EROSION OF NOURISHED BETHANY BEACH IN DELAWARE, USA

Abstract

Erosion of the nourished beach in Bethany Beach in Delaware is examined using available beach profile, wave and tide data during September 2007 to September 2010. The volume of the placed sand with the median diameter of 0.31mm was about 500m(3)/m along the curved shoreline of 1.8 km length. The placed sand volume along this shoreline decreased to approximately 30% for the duration of 2.5 years. The nourished beach was attacked by two severe storms in May 2008 and November 2009. The eroded sand volume above the mean sea level (MSL) was about 70m(3)/m for each of the two storms and emergency repairs were necessary. The recovery after the first storm was about 8m(3)/m above MSL and much smaller than the eroded volume probably because the large placed sand volume resulted in the relatively steep eroded profile below MSL. The numerical cross-shore model with multiple cross-shore lines is used to compute the cross-shore and longshore sediment transport. The decrease of the placed sand volume is found to be caused partly by the increase of the longshore sand transport in the downdrift direction. The beach erosion by the two storms is shown to be caused by the offshore sand transport and the alongshore gradient of the longshore sand transport rate. The small recovery after the first storm is difficult to reproduce without increasing the onshore bed load transport rate by a factor of 2.