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Previous work identified hurricane deposits in the back-barrier lagoon of Laguna Playa Grande (LPG) in Vieques, Puerto Rico, dating back over 5000 years, with periods of increased storm-induced overwash activity attributed to variability in regional hurricane climatology. In 2017, Hurricane Maria made direct landfall on LPG just below category 5 strength, providing the opportunity to revisit the site to improve upon interpretations of storm-induced deposition. Maria caused widespread wave-induced overwash of the barrier beach and extensive mangrove mortality with roughly a 40% reduction in vegetative cover along the barrier. Sediment trapping and overwash deposition occurred within ripped up and broken mangrove debris during the hurricane, which prevented sediments from being carried further landward into the lagoon. Thus, no measurable overwash deposition was observed in the larger, western portion of the lagoon where previous hurricane reconstructions are derived. Significant overwash deposition (1–27 cm thick) was observed in the smaller, eastern portion of the lagoon where human cut paths through the mangroves allowed for unobstructed flow. Early historical photos support 53% reduced vegetation in 1936 followed by revegetation towards present day that prevented sandy overwash deposits from Maria in the western side, explaining the discrepancy between previously observed hurricane overwash record compared to restricted deposition solely to locations with modern footpaths for Maria. Hurricanes occurring during the recovery-revegetation phase of such storms likely result in greater overwash deposition in back-barrier lagoons due to less barrier vegetation relative to periods with robust mangrove vegetation cover. Results highlight the important role of fringing mangrove forests in flood mitigation, and the vulnerability of back-barrier environments to enhanced flooding following both anthropogenic and event-driven vegetation loss. [ABSTRACT FROM AUTHOR]

The marginal ice zone (MIZ) is the area between sea ice and open water, the structure of which is mainly determined by wave and ice interactions. Thus mastering the characteristics of MIZ is of great significance to the Arctic routes opening and the natural resources development. In this paper, the hydroelastic response of ice floes in waves is studied, a three-dimensional numerical wave tank is established based on the computational fluid dynamics technology. The finite volume method and finite element method are respectively utilized for the discrete fluid domain and ice domain. A mapping interface at the junction of the fluid and ice floes domains is created to perform data mapping by the shape function interpolation method and the least square method. This work presents a series of numerical simulations to study the fluid-solid interaction of waves and ice floes. Under the given incident wave parameters, the vertical bending deformation of ice floes with different shapes under the excitation of waves, the effect of ice floes' deformation on the wave field are studied, and the effect of wave overwash on the transmitted wave field is emphasized. Results show that the shape of the ice floes significantly affects its elastic deformation and scattered wave field, and the wave overwash phenomenon attenuates the scattering wave. [ABSTRACT FROM AUTHOR]

Segmented barrier islands can be found in regions with small tidal ranges. In contrast to tidally dominated barriers, where inlet dynamics are thought to control island length scales, the controls on barrier island length scales in wave‐dominated environments have not been quantified. These microtidal barriers typically have a curved shoreline, suggesting the influence of wave‐driven alongshore sediment transport. Microtidal barriers are also typically hydrodynamically isolated from one another, as weak tidal flows limit interactions between adjoining barriers. To better understand the controls on and scales of barrier segmentation in the relative absence of tides, here we develop a theoretical framework to estimate the alongshore length scales at which a barrier will either breach or heal following a disturbance in the barrier morphology. The non‐dimensional framework compares the timescales of overwash (advective) and alongshore sediment transport (diffusive) processes along barrier island chains. We then apply this framework to modern barrier islands in the microtidal Gulf of Mexico using wave hindcast data and the lengths, widths, heights, and lagoon depths measured from remotely sensed geospatial data and topobathymetric data. We find that most of these barriers are currently longer than their critical length scale, often as a result of coastal restoration efforts. Our critical length scale analysis suggests that most of the Gulf of Mexico barriers are vulnerable to segmentation despite coastal restoration efforts intended to protect fisheries and the mainland coasts. Plain Language Summary: Barrier islands provide valuable protection for mainland communities, marine fisheries, and coastal ecology. Restoration efforts are frequently enacted to preserve barrier islands, particularly after storm‐induced breaches segment a barrier into two or more shorter islands. Stable barrier island length scales have been predicted in environments with large tides, where barriers are separated by inlets; however, the characteristic stable length of barriers has not been quantified for environments with small tidal ranges. In these micro‐tidal environments, barriers are primarily shaped by two mechanisms of sediment transport: (a) overwash, which moves sediment from the nearshore environment on top of and behind barriers and (b) wave‐driven alongshore sediment transport, which smooths the shoreline. Here, we present a theoretical framework to evaluate the ability of barrier islands to heal when breached, based on a comparison of timescales for barrier rollover by overwash and shoreline smoothing through alongshore sediment transport. We then apply this analysis to barrier islands in the Gulf of Mexico, finding that many of these barriers are overly long, which suggests that they are vulnerable to segmentation despite (or perhaps due to) coastal restoration. Key Points: We assess the vulnerability of micro‐tidal barriers to segmentation by comparing the timescales of island rollover and alongshore diffusionThe comparison defines a critical length scale over which barriers should be vulnerable to segmenting into smaller islandsMany Gulf of Mexico islands are longer than predicted critical length scales—these long islands tend to be anthropogenically modified [ABSTRACT FROM AUTHOR]

Williams, H.F.L., 2024. Testing digital elevation model of difference (DoD) estimation of washover fan thickness, Matagorda Peninsula, Texas. Journal of Coastal Research, 40(5), 852–859. Charlotte (North Carolina), ISSN 0749-0208. Washover fans are a fundamental component of coastal sediment budgets. While fans can be relatively easily delineated on air photos to obtain fan area, measurement of fan thickness, which is required for volume estimation, is problematic. Field surveys can be conducted, but fieldwork can be labor-intensive, costly, and time-consuming and have access problems. This study used pre- and poststorm LIDAR digital elevation models (DEMs) to create a digital elevation model of difference (DoD), which was then used to estimate the mean thicknesses of 10 Hurricane Harvey washover fans on Matagorda Peninsula, Texas. The accuracy of the LIDAR-derived fan thicknesses was assessed by comparing the LIDAR results to fan mean thicknesses based on pits excavated into fans. Seven out of 10 LIDAR-based thicknesses underestimated pit-based thicknesses, suggesting the presence of systematic bias in the LIDAR-derived DEMs. Underestimation of fan thicknesses was in the range of 36% to 56%. The source of the suspected bias is uncertain; it is possible that compaction of marsh sediments by the washover fans lowered the fans and reduced their apparent thicknesses. It is concluded that pre- and post-storm LIDAR DEMs can be used to estimate washover fan thickness, but in areas of compactible substrates, such as coastal marshes, steps should be taken to identify, evaluate, and address potential bias in the DEM data. [ABSTRACT FROM AUTHOR]

In this paper, the ditching performance of a seaplane model on calm water and a uniform water current coupled with wind was numerically investigated. The overset grid technique was applied to treat the large amplitude of the body motions of the seaplane without leading to mesh distortion. The effects of the initial velocity and the initial pitch angle on the slamming loads and motion responses were investigated for the seaplane's ditching on calm water. A good agreement with the experimental data on the velocity and angle was obtained. Besides ditching on calm water without the water current and wind, three more-complicated conditions were adopted, including the seaplane's ditching on calm water with wind, a water current without wind, and a water current coupled with wind. The accelerations and impact pressures of the seaplane can be influenced by the wind or current. Water splashing and overwashing could be observed during the water entry process, with water overtopping the seaplane head or nose and flowing over the body surface. It can be concluded that the relative motion between the water and the seaplane model should be carefully controlled to avoid possible damages caused by the occurrence of overwashing. [ABSTRACT FROM AUTHOR]

