Barrier island erosion and overwash study -- effect of seawalls. Volume 2

Abstract

This is the second of a pair of reports documenting the effects of storms on beach systemsincluding the presence of seawalls. With the aim of simulating the effects of overwash onbarrier islands with seawalls and characterizing their response, a series of eight experimentswas conducted at the Coastal Engineering Laboratory of the University of Florida. The barrierisland was simulated by a 400 feet wide horizontal crest and an initially uniform mildly-sloped(1:19) beach. The effects of positioning the seawall at two different locations as well as the effectsof various storm surge levels and accompanying overtopping were investigated. Experimentswere conducted with both regular and irregular storm waves. With the seawall located at theslope break between the crest and the sloping beach of the barrier island, and the crest of theseawall just submerged in sand, the effects on the sediment transport process were found to beminimal. For the same position of the seawall but with the crest of the seawall raised above thesurrounding ground level, overtopping caused washover of sand indicating substantial transportin suspension. Increased levels of overtopping tended to accentuate bed profile changes butsupress bar formation (as did irregular waves). Positioning the seawall at the Mean Sea Levelshoreline caused significant scour both immediately landward as well as immediately seawardof the seawall. A prominent scour trough developed further seaward. The longshore bar washighly three-dimensional. It appears that seawalls need to be located adequately landward of theshoreline to discharge their function effectively without adverse effect to the beach. In addition,concerns for safety warrant the presence of an adequate buffer-zone between the seawall andthe upland property. (61 pp.)