Dauphin Island Research Articles

Mobile Observations of Ozone and Aerosols in Alabama: Southeastern US Summer Pollution and Coastal Variability

AbstractWe present a detailed analysis of measurements of ozone (O3) and aerosols collected using multiple platforms and instruments during a 5‐day mini‐campaign in Alabama, for improving our understanding of the chemistry and air quality in the Southeastern United States. Our study features the first on‐road O3 lidar profiling on highways and the coordinated airborne observations that captured O3 and aerosol structures associated with synoptic weather conditions, stratospheric intrusions, wildfire smoke transport, and enhancements in urban areas. The Doppler wind lidar data suggest that the vertical structures and variability of O3 and aerosols at Dauphin Island, a coastal site in the Gulf of Mexico, were related to both the synoptic flow and local‐scale land‐sea breeze circulation. The onshore gulf breeze, which typically started in the afternoon, helped to quickly dilute air pollutants near the surface, resulting in large gradients toward inland areas. In contrast, gradients were much smaller on days without a gulf breeze. O3 and humidity were highly anti‐correlated in the lower free troposphere, but weakly anti‐correlated in the planetary boundary layer (PBL) suggesting more complicated emissions and sink of O3 in the PBL. Our on‐road and airborne measurement strategies to quantify the vertical and horizontal chemical gradients will be useful for future geostationary satellite validation.

Examining the effect of salinity on dolphin mortality using Lagrangian particle tracking in a hydrodynamic model

Numerous dolphins are found dead on beaches and waterways of the Gulf of Mexico and Mississippi Sound every year. In 2019 an unusual mortality event (UME) occurred when 337 deceased bottlenose dolphins were stranded between Louisiana, Mississippi, Alabama, and Florida. According to NOAA, based on observations of skin lesions or distinct ulcerative dermatitis and other internal pathologic findings and the environmental conditions during that period of time, the identified cause of this UME was determined to be protracted exposure to low salinity waters. Dolphin carcasses are often found stranded on beaches days after they die. Consequently, their initial place of death is unknown. In this study, an existing 2D hydrodynamic model (EFDC+ 11.2) of the Mississippi Sound simulated by Armandei et al. (2021) was used to track dolphin carcass movement and to simulate salinity at the time and location of each predicted dolphin's death. Particles within the Lagrangian particle tracking module were used to represent the movement of 19 dolphin carcasses. A large number of particles (virtual dolphin carcasses) were seeded throughout the model domain and tracked for five days. These results were used to hindcast each dolphin's original location of death. The results indicate that the most likely place of death for 12 dolphins stranded on the beaches of Mississippi, along with the two found on Ship and Horn Islands, was west of their stranding location., For both of the dolphins stranded on Dauphin Island, the most likely place of death was in the Mobile Bay area north of where they were found. The remaining three dolphins found dead on barrier islands, most likely originally died in close proximity to their stranding locations. The average simulated salinity of all the dolphins' most probable original place of death was below five except for two cases. These results are compared to the salinity of the Mississippi Sound during Bonnet Carré spillway opening and non-opening dates. The results highlight the significant impact of the spillway's opening on the reduction of salinity and its association with dolphin mortality.

Decision science as a framework for combining geomorphological and ecological modeling for the management of coastal systems

The loss of ecosystem services due to climate change and coastal development is projected to have significant impacts on local economies and conservation of natural resources. Consequently, there has been an increase in coastal management activities such as living shorelines, oyster reef restoration, marsh restoration, beach and dune nourishment, and revegetation projects. Coastal management decisions are complex and include challenging trade-offs. Decision science offers a useful framework to address such complex problems. Here, we provide a synthesis about how decision science can help to integrate research from multiple disciplines (physical and life sciences) with management of coastal and marine systems. Specifically, we discuss the importance of considering concepts and techniques from ecology, coastal geology, geomorphology, climate science, oceanography, and decision analysis when developing conservation plans for coastal restoration. We illustrate the process with several coastal restoration studies. Our capstone example is based on a recent barrier island restoration assessment project at Dauphin Island, Alabama, which included the development of geomorphological and ecological models. We show how decision science can be used as a framework to combine geomorphological and ecological modeling to help inform management decisions while considering uncertainty about system changes and risk tolerance. We also build on our examples through a review of recently developed techniques for spatial conservation planning for land acquisition decisions and the application of adaptive management for sequential decisions.