Constant changes occur in coastal areas over different timescales, requiring observation and modeling. Specifically, modeling morphological changes resulting from short-term events, such as storms, is of great importance in coastal management. Parameter calibration is necessary to achieve more accurate simulations of process-based models that focus on specific locations and event characteristics. In this study, the XBeach depth-averaged model was adopted to simulate subaerial data pre- and post-storms, and overwash phenomena were observed using the data acquired through unmanned aerial vehicles. The parameters used for the model calibration included those proposed in previous studies. However, an emphasis was placed on calibrating the parameters related to sediment transport that were directly associated with overwash and deposition. Specifically, the parameters corresponding to the waveform parameters, wave skewness, and wave asymmetry were either integrated or separated to enable an adequate representation of the deposition resulting from overwash events. The performance and sensitivity of the model to changes in volume were assessed. Overall, the waveform parameters exhibit significant sensitivity to volume changes, forming the basis for calibrating the deposition effects caused by overwashing. These results are expected to assist in the more effective selection and calibration of parameters for simulating sediment deposition due to overwash events. [ABSTRACT FROM AUTHOR]

Although sandy barriers are a key component of many Great Lakes coastal wetlands, little is known about their geomorphic development in response to water-level changes, despite widespread implications for hydrology and ecology. This paper addresses the morphodynamic development of the Sandy Pond barrier, eastern Lake Ontario, which underwent a sudden change in base-level regime with the onset of regulatory practices. The creation of new inlets with coeval closing of pre-existing ones, which had previously been coupled to shifts in base water level > 1 m, has not occurred since restriction of the natural lake-level oscillatory range. Barrier breaching is facilitated by decadal lake-level highs and inlet stability by entrenchment during lake-level fall and lowstand conditions. Water-level extremes have been absent since the 1970s, with stunting of the decadal oscillatory change signature (to < 1 m of variance) and a mean level above that of the pre-regulatory period. Rather than persistence of the cyclic dynamics of barrier-breaching and channel entrenchment (associated with abandonment elsewhere), the new conditions promoted lateral migration of the present channel and the formation of recurved beach ridges along the updrift inlet side. This morphology is not recognized in relict depositional architectures associated with former known inlet positions. While estuarine flushing capacity continues to be maintained (by constant inlet-channel dimensions), there are implications of this new geomorphic trajectory for barrier-landform succession and ecology. [ABSTRACT FROM AUTHOR]

Heffentrager, M.L.; Wasklewicz, T.; Sirianni, H.; Gares, P.; Kernstine, L., and Richter, J., 2023. Reproducibility in coastal physical laboratory experiments: Washover deposits and channel morphology. Journal of Coastal Research, 39(5), 823–835. Charlotte (North Carolina), ISSN 0749-0208. The reproduction of physical laboratory experiments within the field of geomorphology does not occur often, despite the call to reproduce natural landscape experiments as a means of more accurately defining geomorphic changes across a variety of environmental settings. Reproducing published results not only supports prior findings but also builds evidence on geomorphic process across laboratory and natural scales. Overwash, the run-up of ocean water and sediment over a sandy barrier, is both a hazard when it impacts coastal properties and infrastructure and a natural resilience factor as it benefits sustained long-term evolution of the barrier island systems. Here, a coastal physical laboratory experiment that simulates overwash and resulting washover deposits is reproduced. The research objective is to expand the current knowledge of how these deposits form under different laboratory settings. Understanding the influence of the dimensions of a physical laboratory experimental table, sediment variations, and back barrier surfaces can give support to researchers planning future physical coastal laboratory experiments, especially those involving washover deposit formation. The results show that final washover deposit morphologies can be reproduced from similar experimental designs. Variations between the experiments resulted from confining boundaries of experimental tables. The refinement of coastal laboratory experiments, in tandem with fieldwork, enhances the understanding of overwash processes for both scientific research and applied scientific approaches. [ABSTRACT FROM AUTHOR]

Since the field measurement of wave overwash is difficult to obtain during severe storms, a laboratory experiment is conducted to measure the time and space evolution of the overwash and beach profile over the sand dune continuously using high‐speed video image analysis for different wave conditions of irregular waves and solitary waves. Characteristics of the cross‐shore sediment transport and morphology change are discussed from the laboratory experiment data. The inflection point, which marks the shifting turn in dune crest movement during overwash, is determined by analyzing feature points in the variation of dune geometry. It is confirmed that the overwash water volume rate reaches a maximum value when the inflection point occurs. Additionally, the overwash water volume is found to be linearly related to the deposited sediment volume, and the dune erosion volume is proportional to wave energy flux during the overwash process. The presence of secondary dunes is found to significantly alleviate coast erosion during overwash and reduce shoreline retreat by more than 40%, regardless of the height of the secondary dune crest. Furthermore, the advances and limitations in overwash hydrodynamics are discussed. It is recommended that this continuous morphological change monitoring data be combined with a wave resolution model for detailed hydrodynamic simulations to further understand the overwash process. Plain Language Summary: Beach erosion and land inundation occur when wave runup exceeds the crest of dunes during extreme natural hazards. Dunes play a crucial role in coastal defense by reducing sediment loss and maintaining beach stability. To understand the hydrodynamic conditions and morphology change during the overwash process, we set up a dune‐beach experiment in a laboratory flume. With the support of high‐speed camera technology, this study examines the response of dunes during the overwash process and the changes in dune feature points. It is pointed out that dune erosion shows a relationship with wave energy flux and is consistent with shoreline recession. Moreover, secondary dunes can effectively reduce coastal erosion during overwash. Key Points: The experiment reveals that the inflection point in dune crest movement during overwash corresponds to the maximum overwash water rateOverwash water volume is linearly related to deposited sediment volume, while dune erosion volume is proportional to wave energy fluxThe presence of secondary dunes significantly reduced coast erosion during overwash, reducing shoreline retreat by over 40% [ABSTRACT FROM AUTHOR]

Coastal barrier systems are low‐lying environments that bear the brunt of storm impacts, with cumulative and complex consequences for barrier evolution. Most studies of barrier responses to storms examine what happens when water and sediment get driven landward across a barrier from its ocean side. Sherwood et al. (2023, https://doi.org/10.1029/2022jf006934) investigate the effects of overland flow and sediment transport forced across a barrier in the opposite direction—from its sheltered side, seaward. Using high‐resolution imagery of a barrier island observed before and after a hurricane, Sherwood et al. (2023, https://doi.org/10.1029/2022jf006934) show that "outwash" flow across the barrier shifted several times more sediment by volume than is typically reported for beach and dune erosion from onshore forcing. Their findings are remarkable because they are not exceptional: a related survey of barriers along the Atlantic and Gulf Coasts of the USA observed patterns of outwash morphology essentially everywhere. Insights into outwash morphology open exciting questions regarding the overlooked role of storm‐driven seaward sediment transport in barrier dynamics, with important implications for post‐storm barrier recovery and barrier evolution over decades to centuries. Plain Language Summary: Coastal barrier systems—beaches backed by dunes, marshes, and often a lagoon or sound—offer natural protection to coastal floodplains by absorbing some of the physical impacts of storms, including hurricanes. Most studies of storm impacts on coastal barriers investigate changes that occur when a storm drives water and sediment landward, over a barrier from its ocean‐facing side. But storms also drive sediment‐laden flows in the opposite direction: over the back of a barrier from its more sheltered side, toward the ocean. Here, I highlight work by Sherwood et al. (2023, https://doi.org/10.1029/2022jf006934) that details extensive patterns of erosion and accretion from "outwash" flow seaward over a barrier during a hurricane. Added up over time, sediment shifting landward and seaward across a barrier shapes the barrier landscape and affects how the barrier may respond to future storm impacts. Sherwood et al. (2023, https://doi.org/10.1029/2022jf006934) offer findings with important implications for understanding how coastal barriers evolve in space and time, and address a question that is fundamental to many geomorphic systems: when a large volume of sediment moves during an extreme weather event, where does it all go? Key Points: Sediment transport seaward during storms remains largely overlooked in coastal researchSeaward outwash may shift more sediment by volume than onshore‐forcing processes of beach and dune erosionThe complex relationship between storm‐driven seaward transport and barrier recovery opens exciting questions regarding barrier evolution [ABSTRACT FROM AUTHOR]

