Scale-dependent behavior of the foredune: Implications for barrier island response to storms and sea-level rise

The impact of storm surge on a barrier island tends to be considered from a single cross-shore dimension, dependent on the relative elevations of the storm surge and dune crest. However, the foredune is rarely uniform and can exhibit considerable variation in height and width at a range of length scales. In this study, LiDAR data from barrier islands in Texas and Florida are used to explore how shoreline position and dune morphology vary alongshore, and to determine how this variability is altered or reinforced by storms and post-storm recovery. Wavelet analysis reveals that a power law can approximate historical shoreline change across all scales, but that stormscale shoreline change (~10 years) and dune height exhibit similar scale-dependent variations at swash and surf zone scales (<1000 m). The in-phase nature of the relationship between dune height and storm-scale shoreline change indicates that areas of greater storm-scale shoreline retreat are associated with areas of smaller dunes. It is argued that the decoupling of storm-scale and historical shoreline change at swash and surf zone scales is also associated with the alongshore redistribution of sediment and the tendency of shorelines to evolve to a more diffusive (or straight) pattern with time. The wavelet analysis of the data for post-storm dune recovery is also characterized by red noise at the smallest scales characteristic of diffusive systems, suggesting that it is possible that small-scale variations in dune height can be repaired through alongshore recovery and expansion if there is sufficient time between storms. However, the time required for dune recovery exceeds the time between storms capable of eroding and overwashing the dune. Correlation between historical shoreline retreat and the variance of the dune at swash and surf zone scales suggests that the persistence of the dune is an important control on transgression through island migration or shoreline retreat with relative sea-level rise

Variations in Nearshore Bar Morphology: Implications for Rip Current Development at Pensacola Beach, Florida from 1951 to 2004

In 2002, Pensacola Beach was identified by the United States Lifesaving Association as being the most hazardous beach in the continental United States for beach drowning by rip currents. Recent studies suggest that the rip currents at Pensacola Beach are associated with a transverse bar and rip morphology that develops with the migration of the bars and recovery of the beachface following an extreme storm. Combined with an alongshore variation in wave forcing by transverse ridges on the inner-shelf, the bar cycle (of bar response and recovery to extreme storms) is hypothesized to create both rip current hotspots and periods of rip activity. However, it is unknown at what stage, or stages, the bar cycle is associated with the formation of these hotspots and the greatest number of rips. To determine how the accretional rip hazard varies in response to the nearshore bar cycle, this thesis will quantify the alongshore variation in the nearshore bar morphology on Santa Rosa Island from 1951 to 2004. Aerial photographs and satellite images are collected for the study area and nearshore features are digitized in ArcGIS and evaluated using wavelet analysis. Specifically, a continuous wavelet transform is used to the identify times and locations when a transverse bar and rip morphology is present or is in the process of developing. The findings suggest that the rip-scale variation in bar morphology (~100-250m) is superimposed on an alongshore variation consistent with the scale of the transverse ridges (~1000m). From the outer bar to the shoreline, and as the bar migrates landward, the variation becomes increasingly dominated by the rip-scale variation. Hotspots of rip current activity were found consistently between years at Fort Pickens Gate, San Souci, Holiday Inn, Casino Beach, Avenida 18 and Portofino, as clusters of rip-scale variation