All according to plan: Maldevelopment, moral hazard, federal aid, and climate change adaptation on Dauphin Island, Alabama, U.S.A.

Dauphin Island, Alabama is a particularly egregious example of moral hazard and maldevelopment and the policies and practices that allow this to take place. The west end of Dauphin Island offers a clear case study of such practices and illustrates the crucial role that State and local regulation play in curtailing many of the moral hazard effects of federal disaster relief funding in promoting maldevelopment. Where unrestricted, moral hazard can act as an incentive to build or rebuild in at-risk areas, and unlike many other coastal areas, Dauphin Island lacks policies and requirements which restrict such development. Using the richest dataset available for the island and geospatial visualization, this analysis presents Dauphin Island as a case study of the potential consequences of unrestricted, heavily subsidized maldevelopment, and outlines the policies that enabled it to take place.Dauphin Island demonstrates that Federal disaster relief programs other than the National Flood Insurance Program (NFIP) can contribute to maldevelopment. On Dauphin Island, Federal disaster relief from a combination of programs created a continual cycle of rebuilding on the island since 1979. Crucially, local coastal management policy acts as a valve restricting and directing the flow of federal funds. On Dauphin Island, those funds have not been directed towards resiliency, but towards maintaining and rebuilding the most vulnerable portion of the island.Geospatial visualization of current parcel development patterns clearly shows the consequences of these incentives to build and rebuild in areas that are known to be risky in terms of future storms and flooding. In particular, this analysis indicates that the “typical” property on the island is: (1) not owned by island residents, and (2) is a second home either rented out or used seasonally. Thus, a significant portion of the State, Local, and Federal expenditures designed for local disaster relief benefit property owned by off-island and out-of-state residents used for second homes. These high-value, high-risk properties result in the island receiving disproportionately high (FEMA) expenditures.

The Potential of Wave Energy Conversion to Mitigate Coastal Erosion from Hurricanes

Wave energy conversion technologies have recently attracted more attention as part of global efforts to replace fossil fuels with renewable energy resources. While ocean waves can provide renewable energy, they can also be destructive to coastal areas that are often densely populated and vulnerable to coastal erosion. There have been a variety of efforts to mitigate the impacts of wave- and storm-induced erosion; however, they are either temporary solutions or approaches that are not able to adapt to a changing climate. This study explores a green and sustainable approach to mitigating coastal erosion from hurricanes through wave energy conversion. A barrier island, Dauphin Island, off the coast of Alabama, is used as a test case. The potential use of wave energy converter farms to mitigate erosion due to hurricane storm surges while simultaneously generating renewable energy is explored through simulations that are forced with storm data using the XBeach model. It is shown that wave farms can impact coastal morphodynamics and have the potential to reduce dune and beach erosion, predominantly in the western portion of the island. The capacity of wave farms to influence coastal morphodynamics varies with the storm intensity.

How Does Ocean Acidification Affect the Early Life History of Zostera marina? A Series of Experiments Find Parental Carryover Can Benefit Viability or Germination

Elevated partial pressure of carbon dioxide (pCO2) as a concomitant of global climate change may facilitate the establishment of future seagrass meadows and subsequently its benefit could be incorporated into techniques to increase restoration success. In five manipulative experiments, we determined how increased CO2 affects the maturation of flowers, and the development of seeds and seedlings for the foundation species Zostera marina. Experiments tested the development from both seeds collected from non-treated flowering shoots (direct) and seeds harvested from flowering shoots after CO2 exposure (parental carryover). Flowering shoots were collected along the western coast of Sweden near the island of Skafto. The seeds produced were used in experiments conducted at Kristineberg, Sweden and Dauphin Island, AL, United States. Experiments varied in temperature (16, 18°C) and salinity (19, 33 ppt), as well as duration and magnitude of elevated CO2 exposure. Flowering maturation, spathe number, seed production, and indicators of seed quality did not appear to be affected by 39–69 days of exposure to CO2 conditions outside of natural variability (pCO2 = 1547.2 ± 267.60 μatm; pHT = 7.53 ± 0.07). Yet, seeds produced from these flowers showed twofold greater germination success. In another experiment, flowering shoots were exposed to an extreme CO2 condition (pCO2 = 5950.7 ± 1,849.82 μatm; pHT = 6.96 ± 0.15). In this case, flowers generated seeds that demonstrated a fivefold increase in an indicator for seed viability (sinking velocity). In the latter experiment, however, germination appeared unaffected. Direct CO2 effects on germination and seedling production were not observed. Our results provide evidence of a parental CO2 effect that can benefit germination or seed viability, but early benefits may not lead to bed establishment if other environmental conditions are not well suited for seedling development. Outcomes have implications for restoration; CO2 can be supplied to flowering shoot holding tanks to bolster success when the purpose is to redistribute seeds to locations where beds are extant and water quality is adequate.

