A wave-driven jet over a rocky shoal

Abstract

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. Copyright 2010 by the American Geophysical Union.