| Abstract: | The results summarized herein are based on subaerial beach profiles taken on the Atlantic Ocean at Sandbridge, Virginia (USA). The shoreline has experienced an average, historic recession rate of about 2 m/yr for more than 120 years before seawall construction began in 1978. The purpose of this study is to determine whether or not the 16 walled sections increase the existing erosional trend at adjacent, non-walled beaches. Fifteen years of survey data are employed with 8–9 years of data taken before wall construction peaked in 1989. The main focus of these results is on five full wave years of monthly and post-storm survey data taken at 28 locations (16 walled, 4720 m, 62% and 12 non-walled, 2950 m, 38%) since October 1990. Three time scales (historic, seasonal, storms) and three analysis methods were used to address three questions concerning the effects of seawalls on adjacent beaches.It has been determined that volume erosion rates are not higher in front of seawalls (Question No. 1). However, the seasonal variability of the sand volume in front of walls is generally greater than at non-walled locations. Winter season waves drag more sand offshore in front of walls but also summer swell waves pile more sand up against walls in beach rebuilding.Walled beaches were found to recover about the same time as non-walled beaches for both seasonal transitions (winter to summer) and following erosional storm events (Question No. 2).At a few non-walled locations, the sand volume landward of adjacent walls was found to be eroding at a faster rate after wall construction. At some other non-walled locations, the sand volume landward remained constant or increased in time after nearby walled construction. The evidence for Sandbridge beach as a whole was considered inconclusive for Question No. 3.After wall construction, sand trapped behind the wall is not available for transport to adjacent beaches during and after storm events. The loss of this sand volume is felt to be in the initial stage of detection at Sandbridge. More full years of profile data are needed to confirm and quantify the effect. Sand is also trapped beneath the road (baseline) at non-walled locations. The study is continuing. |
