SUSPENDED PARTICULATE MATTER AND DYNAMICS OF THE MFOLOZI ESTUARY, KWAZULU-NATAL - IMPLICATIONS FOR ENVIRONMENTAL-MANAGEMENT

The Mfolozi Estuary on the KwaZulu-Natal coast of South Africa is the most turbid estuary in Natal due to poor catchment management, leading to large quantities of suspended particulate matter (SPM) entering th e estuary from the Mfolozi River. This paper quantities some of the so lute and sediment dynamics in the Mfolozi Estuary where the main docum ented environmental concern is the periodic input of SPM from the Mfol ozi Estuary to the St. Lucia system, causing reduction of light penetr ation and endangering biological productivity in this important nature reserve. Synoptic water level results have allowed reach mean bed she ar stresses and velocities to be calculated for an observed neap tidal cycle. Results indicate that ebb velocities dominate the sediment tra nsport processes in the estuary when fluvial input in the Mfolozi Rive r is of the order of 15-20 m(3) s(-1). Observed and predicted flood ti de velocities are too low (<0.35 ms(-1)) to suspend and transport sign ificant amounts of SPM. Observed results indicate that although the SP M load entering the estuary is dominantly from the Mfolozi River, the Msunduzi River flow plays a major role in the composition of the estua ry's salinity and velocity fields. It is calculated that the Mfolozi E stuary would fill with sedimentin 1.3 years if it was cut off from the sea. The major fluvial flood events help maintain the estuary by peri odically pushing sediment seawards (spit progrades seawards 5 m yr(-1) ) and scouring and maintaining the main flow channel in the estuary. D uring low fluvial flow conditions, tidal flow velocities will become t he dominant control on sediment transport in the estuary. Interchange of SPM between the St. Lucia and Mfolozi estuaries under present condi tions is complicated by the strong transverse velocity shear between t he two systems at their combined mouth. This is creating a salinity-ma intained axial convergence front that suppresses mixing of solutes and SPM between the systems for up to 10 h of the tidal cycle during obse rved conditions.