Observations and predictions of fluid velocities in the surf and swash zones

Surfzone and swashzone water level fluctuations and cross‐shore fluid velocities observed at seven cross‐shore locations for 139 51‐min periods on a low‐sloped, fine‐grained sandy beach are shown to be modeled well by the depth‐averaged, nonlinear shallow water equations. The model, initialized with observations about 150 m from the shoreline in 300 cm water depth, accurately predicts the observed decrease in sea swell wave heights and orbital velocities with decreasing water depth across the surf zone and the observed maxima in infragravity orbital velocities and offshore‐directed mean flows near the outer edge of the swash zone. In the surf zone both the observed and predicted velocity fluctuations are dominated by sawtoothed‐shaped, pitched forward (asymmetrical) sea swell waves. In the swash zone, infragravity waves contribute substantially to the total velocity variance and asymmetry. Shoreward of the outer swash zone, observed and predicted total and infragravity velocity asymmetry decrease onshore, changing sign from positive to negative in the upper swash.