Demand for building sand remains high in the Durban region, although one of its main sources, the lower Mgeni River, is threatened by Inanda Dam. Completed in 1988 at a site some 32 km from the river mouth, the dam may have influenced bedload transport and deposition. The aim of this study was to ascertain whether the dam had affected fluvial sediment delivery to sand extraction sources below the dam, and to predict the likely affects of continued sand mining on the Mgeni estuary. Bridge construction and canalisation of a section of the lower river reach have all influenced the river in recent decades, and the situation is further complicated by a huge flood in 1987.
Re-examination of estuarine cross-sections originally surveyed prior to dam completion, comparison of current estuarine sediment characteristics with those of pre-dam sediments, and a bottom survey of the Inanda Dam itself were key elements in the analysis, conclusions and future predictions .
Mean annual discharge of the Mgeni river is estimated at 323 million m3 per year, and the best estimate of annual sediment load, calculated from repeated bottom surveys of Inanda Dam, was 6,8 million tonnes. Tributaries entering the river below the dam, which together drain 8,9 % of the total catchment, could yield up to 600 000 tonnes per year. Up to 1982 about 800 000 tonnes of building sand was being extracted from the river annually, nut by 1987 this figure had fallen to 130 000 tonnes.
The study concluded that combined effects of the engineering modifications to the river, the 1987 flood and sand mining have changed the nature of the sediment and the estuary considerably. Continued extraction of sand even at low levels will result in estuarine scouring and a new equilibrium form probably within a decade. Complete cessation of sand mining will delay this, but long term scouring will still occur, meaning that future plans for estuarine management must take account of physical dynamism within the system.