Swash on a gently sloping beach

Waves observed in the inner surf and swash zones of a fine grained, gently sloping beach are modeled accurately with the nonlinear shallow water equations. The model is initialized with observations from pressure and current sensors collocated about 50 m from the mean shoreline in about l m depth, model predictions are compared to pressure fluctuations measured at five shoreward locations and to run‐up. Run‐up was measured with a vertical stack of five wires supported parallel to and above the beach face at elevations of 5, 10, 15, 20, and 25 cm. Each 60‐m‐long run‐up wire yields time series of the most shoreward location where the water depth exceeds the wire elevation. As noted previously, run‐up measurements are sensitive to the wire elevation owing to thin run‐up tongues not measured by the more elevated wires. As the wire elevation increases, the measured mean run‐up location moves seaward, low‐frequency (infragravity) energy decreases, and higher‐frequency sea swell energy increases. These trends, as well as the variation of wave spectra and shapes (e.g., wave skewness) across the inner surf zone, are well predicted by the numerical model.