ALONGSHORE MOMENTUM BALANCES IN THE NEARSHORE

The one-dimensional, time-averaged (over many wave periods) alongshore momentum balance between forcing by wind and breaking waves and the b ottom stress is examined with field observations spanning a wide range of conditions on a barred beach. Near-bottom horizontal currents were measured for 2 months at 15 locations along a cross-shore transect ex tending 750 m from the shoreline to 8-m water depth. The hourly averag ed bottom stress was estimated from observed currents using a quadrati c drag law. The wave radiation stress was estimated in 8-m depth from an array of pressure sensors, and the wind stress was estimated from a n anemometer at the seaward end of a nearby pier. The combined wind an d wave forcing integrated ever the entire cross-shore transect is bala nced by the integrated bottom stress. The wind stress contributes abou t one third of the forcing over the transect. Analysis of the momentum balances in different cross-shore regions shows that in the surf zone , wave forcing is much larger than wind forcing and that the bottom dr ag coefficient is larger in the surf zone than farther seaward, consis tent with earlier studies.