The Longshore Transport Enigma and Analysis of a 10-Year Record of Wind-Driven Nearshore Currents

摘要

Previous analysis of a 10-year record of nearshore directional wave spectra collected with an Acoustic Doppler Current Profiler (ADCP) installed outside the surf zone in Melbourne Beach, Florida, unexpectedly revealed that the long-term average wave-induced radiation stress (S-xy) was nearly balanced between northerly and southerly forcing. More than 4 years of wind data collected at the site with a directional anemometer also showed a nearly balanced net longshore wind stress. However, shoreline offsets at nearby jettied inlets clearly indicate a predominant north-to-south net sediment transport. This enigma was investigated by analyzing the nearshore currents measured by the ADCP, and examining their correlation with the wind and incident waves. Significant correlation was found between the depth-averaged wind-driven longshore current and the incident wave conditions; e.g., the average energy-based significant wave height (H) over bar (mo) is typically larger (0.95 m) when the current is directed to the south than when the current is directed to the north (0.73 m). Guided by the classic Coastal Engineering Research Center (CERC) formula for longshore sediment transport, it is found that H-mo(5/2) is significantly more correlated with southerly directed longshore currents (r = 0.47) than northerly currents (r = 0.21). Also, if a "storm'' is defined as whenever H-mo exceeds 1.75 m, 40% of this time, the mean wave direction is out of the northeast quadrant, 33% is from the southeast, and 27% approaches shore-normal. Additionally, during storms, a stronger correlation between S-xy is found with southerly directed wind-driven currents (r = 0.51) than with northerly directed currents (r = 0.32). These findings indicate that net longshore sediment transport may in fact be heavily influenced by the correlation of the local wind with the waves, a feature not included in traditional longshore sediment transport formulas.