Modelling the morphodynamics of gravel beaches during storms with XBeach-G

This paper presents an extension of the XBeach-G numerical model with a sediment transport and morphology module, which includes the effect of groundwater ventilation and flow inertia on sediment transport, to simulate the morphodynamic response of pure gravel beaches and barriers to storms. The morphodynamic XBeach-G model is validated by simulating the morphodynamic response of one laboratory and four natural gravel barriers to 10 separate storm events, where the observed morphodynamic response ranged from berm building to barrier rollover. Model results show that XBeach-G is capable of reproducing the type of morphodynamic response of the barrier well in qualitative and quantitative sense (median BSS 0.75), with higher skill for more energetic storm conditions. Inclusion of acceleration forces on coarse gravel beaches is shown to significantly increase model skill and may be essential in modelling these types of beaches. The effect of varying hydraulic conductivity within estimated and published ranges is shown to be of secondary importance. The range of validation cases and lack of site-specific calibration show that XBeach-G can be applied to predict storm impacts on pure gravel beaches and barriers with reasonable to high confidence for a range of hydrodynamic forcing conditions and barrier response types.