COMPARISON OF WIND AND WAVE FORCING OF LONGSHORE CURRENTS

Wind and wave forcing of longshore currents for varying wind condition s and beach slopes are compared using a numerical model. Waves are ass umed to be locally generated by the same winds and fully arisen, The f inite-depth TMA wind-wave spectrum is used to describe the radiation s tress spectrum. Total wind force acting across the surf zone is propor tional to surf zone width and therefore inversely proportional to beac h slope. Wind force is not important for steep beaches. Total wave for ce is dependent of wave energy flux and wave direction at breaking, wh ich are functions of the wind speed and direction. Wind force becomes increasingly significant relative to wave force, as wind speed and inc ident wind direction approaches parallel to shore. For obliquely onsho re winds and a gentle beach slope (tan beta = 0.01), wind force can be significant, with wind to wave force ratios ranging from 10 to 100% f or winds between 10 and 30 m s-1 and wind and wave directions from 10 to 80-degrees. Thus, under certain wind and wave conditions, wind forc e can be a first order term in the alongshore momentum balance.