Range expansion and complete mitochondrial genome of the highfin blenny (Lupinoblennius nicholsi).

The highfin blenny, Lupinoblennius nicholsi, is a marine fish species reported in reef and rocky inshore habitats with a disjunct distribution in the southern Gulf of Mexico. Overall, there are very few studies on this species and there is a scarcity of molecular resources for genetic comparisons. We set out to report the first mitochondrial genome for L. nicholsi and report a range expansion for the species. An individual of L. nicholsi was collected from the coast of Dauphin Island, Alabama. The mitochondrial genome was sequenced, assembled, and annotated. The fragment corresponding to cytochrome oxidase I (COI) was used to compare this sample to other cryptobenthic species of the Atlantic. Finding a mature individual in the coast of Alabama implies this species has a continuous distribution throughout the northern Gulf of Mexico. The mitochondrial genome of L. nicholsi is 16,416bp in length and comprised of 13 protein coding genes, 22 transfer RNA genes, two ribosomal RNA genes, and a non-coding D-loop. Comparisons using COI support the species is L. nicholsi and separate it from other cryptobenthic fishes found in the area. This study represents the first mitochondrial genome for this L. nicholsi, serving as a reference for future comparative studies with marine fishes. By reporting the range expansion of this species, this study provides insights on the fish diversity of the Gulf of Mexico.

Fourteenth Colony: The Forgotten Story of the Gulf South During America's Revolutionary Era by Mike Bunn

Reviewed by: Fourteenth Colony: The Forgotten Story of the Gulf South During America's Revolutionary Era by Mike Bunn Timothy C. Hemmis Fourteenth Colony: The Forgotten Story of the Gulf South During America's Revolutionary Era. By Mike Bunn. Montgomery: NewSouth Books, 2020. 288 pp. $28.95. ISBN 978-1-5883-8413-3. Mike Bunn's Fourteenth Colony: The Forgotten Story of the Gulf South During America's Revolutionary Era paints a vibrant portrait of the brief history of British West Florida. It is a much-welcomed narrative of the rise and fall of the British in the Gulf South. Often in the story of American history, the events in the Gulf South are an afterthought; however, in this well-researched and well-written account Bunn traces the simple origins of British West Florida to its eventual fall in 1780. Many people are well aware of the thirteen colonies that would eventually become the United States of America. However, many often do not realize there were other British colonies in the New World. Even though it is hotly debated among colonial scholars as to which colony held the title of the fourteenth, Bunn argues that British West Florida fits the description because it was "well used in literature" at the time of the American Revolution (xii). Despite the odd scholarly debate, it does not really affect the history of the Gulf South and its importance to the story of America. Bunn structures his book around the humble beginnings of British West Florida, specifically focusing on Pensacola and Mobile. Early on, West Florida was not a popular colony for the British, but it had immense commercial potential as it was near the lands of the Creek and Choctaw tribes, and that Native American trade sustained the colonial economy. Bunn points out that few British migrants moved to the Gulf South because of its inhospitable weather and tropical diseases. The first settlers tended to gravitate toward ports on the Gulf [End Page 330] of Mexico, like Mobile or Pensacola. But he notes that there were many land claims and grants in the region, especially near the Mississippi River. For example, he tells the story of Anthony Hutchins, who moved his family from the Carolinas to settle in West Florida near Natchez in hopes of obtaining farming lands for a large plantation (77). He demonstrates that Natchez was not a settled metropolis, but rather a frontier outpost that had the appeal of being near the Mississippi River and fertile lands. Despite these positive attractions, Bunn points out that the population of West Florida was relatively small with about 6,000 people in 1779 (83). The author examines daily life in the colony, which included the difficulties of contending with a wide variety of tropical diseases. West Florida became a notoriously hazardous environment to live in, as dangers lurked around every corner. Although this reviewer would have liked more discussion of hurricanes and how they affected the daily life of the colony, this section was enlightening as the environment is often a forgotten aspect of the colonial experience. Bunn also highlights the commercial life in West Florida and explains the agricultural practices of the region. In this chapter, the author mentions the role of slaves in the colony, which has often been overlooked. Additionally, Bunn correctly suggests that the cattle industry flourished in West Florida. Bunn describes Dauphin Island---which today is a popular vacation destination for Alabama beachgoers---as "one big cattle pen" during the colonial period (116). The most familiar part of Bunn's narrative were the chapters on the Revolutionary War. The infamous Willing Raid was a key part of Bunn's narrative, as it was the only time American patriots made an invasion of West Florida. The author demonstrates that this operation was more of a personal vendetta than one of any military importance. The objective of the Willing expedition was one of nuisance and commercial disruption, and it only rallied settlers against the American cause. However, the heart of the book really focuses on Spanish governor of Louisiana Bernardo de Gálvez's military campaign against the British. Here the narrative seems to be more [End Page 331] from...

