Uncertainties in the simulation of bed evolution in recirculating flow area behind groynes

The main characteristic of the flow induced by groynes is the development of a recirculating flow area. An interaction between the recirculating flow and bed mobility leads to the development of a scour hole around the groyne, and a sediment deposition bar downstream. The simulation of the bed evolution around a groyne was investigated by linking a developed sediment transport model (STM-2D) to the FAST-2D flow model. The flow model was modified for the present study to simulate the detailed characteristics of the flow around the groyne. The previous study by the author indicated that the results of the numerical simulations compare well with the experimental data in two cases of the flows in flat-bed channels and in channels with complex-bed topography induced by a groyne.
    The present state of sediment transport relationships, in particular the relationships for calculating the bed-shear stress and the bed-load transport rate, was evaluated as an essential step for the computational simulation of the groynes performance. A depth-averaged version of the bed-shear stress relationship was modified with the inclusion of correction factors for the effects of the local spiral motion and of the local bed topography. Sediment transport relationships of Ackers and White, and of van Rijn (both deterministic and stochastic methods) were adapted. A number of difficulties were encountered in the development and validation of this model, nevertheless, substantial progress was made. This paper discusses the major uncertainties in the simulation process. It is concluded that modification of the conventional bed-shear stress relationship is not sufficient for simulating realistically the bed features behind the groyne. The development of a conceptual method for calculating the bed-shear stresses around groynes is a priority and recommended for future studies.