As sea-level rise (SLR) and human-made interventions affect coastal currents and sediment transport, coastal barriers have become more vulnerable to the effect of storms, hurricanes, and climate variability. The response of each barrier is unique and depends on wave regime, coastline orientation, weather conditions, bathymetry, and type of human-made interventions, among other factors. In the Magdalena River deltaic barrier, located on the Colombian Caribbean coast, coastal erosion has caused the loss of hundreds of square kilometers of critical ecosystems, such as wetlands and lagoons, since the 1960s. This work aims to analyze the short-term drivers behind the observed loss of lagoons, particularly the drivers of lagoon breaching events and subsequent healing along the deltaic barrier. Lagoon breaching events and healings were detected using satellite imagery, and the timing of these events was related to prior local atmospheric, oceanographic, and fluvial conditions. The findings reveal that the dynamics of the lagoons are driven by extreme river discharges and energetic wave conditions associated with storms or hurricanes. Healing is driven by the sediment supplied by littoral currents and average waves punctuated by energetic events. The cumulative effect of breaching and healing has resulted in a deltaic barrier that has rolled over the lagoons, reducing their size over time. These findings provide a better understanding of the forces of coastal retreat and will help inform future management decisions of the coastal zone. [ABSTRACT FROM AUTHOR]

In this paper, the ditching performance of a seaplane model on calm water and a uniform water current coupled with wind was numerically investigated. The overset grid technique was applied to treat the large amplitude of the body motions of the seaplane without leading to mesh distortion. The effects of the initial velocity and the initial pitch angle on the slamming loads and motion responses were investigated for the seaplane’s ditching on calm water. A good agreement with the experimental data on the velocity and angle was obtained. Besides ditching on calm water without the water current and wind, three more-complicated conditions were adopted, including the seaplane’s ditching on calm water with wind, a water current without wind, and a water current coupled with wind. The accelerations and impact pressures of the seaplane can be influenced by the wind or current. Water splashing and overwashing could be observed during the water entry process, with water overtopping the seaplane head or nose and flowing over the body surface. It can be concluded that the relative motion between the water and the seaplane model should be carefully controlled to avoid possible damages caused by the occurrence of overwashing.

Constant changes occur in coastal areas over different timescales, requiring observation and modeling. Specifically, modeling morphological changes resulting from short-term events, such as storms, is of great importance in coastal management. Parameter calibration is necessary to achieve more accurate simulations of process-based models that focus on specific locations and event characteristics. In this study, the XBeach depth-averaged model was adopted to simulate subaerial data pre- and post-storms, and overwash phenomena were observed using the data acquired through unmanned aerial vehicles. The parameters used for the model calibration included those proposed in previous studies. However, an emphasis was placed on calibrating the parameters related to sediment transport that were directly associated with overwash and deposition. Specifically, the parameters corresponding to the waveform parameters, wave skewness, and wave asymmetry were either integrated or separated to enable an adequate representation of the deposition resulting from overwash events. The performance and sensitivity of the model to changes in volume were assessed. Overall, the waveform parameters exhibit significant sensitivity to volume changes, forming the basis for calibrating the deposition effects caused by overwashing. These results are expected to assist in the more effective selection and calibration of parameters for simulating sediment deposition due to overwash events.

Barrier islands are especially vulnerable to hurricanes and other large storms, owing to their mobile composition, low elevations, and detachment from the mainland. Conceptual models of barrier‐island evolution emphasize ocean‐side processes that drive landward migration through overwash, inlet migration, and aeolian transport. In contrast, we found that the impact of Hurricane Dorian (2019) on North Core Banks, a 36‐km barrier island on the Outer Banks of North Carolina, was primarily driven by inundation of the island from Pamlico Sound, as evidenced by storm‐surge model results and observations of high‐water marks and wrack lines. Analysis of photogrammetry products from aerial imagery collected before and after the storm indicate the loss of about 18% of the subaerial volume of the island through the formation of over 80 erosional washout channels extending from the marsh and washover platform, through gaps in the foredunes, to the shoreline. The washout channels were largely co‐located with washover fans deposited by earlier events. Net seaward export of sediment resulted in the formation of deltaic bars offshore of the channels, which became part of the post‐storm berm recovery by onshore bar migration and partial filling of the washouts with washover deposits within 2 months. This event represents a volumetric setback in the overwash/rollover behavior required for barrier transgression, but the new ponds and lowland habitats may provide beneficial habit for endangered species and will likely persist for years. Plain Language Summary: As sea level rises, barrier islands tend to migrate toward land, helped by storms that move sand from the ocean to the back side. In rarer events, such as Hurricane Dorian (2019), storms can transport sand from the back side to the ocean. Using overlapping photos collected from a plane, we created 3‐D elevation maps and stitched‐together photo mosaics of North Core Banks, North Carolina, immediately before and after Hurricane Dorian, and then once a month for 3 months afterward, to document the erosion and recovery of the beach and barrier island. We found major changes. During the storm, abnormally high water levels in Pamlico and Core Sounds flooded the island and created distinct channels as water drained from the sound side into the Atlantic Ocean. This process, called outwash, moved 18% of the island sand into the ocean over the course of a few hours. The maps also showed the initial stages of a recovery process, as beach sand was moved by ocean waves and plugged the channels, creating new, and semi‐permanent, habitats within the barrier island. Events such as Hurricane Dorian may slow the typical migration process of barrier islands and change the landscape for years to come. Key Points: Wind‐driven surge from Pamlico Sound inundated the barrier island of North Core Banks during Hurricane DorianSeaward‐directed flow (outwash) through gaps in the dunes eroded about 18% of the island volume, mostly from antecedent washover depositsThis event represents a setback for barrier transgression but has created beneficial habitat for endangered species [ABSTRACT FROM AUTHOR]

Overwash is the cross‐shore transport of water and sediment from a waterbody over the crest of a sand or gravel barrier beach, and washover is the resulting sedimentary deposit. Washover volume, and alongshore patterns of washover distribution, are fundamental components of sediment budgets for low‐lying coastal barrier systems. Accurate sediment budgets are essential to forecasting barrier system sustainability under future climate‐driven forcing. However, comprehensive surveys of three‐dimensional washover morphology are challenging to deliver. Here, we use the results of a physical experiment, analysis of lidar data, and examples of washover characteristics reported in the literature to develop scaling relationships for washover morphometry that demonstrate volume can be reasonably inferred from planform measurements, for washover in natural (non‐built) and built barrier settings. Gaining three‐dimensional insight into washover deposits from two‐dimensional information unlocks the ability to analyze past aerial imagery and estimate contributions from washover flux to sediment budgets for past storms. Plain Language Summary: When a coastal storm drives water up and over the crest of a beach, sediment gets picked up by the flow and deposited behind the beach. That deposit, which might settle among dunes, on a marsh, or on the streets of a neighborhood, is called washover. Knowing the total volume of washover delivered by a storm event helps coastal managers account for the amount of sediment that moves through a beach system over time, known as a sediment budget. An accurate sediment budget provides an indication of environmental functioning and sustainability. However, washover volume is expensive to measure over large spatial scales—so despite its importance to sediment budgets, washover volume often goes unquantified. What if three‐dimensional washover volume could be estimated from the two‐dimensional shape and size of washover deposits visible in aerial photos? Here, we created washover deposits in a laboratory and compared them to washover in field settings to show that the area of a washover deposit is a good predictor of its volume. This relationship between area and volume unlocks information about past storm deposition captured in aerial photo archives, and enables estimation of washover volume where direct measurement of volume is unavailable or impossible. Key Points: Washover volume can be estimated from washover areaScaling relationships for washover size and shape link measurements from a physical experiment and field examplesEstimation of washover volume from aerial imagery is a step toward quantifying storm‐driven contributions to sediment budgets [ABSTRACT FROM AUTHOR]