Developing bare-earth digital elevation models from structure-from-motion data on barrier islands

Unoccupied aerial systems can collect aerial imagery that can be used to develop structure-from-motion products with a temporal resolution well-suited to monitoring dynamic barrier island environments. However, topographic data created using photogrammetric techniques such as structure-from-motion represent the surface elevation including the vegetation canopy. Additional processing is required for estimating bare-earth elevation, which is critical for understanding the underlying geomorphology of these islands. In this study, we used a vegetation and elevation survey to produce bare-earth digital elevation models from structure-from-motion-derived elevation products for two sites on Dauphin Island, Alabama (USA). One site was exposed to high wave energy and included a mix of beach, dune, and barrier flat habitats that were dominated by supratidal/upland herbaceous vegetation. The second site was exposed to low wave energy and was dominated by intertidal marsh. Aerial imagery was collected in late fall of 2018 and spring of 2019. We tested several machine learning algorithms for predicting and removing elevation bias for vegetated areas using predictors that included spectral indices from unoccupied aerial systems-based multispectral imagery and landscape position information (e.g., relative topography and distance from shore). Models were developed for each site and season. We also explored how well the model from one season generalized to data from a different season for the same site. For developing initial digital surface models, we found that utilizing a minimum bin algorithm, as opposed to interpolation, led to lower elevation bias. For bias removal, Gaussian process regression performed the best and led to a root mean square error for the bare-earth digital elevation models of around 0.10 m for the high energy site and 0.15 m for the low energy site. Compared to the digital surface models, the root mean square error for the bare-earth digital elevation models was reduced by at least 29 percent for the high energy site and 69 percent for the low energy site. For all models, common predictors included surface elevation, vegetation greenness, and distance from the shoreline. The models produced comparable results when trained using data from a different season. The error estimates for all analyses were within published elevation standards for lidar data for vegetated areas. With calibration, this approach could be portable to other areas or data, such as aerial lidar (conventional or unoccupied), to provide an efficient and repeatable framework for monitoring geomorphology or provide baseline elevations for predicting changes to these environments under future conditions.

Storm-Driven Fresh Submarine Groundwater Discharge and Nutrient Fluxes From a Barrier Island

Quantifying and characterizing groundwater flow and discharge from barrier islands to coastal waters is crucial for assessing freshwater resources and contaminant transport to the ocean. In this study, we examined the groundwater hydrological response, discharge, and associated nutrient fluxes in Dauphin Island, a barrier island located in the northeastern Gulf of Mexico. We employed radon (222Rn) and radium (Ra) isotopes as tracers to evaluate the temporal and spatial variability of fresh and recirculated submarine groundwater discharge (SGD) in the nearshore waters. The results from a 40-day continuous 222Rn time series conducted during a rainy season suggest that the coastal area surrounding Dauphin Island was river-dominated in the days after storm events. Groundwater response was detected about 1 week after the precipitation and peak river discharge. During the period when SGD was a factor in the nutrient budget of the coastal area, the total SGD rates were as high as 1.36 m day–1, or almost three times higher than detected fluxes during the river-dominated period. We found from a three-endmember Ra mixing model that most of the SGD from the barrier island was composed of fresh groundwater. SGD was driven by marine and terrestrial forces, and focused on the southeastern part of the island. We observed spatial variability of nutrients in the subterranean estuary across this part of the island. Reduced nitrogen (i.e., NH4+ and dissolved organic nitrogen) fluxes dominated the eastern shore with average rates of 4.88 and 5.20 mmol m–2 day–1, respectively. In contrast, NO3– was prevalent along the south-central shore, which has significant tourism developments. The contrasting nutrient dynamics resulted in N- and P-limited coastal water in the different parts of the island. This study emphasizes the importance of understanding groundwater flow and dynamics in barrier islands, particularly those urbanized, prone to storm events, or located near large estuaries.

