Phase resolving runup and overtopping field validation of SWASH

Time series of storm wave runup and overtopping observed on a sandy beach with a scanning LiDAR are compared with predictions of the phase-resolving numerical model SWASH 1D. SWASH is initialized 300 m offshore (8-m depth) with phase-resolved estimates of shoreward and seaward propagating waves, observed with a co-located pressure sensor-current meter. During 5 h of storm conditions (2.4-m significant wave height, 17-sec peak period, high tide) swash zone bed level erosion of 80 cm was observed with the LiDAR and included in SWASH simulations. Model offshore bathymetry is an ensemble of historical surveys. SWASH-predicted and LiDAR-observed runup time series are in-phase and coherent in both the sea-swell and infragravity frequency bands. Overtopping was intermittent and occurred only for the largest runups that were at suboptimal viewing angles. SWASH overpredicted by a factor of two the number of overtopping events observed with LiDAR and a single point pressure sensor. Phase-coupling between infragravity and sea swell waves at the offshore boundary and shoreline erosion both significantly affect model runup and overtopping. SWASH prediction misfits of 5%–6% in runup sea-swell and infragravity heights are encouragingly small given the uncertainty in underwater bathymetry, 1D dynamics assumption, and default model representations of wave breaking and bottom friction.