In the marginal ice zone (MIZ), where ocean waves and sea ice interact, waves can produce flows of water across ice floe surfaces in a process known as wave overwash. Overwash potentially influences wave propagation characteristics, floe thermodynamics, and floe surface biological and chemical processes. However, the extent of the MIZ affected by overwash and its dependence on prevailing wave and ice conditions is unknown. In this paper, we propose a model of overwash extent caused by irregular incoming waves into a MIZ consisting of a random floe field. We validate the overwash extent model against laboratory experiments. We use the model to study mild to extreme incoming waves to floe field characteristics of the spring–summer ice retreat and autumn–winter ice advance and with compact ice edges. Overwash is typically predicted to extend a few kilometers and is generally greater for the autumn–winter advance than the spring–summer retreat. The model predictions provide a basis for improved understanding of the impacts of ocean waves on the ice cover. We also apply the model to incoming waves and a floe field with a diffuse ice edge representative of conditions during a field experiment, predicting overwash extents up to 16 km. During the field experiment, the wave and ice floe properties were intermittently monitored by a camera system, demonstrating how the sparse field data available on overwash can be advanced. Plain Language Summary: Wave overwash is the flow of water across the surface of a thin floating body due to passing waves. It is an important process in the marginal ice zone (MIZ) where sea ice floes and ocean waves interact. Overwash reduces the energy of ocean waves, removes snow from, and wets the sea ice surface, which can drive growth or melting. Water left on the sea ice surface can host organisms and affects chemical processes taking place there. Overwash is pervasive in laboratory experiments where it has been measured and studied. In contrast, there are few reports of overwash in the field and the extent of its occurrence has not been measured. We propose a model to predict the region of the MIZ in which overwash occurs. We use the model to compare overwash extents for MIZs associated with the spring–summer ice retreat and autumn–winter ice advance for calm to energetic wave conditions, including the conditions observed during a winter Antarctic icebreaker voyage. For field conditions, the model predicts that overwash typically extends at least several kilometers into the MIZ, indicating that it likely contributes to the ice edge evolution. Key Points: A mathematical model is proposed for the extent of wave overwash of ice floes in the marginal ice zone (MIZ) based on coupled floe–wave motionsFrequent overwash is predicted up to 16 km from the ice edge during a voyage into Antarctic MIZ composed of pancake iceWave overwash is predicted to be a persistent feature of the outermost MIZ, likely contributing to the evolution of the ice edge [ABSTRACT FROM AUTHOR]

As sea-level rise (SLR) and human-made interventions affect coastal currents and sediment transport, coastal barriers have become more vulnerable to the effect of storms, hurricanes, and climate variability. The response of each barrier is unique and depends on wave regime, coastline orientation, weather conditions, bathymetry, and type of human-made interventions, among other factors. In the Magdalena River deltaic barrier, located on the Colombian Caribbean coast, coastal erosion has caused the loss of hundreds of square kilometers of critical ecosystems, such as wetlands and lagoons, since the 1960s. This work aims to analyze the short-term drivers behind the observed loss of lagoons, particularly the drivers of lagoon breaching events and subsequent healing along the deltaic barrier. Lagoon breaching events and healings were detected using satellite imagery, and the timing of these events was related to prior local atmospheric, oceanographic, and fluvial conditions. The findings reveal that the dynamics of the lagoons are driven by extreme river discharges and energetic wave conditions associated with storms or hurricanes. Healing is driven by the sediment supplied by littoral currents and average waves punctuated by energetic events. The cumulative effect of breaching and healing has resulted in a deltaic barrier that has rolled over the lagoons, reducing their size over time. These findings provide a better understanding of the forces of coastal retreat and will help inform future management decisions of the coastal zone.

The morphodynamic functioning of the Sillon de Talbert gravel barrier spit is analysed using a high‐frequency survey carried out between September 2012 and December 2019. It is based on beach profile measurements along two transects, modelling offshore wave data (WW3), tide gauge records, and shallow waves and water levels recorded in the intertidal zone. A barrier retreat of −23 to −30 m over the 7‐year survey (i.e. −3.3 to −4.3 m yr−1) is measured. This retreat is not related to long‐term sea level rise (macroscale of 102–103 yr), but to mesoscale (100–102 yr) morphogenic events combining storm wave and high spring tide. Over 87–90% of the barrier retreat is due to three significant events (1–2 February 2014, 9 February 2016, and 3 January 2018). The storm impact scale model of Orford and Carter is tested. The estimation of the wave runup for the calculation of extreme water levels [i.e. peak overflow elevation (Oe) component] is based on the calibration of an equation performed from in‐situ measurements of the swash elevation. The flow depth (Od,q) overtopping the crest of the barrier (Bh) is thresholded by taking into account the morphological response of the barrier in order to define regimes corresponding to overtopping, discrete overwash, and sluicing overwash. While the Orford and Carter model is generally successful in reproducing the morphodynamic evolution of Sillon de Talbert, the wave energy flux (F) must be considered as an additional parameter in order to improve the fit of the model, so far as it contributes in some cases to change the morphodynamic regime. Thus, the wave energy flux constitutes a key component in the quantification of the water flow across the barrier (Od,q) corresponding to the hydrodynamic forcing of the model, which becomes (Od,F). [ABSTRACT FROM AUTHOR]

Modern microfossil distributions reflect site‐specific habitats and provide an opportunity to assess sediment transport pathways in the nearshore environment. When applied to overwash deposits in the geological record, they provide insight into sediment provenance and transport, factors important for understanding patterns of frequency and intensity of past storms and tsunamis. Modern distribution studies are rare and often the first established ones occur immediately after an overwash event as part of a post‐event field survey. This is problematic because it is unclear what effect overwash events have on nearshore microfossil assemblages and what time interval is necessary for them to return to pre‐event conditions. This study documents the impacts of Hurricane Irma on nearshore sediments off the coast of Anegada, British Virgin Islands, using distributions of Homotrema rubrum, an encrusting foraminifer with a defined provenance in coral reefs. At four sampling intervals spanning two years, from six months pre‐Hurricane Irma to eighteen months after, surface sediment was collected from three transects on the northern and southern shores of the island. Partitioning Around Medoids cluster analysis revealed that Hurricane Irma introduced an influx of well‐preserved fragments into the reef flat and made the sediments more uniform, limiting the foraminifer's utility as a known sediment transport indicator. The mixing of sediments along the two northern transects (reef proximal) persisted for seven to eighteen months before returning to near pre‐hurricane conditions. However, the southern transect (absence of reef), where Homotrema rubrum concentrations are significantly less, failed to recover within the time period assessed by this study, indicating a variable recovery period between Atlantic Ocean and Caribbean Sea facing shorelines. Results from this study suggest that a waiting period of at least eighteen months after a major storm is recommended before collecting surface sediment from the nearshore environments of reef‐dominated coastlines. [ABSTRACT FROM AUTHOR]

Coastal overwash is a natural phenomenon that commonly occurs during storm events and can cause considerable changes in nearshore morphology within a short time. In this study, a complete set of empirical overwash transport algorithms is developed and introduced into a phase-averaged wave-current-sediment transport coupling model that integrates the Finite-Volume Community Ocean Model (FVCOM) and the Simulating Waves Nearshore (SWAN) model. The resulting morphological evolution model can simulate coastal overwash. Validation against the data obtained from multiple sets of laboratory overwash experiments demonstrates that the model performs relatively well in simulating morphological changes caused by runup overwash and inundation overwash under different hydrodynamic and beach profile conditions. The sensitivity of each empirical coefficient in the overwash transport algorithms is comprehensively analyzed. The effects of each coefficient on the output of the model are discussed, and a recommended value range is provided for each coefficient. [ABSTRACT FROM AUTHOR]

