Depth‐Resolved Surfzone Wave and Roller Transformation

Abstract

The vertical structures of cross‐shore wave, roller, and other momentum forcing terms are examined using a two‐phase Reynolds‐averaged Navier‐Stokes model driven with field and laboratory measured waves and bathymetry. Modeled wave height, setup, and undertow agree well with field observations along a cross‐shore transect extending from the shoreline to about 5‐m water depth, and with measurements in a wave flume. Momentum flux carried by wave, roller, vertical turbulence transfer, pressure slope, and vertical inertia primarily balance near the surface, while wave, vertical turbulence transfer, vertical inertia, and setup primarily balance in the mid‐to lower‐water column. Turbulence transports momentum downward, while vertical inertia transfers momentum upwards. Combined wave and roller forcing agrees with a linear wave theory parametrization offshore of the sandbar and on a near‐planar beach, and combined wave, roller, and vertical inertia agrees with linear theory onshore of the sandbar. Roller thickness is related to local wave height, similar to previous studies. Sub‐surface offshore‐directed mean currents (undertow) can be estimated from the mid‐to lower‐water column momentum balance. Turbulence production dominates the near‐surface turbulence‐energy‐flux balance, and its penetration depth in the sandbar trough is correlated with local wave height.