Comparison of wind and wave forcing of longshore currents

Abstract

Wind and wave forcing of longshore currents for varying wind conditions and beach slopes are compared using a numerical model. Waves are assumed to be locally generated by the same winds and fully arisen. The finite-depth TMA wind-wave spectrum is used to describe the radiation stress spectrum. Total wind force acting across the surf zone is proportional to surf zone width and therefore inversely proportional to beach slope. Wind force is not important for steep beaches. Total wave force is dependent of wave energy flux and wave direction at breaking, which are functions of the wind speed and direction. Wind force becomes increasingly significant relative to wave force, as wind speed and incident wind direction approaches parallel to shore. For obliquely onshore winds and a gentle beach slope (tan β = 0.01), wind force can be significant, with wind to wave force ratios ranging from 10 to 100% for winds between 10 and 30 m s −1 and wind and wave directions from 10 to 80°. Thus, under certain wind and wave conditions, wind force can be a first order term in the alongshore momentum balance.