Shrubs are common – and presently expanding – across coastal barrier interiors (the land between the foredune system and back‐barrier bay), and have the potential to influence barrier morphodynamics by obstructing cross‐shore overwash flow. The ecological and geomorphological consequences of ecomorphodynamic couplings of the barrier interior, however, remain largely unexplored. In this contribution, we add an ecological module of shrub expansion and mortality to a spatially‐explicit exploratory model of barrier evolution (Barrier3D) to explore the effects of shrub‐barrier feedbacks. In our model simulations, we find that the presence of shrubs significantly alters barrier morphology and behavior. Over timescales of decades to centuries, barriers with shrubs (relative to those without) tend to be narrower, migrate landward more slowly, and have a greater proportion of subaerial volume distributed toward the ocean‐side of the barrier. Shrubs also tend to increase the likelihood of discontinuous barrier retreat, a behavior in which a barrier oscillates between periods of transgression and relative immobility, because shrubs induce prolonged periods of barrier immobility by obstructing overwash flow. However, shrubs can increase barrier vulnerability to drowning by preventing periods of transgression needed to maintain barrier elevation relative to rising sea levels. Additionally, physical barrier processes influence shrub expansion in our simulations; we find that greater dune erosion and overwash disturbance tends to slow the rate of shrub expansion across the barrier interior. Complementing recent observational studies of barrier islands in Virginia, USA, our results suggest that interior ecology can be a key component of barrier evolution on annual to centurial timescales. Plain Language Summary: Shrubs are common on coastal barriers, typically found sheltered behind dunes. During large storms, sand from the beach is washed into the barrier interior and beyond in a process called overwash. Overwash tends to cause the barrier to gain elevation relative to sea level, and also move the whole barrier landward. Shrubs, however, can obstruct this flow across a barrier. Here we explore how shrubs blocking overwash can impact barrier change, and how barrier change affects the growth of shrubs. We do this by adding new code to represent shrubs to an existing computer model of barrier evolution. Results from the computer model suggest that barriers with shrubs are typically narrower and tend to move landward more slowly compared to barriers without shrubs. Additionally, barriers with shrubs are more likely to move landward in a stop‐and‐go manner, switching between periods of fast landward motion and no motion. Therefore, barriers with shrubs are less likely to smoothly and continuously move landward. We also find that shrubs expand more slowly across barriers when storms cause significant dune erosion and overwash disturbance. Our work suggests that including shrubs in barrier models helps us better understand the ecological and geomorphological change of barrier systems. Key Points: Modeled barriers with shrubs tend to be narrower, migrate landward more slowly, and be more strongly skewed volumetrically toward the oceanShrubs increase the likelihood of discontinuous barrier retreat and, under some conditions, increase barrier vulnerability to drowningShrubs expand more slowly across barriers experiencing greater dune erosion and overwash disturbance (and vice versa) [ABSTRACT FROM AUTHOR]

Many of the world's beaches are embayed, but while a large body of work addresses the geomorphology of pocket beaches in oceanic settings, little is known about urban analogs, especially within the Great Lakes of North America. Groins and jetties shelter these systems from direct interaction with littoral processes, which elsewhere can influence how changes in lake level, winter-ice cover, and wave climate impact beach evolution. We address the direct controls of these forcing parameters on beach morphodynamics over a 33-yr period at North Point Beach, which is confined to an engineered 'container' along Lake Michigan's wave-dominated SW margin. Analysis of near-annual beach change suggests lake-level change is the dominant geomorphic driver over inter-annual to decadal timeframes, with winter ice playing a secondary role. Pocket-beach shoreline positions were found to be unreliable indicators of sand volumetric changes. Lake-level rise facilitated shoreline retreat and overwash-induced beach accretion while high lake levels created the accommodation for additional sands to enter the embayment. This is important for coastal managers to consider when developing mitigation strategies for ongoing lake-level fluctuations and anticipated regional climate impacts. This foundational assessment has implications for embayed beaches of the greater Chicago coastal margin (n > 20), where many other site-specific variables (e.g., groin orientations and shoreline aspect) may factor into nearshore and onshore beach morphodynamics. Continued research into urban pocket beaches of the Great Lakes stands to offer useful information on the impacts of littoral fragmentation on coastal sediment routing during different lake-level phases and degrees of littoral interconnectivity. [ABSTRACT FROM AUTHOR]

Williams, H.F.L. and Rains, B.J., 2022. Effect of barrier height on magnitude and character of Hurricane Harvey washover fans, Matagorda Peninsula, Texas, U.S.A. Journal of Coastal Research, 38(1), 133–139. Coconut Creek (Florida), ISSN 0749-0208. This study uses topographic profiles, washover fan volumes, and shoreline retreat rates to explore relationships between barrier height, erosion, and storm washover sedimentation. Pre- and post-Hurricane Harvey topographic profiles were created along 15 transects using Bare Earth LIDAR (2016) and transit level surveys (2019). Washover fan thicknesses and areas were measured to estimate washover fan volumes. An inverse relationship was found between washover fan volume and pre- and poststorm barrier heights. One section of shoreline had a prestorm height of about 3 m, which almost completely blocked overwash, but may have contributed to a large increase in shoreline erosion. In contrast, a section of shoreline with a low prestorm barrier generated relatively large washover fans, but experienced lower shoreline retreat and was the only section to experience beach and dune recovery in the year after landfall. Shoreline retreat was further quantified between 2014 and 2018 using Google Earth imagery and LIDAR data from 2014, 2016, 2017, and 2018 to track migration of the shoreline. The entire shoreline in the study area is undergoing relatively rapid retreat, but the results suggest that Hurricane Harvey increased erosional retreat rates by up to 195%. The Colorado River Jetty borders the study area and may have acted as a barrier to wave erosion, contributing to enhanced marsh aggradation at marsh sites in close proximity to the jetty and promoting shoreline recovery in the poststorm period. The research results provide valuable information on the interaction of hurricane storm surge with natural and anthropogenic barriers, beach and dune erosion, and marsh aggradation along the coast of Texas. [ABSTRACT FROM AUTHOR]

Florida's Big Bend region hosts the second largest concentration of breeding American Oystercatchers in the state, but reproductive success is low. Nest site characteristics and predation were examined to determine their influence on survival of nests and broods at two areas in the southern Big Bend (Cedar Key and Barge Canal). The probability of a nest surviving in Cedar Key was low (x̄ = 0.25, CI = 0.13–0.41) and limited by nest overwash (46% of known nest attempts); survival of nests at Barge Canal was much higher (x̄ = 0.45, CI = 0.31–0.58). However, 40% of chicks that survived to fledge (35 days) at Barge Canal died before reaching independence (60 days). Raccoon presence and hatch date were negatively correlated with brood survival at Barge Canal. Finally, chicks at Barge Canal weighed less and were smaller compared to chicks at an Atlantic Coast site, which may be related to low abundance of live oysters within 100 m of their nest sites. Efforts to enhance oystercatcher reproductive success may require different approaches for each site: habitat restoration to increase elevation of nest sites in Cedar Key and reduction of predators at Barge Canal. [ABSTRACT FROM AUTHOR]

Williams, H.F.L. and Liu, K.-b., 2021. Testing XRF discrimination of marine and terrestrial flood deposits in southeastern Texas coastal marshes. Journal of Coastal Research, 37(6), 1081–1087. Coconut Creek (Florida), ISSN 0749-0208. This study tests the ability of X-ray fluorescence (XRF) analysis to discriminate between storm-generated marine and terrestrial sediment beds in coastal wetlands. Cluster analysis was applied to the elemental compositions of a sediment core and a monolith collected from McFaddin National Wildlife Refuge in southeastern Texas. The core contains washover sediment beds previously attributed to hurricanes Ike, Rita, Carla, and Audrey. The monolith contains a terrestrial flood deposit attributed to Hurricane Harvey and a washover sediment bed attributed to Hurricane Ike. Clustering correctly distinguished between washover, flood, and marsh deposits. This level of discrimination was not achieved by previous studies based on loss-on-ignition and microfossil analyses because sediment from these different sources can have similar moisture, organic, carbonate, texture, and microfossil characteristics. Cluster analysis places every centimeter of a sediment core into a cluster, providing a more precise means of evaluating the relative contributions of marine and terrestrial sources. In the study area, for example, cluster analysis suggests that 50.5% of coastal marsh sediment was derived from marine sources. The XRF technique used in this study is a promising tool for discrimination of marine and terrestrial sediment sources. Further research is warranted to apply this technique in other coastal environments subject to marine and terrestrial sediment inputs. [ABSTRACT FROM AUTHOR]

