Observations of wave breaking and surf zone width from a real-time cross-shore array of wave and current sensors at Duck, NC

Abstract

Data from a cross-shore array of acoustic sensors at the US Army Corps of Engineers Field Research Facility is examined for evidence of wave transformation and longshore currents across the surf zone by comparing several events in 2009–10 with different wave statistics. Hurricane Bill (Hs = 3+ m, Tp = 18 s) was a long-period wave event with strong evidence of non-linear wave transformations, and a track that was offshore such that the coast received very little wind. A strong depth-uniform longshore current was observed at the 5 and 6 m sites (up to 1.8 m/s) that was in-phase with the wave energy. Weak currents were measured at the 8 and 11 m depth sites, indicating that the limit of the surf zone extended to between 6 and 8 m depth. Hurricane/Nor'easter Ida (Hs = 5+ m, Tp = 12 s) was a typical large wave event in the fall, with strong winds (wind-sea a major wave component) and rotating wind direction. Hurricane Earl (Hs = 4+ m, Tp = 15 s) was the first major wave event with all sensors in place, since the sensors at the 2 and 3 m depths were added in August 2010. For the selected events we present the observations of wave evolution across the surf zone. The offshore extent of wave breaking was determined from Argus Station imagery by analyzing pixel intensity for time exposure images along cross-shore transect. Surf zone widths are compared to the estimated extent of breaking by comparing wave energy across the array and the magnitude of the longshore current. The alongshore momentum balance was estimated to determine the contribution of radiation stress gradients to observed longshore current. The results provide a comparison of the seaward limit of the surf zone and width of the wave-driven current for different wave forcing conditions.