The influence of incident waves on runup

Abstract

Wave runup is generated by energy which remains after wave breaking and travels farther to the coast in the form of a bore. It can be seen as a thin wedge of water running up the beach face (Brocchini and Baldock, 2008). Under storm conditions runup is responsible for beach and dune erosion and accurate runup predictions are therefore required (Ruggiero et al., 2001; Stockdon et al., 2005). For runup and its components, the time-mean setup component and the time-varying swash component, empirical parameterizations have been developed in the past, but they cannot be validated for storm conditions due to a lack of data (Stockdon et al., 2005). The data gap can be filled by numerically simulated runup, for example with the process-based XBeach model. XBeach is a depth-averaged model which predicts nearshore hydrodynamics and can be used in a phase-averaged or a phase-resolving mode. However, both the significant incident and infragravity swash is underpredicted by the phase-averaged XBeach Surfbeat model (Palmsten and Splinter, 2016; Stockdon et al., 2014), which does not resolve incident wave motions. In order to predict runup under storm conditions with confidence the performance of XBeach under mild conditions should be assessed. Here runup simulated with XBeach Surfbeat and the phase-resolving XBeach Non-hydrostatic for the intermediate reflective beach of Duck was compared to measurements of the SandyDuck'97 experiment, where mild offshore conditions were present. A 2DH model was set up using measured bathymetry and forced with measured frequency-directional spectra. The hydrodynamics responsible for a difference in runup prediction were investigated and their origin in the cross shore was identified. It was shown that the prediction of significant incident and infragravity swash can be improved by using the phase-resolving XBeach Non-hydrostatic model instead of the XBeach Surfbeat model, while performance for setup remains similar. Incident swash predictions are improved by resolving the incident wave motions. The major part of the improvement in infragravity swash predictions is driven by differences in infragravity wave transformation between the two XBeach models. A small part also originates within the swash zone, for which incident bore merging can be a possible explanation. The difference in infragravity wave height predictions between the two XBeach models mainly develops in the surf zone where a different response to directional spreading and different degrees of shoaling most likely can explain the difference in infragravity wave height. Against expectations no correlation with the groupiness of the incident waves or with the phase difference between wave group and infragravity wave was found. A small part of the difference in infragravity wave height predictions is already present near the offshore boundary and probably results from interaction processes between high and low frequency wave boundary conditions. It can thus be said that on intermediate reflective beaches, where both incident and infragravity waves play a role, the resolving of incident wave motions is a necessity to predict runup accurately. In these situations the phase-resolving XBeach Non-hydrostatic model should therefore be used, instead of the phase-averaged XBeach Surfbeat model. Here only an intermediate reflective beach and a small range of energetic conditions were included. More types of beaches and storm condition forcing should be investigated to be able to validate the empirical parameterizations but also to further indicate applicability ranges of the two XBeach models. Also, more attention should be paid to hydrodynamic differences at the boundary and in the surf zone in order to find more conclusive reasons for differences between the two XBeach models.