Barrier islands and their backbarrier saltmarshes have a reciprocal relationship: aeolian and storm processes transport sediment from the beaches and dunes to create and build marshes along the landward fringe of the island. In turn, these marshes exert a stabilizing influence on the barrier by widening the barrier system and forming a platform onto which the island migrates, consequently slowing landward barrier migration and inhibiting storm breaching. Here, we present a novel framework for applying these natural interdependencies to managing coastal systems and enhancing barrier-island resilience. Further, we detail application of these principles through a case study of the design of a marsh creation project that showcases the interdisciplinary engagement of scientists, engineers, stakeholders, and policymakers. Specifically, we describe: (1) the ecologic, sedimentologic, stratigraphic, and morphologic data obtained from the southern 4 km of Cedar Island (Virginia, United States) and nearby backbarrier tidal channels, tidal flats, and flood-tidal deltas, and (2) the use of those data to develop an engineering and design plan for the construction of a high (46 ha) and low (42 ha) fringing marsh platform located behind the island, proximal to a former ephemeral inlet. Additionally, we chronicle the process used to narrow five initial alternative designs to the optimal final plan. This process involved balancing best-available existing science and models, considering design and financial constraints, identifying stakeholder preferences, and maximizing restoration benefits of habitat provision and shoreline protection. Construction of this marsh would: (1) provide additional habitat and ecosystem benefits, (2) slow the rapid migration (up to 15 m/yr at present) of the barrier island, and (3) hinder island breaching. Ultimately, this project – presently at the final design and permitting stage – may enhance the storm and sea-level rise resilience of the island, backbarrier marshes and lagoons, and the mainland town community; and provide an example of a novel science-based approach to coastal resilience that could be applied to other global barrier settings.

Barrier islands and spits tend to migrate landward in response to sea‐level rise through the storm‐driven process of overwash, but overwash flux depends on the height of the frontal dunes. Here, we explore this fundamental linkage between dune dynamics and barrier migration using the new model Barrier3D. Our experiments demonstrate that discontinuous barrier retreat is a prevalent behavior that can arise directly from the bistability of foredune height, occurring most likely when the storm return period and characteristic time scale of dune growth are of similar magnitudes. Under conditions of greater storm intensity, discontinuous retreat becomes the dominant behavior of barriers that were previously stable. Alternatively, higher rates of sea‐level rise decrease the overall likelihood of discontinuous retreat in favor of continuous transgression. We find that internal dune dynamics, while previously neglected in exploratory barrier modeling, are an essential component of barrier evolution on time scales relevant to coastal management. Plain Language Summary: Barrier islands and spits tend to move landward in response to sea‐level rise and storms when sediment from the beach is washed into the barrier interior and beyond, but tall dunes at the front of a barrier can prevent this process from happening for all but the largest storms. Here, we explore the interactions between dunes, storms, and barrier migration with a new model (Barrier3D). Our model shows that barriers migrate when dunes are low but are stationary when dunes are tall. Over decades to centuries, repeated cycles of dune loss and regrowth lead to sporadic (or "discontinuous") barrier retreat. Additionally, we find that sporadic behavior may become less common in the future in response to rising sea levels, while increasing storm intensity will change which environments are most likely to experience discontinuous retreat. These findings emphasize the importance of dune dynamics in controlling barrier evolution. Barrier migration over decadal timescales ‐ timescales relevant to coastal communities and managers ‐ is commonly modeled as a constant background process, but our results suggest that is not always the case. Key Points: Dune height bistability drives alternating periods of barrier immobility and transgression over decades to centuriesSimulations suggest discontinuous (continuous) retreat will become less (more) common with greater sea‐level rise rates and storm intensityDune dynamics are a critical driver of barrier evolution on time scales relevant to coastal management [ABSTRACT FROM AUTHOR]

Waves and water level setup during storms can create overwashing flows across barrier islands. Overwashing flows can cause erosion, barrier breaching, and inlet formation, but their sediments can also be deposited and form washover fans. These widely different outcomes remain difficult to predict. Here we suggest that a breach develops when the sediment volume transported by overwashing flows exceeds the barrier subaerial volume. We form a simple analytical theory that estimates overwashing flows from storm characteristics, barrier morphology, and dune vegetation, and which can be used to assess washover deposition and breaching likelihood. Our theory suggests that barrier width and storm surge height are two important controls on barrier breaching. We test our theory with the hydrodynamic and morphodynamic model Delft3D as well as with field observations of 21 washover fans and 6 breaches that formed during Hurricane Sandy. There is reasonable correspondence for natural but not for developed barrier coasts, where traditional sediment transport equations do not readily apply. Our analytical formulations for breach formation and overwash deposition can be used to improve long‐term barrier island models. Plain Language Summary: Storms can have big impacts on coastlines. Beaches and dunes can be eroded, their sands can be blown inland, coverings houses and streets. Sometimes, there is so much erosion that new channels have formed that breach the island and make a new connection between the ocean and the lagoon. Here, we develop and test a new theory to predict these outcomes ahead of time. We find that high storm surges along narrow dunes are most vulnerable to breaching. Key Points: New analytical theory compares overwashing flow against barrier volume to predict breaching and washover depositionWe test our theory against Delft3D simulations and Hurricane Sandy observations: vegetation and island elevation help to prevent breachingDeveloped barrier islands do not follow predicted trends, suggesting alternative controls on overwashing and breaching [ABSTRACT FROM AUTHOR]

Abstract Extreme geohazard events can change landscape morphology by redistributing huge volumes of sediment. Event‐driven sediment deposition is typically studied in unbuilt settings – despite the ubiquity of occurrence and high economic cost of these geohazard impacts in built environments. Moreover, sedimentary consequences of extreme events in built settings tend to go unrecorded because they are rapidly cleared, at significant expense, from streets and roads to facilitate emergency response. Reducing disaster costs requires an ability to predict disaster impacts, which itself requires comprehensive measurement and study of the physical consequences of geohazard events. Here, using a database of poststorm aerial imagery, we measure plan‐view geometric characteristics of sandy washover deposits in built and unbuilt settings following five different hurricane strikes along the Atlantic and Gulf Coasts of the US since 2011. We identify systematic similarities and differences between washover morphology in built and unbuilt environments, which we further explore with a simplified numerical model. Our findings suggest that spatial characteristics of the built environment (termed “fabric”) – specifically, the built fraction of the depositional zone – exerts a fundamental control on the form of large deposits. Accounting for the influence of built fabric on the morphodynamics of flow‐driven geohazards is a tractable step toward improved forecasts of hazard impacts and disaster risk reduction.

The Reddish Egret (Egretta rufescens) is considered a species of conservation concern throughout its range, primarily due to high rates of coastal land loss at key nesting locations. Nest and brood survival, causes and timing of nest failures, as well as nest site characteristics and site selection were studied for three breeding seasons (2016-2018) on a natural marsh island in southwestern Louisiana, USA. Of 110 nesting attempts, 58.2% hatched at least one egg. During incubation, overall daily survival rate was 0.979 (± 0.003 SE), corresponding to 56.4% success from egg laying to hatching. For broods, daily survival rate was 0.993 (± 0.002 SE), or 72.9% success from hatching to fledging. Overwash was identified as the primary cause of known nest and brood loss, accounting for 48.9% and 27.8% of all nest and brood failures, respectively. Overall productivity for breeding pairs within the study area was 1.06 chicks/pair for all years. Data suggests that overwash from extreme high tides and wind can significantly contribute to lower reproductive success of breeding pairs but the lack of mammalian predators on the island may contribute to increased nest success, especially in years that do not experience high water levels. [ABSTRACT FROM AUTHOR]

