Alongshore momentum balances in the nearshore

TLDR

The study examines the one‑dimensional, time‑averaged along‑shore momentum balance between wind, breaking waves, and bottom stress using field observations on a barred beach. Near‑bottom horizontal currents were measured over two months at 15 sites along a 750‑m transect, and hourly bottom stress, wave radiation stress, and wind stress were estimated from currents, pressure sensors, and an anemometer, respectively. The integrated wind and wave forcing balances the bottom stress across the transect, with wind contributing about one third, and in the surf zone wave forcing dominates while bottom drag is higher, matching earlier studies.

Abstract

The one‐dimensional, time‐averaged (over many wave periods) along‐shore momentum balance between forcing by wind and breaking waves and the bottom 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 extending 750 m from the shoreline to 8‐m water depth. The hourly averaged bottom stress was estimated from observed currents using a quadratic 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 an anemometer at the seaward end of a nearby pier. The combined wind and wave forcing integrated over the entire cross‐shore transect is balanced by the integrated bottom stress. The wind stress contributes about 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 drag coefficient is larger in the surf zone than farther seaward, consistent with earlier studies.