Vertical flow structure during Sandy Duck: observations and modeling

Abstract

Observations of the vertical distribution of time-averaged cross-shore and alongshore flows during the Sandy Duck field experiment are compared with model predictions to assess the parameters governing the flow behaviour. The measurements were obtained with a vertical stack of eight two-component current meters, with the lowest and highest sensor at, respectively O(0.1) and O(2.7) m, above the bed. Observations under breaking wave conditions within the surfzone show that the maximum return flow velocities occur in the lower part of the water column, consistent with laboratory observations. Under non-breaking conditions outside the surfzone the maximum return flow velocities are observed closer to the water surface, again in line with laboratory results. Analogous to previous observations the measured longshore current velocity profiles are logarithmic under non-breaking conditions and become more depth-uniform under breaking conditions. The model description of the vertical structure of the flow includes the presence of wind stresses, wave stresses, pressure gradients, turbulent eddy viscosity and a wave boundary layer. The model utilizes parabolic shape functions to describe the vertical distribution of the turbulent eddy viscosity in the middle layer and within the bottom boundary layer. Eddy viscosity is enhanced in regions where turbulence is produced, i.e. near the surface in the case of breaking waves and within the bottom boundary layer. Estimates of the wave-breaking-induced turbulent eddy viscosity and bottom friction are obtained by minimizing the model–measurement discrepancies. Predictions utilizing calibrated expressions for both the turbulent eddy viscosity and bottom friction are in general agreement with the observations, provided the wave transformation and associated mass flux are modeled correctly and a parabolic eddy viscosity distribution is used. Using a piecewise constant eddy viscosity distribution generally results in a degrading of the agreement between measurements and model results.