A Subtle Beyond


 
 
 In July 2013, mother and daughter Lihie and Ruti Talmor travelled to Alabama, Mississippi, and Tennessee, walking in the footsteps of a young, not- yet-famous Elvis Presley. The journey was a quest for something much larger than him. Using Roland Barthes’ notion of the punctum, Ruti investigates how Lihie Talmor’s photogravure of Dauphin Island shows us how the layering of time and loss creates the meaning of place.
 
 

Environmental factors and occurrence of horseshoe crabs in the northcentral Gulf of Mexico.

This study provides regional-scale data on drivers of horseshoe crab (Limulus polyphemus) presence along the northcentral Gulf of Mexico coast and has implications for understanding habitat suitability for sparse horseshoe crab populations of conservation concern worldwide. To collect baseline data on the relationship between environmental factors and presence of horseshoe crabs, we surveyed four sites from the Fort Morgan peninsula of Mobile Bay, Alabama (AL) to Horn Island, Mississippi (MS). We documented number, size and sex of live animals, molts, and carcasses as metrics of horseshoe crab presence and demographics for two years. Data were compared to in situ and remotely sensed environmental attributes to assess environmental drivers of occurrence during the time of study. Overall, greater evidence of horseshoe crab presence was found at western sites (Petit Bois and Horn Islands) compared to eastern sites (Dauphin Island, Fort Morgan peninsula), mediated by a combination of distance from areas of high freshwater discharge and interannual variation in weather. Higher sex ratios also were found associated with higher occurrence, west of Mobile Bay. Land cover, particularly Bare Land and Estuarine Emergent Wetland classes that are common to western sites, was most predictive of live animal and to some extent carcass occurrence. Our findings suggest that small-scale variation in habitat quality can affect occurrence of horseshoe crabs in sparse populations where density is not a limiting factor. Data from molts and carcasses were informative to supplement live animal data and may be useful to enhance ecological assessment and support conservation and management in regions with sparse populations.

Development and Application of an Empirical Dune Growth Model for Evaluating Barrier Island Recovery from Storms

Coastal zone managers require models that predict barrier island change on decadal time scales to estimate coastal vulnerability, and plan habitat restoration and coastal protection projects. To meet these needs, methods must be available for predicting dune recovery as well as dune erosion. In the present study, an empirical dune growth model (EDGR) was developed to predict the evolution of the primary foredune of a barrier island. Within EDGR, an island is represented as a sum of Gaussian shape functions representing dunes, berms, and the underlying island form. The model evolves the foredune based on estimated terminal dune height and location inputs. EDGR was assessed against observed dune evolution along the western end of Dauphin Island, Alabama over the 10 years following Hurricane Katrina (2005). The root mean square error with EDGR (ranging from 0.18 to 0.74 m over the model domain) was reduced compared to an alternate no-change model (0.69–0.96 m). Hindcasting with EDGR also supports the study of dune evolution processes. At Dauphin Island, results suggest that a low-lying portion of the site was dominated by overwash for ~5 years after Katrina, before approaching their terminal height and becoming growth-limited after 2010. EDGR’s computational efficiency allows dune evolution to be rapidly predicted and enables ensemble predictions to constrain the uncertainty that may result if terminal dune characteristics are unknown. In addition, EDGR can be coupled with an external model for estimating dune erosion and/or the long-term evolution of other subaerial features to allow decadal-scale prediction of barrier island evolution.

Sensitivity of Storm Response to Antecedent Topography in the XBeach Model

Antecedent topography is an important aspect of coastal morphology when studying and forecasting coastal change hazards. The uncertainty in morphologic response of storm-impact models and their use in short-term hazard forecasting and decadal forecasting is important to account for when considering a coupled model framework. This study provided a methodology to investigate uncertainty of profile response within the storm impact model XBeach related to varying antecedent topographies. A parameterized island Gaussian fit (PIGF) model generated an idealized baseline profile and a suite of idealized profiles that vary specific characteristics based on collated observed LiDAR data from Dauphin Island, AL, USA. Six synthetic storm scenarios were simulated on each of the idealized profiles with XBeach in both 1- and 2-dimensional setups and analyzed to determine the morphological response and uncertainty related to the varied antecedent topographies. Profile morphologic response tends to scale with storm magnitude but among the varied profiles there is greater uncertainty in profile response to the medium range storm scenarios than to the low and high magnitude storm scenarios. XBeach can be highly sensitive to morphologic thresholds, both antecedent and time-varying, especially with regards to beach slope.

