Estimating runup with limited bathymetry

Abstract Wave runup estimates are used in erosion and overtopping models, and in coastal structure design. However, runup depends on often incompletely known surf and swash bathymetry. The many existing runup parameterizations characterizing bathymetry with only the foreshore (swash zone) beach slope β f are necessarily of limited accuracy. Here, an empirical model relating runup to incident wave spectra is extended to include an effective, mid-surfzone slope, β eff , that depends on the cross-shore location of the midpoint of breaking-wave dissipation. The empirical model is trained using numerical simulations (SWASH) of 138 hindcast historical storm waves, two different offshore infragravity wave boundary conditions, and 24 representative eroded beach bathymetries from a southern California beach. The model is tuned for the swell waves and concave up (sometimes barred) depth profiles characteristic of the study region. Consistent with their generation by surfzone-wide processes, setup and infragravity runup depend more strongly on surfzone β eff than on foreshore β f . In contrast, sea-swell runup depends more strongly on shoreline processes, and β f is more important than β eff . Empirical model accuracy is improved by including both β eff and β f .