Flood alleviation and restoration on the Lourens river, Somerset west, South Africa

Abstract

Somerset West and Strand in the Western Cape, South Africa, were developed on the Lourens River floodplain. This hardened the catchment and reduced the capacity of the river to transport and store floodwaters. The result was recurrent flooding of residential and industrial areas and a fear that this could lead to loss of human life. In response to these concerns, the City of Cape Town implemented flood alleviation measures with a ‘soft’ engineering approach that incorporated geomorphological and ecological principles into their design. This was one of the first engineer/ecologist collaborations in South Africa, which attempted to make better decisions for the river ecosystem within the constraints imposed on it by urban development; and in so doing to create a self-sustaining river that requires little ongoing manipulation. The aim of this dissertation was to assess the extent to which ecological considerations were incorporated into the flood alleviation works on the Lourens River and whether this improved physical habitat and the diversity of riverine biota. Physical habitat was mapped from 1:50 000 topographic maps and aerial imagery in a GIS, and cross-sectional profiles, diversity of hydraulic biotopes and subtsrate composition were surveyed in the field.