First Pliocene fish otolith assemblage from the Gulf Coastal Plain, Dauphin Island, Mobile County, Alabama, USA

ABSTRACT The first Pliocene teleostean otoliths from the Gulf Coastal Plain are described from Dauphin Island, Mobile County, Alabama, USA. The 250 otolith specimens, mainly sagittae, were obtained from shell hash dredged from an offshore Pliocene unit deposited onshore as part of recent beach nourishment. Analysis of the otoliths revealed 22 species from 11 teleostean families. Sciaenid fishes dominate the assemblage and account for 79.6% of the total specimens, while the other 10 families (including indeterminates) comprise approximately 20% of the total. Comparison to modern biological and ecological requirements for the fishes represented by the otoliths suggest a shallow (0–20 m) neritic environment with some brackish and rare deep-water influence. Several of the most abundant taxa indicate shallow, soft bottom estuarine habitat with upstream oligohaline creeks (0.5 to 5.0 ppt salinity) serving as primary nursery area. However, the percentage of marine-only species may indicate more interaction with open estuary and shallow coastal areas and subtropical waters. The lack of invertebrate settlement on the otoliths likely indicates limited surface residence-time. All 22 taxa recovered from the Dauphin Island site are highly significant since they represent the first Pliocene occurrences of these taxa within the Gulf Coastal Plain of North America.

Estimating blue crab (Callinectes sapidus) larval release sites in the Gulf of Mexico using an oceanographic particle-tracking model

Small, widely-dispersing, pelagic larvae are difficult to monitor through direct observation, but biophysical models can provide greater understanding of dispersal patterns. Here, we used a technique of reversing time in a biophysical model to estimate larval release sites of the blue crab, Callinectes sapidus, in the northern Gulf of Mexico by backtracking from known locations of postlarval settlement. We modeled surface ocean movement in reverse time from four settlement sites (Grand Isle, Louisiana; Ocean Springs, Mississippi; Dauphin Island, Alabama; and Pensacola, Florida). In the Gulf of Mexico, blue crab fisheries are managed at the state level. We found on average 73% of the larvae were traced back to larval release sites in the same state as settlement. Barrier island release sites accounted for 11% of settlement at Grand Isle and 17% at Dauphin Island. Less than 0.1% of larvae that settled in Grand Isle were tracked back to a known blue crab spawning area located to the south west, Ship Shoal. The distance traveled by larvae was highest in Pensacola and Grand Isle. The coefficient of variation of distance traveled was highest in Dauphin Island. The average distance traveled to each settlement location ranged from 31 to 124 km. Distance traveled explained 24% of empirical settlement numbers (F1,89 = 28.82, P < 0.001;R2 = 0.24). Settlement success was higher at release sites predicted by this study than at randomly generated sites suggesting female selection of larval release sites.

Geomorphic changes measured on Dauphin Island, AL, during Hurricane Nate

Storm surge and waves from Hurricane Nate in 2017 resulted in large overwash and inundation regions on Dauphin Island, Alabama. The overwash event consisted of the transport of water and sediment over the beach, dune, and barrier island system. Seven transects were established to measure pre- and post-storm survey profiles. Nine wave and water level sensors were deployed in an overwash region and captured the overwash conditions including time-varying water levels and waves. All transects experienced a net loss of sediment from the subaerial region surveyed and a range of inundation and sediment overwash. The results highlight the limits of empirical estimates for evaluating the exposure of backdune regions to overwash and inundation.

