Post-dam sediment dynamics below the Inanda Dam at the Mgeni estuary, KwaZulu Natal (South Africa)

Abstract

The Inanda Dam, situated some 32 to 35km upstream of the Mgeni River estuary on the Indian Ocean 5 km north of Durban, was constructed between 1984 and 1989. This impoundment deprived the downstream section of a great volume of water and sediment supply, initiating significant downstream changes. This situation is compounded by sand winning which directly extracts about 210,000 tonnes of sediment from the Lower Mgeni further depriving the estuary of sediment. A 1997 study predicted that assuming a continuous competent discharge and low contribution of sediments from the tributaries, the channel would gradually scour. Other predictions included a reduction in the total sediments reaching the estuary, continued flushing of existing channel sediments downstream towards the estuary, site-specific channel bed erosion at times of peak water release, gradual build up of sediments near the estuary mouth, gradual fining of bed-load channel and estuarine sediments, and ongoing re-establishment of the central island. The main aim of this study was thus to investigate downstream changes in the Mgeni river estuary below the Inanda dam with regards to sediments, water discharge and channel morphology from 1997 to test these assertions. Results show a decrease in competent discharge below the Inanda dam since 1997, resulting in a corresponding decrease in sand and an increase in mud fractions, with the mud content being associated predominantly with heavy minerals. This is reflected in the poorly sorted sediment. The plotting of cross-sectional survey revealed site-specific erosion, as well as estuarine bank failure to be an on-going process, and indicate points of bed scouring and accretion. A number of reasons have been identified as geomorphological explanations for the changes since 1997. These include among others, a lower hydraulic gradient at the estuary, occasional minor flood events, a weak bank material composition along some portions of the estuarine bank, the Tnanda dam , which impounds coarse sediments and large volume of water and increased tidal activity at the Mgeni estuary. It is concluded that although some of the probable responses predicted in 1997 have happened, sufficient relaxation time may not yet have elapsed for others to become evident.