4.5 Scheme Elements

Key elements of the scheme (recharge and abstraction) are illustrated in Figure 18. It is useful to note the points below when viewing Figure 18 (Water Scarcity Solutions, 2015):

• the dam and associated storage in which the silt from flood waters can be allowed to settle;
• the dam is constructed without a foundation cut-off wall extending to bedrock thus permitting groundwater throughflow;
• a multi-level off-take pipe to allow transfer of clear water to the infiltration beds;
• two large infiltration areas downstream of the dam; and
• silt removal around the abstraction tower (Figure 18b).

Figure 18 – Omdel Scheme: a) schematic of the recharge and abstraction process (Water Scarcity Solutions, 2015); and b) photo of silt removal around abstraction tower (Mostert and Matengu, 2019).

When the dam fills with water, it is allowed to stand for 9 weeks so that silt settles out. Then, clean surface water is skimmed off by an abstraction tower and flows through a pipe under the wall to four infiltration ponds (Figure 19). These are operated in a routine cycle of infiltrating, drying, scraping and refilling (Zeelie, 2004).

Figure 19 – a) Photograph of two Omdel infiltration basins. b) Map of Omdel basins. (Zeelie, 2004).

During flooding, the artificial recharge ranged between 52 to 89 percent of the surface water flow. Despite the infrequent flooding, infiltration through the basins increased the annual recharge from 5.8 to 7.9 Mm³/year and the estimated sustainable yield increased from 2.8 to 4.6 Mm³/year (Mostert and Matengu, 2019). The capital cost of the project was US$ 16.8 million, resulting in a very favorable unit cost of water of US$ 0.25/m³ (Water Scarcity Solutions, 2015).

Because of demand pressures and uncertainty over the sustainable yield, the aquifer had been operated at an average withdrawal of 6.3 Mm³/year since artificial recharge implementation. This resulted in an overdraft, as reflected in declining aquifer water levels. The large aquifer storage sustained the scheme and for better access to the available storage, 17 additional production boreholes were drilled during this period.