Extreme geohazard events can change landscape morphology by redistributing huge volumes of sediment. Event‐driven sediment deposition is typically studied in unbuilt settings – despite the ubiquity of occurrence and high economic cost of these geohazard impacts in built environments. Moreover, sedimentary consequences of extreme events in built settings tend to go unrecorded because they are rapidly cleared, at significant expense, from streets and roads to facilitate emergency response. Reducing disaster costs requires an ability to predict disaster impacts, which itself requires comprehensive measurement and study of the physical consequences of geohazard events. Here, using a database of poststorm aerial imagery, we measure plan‐view geometric characteristics of sandy washover deposits in built and unbuilt settings following five different hurricane strikes along the Atlantic and Gulf Coasts of the US since 2011. We identify systematic similarities and differences between washover morphology in built and unbuilt environments, which we further explore with a simplified numerical model. Our findings suggest that spatial characteristics of the built environment (termed "fabric") – specifically, the built fraction of the depositional zone – exerts a fundamental control on the form of large deposits. Accounting for the influence of built fabric on the morphodynamics of flow‐driven geohazards is a tractable step toward improved forecasts of hazard impacts and disaster risk reduction. Plain‐language Summary: Many kinds of hazardous extreme events – floods, landslides, and volcanic activity – send flows laden with sediment coursing into built environments. For example, when hurricanes strike built‐up areas of low‐lying coastline, huge volumes of sand get channeled down streets and between buildings, requiring expensive emergency clean‐up. Patterns of hurricane‐driven deposition in the fabric of built settings have been described but rarely measured. We measure hurricane deposits in built and unbuilt environments and find systematic similarities and differences between the two types of setting. Our findings suggest that assessment and mitigation of disaster risk in built environments prone to flow‐driven hazards could be improved by accounting for the effect of built fabric on sediment dynamics. Key Points: We measure geometric characteristics of sandy washover deposits in built and unbuilt coastal environments following hurricane strikesWe quantify systematic similarities and differences between washover morphology in built and unbuilt environmentsOur findings suggest that the "fabric" of the built environment exerts a fundamental control on large washover deposit form [ABSTRACT FROM AUTHOR]

Talavera, L.; del Río, L.; and Benavente, J., 2020. UAS-based high-resolution record of the response of a seminatural sandy spit to a severe storm. In: Malvárez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 679-683. Coconut Creek (Florida), ISSN 0749-0208. Human infrastructures in barrier islands restrict the natural storm-induced overwash needed by these systems to evolve, further increasing their vulnerability in the medium and long term. For this reason, accurate overwash measurements in coastal environments subject to varying degrees of occupation are needed. In this work, two Unmanned Aerial System (UAS) flights were performed over two distinct sectors of a sandy spit located in Cádiz (SW Spain) prior and after the landfall of storm Emma. This storm was a high-energy event that lasted 153 hours,with an average significant wave height of 4 m and a storm peak of 6.81 m., and water levels (tide and surge) of up to 4 m. The images derived from the UAS flights were processed with Structure From Motion (SfM) algorithms using Pix4D software, resulting in high-resolution mosaics and Digital Elevation Models (DEMs) of the study area. These allowed (1) the digitization of the washover deposits in both sectors before and after the storm, and (2) the computation of DEMs of Difference (DoDs), which in turn allowed the analysis of volumetric estimates of erosion/accretion triggered by the significant overwash. The results revealed a nonuniform coastal response between both sectors. The area subject to higher degree of development showed a clear restriction in the onshore delivery of sediment during overwash due to the presence of a channel and a road, the coalescence of the nine pre-existing washovers into a unique deposit of 77546 m2, and much higher damage extents and associated recovery costs. In the natural sector, the overwash processes reactivated the three pre-existing washovers (which enlarged their areas up to 200%) with no significant damages reported. These findings serve as examples of coastal responses to future similar hydrodynamic conditions, and suggest that this spit will likely evolve following different trajectories in the long-term. [ABSTRACT FROM AUTHOR]

Storm surge, a major disaster in coastal areas, is generated by tropical cyclones. Strong currents and waves during storm surges often cause sediment transport and beach erosion, significantly damaging the coastal ecosystem. This paper presents a flow-wave-sediment-morphodynamic coupled model to predict morphodynamic changes caused by overwash during storm surges. In the model, the flow field is calculated using the generalized shallow water equations coupled with the Simulating Waves Nearshore (SWAN) model, while sediment transport and bed changes are computed using a non-equilibrium total-load sediment transport model. The model solves the governing equations by employing the explicit finite-volume approach based on a rectangular mesh. The Godunov-type central upwind scheme is adopted to calculate the interface fluxes, thereby solving the complex Riemann problem. A series of numerical experiments considering wave-induced longshore current and short-term bed erosion are conducted to validate the established model and demonstrate its ability to simulate waves, flow fields, and bed changes during storm surge. Specifically, we used the model to simulate the morphological evolution of the Santa Rosa barrier island caused by Hurricane Ivan's storm surge, and the results indicate that the model accurately predicts the real-case erosion process caused by storm surge and overwash during the hurricane. The simulated results clearly explain overwash development, including washover fans, foredune erosion, and back-barrier deposition. It demonstrates that the front dune significantly eroded with the Santa Rosa barrier island coastline retreating by a maximum of about 80 m during the storm surge. More than three breaches with a width of up to 135 m were formed on the sandy barrier due to wave overwash, while the topography at the breach changed significantly. • A new flow-wave-sediment-morphodynamic coupled model is developed to predict morphodynamic changes caused by overwash during storm surges. • A series of numerical experiments conducted using the new model demonstrates its ability and it successfully predicts the overwash erosion of the Santa Rosa barrier island during Hurricane Ivan's storm surge. [ABSTRACT FROM AUTHOR]

Napatree Point, an isolated barrier in southern Rhode Island, provides a case study of barrier spit migration via storm driven overwash and washover fan migration. Documented shoreline changes using historical surveys and vertical aerial photographs show that the barrier had little in the way of net change in position between 1883 and 1939, including the impact of the 1938 hurricane. The barrier retreated rapidly between 1945 and 1975, driven by both tropical and extra-tropical storms. The shoreline position has been largely static since 1975. The removal of the foredune during the 1938 hurricane facilitated landward shoreline migration in subsequent lower intensity storms. Dune recovery following the 1962 Ash Wednesday storm has been allowed due to limited overwash and barrier migration over the last several decades. Shoreline change rates during the period from 1945–1975 were more than double the rate of shoreline change between 1939 and 2014 and triple the rate between 1883 and 2014, exceeding the positional uncertainty of these shoreline pairs. The long-term shoreline change rates used to calculate coastal setbacks in Rhode Island likely underestimate the potential for rapid shoreline retreat over shorter time periods, particularly in a cluster of storm activity. While sea-level rise has increased since 1975, the barrier has not migrated, highlighting the importance of storms in barrier migration.

Sea level rise increases the pressure on many coastlines to retreat landwards which will lead to coastlines previously held in position through management, being allowed to retreat where this is no longer affordable or sustainable. Barrier beaches have historically rolled back in response to different hydrodynamic events and sea level rise, but very little is known as to how quickly and how far roll-back is going to occur once management has ceased. Data from more than 40 topographical surveys collected over 7 years along the 1.5 km long, almost swash-aligned shingle barrier at Medmerry (southern England) are used together with hydrodynamic data in a wide-ranging assessment of barrier roll-back. This study shows that roll-back is progressing through time along the barrier in downdrift direction in response to a gradual reduction in cross-sectional area through longshore transport. The Barrier Inertia concept provides a practical means to assess stability/instability for events experienced, but also a tool to assess the short- to medium term risk to the coast downdrift of the immediate study area where flood risk still needs to be managed. Roll-back is influenced particularly by the creation of an artificial tidal breach and removal of its sediment, the elevation of the underlying marsh and clay sediments, the number and severity of storms experienced and the presence of legacy groynes; roll-back has exceeded modelled predictions and expert judgement by an order of magnitude.

