Report on St. Lucia estuary siltation

Abstract

The St. Lucia Lake is situated on the Zululand coast of Natal (see figure 1) and is connected to the sea by means of an estuary. The mouth of the estuary has been subject to steady deterioration. It is evident that early in this century a small bay existed in the area between the present mouths of the estuary and t h e Umfolozi River, with the estuary from the lakes discharging into the northern area of the bay and the Umfolozi River into the southern area. Many factors such as littoral driftt waves and tidal action contributed to the silting up of the bay and eventually to the closure of both the Umfolozi River, and the estuary in 1951. It can be assumed that the deterioration of the outlet was accelerated, especially during the past thirty years, by the development of the Umfolozi flats under sugar cane. To allow this development, large drains were cut through the former low lying swamps which acted as "filter beds" for the silty water during floods. The result was that silt was carried into the bay and added to the problem of siltation by littoral processes. The contact and mixing of the river waters with the sea water accelerated the rate of silt deposition so that at first the bay became shallower and the area of silt deposition gradually extended further up the estuary. The latter reached such a stage that the pont plying across the estuary at the site of the present bridge could no longer be operated because of insufficient water depth. An earth causeway had to be thrown across to maintain access between the st. Lucia village and the mainland, pending the construction of a permanent bridge? plans for which were immediately started. The decreased interchange of tidal water accelerated the siltation process and led to the eventual closing. It is stated that whereas in 1938 the mouth was 500ft wide at low water, in 1948 it was only 150ft wide and in June 1951 the mouth closed)*.