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Coupled hydrodynamic modeling and sediment core analysis was used to investigate Hurricane Rita hydrodynamic conditions and associated sediment dynamics in Sister Lake, a shallow coastal lake in Terrebonne Basin, Louisiana. Tropical cyclone impacts on wetland, terrestrial, and shelf systems have been previously studied and reasonably delineated, but little is known about the response of coastal lakes to storm events. This initial investigation of tropical cyclone impacts on a shallow coastal lake clarifies sediment transport and deposition patterns in a geologically complex deltaic region. Modeling results from Hurricane Rita forcing conditions hindcast a maximum storm surge elevation of approximately 1.1 m and a significant wave height of 1.0 m in Sister Lake. Bed shear stresses across almost the entire model domain leading up to Hurricane Rita's landfall were above the critical value causing erosion of fine-grained bottom sediments, and quickly decreased in the western portion during Rita's landfall, indicating significant deposition in this western portion of the lake. The ideal event sedimentation unit that would result from the storm conditions hindcast from the numerical model was corroborated with stratigraphy identified in box cores; sedimentary units with an erosional base overlain by recently deposited silty material topped by clays. This study provides a fundamental understanding of lake bottom characteristics and impacts of storm-related physical processes on erosion and deposition.

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.

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.

STONE, G.W.; PEPPER, D.A.; XU, J., and ZHANG, X., 2004. Ship Shoal as a prospective borrow site for barrier island restoration, coastal south-central Louisiana, USA: numerical wave modeling and field measurements of hydrodynamics and sediment transport. Journal of Coastal Research, 20(1), 70‐89. West Palm Beach (Florida), ISSN 07490208. 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.

Hydrodynamic and sediment transport measurements from instrumentation deployed during a 54-day winter period at two sites on the Louisiana inner shelf are presented. Strong extratropical storms, with wind speeds of 7.8 to 15.1 m s–1, were the dominant forcing mechanism during the study. These typically caused mean oscillatory flows and shear velocities about 33% higher than fair weather (averaging 12.3 and 3.2 cm s–1 at the landward site, and 11.4 and 2.7 cm s–1 at the seaward site, respectively). These responses were coupled with mean near-bottom currents more than twice as strong as during fair weather (10.3 and 7.5 cm s–1 at the landward and seaward sites, respectively). These flowed in approximately the same direction as the veering wind, causing a net offshore transport of fine sand. Weak storms were responsible for little sediment transport whereas during fair weather, onshore sand transport of approximately 25–75% of the storm values appears to have occurred. This contradicts previous predictions of negligible fair-weather sediment movement on this inner shelf.

The dispersal behavior of river sediment plumes from the Mississippi and Atchafalaya rivers were investigated using a 3-D hydrodynamic and sediment transport model implemented for the Louisiana inner shelf to demonstrate the application of numerical models, field data and satellite images to study river sediment plumes for the area. The key tasks of calibration and skill assessment of the model were performed using vertical current profile data from ADCP’s deployed at WAVCIS coastal observing stations. Seasonal hydrodynamic features, including inertial oscillations, were also considered for fine tuning of the calibration parameters. Plume dispersion patterns, computed from the sediment transport model, were verified using satellite-derived suspended sediment concentration data. Sediment transport characteristics on the innershelf were simulated for a typical spring season river plume discharge scenario and also when significant sediment resuspension occurred along the shelf, resulting from waves generated during the passage of cold fronts. The results show that the river plumes were significantly influenced by the prevailing spring hydrodynamics along the Louisiana shelf and that the waves also facilitated the resuspension and the transport of fine grained sediments.

In this study we implement the recently developed unstructured mesh SWAN for simulating wave fields during hurricanes Gustav and Ike, which made landfall along the Gulf coast of the United States during September 2008. The model is validated using NDBC buoy observations offshore and data from several WA VCIS stations along the Louisiana coast From a fundamental research perspective, the numerical output from the unstructured version of SWAN, spectral as well as bulk wave parameters, when compared to in situ observations, show excellent agreement Our study results also imply that targeted sand mining from Sabine Bank, southwest Louisiana, would lead to negligible variations in the nearshore wave field; however, extensive dredging may render the coast vulnerable to higher energy waves and, therefore, more erosion and littoral sediment transport The conclusions from our study are also useful for designing viable dredging plans for the offshore sand deposits, such as Sabine Bank, for supporting coastal restoration and maintenance projects proposed along the northern Gulf of Mexico coast.

