597 research outputs found

    Sediment Impact on the Formation of Hypoxic Waters in the Northern Gulf of Mexico: A Synthesis of Sediment Texture, Composition, Erodibility and Transport

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    Annual hypoxic events have been found to occur over the past several decades in the northern Gulf of Mexico (nGOM) and have prompted researchers to begin studying the mechanisms that control hypoxia formation so they may advise policy makers on the appropriate mitigating responses. This has led to the development of 3-dimensional modeling systems that incorporate marine physical, biological, geological, and chemical processes that may impact the formation and duration of hypoxic regimes in the nGOM. This study used field, laboratory, and modeling techniques to examine how sediment may be eroded from the seabed and where/how it is transported across the nGOM. Analysis of sediment texture, composition, and erodibility through field studies and Regional Ocean Model System (ROMS) simulations have shown that spatial variability in sediment grain size and erodibility relates mostly to the proximity to the major river deltas (Mississippi and Atchafalaya) and to the remnants of historic shifts in the Mississippi deltaic lobe system. Temporal variability stems mostly from changes in seasonal weather patterns, with more energetic weather in winter and spring setting up an active bottom boundary layer (BBL) which agitates seabed sediment and therefore increases its erodibility, compared to summer quiescent periods that can allow for seabed consolidation due to a low-energy BBL. This study has also found evidence that there is an organically enriched flocculent layer of material at the water-sediment surface that is highly erodible. Based on comparisons of model simulations and experiments, the shear stress levels during the quiescent periods may be strong enough to resuspend this material and reintroduce it into the lower water column where it may be decomposed by bacteria. Modeling studies have illustrated that understanding the interactions among physical, chemical, biological, and geological dynamics is a challenging but necessary in order to determine what mitigating practices will be most beneficial. This study only examines a part of the complex hypoxic water system, but it provides a stepping stone for future studies that examine the complex interaction of the different regimes that influence hypoxia formation

    Contribution of hurricane-induced sediment resuspension to coastal oxygen dynamics

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    © The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Scientific Reports 8 (2018): 15740, doi:10.1038/s41598-018-33640-3.Hurricanes passing over the ocean can mix the water column down to great depths and resuspend massive volumes of sediments on the continental shelves. Consequently, organic carbon and reduced inorganic compounds associated with these sediments can be resuspended from anaerobic portions of the seabed and re-exposed to dissolved oxygen (DO) in the water column. This process can drive DO consumption as sediments become oxidized. Previous studies have investigated the effect of hurricanes on DO in different coastal regions of the world, highlighting the alleviation of hypoxic conditions by extreme winds, which drive vertical mixing and re-aeration of the water column. However, the effect of hurricane-induced resuspended sediments on DO has been neglected. Here, using a diverse suite of datasets for the northern Gulf of Mexico, we find that in the few days after a hurricane passage, decomposition of resuspended shelf sediments consumes up to a fifth of the DO added to the bottom of the water column during vertical mixing. Despite uncertainty in this value, we highlight the potential significance of this mechanism for DO dynamics. Overall, sediment resuspension likely occurs over all continental shelves affected by tropical cyclones, potentially impacting global cycles of marine DO and carbon.Support for J. Moriarty was provided by the USGS Mendenhall Program

    Landscape-Scale Analysis Of Wetland Sediment Deposition From Four Tropical Cyclone Events

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    Hurricanes Katrina, Rita, Gustav, and Ike deposited large quantities of sediment on coastal wetlands after making landfall in the northern Gulf of Mexico. We sampled sediments deposited on the wetland surface throughout the entire Louisiana and Texas depositional surfaces of Hurricanes Katrina, Rita, Gustav, and the Louisiana portion of Hurricane Ike. We used spatial interpolation to model the total amount and spatial distribution of inorganic sediment deposition from each storm. The sediment deposition on coastal wetlands was an estimated 68, 48, and 21 million metric tons from Hurricanes Katrina, Rita, and Gustav, respectively. The spatial distribution decreased in a similar manner with distance from the coast for all hurricanes, but the relationship with distance from the storm track was more variable between events. The southeast-facing Breton Sound estuary had significant storm-derived sediment deposition west of the storm track, whereas sediment deposition along the south-facing coastline occurred primarily east of the storm track. Sediment organic content, bulk density, and grain size also decreased significantly with distance from the coast, but were also more variable with respect to distance from the track. On average, eighty percent of the mineral deposition occurred within 20 km from the coast, and 58% was within 50 km of the track. These results highlight an important link between tropical cyclone events and coastal wetland sedimentation, and are useful in identifying a more complete sediment budget for coastal wetland soils

