Appropriate technology options for managing drainage flows from low cost peri urban settlements in South Africa.

Abstract

Inadequate drainage in dense peri-urban settlements in South Africa is a significant problem endangering the public and environmental health and of particular concern are the downstream watercourses, which are a source of drinking water supply, a scarce resource. The objective of this research was to identify appropriate solutions within the limited scope of technical and financial feasibility with reference to Alexandra west bank as a case study area. The findings show that three physical site conditions hamper the application of onsite drainage approaches in Alexandra west bank Township, the case study area. They are: Congestion, due to haphazard development patterns, High drainage flow generation resulting from high population densities and the predominantly impermeable surface area due to intensive site development, and Poorly draining soils Congestion, high densities and intensive site development are characteristics common to low-income settlements in South Africa, and they result in lack of space availability for storage facilities, and interference with nature’s ability to retard, retain and infiltrate significant quantities of the storm runoff flows. Poor soil drainage capabilities, which is more specific to study area would result in a slow rate of exfiltration of drainage flows that would in turn cause ponding and the associated health hazards.

Estimates of drainage flows generated from the study area as determined from field observations, flow measurements and computer simulation techniques indicate that if the minimum rate of production of just the wastewater component of the drainage flows is taken, which is approximately 37m3/ha/day, it exceeds the rate recommended for safe onsite management of drainage flows by almost four times.

Three off-site drainage system arrangements were compared on the basis of the cost of outfall pipe drains sized according to conservative design procedures, and it was found that the combined sullage and storm water drains with separate sewage (black water) drainage system arrangement is more economical than the commonly practiced approach of separate storm and combined sewage and sullage drainage system arrangement.