Observations of nearshore circulation: Alongshore uniformity

[1] Nearshore circulation, observed for 4 months on a 200-m-long stretch of natural beach during the SandyDuck field experiment, is shown to be alongshore uniform. An alongshore momentum balance between (wind and wave) forcing and bottom stress, cross-shore integrated between the shoreline and approximately 4 m water depth, holds on each of five instrumented cross-shore transects (skill 0.87). The corresponding five best fit drag coefficients are similar, consistent with the assumption that terms in the momentum balance associated with alongshore nonuniformity are negligible. In addition, the alongshore nonuniformity of the circulation and bathymetry were examined at five cross-shore locations. Except near the shoreline, the circulation and bathymetry were rarely strongly alongshore nonuniform, and the circulation nonuniformities were usually no larger than expected from current-meter noise alone. Near the shoreline, the bathymetry was more irregular and the circulation was often detectably nonuniform, although no relationship between bathymetric and circulation nonuniformities was found. The closure of the alongshore momentum balances on cross-shore transects, and the observed alongshore uniformity of the circulation on four of five alongshore transects, demonstrates that the simplified dynamics of alongshore uniform circulation are valid during the experiment.