent on groundwater sources to supply in its water demand.

Production of potable water from the OMDEL aquifer has reached the sustainable yield of this resource. To enhance groundwater recharge to the aquifer the Department of Water Affairs (DWA) commissioned the construction of the OMDEL dam, which was completed in 1994. Subsequently spreading-type infiltration basins were established to artificially recharge the aquifer using water retained by the dam. Two flood events, both resulting in inflows of 18 Mm3 into the dam during the 1996/97 and 1999/2000 rainy seasons, presented the first opportunities to implement operation and management of the artificial recharge scheme.

In conjunction with NamWater (water supply company of Namibia), Royal Haskoning has carried out a co-operation project entitled "Re-evaluation and management models for the coastal aquifers (Kuiseb and OMDEL aquifers) of the Central Namib Area". As part of this project a hydrogeological system analysis was carried out, resulting in two models, one for the Kuiseb aquifer and one for the OMDEL aquifer.

The OMDEL model is used for a review and re-assessment of the current operation and management of the OMDEL artificial recharge scheme. The model provides information about the extend of spreading of infiltrating water in the aquifer and if it can be recovered for production. The model is also used for water balance calculations, important for identifying possible other locations suitable for artificial recharge and to optimise the current scheme. Water balance calculations of the entire OMDEL aquifer are used determine the short and long term sustainable yield.

How it was done

A water balance calculation with a groundwater model can only be reliable when the infiltration basins and the dam are properly schematised. The way they are schematised is strongly depending on their operation and management. When operational a constant water level is maintained in the infiltration basins.

The water levels in the dam are also known or for a future scenario can be calculated from an inflow volume. For this reason both the dam and the infiltration basins are incorporated in the topsystem of the model (see illustration). Which has as a major advantage that the model can be calibrated not only using measured groundwater levels but infiltration fluxes as well. For the calculated infiltration fluxes (water balance function) can be compared to measured inflow into the basins and seepage from the dam. Because the main modelling objective is to simulate aquifer response in time a transient model is used, making it possible to use changing water levels with time.

Results
The OMDEL aquifer model for the first time afforded an inter-relational approach to the dynamics and behaviour of the aquifer and interdependent hydrogeological components. Moreover the simulation (transient calculation) of the 1996/97 rainy season provided valuable information concerning aquifer response to a dam fill and subsequent artificial recharge.

It is envisaged to utilise the OMDEL aquifer model to simulate and predict specific operational and management scenarios, thereby contributing to the optimal and sustainable utilisation of the involved aquifer.
 
  Location of the artificial ercharge scheme
 
TRIWACO-TOPSYSTEM applied for artificial recharge basins

 

 
   
 
When active the basin has a controlled (fixed) water level and an infiltration resistance of several days. triwaco automatically adapts the flux depending on saturated or unsaturated conditions beneath the basins. When inactive the controlled water level is set below the drainage level (infiltration in that case will not occur). The infiltration- and drainage resistances are defined by polygons given the specified values.

When the groundwater level rises above the bottom of the basin then, depending on the controlled water level, drainage may occur (drainage resistance is several days). Whether a basin is active or inactive at a specific time or period is defined by the user.