ABSTRACT

The Zululand coast extends 320 km SSW from the Mozambique border to the Tugela River, and is backed by a tapering coastal plain which, though 80 km wide in the north, pinches out 40 km north of the Tugela. The coast consists of long barrier beaches, surmounted by sand dunes up to 180 m high, behind which lie extensive swamps, lagoons, and old dune ridges. The present barrier has grown in response to high energy and material transfers during and since the Holocene transgression, Nevertheless, older nearshore sediments, dune sands, and swamp deposits indicate Pleistocene sea-level oscillations across this coast, and show that the present barrier is superimposed upon remnants of a major Pleistocene barrier system represented by Port Durnford Beds.

Shorezone dynamics are conditioned by prevailing northeasterly and southwesterly winds, by complex nearshore and coastal circulation systems, and by seasonally high sediment discharges from neighboring watersheds. Nearshore circulation patterns, consisting mainly of asymmetric cells, generate longshore currents exceeding 1 m/sec. Inner and outer breaker zones reflect offshore bars. Beyond the nearshore circulation, reversing coastal currents related to the Agulhas Current system flow parallel to the shore with surface velocities of 0.2 to 0.8 m/sec, reduced by a factor of 0.4 near the seabed. During the winter dry season, strong littoral drift creates barrier beaches across estuaries. During the summer rainy season, these barriers are breached and vast sediment plumes extend seaward and alongshore. The Tugela brings 10.5 x 106 tons of sediment to the coast annually.

Behind the coastal barrier lie Kosi Bay, Lake Sibayi, Lake St. Lucia, and Richards Bay, remnants of once extensive lagoons reduced by infilling and segmentation. As Holocene barriers became more complete, drainage outlets were plugged with sediments and lagoons became vast sediment traps. Holocene sedimentation patterns are complex and impressive. Furthermore, present sedimentation rates for Lake St. Lucia are three times the mean rate for the past 1825000 yrs, largely due to accelerated erosion from mis-used Zululand watersheds. Reedswamp encroachment, spit growth, lagoon segmentation, and high salinities now pose serious threats to the survival of aquatic life.