Depth of activation of sediment by plunging breakers on a steep sand beach

Field data were collected over a lunar tidal cycle on a meso-tidal medium-sand estuarine beach in Delaware Bay, USA to document the relationship between wave height and depth of sediment activation in a low-wave-energy environment. Dominant energy was at the frequencies of locally-generated waves and ocean swell. Significant wave heights ranged from 0.06 m to 0.52 m. Periods of locally-generated waves averaged 4.0 s during strong onshore winds and 2.6 s during low-speed winds. Waves on the foreshore were always plunging. The foreshore maintained a slope of approximately 6.0° throughout the study period. Mean grain size at mid-foreshore was 0.46 mm. The greatest net surface-elevation change in response to storm erosion and post-storm recovery was 0.26 m. Results confirm previous observations that depths of sediment activation are greater for steeper beaches. The spatial variability in depths of activation across the foreshore is comparable to results of tidal cycle studies, where the ratio of depth of activation to wave height is great because a large percentage of the foreshore comes under the influence of the breakers. The ratio of depth of activation to wave height on the portion of the foreshore that is not transgressed by the breakers is lower and comparable to results of short-term investigations, provided that the influence of beach slope is considered in the calculation of breaking wave height.