Estimation and validation of sediment properties by normal-incidence echosounder

Broad implementation of normal-incidence echo sounding has generated a need for sediment characterization models. These models may utilize the magnitude, phase, and/or envelope shape of reflected acoustic waveforms to infer physical, geoacoustic, and engineering properties of ocean sediment. Normal-incidence echo soundings enable the remote survey of vast underwater areas at a fraction of the cost of ground truth sampling methods, i.e., cores and grabs. In collaboration with the Dauphin Island Sea Lab, acoustic data was collected in the Petit Bois Pass off of the Alabama coast. The magnitude, pulse width, and skew ratio of the matched filter output were measured and quantified as bottom loss, full-width at half-max, and the ratio of half-width at half-max, respectively. Bottom loss measurements were used to determine the index of impedance which, through the application of empirically based regression equations, was then used to calculate sound speed ratio, density, mean grain size, and porosity of the sediment along the survey track. The estimated values of sediment properties are generally in agreement with those measured in the laboratory by ground truth survey. [Work supported by the Office of Naval Research, Ocean Acoustics.]Broad implementation of normal-incidence echo sounding has generated a need for sediment characterization models. These models may utilize the magnitude, phase, and/or envelope shape of reflected acoustic waveforms to infer physical, geoacoustic, and engineering properties of ocean sediment. Normal-incidence echo soundings enable the remote survey of vast underwater areas at a fraction of the cost of ground truth sampling methods, i.e., cores and grabs. In collaboration with the Dauphin Island Sea Lab, acoustic data was collected in the Petit Bois Pass off of the Alabama coast. The magnitude, pulse width, and skew ratio of the matched filter output were measured and quantified as bottom loss, full-width at half-max, and the ratio of half-width at half-max, respectively. Bottom loss measurements were used to determine the index of impedance which, through the application of empirically based regression equations, was then used to calculate sound speed ratio, density, mean grain size, and porosity of the se...

Variability of the geoacoustic properties of infauna-bearing marine sediments

Infauna influence sediment physical parameters, such as porosity, grain size, and pore fluid properties, through bioturbation and other biological processes. To investigate the effects of the resulting sediment inhomogeneity, in situ measurements of compressional and shear wave propagation were acquired in Petit Bois Pass, west of Dauphin Island, Alabama, USA. After completion of the acoustic measurements, diver core samples were collected from between the sources and receivers to characterize the physical properties of the seabed along the acoustic propagation paths. The collected sediments were analyzed for infauna biomass and geotechnical properties to support acoustic propagation modeling. The acoustic data are analyzed in the context of infaunal functional groups, which are based on acoustically significant traits such as body type (hard/soft and large/small) and infaunal activity (sediment mixing or tube building). While existing sediment acoustics models do not explicitly account for effects of biology, they do allow for parameterization of various physical properties, which can be modified by the presence of biological organisms and bioturbation. Results from the in situ acoustic measurements and core analysis will be compared with such models to gain insight into the acoustical effects of infauna. [Work supported by ONR.]

Late Quaternary evolution and stratigraphic framework influence on coastal systems along the north-central Gulf of Mexico, USA

Coastal systems in the Gulf of Mexico are threatened by reduced sediment supply, storm impacts and relative sea-level rise (RSLR). The geologic record provides insight into geomorphic evolution thresholds to these forcing mechanisms to help predict future barrier evolution in response to climate change. This study synthesizes ∼2100 km of geophysical data, 700 + sediment cores, and 62 radiocarbon dates to regionally map two lowstand sequence boundaries, multiple ravinement surfaces and fourteen depositional facies demonstrating stratigraphic and antecedent topographic influences on coastal evolution. The Mississippi-Alabama (MSAL) barriers are anchored by a marine isotope stage (MIS) 5e section of Dauphin Island coupled with an MIS 2 surface gradient change. Sand for the modern MSAL barriers were largely sourced through Holocene transgressive ravinement of relict valley fill deposits, providing up to 300 × 106 m3 of sand. Mud-filled MIS 2 tributaries correspond to areas of repeated storm breaching or tidal inlets.A Holocene geomorphic evolutionary model was created for Petit Bois and Dauphin Islands, highlighting RSLR rates, changes in sediment supply and the antecedent geologic framework. As the MIS 2 surface was flooded, tidal/wave scour supplied sand to migrating marine shoals. These transgressing shoals converted drowned paleovalleys to estuaries ∼9ka BP. Islands formed in their modern positions ∼6ka BP, when sediment supply was high and RSLR rates were 2 mm/yr. Between ∼4ka-1750 CE, islands prograded from reduced RSLR rates of 1-0.4 mm/yr and sufficient sand supply from alongshore/inner shelf sources. Currently, the islands experience 3.74 mm/yr of RSLR and reduced sediment supply, resulting in barrier degradation.