A wave‐driven jet over a rocky shoal

Field observations and model simulations are presented of flow generated by waves breaking over a shoal at the entrance to a shallow bay. The shoal is composed of a series of steep and narrow bedrock ridges with depths of 2–8 m at the ridge crests. Observations from instruments on the shoal indicated peak significant wave heights during a storm event were 2.4–4.0 m across the observation sites; this spatial variability is due to wave breaking over the ridges. The 2‐D depth‐averaged hydrodynamic model Delft3D, coupled to the wave model SWAN, was used to simulate the waves and wave‐driven flow over the shoal and throughout the entire bay with a nested fine grid (5 m resolution) to resolve the shoal bathymetry. The model predicts a well‐defined jet behind the shoal, with mean axial speeds of 0.4–0.7 m/s. The observations indicate maximum speeds behind the shoal of 0.3–0.4 m/s, exceeding the maximum tidal current speed by more than a factor of 4, that are consistent in timing and direction with the model predictions. The model overpredicts wave breaking over the steep slopes and as a result the current speeds are overestimated.