Modelling sand bypass schemes on the KwaZulu-Natal coastline.

Abstract

Coastal structures such as breakwaters cause a disruption of longshore sediment transport along coastlines. The result of this disruption creates sand accumulation up-drift and beach erosion down-drift of these structures. Therefore, sediment bypass schemes are implemented by dredging the sand out of the sand trap up-drift of the structures and nourishing the beach down-drift of them. The beach north of the Richards Bay harbour entrance in KZN, South Africa was used as a case study to model and compare alternative nourishment schemes to alleviate chronic beach erosion due to disruption of the longshore sediment supply. This study used the Delft3D 2DH sediment transport models to investigate the nourishment schemes and a calibration study was done to test the capability of the models to maintain a theoretical equilibrium profile over a long term simulation. Subsequently the model was used to investigate and compare three nourishment schemes at a case study site over a period of a year to determine the beach response to the nourishment. The sediment budget for the nourishment schemes was limited to 1 000 000 m3 per year. The first scheme comprised of a continuous steady nourishment throughout the year and the second scheme was a bulk nourishment where the sediment is dumped onto the beach at the maximum dredging capacity, in this case 10 000 m3/day. The last was a bimonthly sediment nourishment scheme. The model calibration results revealed that a single wave related transport factor governs the cross-shore movement direction. A single set of parameters does not produce offshore sediment movement during large wave events and onshore movement during smaller wave events as observed in reality. Therefore, the model was unable to reproduce a quasi-equilibrium behaviour unless the cross-shore transport factors are allowed to vary as a function of wave height. It was possible to define a cross-shore factor within the Van Rijn transport model that limited the cross-shore movement over a long term morphological simulation resulting in only the longshore transport affecting the morphology within the model. This model setup was used for the case study since a lack of sediment supply was the main focus of this study. The continuous steady nourishment results showed a natural longshore shore movement of sediment down-drift of the harbour entrance and a uniform beach width increase along the entire beach. The bimonthly nourishment closely emulated the continuous nourishment resulting in a net increase of beach width along the modelled coastline. The bulk nourishment revealed significant differences to the previous cases. The sheltering effect of the northern breakwater kept the main recreational beach in a nourished state while the northern beach outside of the breakwater’s shadow-zone returned to its initial sand starved state.