Geomorphological impacts of Inanda Dam on the Mgeni estuary, north of Durban, South Africa)

Abstract

Completed in 1988 at a site some 32 km from the Mgeni river mouth, the Inanda Dam may have influenced bed load transport and deposition. The aims of this study were to ascertain whether the dam had affected fluvial sediment delivery to the lower channel, whether the sediment characteristics and bedform of the estuary had been modified and, within this context, to predict the likely effects of continued sand mining on the Mgeni estuary. Re-examination of estuarine cross sections originally surveyed prior to dam completion, comparison of present estuarine sediment characteristics with those of pre-dam sediments and a bottom survey of the Inanda Dam itself were key elements in the analysis. Mean annual discharge of the Mgeni river has reduced by 4% since dam construction and changes in discharge regime caused by the water release policy resulted in an increase in competent discharge events. Retention of sediment behind the dam wall had led to reduction in available downstream sediment. Sediment volume in the estuary had not changed significantly since 1989, although sediment calibre is now much finer. These changes will probably signal the end of sand mining in the lower river and will ultimately cause the estuary to deepen as residual pre-dam sediment is flushed through and not replaced by new material from upstream. Since Mgeni sediment is a source of material for natural beach nourishment, it is likely that beaches will become narrower.