    Patterns and Pathways of Wetland Sedimentation and Landscape Change in Coastal Louisiana

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    Coastal Louisiana wetlands exist in a dynamic physical environment and retracted dramatically in the last century. Here I examine the spatial and temporal variability of this landscape with an emphasis on the interactions between anthropogenic landscape modifications and geological processes. The Mississippi River watershed underwent drastic changes during the past 200 years, beginning with widespread land clearing and, later, large-scale reservoir construction. These modifications caused increases in suspended sediment concentrations, then sharp decreases, and have remained relatively stable since 1960. I show how changes in land area of the Mississippi River birdfoot delta reflect these fluctuations, and that they are distinct from the timing of land losses elsewhere along the coast. The deposition of inorganic sediments elsewhere along the coast is driven primarily by marine processes. I quantified the total amount and spatial distribution of mineral sediment following recent hurricanes and found that Hurricanes Katrina, Rita, and Gustav deposited an estimated 68, 48, and 21 million metric tons (MMT), respectively. I used the observed sediment deposition patterns away from the coast and storm track to estimate a long-term tropical cyclone sedimentation rate (5.6 MMT/yr) for coastal Louisiana wetlands, which accounts for the majority of inorganic sediments in soils of the abandoned delta lobes and chenier plain. I applied geographically weighted regression as a supplement to a traditional regression of geological and anthropogenic factors to further explore patterns of landscape variability. I found that the patterns of interior wetland loss are strongly related to the density of dredged canals, and that this relationship varies spatially. Canals closer to the coast, for example, are more strongly correlated to land loss than those found further inland. The research presented here raises new questions about how physical, chemical, and biological systems interact and regulate coastal systems, and how these driving factors can vary considerably over relatively short distances. The success of coastal restoration in Louisiana and elsewhere will be greatly aided if this spatial variability and remaining scientific uncertainties are included in planning and implementation schemes

    Satellite Assessment of Bio-Optical Properties of Northern Gulf of Mexico Coastal Waters Following Hurricanes Katrina and Rita

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    The impacts of major tropical storms events on coastal waters include sediment resuspension, intense water column mixing, and increased delivery of terrestrial materials into coastal waters. We examined satellite imagery acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) ocean color sensor aboard the Aqua spacecraft following two major hurricane events: Hurricane Katrina, which made landfall on 29 August 2005, and Hurricane Rita, which made landfall on 24 September. MODIS Aqua true color imagery revealed high turbidity levels in shelf waters immediately following the storms indicative of intense resuspension. However, imagery following the landfall of Katrina showed relatively rapid return of shelf water mass properties to pre-storm conditions. Indeed, MODIS Aqua-derived estimates of diffuse attenuation at 490 nm (K_490) and chlorophyll (chlor_a) from mid-August prior to the landfall of Hurricane Katrina were comparable to those observed in mid-September following the storm. Regions of elevated K_490 and chlor_a were evident in offshore waters and appeared to be associated with cyclonic circulation (cold-core eddies) identified on the basis of sea surface height anomaly (SSHA). Imagery acquired shortly after Hurricane Rita made landfall showed increased water column turbidity extending over a large area of the shelf off Louisiana and Texas, consistent with intense resuspension and sediment disturbance. An interannual comparison of satellite-derived estimates of K_490 for late September and early October revealed relatively lower levels in 2005, compared to the mean for the prior three years, in the vicinity of the Mississippi River birdfoot delta. In contrast, levels above the previous three year mean were observed off Texas and Louisiana 7-10 d after the passage of Rita. The lower values of K_490 near the delta could be attributed to relatively low river discharge during the preceding months of the 2005 season. The elevated levels off Texas and Louisiana were speculated to be due to the presence of fine grain sediment or dissolved materials that remained in the water column following the storm, and may also have been associated with enhanced phytoplankton biomass stimulated by the intense vertical mixing and offshore delivery of shelf water and associated nutrients. This latter view was supported by observations of high chlor_a in association with regions of cyclonic circulation

