Barrier island erosion and overwash study - volume 1

Abstract

This is the first of a pair of reports documenting the effects of storms on barrier island systems.The present report (Volume 1) investigates storm effects on natural island conditions whereasVolume 2 addresses the effects of seawalls. With the aim of simulating the effects of overwashon barrier islands and characterizing their response, a series of nine experiments was conductedat the Coastal Engineering Laboratory of the University of Florida. The barrier island wassimulated by a 400 feet wide (prototype units) horizontal crest and an initially planar (1:19)beach. The effects of various storm surge levels and accompanying overtopping were investigated.Experiments were conducted with both regular and irregular storm waves. Regularwaves without overtopping caused the formation of a substantial berm in the swash zone and aprominent longshore bar offshore. Increasing degrees of overtopping resulted in substantial lossof sand from the barrier island system. The longshore bar was considerably more subtle for thehighest water level tested (11.5 ft. above mean sea level). Simulation of a storm-surge hydrographwith rising and falling water levels indicated that the presence of the bar tends to occuronly during a relatively steady or slowly changing water level. The experiments with irregularwaves were conducted with reasonably similar wave heights and carrier periods as those withregular waves. The major difference was in the characteristics of the longshore bar response. Incomparison with cases with regular waves, the bar was less distinct without overtopping, subtlewith minimal overtopping and absent in cases with substantial overtopping. These experimentsseem to indicate that offshore bars are simply break-point bars which require a fairly steadybreak-point and undertow (return of mass transport) for optimal formation. (Document has 84 pages.)