Beach Morphodynamics influenced by an ebb-tidal delta on the north Florida Atlantic coast

Tidal inlets interrupt longshore sediment transport, thereby exerting an influence on adjacent beach morphology. To investigate the details and spatial extent of an inlet's influence, we examine beach topographic change along a 1.5 km coastal reach adjacent to Matanzas Inlet, on the Florida Atlantic coast. Analyses of beach morphology reveal a behavioral change between 0.64 and 0.86 km from the inlet channel centerline, interpreted to represent the spatial extent of inlet influence. Beyond this boundary, the beach is narrow, exhibits a statistically significant inverse correlation of shoreline position with offshore wave conditions, and has a uniform alongshore pattern in temporal behavior, as determined from empirical orthogonal function (EOF) analysis. On the inlet side of the boundary, the beach experiences monotonic widening (with proximity to the inlet), lacks spatial consistency in correlation between shoreline position and wave conditions, and exhibits an irregular pattern in spatial EOF modes. We augment the field observations with numerical modeling that provides calculations of wave setup and nearshore current patterns near the inlet, highlighting the effects of the ebb-tidal delta on the assailing waves. The modeling results are verified by a natural experiment that occurred during May 2009, when a storm-produced sedimentary mass accreted to the lower beach, then subsequently split into two oppositely directed waves of sediment that migrated away from the initial accretion site in the subsequent months. Our results suggest that the ebb-tidal delta produces a pattern of wave setup that creates a pressure gradient driving an alongshore flow that opposes the longshore currents derived from breaking of obliquely oriented incident waves. The resulting recirculation pattern on the margin of the ebb-tidal delta provides a mechanism through which the inlet influences adjacent barrier island beach morphology. Copyright (c) 2016 John Wiley & Sons, Ltd.