Identifying Hotspots of Rip Current Activity Using Wavelet Analysis at Pensacola Beach, Florida

Abstract

Rip currents at Pensacola Beach in northwest Florida are associated with a transverse bar and rip morphology that develops as the nearshore bars migrate landward and weld to the beachface. As shown in a number of recent studies, the nearshore bar morphology at Pensacola Beach varies alongshore in response to a ridge and swale bathymetry on the inner-shelf, creating "hotspots" of rip current activity. However, it remains unclear whether these recently identified "hotspots" were present before Hurricane Ivan and if the development of the rip system is a response to the bar cycle. Wavelet analysis of aerial and satellite imagery from 1951 to 2004 was used to quantify the alongshore variation in bar morphology and identify when and where transverse bar and rip morphology develops in response to the storm-driven bar cycle. The innermost bar varies alongshore at a scale (of ~1000 m) that is consistent with the ridge and swale bathymetry, being closest to the beachface immediately landward of the deeper swales. As the bar migrates landward in the swales, it develops a variation with a length scale (of ~200 m) that is consistent with the spacing between rip currents. The transverse bar and rip morphology is found to preferentially develop at the primary beach access points where most drownings and rescues have historically taken place. The lack of a transverse bar and rip morphology landward of the ridges suggests that the bar cycle is rarely completed (with bar welding to the beachface) at those locations before the next hurricane or tropical storm moves it offshore.