MANAGEMENT AND SEDIMENT DYNAMICS OF THE ST-LUCIA ESTUARY MOUTH, ZULULAND, SOUTH-AFRICA

St. Lucia Estuary is on the subtropical, predominantly microtidal Zulu land coast of South Africa. Lake St. Lucia's surface area fluctuates b etween 420 and 215 km2 and has a mean depth of less than 1 m. The 21-k m-long narrows connects Lake St. Lucia with the Indian Ocean. Tidal ef fects penetrate 14 km up the narrows. The St. Lucia system has changed substantially since the 1930s due to bad farming techniques within it s catchment. Large amounts of sediment were deposited in the estuary m outh, resulting in relocation of the Mfolozi River mouth to the south at Mapelane. The St. Lucia catchment was subjected to two devastating floods in the last ten years: Cyclone Domoina during February 1984 and the September 1987 cutoff low flood. After floods scoured out the est uary, marine sand advanced up the estuary at a rate of 1200 m/y as a s eries of flood-tidal deltas. Over 600,000 m3 of sediment accumulated i n the St. Lucia Estuary mouth from February 1988 to November 1989. Of this amount, 466,000 m3 of sediment was removed by dredging, although this has not stopped the shoaling. During high rainfall years, the est uary mouth is able to maintain an open outlet to the sea, but as lake levels drop, shoaling causes the mouth to constrict and eventually clo se. Without the dredging program the mouth would ultimately close duri ng low rainfall years, causing management problems.