The flow and sediment of the Mlalazi estuary in Kwazulu-Natal, South Africa
The catchments and estuaries are fragile systems that are prone to serious degradation from many different anthropogenic impacts (e.g. flow abstraction/diversion, sand mining, soil erosion etc.). In South Africa, many of the anthropogenic impacts on estuarine systems are generally derived from subjective expert opinions that are not supported by hydrodynamic data (Rasifudi, 2019). River flow is one of the main factor that control the dynamics of many estuaries worldwide (Zhou et al., 2014). It is important to understand the water flow depth, direction and velocity in estuaries since they affect and control the erosion, transport and deposition processes of alluvial sediment and nutrients. The Mlalazi Estuary is one of the best conserved estuaries situated along the eastern shoreline of the Kwa-Zulu Natal Province of the Republic of South Africa (RSA). This Estuary is classified as a Temporarily Open/Closed Estuary (TOCE). There is limited observe hydrological data that can be use to improve the decision making process within the Mlalazi Estuary in future. Therefore, a numerical model is useful tool to derive best estimates of flow dynamics and sediment transport. This study investigated the flow dynamics associated with fluvial events within the Mlalazi Estuary resulting from Q2yr, Q10yr, Q20yr, Q50yr and Q100yr flood return periods. The hydraulic modelling software, HEC-RAS (version 5.0) flow model, aided by GIS (HEC-geoRAS), was used to obtain estimates of the flow velocity and water level (depth) within the Mlalazi Estuary and floodplain. Available geometric data (DEM + bathymetry) was used to generate 174 river cross-section profiles at different interval spaces ranging from 60m to 200m along the Mlalazi Estuary. The Mlalazi run-off data simulated from the hydrologic model HEC-HMS (Rasifudi, 2019) was used as an upstream boundary condition. The model was calibrated and validated using the historical data from the 1987 flood survey and continuous water level data from monitoring sites established in 2015 by the Hydrological Research Unit of the University of Zululand. The hydraulic model gave satisfactory performance statistics for high flood events during calibration and validation periods. The model overestimated the Q2yr stage and velocity event in the upper estuary channel and underestimated the stage and velocity in the lower estuary channel. The simulated velocity and Hjulström-Sundborg diagram were used to analyse the fluvial sediments distribution in the estuary channel and floodplains. The inundation maps of different storm sizes revealed that erosion occurs mainly in the active estuary channel and deposition of alluvial sediment takes place on the floodplains. The study concluded that a physically based, numerical flow model is best method for providing reliable estimates for hydrodynamic data and information in estuaries with limited observed data like the Mlalazi Estuary. The study will also provide essential flow information needed to set up the Mlalazi Mouth Model, which will help to determine the ‘flow reserve’ of the Mlalazi Estuary.
Hydrodynamics, Model calibration, Model validation
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