    Bottom boundary layer physics and sediment transport along a transgressive sand body, Ship Shoal, south-central Louisiana: implications for fluvial sediments and winter storms

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    Ship Shoal, a shore-parallel sand body, was recently recognized as having a unique physical and biological environment and also as a potential sand resource for coastal restoration in coastal Louisiana. Little is known regarding such dynamics, in concert with fluvial sediments and winter storms, influenced in unique ecosystems, and likely in future potential sand mining. This dissertation addresses such the morphodynamics and sedimentary processes and their implications for the mining from the shoal using field measurements and numerical modeling studies. During the winter-spring season, fluvial sediment plumes shifted from the prevailing west to southeast during the post-frontal phases, resulted in accumulation of fluid mud on the eastern flank of the shoal and consequent shoal sediment heterogeneity during the spring of 2006; this fluid mud layer strongly interacted with storm waves and currents through the processes of sediment re-suspension, vertical mixing, and hindered settling and redistribution. Studies during winter 2008 represented dynamics dominated by non-cohesive bottom material and hence followed the conventional approaches. State-of-the-art numerical models for waves, currents and transport provided reasonably well estimation for the study area and showed changes in wave transformation, current variability, and sediment transport for various hypothetical post-dredging scenarios. Sediment re-suspension intensity showed spatial differences along the shoal: high on the western flank of the shoal and a decrease toward the eastern shoal due to the change in shoal bathymetry. The results indicated a favor for the fluid mud accumulation on the eastern flank of the shoal, corroborated by in-situ measurements. Data suggest that Ship Shoal appears to have recurring sandy and muddy bottoms depending on the amount of storm-induced sediment reworking and fluvially-derived sediments. The fluid mud on the shoal seems to be patchy and does not remain in place as permanently consolidated mud, given the frequency of winter storms and the dispersal shifts. Numerical simulations suggest that targeted small-scale mining would not significantly alter the hydrodynamics and sediment transport over the shoal. Dredging from the eastern flank of the shoal may give rise to lesser impacts than that from the middle and western flank of the shoal. This suggestion is consistent with that from our collaborative biological study

    Numerical Simulations of Wind Effects on Wave Nonlinearity and Hurricane-Induced Sediment Transport on Louisiana Coast

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    The objective of this study is to model wind effects on wave nonlinearity and the sediment suspension, transport and redistribution caused by hurricanes. The following questions are addressed through numerical simulations: (1) How do winds affect wave triad interactions and wave shape in the shallow water? (2) What is the role of hurricanes in coastal landscape evolution? Do they create more erosion or deposition? (3) Where does the observed post-hurricane deposition on coastal wetlands come from? First, wind effects were incorporated into a Boussinesq-type wave model, and evolution equations were derived for triad interactions with winds. Second, a coupled modeling system for hurricane waves, storm surge, and sediment transport was developed for the Louisiana coast. Third, the modeling system was extended to three dimensions (3D), and the impact of barrier islands on hurricane-induced sediment redistribution was evaluated using the 3D model. The Boussinesq model and the evolution equations together illustrated why following (opposing) winds can enhance (suppress) triad interactions and how the wave shape varies due to the nonlinear wave-wave interactions. The process-based modeling system for coastal Louisiana demonstrated that a major hurricane event has the ability to deliver a considerable amount of sediment to the coastal wetlands, and estimated that Hurricane Gustav (2008) delivered 25.6 million metric tons of sediment to the wetlands in the Terrebonne and Barataria Basins, and most of the observed sediment accretion (97.3% for Terrebonne and 99.8% for Barataria) came from the estuaries. The net deposition on wetlands was 21% smaller in the 3D model than the results from the 2D model using the same sediment properties, while the finding that the hurricane-induced deposition came from erosion in the coastal bays held true regardless of the dimensionality of the model. The deterioration of barrier islands affected the maximum surge level, wave heights and sediment transport in the protected estuaries, but the net effect on sediment fluxes from the continental shelf to the bays and from the bays to wetlands varied by location. Numerical experiments suggested that the contribution from marine sediment to wetland deposition would still be very small even when the barrier islands were severely degraded

    Subaqueous, hurricane-initiated shelf failure morphodynamics along the Mississippi River Delta Front, north-central Gulf of Mexico

