Effects of breaking wave induced turbulence within a longshore current model

Abstract

A longshore current model which includes a modification of the bottom stress term due to the effects of breaking-wave induced turbulence is developed and applied to field data from both barred and planar beaches. This turbulence is postulated as producing a vertical mixing which alters the near-bottom vertical profile of the longshore current. As a result, the bottom fiction coefficient, c f, used to relate the free stream current velocity to the bottom stress is modeled as consisting of two components, c fl, a spatially constant value which is assumed to be related to bottom characteristics and c fr, which is dependent upon breaking-wave induced near-bottom turbulence levels. Employing a one-dimensional turbulent kinetic energy equation to model this breaking-wave induced turbulence, a spatially varying bottom friction coefficient is obtained. The spatially constant c fl is estimated based on data taken seaward of the surf zone, where the wind stress is assumed to be balanced by the bottom stress. The concept is demonstrated without the inclusion of horizontal mixing in the longshore current model formulation. The model predicted cross-shore profiles of longshore current show improved agreement with observations compared with treatments using constant c f values.