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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.

The formation and spatial evolution of Chandeleur Island, Louisiana, has been investigated extensively during the past several decades. No significant evaluation of the longshore sediment system, which is instrumental in the island's evolution and morphodynamic maintenance, has been completed. This paper provides the first quantitative description of the longshore transport system that operates along this transgressive, overwash-dominated barrier island system. The net longshore sediment transport system was investigated via the wave refraction model, WAVENRG, which provided estimates of the transport volumes and drift directions alongshore. Surficial samples were collected from the foredune, midtide and step environments in an effort to characterize the sediments along the island and determine if textural or compositional trends have developed in response to a predicted longshore sediment transport system. Data obtained during this research indicate that the longshore transport system along Chandeleur Island is characterized by a bidirectional drift system, with drift directed both north and south from a nodal point located in the south-central portion of the barrier island. Analysis of the predicted transport volumes indicates that the degree of wave refraction, and therefore the breaker angle, is more instrumental in controlling the alongshore volume rate of sediment transport than the breaker wave heights. Additionally, a larger magnitude of sediment transport is predicted in the southern portion of the barrier, which is in a greater state of deterioration than the north and central portion of the island. This apparent contradiction indicates that factors such as a variable subsidence rate along the island are contributing to the alongshore geomorphology. No significant textural or compositional trends were identified alongshore. This absence of granulometric trends is attributed to the lack of variability of the sediments that comprise the barrier and the frequency of overwash events which occur on this low-profile island.

Over the past three to four decades, there has been a growing awareness of the important controls exerted by large-scale meteorological events on coastal systems. For example, definitive links are being established between short-term (timescales of 5-10 years) beach dynamics and storm frequency. This paper assesses temporal variability of coastal storms (both tropical and extratropical) and the wave climatology in the North Atlantic Basin (NAB), including the Gulf of Mexico. With both storm types, the empirical record shows decadal scale variability, but neither demonstrates highly significant trends that can be linked conclusively to natural or anthropogenic factors. Tropical storm frequencies have declined over the past two or three decades, which is perhaps related to recent intense and prolonged El Ninos. Some forecasts predict higher frequencies of tropical storms like that experienced from the 1920s to the 1960s to occur in coming decades. Results from general circulation models (GCMs) suggest that overall frequencies of tropical storms could decrease slightly, but that there is potential for the generation of more intense hurricanes. These data have important implications for the short-term evolution of coastal systems. There is strong suggestion that extratropical systems have declined overall over the past 50-100 years, but that there is an increase in frequency of very powerful storms, especially at higher latitudes. Both ENSO and the North Atlantic Oscillation (NAO) are shown to have associations with frequencies and tracking of these systems. These empirical results are in general agreement with GCM forecasts under global warming scenarios. Analyses of wave climatology in the NAB show that the last two to three decades have been rougher at high latitudes than several decades prior, but this more recent sea state is similar to conditions from about 100 years ago. The recent roughness at sea seems to be related to high NAO index values, which are also expected to increase with global warming. Thus, when coupled to an anticipated continued rise in global sea level, this trend will likely result in increasing loss of sediment from the beach-nearshore system resulting in widespread coastal erosion.

Results are presented from the deployment of three bottom-mounted instrumentation systems in water depths of 6–9 m on the sandy inner shelf of Louisiana, USA. The 61-day deployment included nine cold front passages that were associated with large increases in wind speed. Two of the most energetic cold front passages were characterized by distinct meteorological, hydrodynamic, bottom boundary layer, and sedimentary responses and may potentially be treated as end-member types on a continuum of regional cold front passages. Arctic surges (AC storms) have a very weak pre-frontal phase followed by a fairly powerful post-frontal phase, when northeasterly winds dominate. Migrating cyclones (MC storms) are dominated by a strong low-pressure cell and have fairly strong southerly winds prior to the frontal passage, followed by strong northwesterly winds. On the basis of measurements taken during this study, AC storms are expected to have a lower average significant wave height than MC storms and are dominated by short-period southerly waves subsequent to the frontal passage. Currents are weak and northerly during the pre-frontal phase, but become very strong and southwesterly following the passage. Sediment transport rate during AS storms was not as high as during MC storms, and the mean and overall direction tended to be southwesterly to westerly, with low-frequency flows producing easterly transport, and wind-wave flows producing southeasterly transport. MC storms had the most energetic waves of any storm type, with peaks in significant wave height occurring during both the pre- and post-frontal phases. The wave field during MC storms tended to be more complex than during AS storms, with an energetic, northerly swell band gradually giving way to a southerly sea band as the post-frontal phase progressed. Currents during MC storms were moderate and northerly during the pre-frontal phase, but became much stronger and southeasterly during the post-frontal phase. Shear velocity was high during both the pre- and post-frontal phases of the storm, although sediment transport was highest following the frontal passage. Mean and overall sediment transport was directed southeasterly during MC storms, with low-frequency and wind-wave flows producing northerly transport. In summary, the data sets presented here are unique and offer insight into the morphosedimentary dynamics of mid-latitude, micro-tidal coasts during extratropical storms.