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    Seafloor instability along the Mississippi River Delta Front (MRDF) gained renewed attention with the landfall of Hurricanes Ivan (2004) and Katrina (2005). Traditional root causes for MRDF shelf failure were exacerbated by sea-state conditions associated with these severe tropical cyclones and their interaction with the seafloor. These conditions were characterized by large waves, long wave periods and wave-induced turbulence in the bottom boundary layer and throughout the water column. An evaluation of local and regional MRDF bathymetry data revealed substantial changes in seafloor elevation and the immediate subsurface sediment profile, hypothesized as the end result of cyclic wave-seafloor interaction, seafloor scour and failure, and the re-initiation of antecedent seafloor slides and subsequent sediment re-deposition. Observed bulk wave and bottom layer conditions during Hurricanes Ivan and Katrina, during which significant wave height and wave period exceeded 15 m and 12 sec, respectively, were used to calibrate a series of MIKE 21 numerical wave models. Once calibrated, hindcasts were generated for earlier MRDF hurricanes dating from 1965. Spectral frequency data indicated long-period, often bimodal MRDF wave effects up to 48 hours prior to storm arrival. Lithologic and geotechnical parameters indicate widely varying shear strengths and safety factors, with higher shear stresses coincident with the 25-m isobath. Safety factors decreased in tandem with hurricane approach both prior to and after peak conditions. One-dimensional sediment failure modeling, calibrated to past seafloor failures, indicated variable ranges of mudslide length, ranging up to several kilometers. A composite risk framework was constructed that employed various triggering, revealing and predisposition danger factors, a statistical analysis of elements at risk, and a vulnerability assessment to identify likely scenarios for future hurricane-initiated seafloor failure. A top tier of historical storms, including Hurricanes Ivan, Camille and the 1856 Last Island Hurricane, was risked as most prone to failure; a secondary tier included Hurricanes Katrina, Opal, Carmen and the 1915 New Orleans Hurricane. Five hypothetical future hurricanes of varying intensity were then used to help characterize potential MRDF seafloor response. Areas at highest risk included those characterized by steep slopes, rapid sedimentation rates, and lengthened temporal exposure to severe hurricane conditions

    Geomorphic Evolution of Caminada Pass in Southeast Louisiana.

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    Tidal inlets play a significant role in barrier island sustainability along the barrier islands of southern Louisiana. With increasing tidal prism, major changes are taking place within and adjacent to the inlets. The purpose of this thesis is to examine how Caminada Pass, a tidal inlet along the Caminada-Moreau headland, has evolved through time. Fundamental to this effort is evaluating which processes have contributed toward inlet evolution and what is the response of the inlet-bordering barrier island shorelines of Grand Isle and Elmer’s Island. This effort summarizes previous results and utilizes published bathymetric data, aerial photographs, vector shorelines, satellite images, and seafloor grab samples. The intent of this research is to document the variety of data that are available for future studies of Caminada Pass, an evaluation of long and short-term changes to the system, and an overall better understanding of the inlet dynamics of Caminada Pass

    Numerical Model of Geochronological Tracers for Deposition and Reworking Applied to the Mississippi Subaqueous Delta

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    Measurements of naturally occurring, short-lived radioisotopes from sediment cores on the Mississippi subaqueous delta have been used to infer event bed characteristics such as depositional thicknesses and accumulation rates. Specifically, the presence of Beryllium-7 (Be-7) indicates recent riverine-derived terrestrial sediment deposition; while Thorium-234 (Th-234) provides evidence of recent suspension in marine waters. Sediment transport models typically represent coastal flood and storm deposition via estimated grain size patterns and deposit thicknesses, however, and do not directly calculate radioisotope activities and profiles, which leads to a disconnect between the numerical model and field observations. Here, observed radioisotopic profiles from the Mississippi subaqueous delta cores were directly related to a numerical model that represented resuspension and deposition using a new approach to account for the behavior of short-lived radioisotopes. Appropriate selection of parameters such as the bioditfusion coefficient, sediment accumulation rate, and radioisotopic source terms enabled a good match between the modeled and observed cores. Comparisons of modelled profiles with geochronological analytical models that estimate accumulation rate and flood layer thickness revealed potential avenues for refining these tools, and highlight the importance of constraining the biodiffusion coefficient
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