Modelling storm hydrodynamics on gravel beaches with XBeach-G

In this paper we present a process-based numerical model for the prediction of storm hydrodynamics and hydrology on gravel beaches. The model comprises an extension of an existing open-source storm-impact model for sandy coasts (XBeach), through the application of (1) a non-hydrostatic pressure correction term that allows wave-by-wave modelling of the surface elevation and depth-averaged flow, and (2) a groundwater model that allows infiltration and exfiltration through the permeable gravel bed to be simulated, and is referred to as XBeach-G. Although the model contains validated sediment transport relations for sandy environments, transport relations for gravel in the model are currently under development and unvalidated. Consequently, all simulations in this paper are carried out without morphodynamic feedback. Modelled hydrodynamics are validated using data collected during a large-scale physical model experiment and detailed in-situ field data collected at Loe Bar, Cornwall, UK, as well as remote-sensed data collected at four gravel beach locations along the UK coast during the 2012-2013 storm season. Validation results show that the model has good skill in predicting wave transformation (overall SCI 0.14-0.21), run-up levels (SCI < 0.12; median error < 10%) and initial wave overtopping (85-90% prediction rate at barrier crest), indicating that the model can be applied to estimate potential storm impact on gravel beaches. The inclusion of the non-hydrostatic pressure correction term and groundwater model is shown to significantly improve the prediction and evolution of overtopping events.