Lodewijk de Vet

Sediment transport formulae relate the movement of sand particles to parameters like the flow velocity and the sediment particle size. It is common knowledge that these formulas have a restricted predictive skill, even for the conditions they are designed for. The majority of transport equations are calibrated on rivers and coastal areas with small flow velocities and the performance of these equations at large velocities (1-10 m/s) is investigated only to a limited extent (Van Rhee, 2010).

During storm events, water level set-up and large waves can result in large flow velocities and morphological changes can be substantial. The above figure shows the devastating impact of hurricane Sandy (October 2012) on the morphology of Fire Island, New York. For the modelling of overwash and breaching, it is important to investigate what processes occur at high flow velocities and how sedimen  transport models can be improved with this knowledge. In this way, more realistic predictions of storm impact can be made and the effectiveness of protective measures can be estimated.

XBeach (Roelvink et al., 2009) is a numerical, depth averaged, model that solves coupled 2DH equations for long wave propagation, short wave energy, flow, sediment transport and bottom changes, for varying wave and flow boundary conditions. Regarding large flow velocities, the sediment transport can be modified by limiting the Shields parameter in the sediment stirring formulations to approximately one.

The rationale behind limiting the sediment transport by defining a maximum Shields parameter value, is based on the assumption that sediment transport rates vary only linearly with the flow velocity once sheet flow conditions are met. In this method, the equilibrium sediment concentration is constant for high flow velocities, which is not necessarily correct (McCall et al., 2010). Additional research is required to validate and/or improve the XBeach model for overwash and breaching conditions at which high flow velocities can occur and limit unwanted effects at lower flow velocities.

McCall, R. T., van Thiel de Vries, J. S. M., Plant, N. G., van Dongeren, A. R., Roelvink, J. A., Thompson, D. M., and Reniers, A. J. H. M. (2010). Two-dimensional time dependent hurricane overwash and erosion modeling at santa rosa island. Coastal Engineering, 57:668--683.

Van Rhee, C. (2010). Sediment entrainment at high flow velocity. Journal of Hydraulic Engineering, 136:572--582.

Roelvink, J. A., Reniers, A. J. H. M., van Dongeren, A. R., van Thiel de Vries, J. S. M., McCall, R. T., and Lescinski, J. (2009). Modelling storm impacts on beaches, dunes and barrier islands. Coastal Engineering, 56(11--12):1133 – 1152.