Morphodynamics over a shore-parallel sand shoal off south-central Louisiana, USA, have been recognized as complex given the occasional infusion of fine sediments, frequent winter storm passage, and complex shoal bathymetry. Results from field surveys and numerical model studies unveiled spatially-varying morphodynamics; Occasional infusion of fine sediments (i.e. fluid mud) created sediment heterogeneity on the shoal; the bottom sediments on the shoal further interacted with storm-induced hydrodynamics. Shallower depths on the western flank of the shoal had high sediment re-suspension, energetic flow velocity, and resultant high sediment transport; the result favored exposure of sandy material on the bottom; the eastern flank of the shoal, located in deeper water, experienced the accumulation of fine sediments. The results had potential implications for some benthic biological variables that spatially change across the shoal. Our results suggest complex bio-physical interaction with uncertainties and further implications for potential future sand mining for restoring rapidly deteriorating Louisiana barrier islands, essential for protecting wetlands along coastal Louisiana.

The landfall of Hurricane Rita in 2005 significantly altered physical and morphological characteristics along the southwestern Louisiana coastline and interior marshes. An excessive storm surge event linked to Rita resulted in widespread, storm-induced overwash deposits in Cameron and Vermilion Parishes, Louisiana. These deposits superseded prior deposits and were aligned parallel to predominant flow patterns associated with the storm's counter-clockwise circulation. These responses became more pronounced farther west and culminating at eyewall impact. Excessive salinity levels resulting from salt-water intrusion severely impacted local vegetation several kilometers inland and over the longer term may exacerbate the loss of vegetation that retards coastal erosion and attenuates waves and storm surge during high-energy events. Massive water levels introduced onshore by surge remained locally ponded several weeks after the hurricane; however, morphological patterns along the coastline had begun facilitating the return flow of water and sediment to the Gulf of Mexico.

Increasing focus has been placed on the critical role the inner-shelf plays in coastal sediment dynamics. During storms, bottom sediment may be mobilized by waves and currents and transported along or across the inner-shelf, affecting the sediment budget of the adjacent coast. During a twelve-day period that included two cold front passages, we measured waves and near-bed currents on the Louisiana inner-shelf (depth ~ 8m) using a sophisticated bottom-mounted instrumentation system. We then calculated bottom boundary layer parameters using wave-current interaction models, including those of Grant and Madsen (1986) and Glenn and Grant (1987). Finally, we predicted sediment transport by assuming steady state turbulent diffusion within and above the wave boundary layer. Results indicate that the second front (Event 2) was the more energetic of the two, and maximum significant wave height (1.33 m), and current speed (0.22 ms-1), occurred during this time. Additionally, mean current-induced shear velocity (1.9 cms-1) and wave-current shear velocity (4.4 cms-1) were highest during this event's frontal and prefrontal stages (respectively). During the postfrontal stage, currents were strong and well organized although combined shear velocities were low as a result of reduced wave height. Predicted sediment transport varied considerably in direction and magnitude throughout the deployment, but was highest (0.13-0.14 gcm-1s-1 SE) during the prefrontal and frontal stages of Event 2. Fair weather transport was low and to the west. We conclude that cold fronts are an important mechanism for sediment movement on the Louisiana inner-shelf, although the associated transport direction and magnitude require further quantification.

Just over a year after the landfall of Hurricane Ivan, scientists have now had an opportunity to evaluate a variety of oceanographic and geologic responses to this storm. Hurricanes Ivan, Katrina,and Rita are among the most powerful hurricanes recently to enter the Gulf of Mexico. Although it weakened from a very powerful Category 5 hurricane to a Category 3 before making landfall along the Alabama coast, Hurricane Ivan devastated the coasts of northwestern Florida and Alabama on 16 September 2004. This article summarizes what researchers have learned about Hurricane Ivan as it moved into the Gulf and made landfall along the northeastern Gulf of Mexico coast. The article focuses on storm meteorology, sea state,shelf circulation, and sediment transport on the shelf and along the coast.