Data are presented indicating the complexity and highly variable response of beaches to cold front passages along the northern Gulf of Mexico, in addition to the impacts of tropical cyclones and winter storms. Within the past decade, an increase in the frequency of tropical storms and hurricanes impacting the northern Gulf has dramatically altered the long-term equilibrium of a large portion of this coast. A time series of net sediment flux for subaerial and nearshore environments has been established for a section of this coast in Florida, and to a lesser extent, Mississippi. The data incorporate the morphological signature of six tropical storms/hurricanes and more than 200 frontal passages. Data indicate that (1) barrier islands can conserve mass during catastrophic hurricanes (e.g., Hurricane Opal, a strong category 4 hurricane near landfall); (2) less severe hurricanes and tropical storms can promote rapid dune aggradation and can contribute sediment to the entire barrier system; (3) cold fronts play a critical role in the poststorm adjustment of the barrier by deflating the subaerial portion of the overwash terrace and eroding its marginal lobe along the bayside beach through locally generated, high frequency, steep waves; and (4) barrier systems along the northern Gulf do not necessarily enter an immediate poststorm recovery phase, although nested in sediment-rich nearshore environments. While high wave energy conditions associated with cold fronts play an integral role in the evolution and maintenance of barriers along the northern Gulf, these events are more effective in reworking sediment after the occurrence of extreme events such as hurricanes. This relationship is even more apparent during the clustering of tropical cyclones. It is anticipated that these findings will have important implications for the longer term evolution of barrier systems in midlatitude, microtidal settings where the clustering of storms is apparent, and winter storms are significant in intensity and frequency along the coast.

The Holocene New Orleans Barrier Complex, now buried by the St. Bernard delta of the Mississippi River, provides an excellent example of barrier deposition fed by a nearshore sediment source. This reworking and onshore transport was initiated by a sudden change in the shelf equilibrium profile caused by a sea-level fall about 4100 yr BP. Here we present a new model of barrier formation which does not invoke an Shepard-type Holocene sea-level curve nor the supply of sediment from a longshore source. The Holocene New Orleans Barrier Complex consists of fine-grained, locally cross-bedded, quartz sand that contains Ophiomorpha nodosa burrows and disarticulated mollusks, primarily marine, buried beneath up to 4 m of silty mud of the St. Bernard Lobe. This barrier island and shoal deposit overlies interbedded, fine-grained sand and mud containing marine mollusks, some articulated, that is interpreted to be a nearshore shelf deposit. Its deposition took place between 5500 and 4200 yr BP (14C), based on individual dates on seven articulated and seven disarticulated shells. The barrier formation is effectively limited to a several-hundred-year window approximately 4000 yr BP by the 3800 yr BP Rangia sp. shells from the immediately overlying St. Bernard Lobe delta-plain deposits and the buried 3900–3500 yr BP Linsley archeological site, situated on a more gulfward distributary levee. In this paper we present a new depositional model on the New Orleans Barrier. The barrier complex contains an abundance of large mollusk shells that have been reworked to the extent that 14 individual shells yield a 2500-year range, 6000–3500 yr BP. An older, nearby source is required. The current model of a spit/shoal complex migrating westward from an eroding eastern Pleistocene headland probably cannot account for the deposition of large reworked shells given the effects of abrasion and selective size sorting over approximately 50 km of longshore transport. Furthermore, this model demands transport rates of millions of cubic meters per year for the present northern Gulf coast which are at least an order of magnitude higher than its highest known rates. We postulate a nearby shell and sand source that is subjacent and offshore rather than adjacent and littoral. We propose it to be the underlying nearshore shelf deposit that could be mobilized by a brief fall of sea level and carried landward. The barrier complex and the uppermost nearshore shelf deposit have markedly different net deposition rates. The upper 25 cm of the nearshore shelf deposit were deposited over 800 years, based on ages of articulated marine pelecypods. A 20-km-long, 3-km-wide and 4-m-thick segment of the barrier complex was deposited in less than 300 years. A 10-cm-thick lag pavement of bryozoan- and oyster-encrusted mollusk shells that comprises the nearshore shelf deposit beneath the northern edge of the barrier complex is evidence for an essentially zero net deposition rate. The current interpretation of a conformable, progradational relationship between these two units is rejected in favor of a disconformable contact. The hiatus across this disconformity must be less than the several-hundred-year duration of barrier complex deposition. The positioning of the shallower-water barrier complex disconformably over the deeper-water nearshore shelf deposits indicates a sea-level fall.

The morphosedimentary maintenance of a 225 km stretch of coast along the northeast Gulf of Mexico, from Grayton Beach, Florida, to Morgan Point, Alabama, has been interpreted previously within the framework of a unidirectional, integrated, monotonic longshore drift model, with a single headland source of sediment located east of Grayton Beach. Net longshore transport and the granulometry and composition of some 2000 foredune, beach and step samples indicate a cellular net drift system, supplied by three independent sources of sediment. One source is provided by the Pleistocene barrier island complex along Grayton-Mirimar Beach, the second at Pensacola Beach on Santa Rosa Island, and the third is onshore transport across the inner shelf between Pensacola, Florida, and Morgan Point, Alabama. The Pleistocene “headland” and Pensacola Beach supply two cells along Santa Rosa Island, whereas onshore transport from the low gradient inner shelf supplies sediment to three cells along the largely accretional beach-ridge-dominated coast from Pensacola Pass to Morgan Point. Drift cells along this coast experience negligible net sediment exchange. These findings have significant implications for both the late Holocene evolution and the morphodynamic maintenance of this coast.