MORPHODYNAMICS OF NEARSHORE BAR ALONG BARRIER ISLAND BEACHES, WEST-CENTRAL FLORIDA

The nearshore bar is one of the most dynamic features in coastal environments. Quantifying the generation, migration, and morphology of the nearshore bar is essential to the understanding and modelling of complex coastal morphodynamics. A sandbar extends along most of the barrier island beaches of West-Central Florida, making this coast an ideal location to study the morphodynamics of nearshore bars. The objective of this study is to examine the factors controlling nearshore bar dynamics along three barrier islands based on time-series beach-profile measurements. Monthly beach surveys of 125 profiles, extending to roughly the short-term closure depth of about 3 m, were conducted along the 35 km coast from 2006 to 2008. The study area encompasses three barrier islands, Sand Key, Treasure Island, and Long Key, along the micro-tidal Pinellas County coastline. Morphologic characteristics of the study area include 1) a shoreline orientation change of 65 degrees from northwest to southwest; 2) a slightly deeper inner continental shelf in the northern portion of the study area; and 3) a regional southward longshore sediment transport interrupted by two tidal inlets. Waves in the study area are typically sea-type generated by local wind conditions. High swells from distal storms occur rarely during the hurricane season.

The regional sandbar dynamics are controlled by the aforementioned spatial and temporal variations in wave climate and nearshore morphology. In this study, the offshore bar became more prominent after high-energy events, such as the passages of tropical storms and cold fronts. For newly nourished monotonic beaches (i.e., without a bar), a bar formed in response to the first storm following nourishment. For beaches with an existing bar, the bar migrated offshore and the depth of the bar crest decreased as a result of the first storm after nourishment. During lower wave-energy conditions, an onshore migration of the bar and infilling of the trough was measured. Although a similar trend in bar migration was observed regionally, the magnitude of the bar response varied at the different locations. For example, the distance, width and crest height of the bar were influenced by variations in nearshore bathymetry, shoreline orientation, and proximity to tidal inlets.