Wave-Group Resolving vs Wave-Resolving Modeling of Surf and Swash Processes

Abstract:

Numerical modeling of beach and dune erosion, overwashing and breaching has gained much from inclusion of wave-group related infragravity motions in models such as XBeach (Roelvink et al, 2009). The main assumption in this model is that on the upper beach the incident-band, short waves are to a large extent dissipated, whereas infragravity wave motions have more oomph and are the ones making it to the dune foot and even over it. It is then justified to resolve the variations in short-wave energy and resulting long-wave motions, while parameterizing the short wave motions. This model has been applied successfully in many cases, both lab and field, concerning sandy beaches and dunes. However, as the sand gets coarser and beaches steeper, more and more incident wave energy is found in the swash, and at some point the parameterizations and associated coefficients start dominating the process. For gravel beaches, McCall et al (2014) have made use of a wave-resolving mode of XBeach, which makes use of a one-layer, nonhydrostatic approach developed by Zijlema et al. (2011). They have included a groundwater model and have shown that both the infiltration-exfiltration processes and the incident-band swash are important in getting the swash hydrodynamics on gravel beaches right. This work is continuing with promising results for morphodynamic response during extreme events. At the same time, we are investigating the skill of both approaches for wave runup and overtopping and are testing the morphodynamic behavior of the wave-resolving model in comparison with data and the original XBeach. So far, at the sandy end of the spectrum, both approaches give good and very similar results. In our presentation we will highlight some of these results and will present a sensitivity study where both approaches will be run and compared for a range of coastal profiles, including hard end structures. This will allow us to give clear guidelines for when to use the (much more computer-intensive) wave-resolving mode.