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Aspects of northern Gulf of Mexico (NGOM) (Louisiana, Mississippi, Alabama, and Florida panhandle) processes and barrier islands that are pertinent to their geomorphologic response are contrasted with the broader knowledge base summarized by SCHWARTZ (1973) and LEATHERMAN (1979, 1985). Salient findings from studies documenting the short-term (storm-induced; timescales of hours, days, and weeks) and long-term (timescales of years, decades, and centuries) response of barrier island systems in the NGOM are synthesized into a conceptual model. The conceptual model illustrates the hypothetical evolution of three barrier island morphologies as they evolve through a typical Category 1-2 hurricane, including poststorm recovery (days to weeks) and long-term evolution (years to decades). Primary factors in barrier island geomorphologic response to storms, regardless of location, are the elevation of the island relative to storm (surge plus setup) elevation, and duration of the storm. Unique aspects of the NGOM barrier islands, compared with knowledge summarized for other barrier types, include (1) storm paths, wind speed, and large bays that create the potential for both Gulf and bayshore erosion and (2) in Louisiana and Mississippi, the potential for loading of the underlying substrate by the barrier island, which, through time, increases consolidation, relative sea level rise, overwash, morphologic change, and migration. We recommend that design of large-scale beach restoration projects incorporate the potential for (1) time-dependent consolidation of the underlying sediment due to project loading and future migration, (2) Gulf and bayshore erosion and overwash, and (3) eolian transport toward the Gulf from north winds. [ABSTRACT FROM AUTHOR]

Ship Shoal, a transgressive sand body located at the 10 m isobath off south-central Louisiana, is deemed a potential sand source for restoration along the rapidly eroding Isles Dernieres barrier chain and possibly other sites in Louisiana. Through numerical wave modeling we evaluate the potential response of mining Ship Shoal on the wave field. During severe and strong storms, waves break seaward of the western flank of Ship Shoal. Therefore, removal of Ship Shoal (approximately 1.1 billion m3) causes a maximum increase of the significant wave height by 90%–100% and 40%–50% over the shoal and directly adjacent to the lee of the complex for two strong storm scenarios. During weak storms and fair weather conditions, waves do not break over Ship Shoal. The degree of increase in significant wave height due to shoal removal is considerably smaller, only 10%–20% on the west part of the shoal. Within the context of increasing nearshore wave energy levels, removal of the shoal is not significant enough to cause increased erosion along the Isles Dernieres. Wave approach direction exerts significant control on the wave climate leeward of Ship Shoal for stronger storms, but not weak storms or fairweather. Instrumentation deployed at the shoal allowed comparison of measured wave heights with numerically derived wave heights using STWAVE. Correlation coefficients are high in virtually all comparisons indicating the capability of the model to simulate wave behavior satisfactorily at the shoal. Directional waves, currents and sediment transport were measured during winter storms associated with frontal passages using three bottom-mounted arrays deployed on the seaward and landward sides of Ship Shoal (November, 1998–January, 1999). Episodic increases in wave height, mean and oscillatory current speed, shear velocity, and sediment transport rates, associated with recurrent cold front passages, were measured. Dissipation mechanisms included both breaking and bottom friction due to variable depths across the shoal crest and variable wave amplitudes during storms and fair-weather. Arctic surge fronts were associated with southerly storm waves, and southwesterly to westerly currents and sediment transport. Migrating cyclonic fronts generated northerly swell that transformed into southerly sea, and currents and sediment transport that were southeasterly overall. Waves were 36% higher and 9% longer on the seaward side of the shoal, whereas mean currents were 10% stronger landward, where they were directed onshore, in contrast to the offshore site, where seaward currents predominated. Sediment transport initiated by cold fronts was generally directed southeasterly to southwesterly at the offshore site, and southerly to westerly at the nearshore site. The data suggest that both cold fronts and the shoal, exert significant influences on regional hydrodynamics and sediment transport. [ABSTRACT FROM AUTHOR]

Abstract: Barrier islands that overlie a compressible substrate, such as islands in deltaic environments or those that overlay mud or peat deposits, load and consolidate the underlying subsurface. Through time, the elevation and aerial extent of these islands are reduced, making them more susceptible to future inundation and overwash. Sand washed over the island and onto back-barrier marsh or into the bay or estuary begins the consolidation process on a previously non-loaded substrate, with time-dependent consolidation as a function of the magnitude of the load, duration of load, and characteristics of the substrate. The result is an increase in the overwash, migration, breaching, and segmentation of these islands. This research developed a two-dimensional (cross-shore) numerical model for evolution of a sandy barrier island that spans durations of years to decades as a function of erosion, runup, overwash, migration, and time-dependent consolidation of the underlying substrate as a function of loading by the island. The model was tested with field data and then applied to evaluate the effects of a compressible substrate on long-term barrier island evolution. Results illustrate that barrier islands overlying a compressible substrate are more likely to have reduced dune elevation due to consolidation, incur overall volumetric adjustment of the profile to fill in compressed regions outside the immediate footprint of the island, and experience increased overwash and migration when the dune reaches a critical elevation with respect to the prevalent storm conditions. [Copyright &y& Elsevier]