Short Term Fine Grain Swash Zone Bed Response to Broad Banded Forcing

An experimental analysis of the vertical change of the swash zone in a fine sand beach over a three-hour time period with an irregular wave system was performed. The data collection was facilitated by a collaboration of researchers involved in the Bardex II experiment at the Delta Flume in Vollenhove during Spring 2012. The goal of Bardex II is to develop a better understanding of sediment transport and beach morphology in a natural wave environment. The data addressed here was collected using a Vectrino II located in the outer edge of the swash zone. The Vectrino II utilizes Doppler shift to resolve velocities at a rate of 100 Hz over a vertical range of 31 mm with 1 mm resolution. The velocity and amplitude resolution of this effort enables estimates of instantaneous bed stresses and millimeter scale bed elevation fluctuations. The results reveal bed elevation fluctuations of 20 to 30 mm for group excursions. Bed fluctuations range from 5 to 20 mm for single swash excursions. The large variability in the bed elevation for a single swash event calls to attention inconsistencies that could arise in the widely used beach surveying methods.