Ebb-Tidal Delta Dynamics for a Tide-Dominated Barrier Island

Abstract

The interaction of wave and tidal processes in the vicinity of tidal inlets that bound barrier islands may yield depositional features that have significant, and even controlling, influence on barrier island morphology. Ebb-tidal deltas formed at the updrift end of barrier islands protect a portion of the island from incoming wave energy and produce refractive effects that determine longshore current magnitude and direction. These shoal areas also provide sediment reservoirs for the natural nourishment of

Marco Island, Florida is a tide-dominated morphology despite its microtidal range. The refractive effects of the updrift shoal are sufficient to produce longshore current divergence and formation of a nodal zone. The ebb-tidal delta not only effects morphology to its lee and near-downdrift beaches but also is a dominant control for shoreline movement along the entire six kilometers of beach. Shoreline movement trends were established by analyzing an air photo time series that included fifteen years of record within a 41 year (1940–1981) time span. Eight years of photographic coverage during the 1971–1981 period permitted determination of an area budget for the island. In this study, measured areal shoreline change was used as a surrogate for sediment gain/loss. Relative shoreline movement trends were established that showed cause and effect relationships of various parts of the Marco Island shoreline.

The resultant conceptual process model developed is believed to be valid for barrier islands bounded by ebb-tidal deltas. Interpretation of the inlet morphology can allow prediction of shoreline movement trends and emphasizes the consequences that may result if dredging or natural processes significantly alter the extent of these shoal