Sand Rivers

Non-perennial rivers characterize Namibia’s hydrology. Because of the country’s arid climate, potential evaporation is almost six times greater than average rainfall, with only approximately 2 percent of rainfall becoming runoff and 1 percent entering the groundwater. Periodic floods recharge the sand river channel of ephemeral rivers and only occasionally reach the Atlantic (Heyns et al., 1998). The catchment of the Omaruru River covers 15,700 km², of which the mountainous source area receives 200 to 450 mm/year of rainfall. Most floods originate in the mountains. The flood waters recharge the sand river channel (Figure 16) as they move downstream and finally reach the ocean if rainfall continues long enough. Much slower flow also takes place as groundwater throughflow in the sand river aquifer (Stengel, 1966). A major challenge is the very infrequent flood events in the lower part of the catchment. In a 22-year period from 1994 (when the scheme was completed) to 2016, there were six periods with flooding in the lower catchment and only three of these with a significant impact on aquifer recharge.

Figure 16 – Ephemeral Omaruru River (Baumeler, 2018) CC BY–SA 4.0.

The Omaruru River, like all westward flowing rivers, carries a high silt load. Previous research had indicated that natural recharge in the system was decreasing due to silt sealing the river bed. As a result, most of the water was flowing over the silt and out to sea. Therefore, it was proposed that the flood waters be contained for a time in a holding dam, 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. This required careful application of operational rules (Water Scarcity Solutions, 2015) as follows:

• water is released when suspended solids are less than 20 mg/L;
• maximum hydrostatic head is maintained at all infiltration beds;
• infiltration beds are cleared of sediment twice yearly; and,
• water levels at infiltration basin observation boreholes are continuously monitored.