Sediment transport dynamics in South African estuaries

Beck, Julia S. (Stellenbosch : University of Stellenbosch, 2005-12)

Thesis (PhD (Civil Engineering))--University of Stellenbosch, 2005.


Estuaries are complex water bodies and differ considerably from fluvial river systems. In estuaries the flow reverses regularly due to the tidal currents and flow depths depend primarily on the tides and not the flow. An estuary has two sources of sediment: the river during floods and the ocean that supplies marine sediment through littoral drift which is transported by tidal currents into the estuary. Oversimplified models cannot be used to investigate the hydrodynamics and geomorphology of an estuary due to its complexity. Sedimentation of South African estuaries has created several environmental and social problems. Sediment transport imbalances have been caused by changes in the river catchments such as increased sediment yields and flood peak attenuation due to dam construction. Historically floods used to flush estuaries to maintain the long-term sediment balance in the river-estuary system, but with reduced flood peaks, sediment transport capacities at the estuaries are reduced and flushing efficiency decreased, resulting in marine transport dominating in many estuaries. Two-dimensional (horizontal, 2DH) numerical models have been found to be appropriate tools for studying hydro- and sediment dynamics in SA estuaries. The modelling shows that the sediment balance in the estuary relies on a delicate balance between dominant flood and ebb flows. Although the models performed very well, there are still additional processes to include such as time varying roughness changes and cohesive sediments. For long-term and long reach simulations, onedimensional (or quasi-two-dimensional) models will also be required in future. Mathematical modeling can be used to simulate the flushing of sediments during floods, but attempts should be made to calibrate these models when adequate field data become available in the future. The modelling has shown that floods play a very important part in estuarine sediment transport processes. Physical modelling was undertaken of the breaching of an estuary mouth. The main aim was to illustrate the merits of breaching at higher water levels as well as to investigate the changes in the mouth during breaching. The data obtained from the experiments were used to calibrate and verify a mathematical model. Mathematical modelling of the breaching process at the Klein River estuary confirms what has been observed during numerous breachings in the field, i.e. that breaching at higher water levels and towards the southeast side is more effective. Sediment transport by both waves and currents was investigated. It was found that with increasing wave and stream power, sediment transport rates would increase if both waves and currents travelled in the same direction. In contrast, it seems that with the current direction opposing that of the waves, Hydraulics of Estuarine Sediment Transport Dynamics in South Africa iii greater wave heights resulted in lower sediment transport rates. A new sediment transport equation, based on stream power, wave power, as well as sediment size was calibrated and verified, and compared to the well-known Bijker formula.

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