Beach sand supply and transport at Kunduchi, Tanzania, and Bamburi, Kenya

Abstract

Beach-head erosion of sandy beach plains in eastern Africa threatens tourism-related infrastructure and the livelihoods of beach plain users. The nature and drivers of physical shoreline change at Kunduchi, near Dar es Salaam, and Bamburi, near Mombasa, are described with analyses of beach sand transport through the annual monsoon cycle and the provenance and sustainability of beach sand supply. Time-series records of wind-vectors at Dar es Salaam and Mombasa show similar averaged patterns. Because of the contrasting alignments of these coasts, the net wind-wave driven longshore transport at Kunduchi (NNW-trending) is north-northwestwards, while at Bamburi (NNE-trending) there is little net transport over the cycle. At Bamburi, the beaches are recharged reef/platform-derived calcium carbonate sand and siliciclastic sand discharged from the hinterland via tidal channels. At Kunduchi, recharge is mostly river-borne siliciclastic sand, but river sand mining threatens natural replenishment. At both sites eroding beach plain deposits contribute siliciclastic sand. Structural responses to maintain beaches protecting susceptible shores – mostly seawalls at Bamburi and groyne fields at Kunduchi – have high capital and maintenance costs, degrade the coastal amenity and may exacerbate erosion. At Kunduchi, beach maintenance is further jeopardised by unchecked illegal river sand mining.