New insights into the physical drivers of wave runup from a continuously operating terrestrial laser scanner

A dune-mounted terrestrial laser scanner was used to investigate the physical drivers of runup on an intermediate beach under a variety of wave conditions. Specifically, the laser is automated to collect hourly, simultaneous observations of beach morphology and hydrodynamics in the swash and inner surf-zone. A strong tidal signal in de-tided runup statistics is observed such that the 2% exceedence runup elevation and mean swash elevation is higher at high tide than at low tide for the same given wave conditions. This is similar to the significant incident band wave height (Hs_in) at the base of the foreshore, which also showed a similar strong tidal dependence. In fact, our results suggest that mean swash elevation and incident band swash scale well with Hs_in, though correlations are slightly improved when beach foreshore slope is included in predictions of incident band swash (r² = 0.73 vs 0.69). In contrast, the Stockdon et al., 2006 relationship, which is based on beach foreshore slope, offshore wave height, and wavelength, only explained 53% of the variance in our data even when beach slope was known. Similar to Stockdon et al., we found infragravity band swash was dependent on deep water wave height and wave length. These data suggest that increased or decreased breaking over the sandbar at low and high tides, respectively, may filter the amount of energy left in the incident band to be transferred to runup at the shoreline. The implications of this sandbar filtering are that up-to-date bathymetry or a quantification of dissipation across the surfzone is necessary to produce accurate real-time runup predictions.