Barrier island response to sea‐level rise depends on their ability to transgress and move sediment onto and behind the barrier, either through flood‐tidal delta deposition or via overwash. Our understanding of these processes over decadal or longer timescales, however, is limited. Here we use a recently developed barrier island model (BRIE) to better understand the interplay between tidal dynamics, overwash fluxes, and sea‐level rise on barrier coasts and barrier island stratigraphy. Model results suggest that in microtidal environments with large alongshore sediment transport fluxes, tidal inlets are ephemeral and migrate rapidly. These conditions lead to effective deposition of flood‐tidal deltas and allow inlets to constitute most of the landward sediment flux. Whether barrier islands can survive sea‐level rise depends on the combined landward sediment flux from overwash and flood‐tidal delta deposition, likely making barrier islands with artificially stabilized inlets (via jetty construction or maintenance dredging) more vulnerable to sea‐level rise. Key Points: Flood‐tidal deltas built by ephemeral and rapidly migrating tidal inlets contribute significantly to barrier landward movement in response to sea‐level riseAdditional landward sediment flux from tidal inlets can help barrier islands keep pace with sea‐level riseBarrier stratigraphy is not always a good indicator of formative landward sediment fluxes [ABSTRACT FROM AUTHOR]

Hydrodynamic processes, such as fluctuating water levels, waves, and currents, shape coastlines across timescales ranging from minutes to millennia. In large lacustrine systems, such as the Laurentian Great Lakes, the role of water level in driving long-term (centuries to millennia) coastal evolution is well understood. However, additional research is needed to explore short-term (weeks to months) beach geomorphic response to fluctuating water level. Developing a process-focused understanding of how water level fluctuations shape coastal response across these shorter time scales is imperative for coastal management. Here, we present measurements of geomorphic response along a lacustrine beach ridge plain to seasonal water level fluctuations during a decadal high-stand in Lake Michigan water level. Frequent topographic change measurements revealed high spatial and temporal variability in geomorphic response to rising lake level. Sites immediately downdrift of shore protection began to erode immediately as lake level increased. The co-occurrence of peak seasonal lake levels and a modest increase in wave energy resulted in erosion and overwash at sites that resisted erosion during the initial seasonal rise in lake level. None of the sites in this study returned to their initial morphology following seasonal lake level rise. Given that peak water levels were nearly identical in 2017 and 2018, yet the majority of erosion at our sites occurred in 2017, we postulate that erosion associated with seasonal lake level rise is primarily a function of the change in annual maximum water level from year to year, rather than solely the elevation of the water level. [ABSTRACT FROM AUTHOR]

The morphodynamics and structure of barriers with persistent tidal inlets have been well studied. In contrast the stratigraphy and functioning of barrier systems with ephemeral inlets is poorly understood. This article examines the barrier‐inlet systems of two intermittently closed open lagoons or temporarily open closed estuaries on the east coast of South Africa. Multiple geophysical surveys using ground‐penetrating radar (GPR) were correlated with exposed sections of the barrier where inlet formation revealed the internal stratigraphy. Stratigraphic observations were placed in the context of the contemporary wave dynamics and mesoscale geomorphic evolution. The integrated databases reveal an absence of migrating channel features. Instead the stratigraphy is dominated by landward dipping sheets of alternating high‐ and low‐amplitude reflectors. These correlate with gravel, shell debris and heavy mineral‐lined beds formed by overwash processes. Where ephemeral inlet structures are preserved in the stratigraphy, their fills comprise aggrading, high‐amplitude reflectors, linked to washover infilling of the inlet mouth. Multiple small channels in the more distal portions of the barrier in georadar stratigraphy are related to channelized washover flow. These barriers often breach during high swell and are subsequently sealed during fairweather wave conditions. Time series analysis of waves and satellite imagery shows a link between storms from the south and breach events. This is consistent with the truncations in subsurface images and inferred barrier lowering by overwash channelling. These barriers experience quasi‐stable oscillations in their landward and seaward shore position, punctuated by periods of barrier rollover associated with the most intense storms. As overwash is responsible in part for both the constructive and destructive phases of the barrier, these barriers have low preservation potential. Persistent rollover driven by overwashing will terminate once accommodation space is eliminated and the barriers are eroded by storm activity. © 2019 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Williams, H.; Van Hoang, L.; Elliott, P.; Nguyen, H.H., and Manh, H., 2019. A tentative record of mid-Holocene sea-level highstand and barrier overwash from the Cam River mouth, Vietnam. Journal of Coastal Research, 35(4), 852–860. Coconut Creek (Florida), ISSN 0749-0208. A freshwater coastal marsh near the mouth of the Cam River in Northern Vietnam stands 2–3 m above mean sea level and is bordered by a coastal barrier that reaches about 6 m above mean sea level. A core from the marsh contains a 14-cm-thick sand and shell layer. The presence of abundant shell fragments suggests inland transport of littoral sediment, and the sand layer is tentatively identified as a washover deposit. The coast of the study area contains a beachrock standing above the modern beach and reaching to ∼4 m above mean sea level. A tentative explanation of this beachrock is that it represents a beach that formed during a mid-Holocene 2–3-m highstand, evidence for which has been reported from Thailand, Malaysia, Singapore, and Vietnam. Possible explanations of the sand and shell layer include the following: it is a late-Holocene tropical cyclone washover deposit, formed by a large storm surge that overtopped the coastal barrier; or it is a late-Holocene tsunami deposit formed when a tsunami wave, probably from a nearby source, overtopped the coastal barrier. The lack of other distinctive washover deposits in the core suggests that overwash of a magnitude that formed the relatively thick sand and shell layer is a rare event at this site within the last 3000 years. Given the tentative but intriguing results of this study, further research at this site is warranted to better define the stratigraphy and age of the apparent washover deposit. [ABSTRACT FROM AUTHOR]

With global warming, the ice-covered areas in the Arctic are being transformed into open water. This provides increased impetuses for extensive maritime activities and attracts research interests in sea ice modelling. In the polar region, ice sheets can be several kilometres long and subjected to the effects of ocean waves. As its thickness to length ratio is very small, the wave response of such a large ice sheet, known as its hydroelastic response, is dominated by an elastic deformation rather than rigid body motions. In the past 25 years, sea ice hydroelasticity has been widely studied by theoretical models; however, recent experiments indicate that the ideal assumptions used for these theoretical models can cause considerable inaccuracies. This work proposes a numerical approach based on OpenFOAM to simulate the hydroelastic wave-ice interaction, with the Navier-Stokes equations describing the fluid domain, the St. Venant Kirchhoff solid model governing the ice deformation and a coupling scheme to achieve the fluid-structure interaction. Following validation against experiments, the proposed model has been shown capable of capturing phenomena that have not been included in current theoretical models. In particular, the developed model shows the capability to predict overwash, which is a ubiquitous polar phenomenon reported to be a key gap. The present model has the potential to be used to study wave-ice behaviours and the coupled wave-ice effect on marine structures. [ABSTRACT FROM AUTHOR]

Gravel beaches and barriers form a valuable natural protection for many shorelines. The paper presents a numerical modelling study of gravel barrier beach response to storm wave conditions. The XBeach non-hydrostatic model was set up in 1D mode to investigate barrier volume change and overwash under a wide range of unimodal and bimodal storm conditions and barrier cross sections. The numerical model was validated against conditions at Hurst Castle Spit, UK. The validated model is used to simulate the response of a range of gravel barrier cross sections under a wide selection of statistically significant storm wave and water level scenarios thus simulating an ensemble of barrier volume change and overwash. This ensemble of results was used to develop a simple parametric model for estimating barrier volume change during a given storm and water level condition under unimodal storm conditions. Numerical simulations of barrier response to bimodal storm conditions, which are a common occurrence in many parts of the UK were also investigated. It was found that barrier volume change and overwash from bimodal storms will be higher than that from unimodal storms if the swell percentage in the bimodal spectrum is greater than 40%. The model is demonstrated as providing a useful tool for estimating barrier volume change, a commonly used measure used in gravel barrier beach management.

Gravel barriers represent physiographic, hydrographic, sedimentary, and ecological boundaries between inshore and open marine offshore environments, where they provide numerous important functions. The morphosedimentary features of gravel barriers (e.g., steep, energy reflective form) have led to their characterization as effective coastal defense features during extreme hydrodynamic conditions. Consequently, gravel barriers have often been intensively managed to enhance coastal defense functions. The Blakeney Point Barrier System (BPBS), U.K., is one such example, which offers the opportunity to investigate the impact of alternative management regimes under extreme hydrodynamic conditions. The BPBS was actively re-profiled along its eastern section from the 1950s to the winter of 2005, whilst undergoing no active intervention along its western section. Combining an analysis of remotely sensed elevation datasets with numerical storm surge modeling, this paper finds that interventionist management introduces systemic differences in barrier morphological characteristics. Overly steepened barrier sections experience greater wave run-up extents during storm surge conditions, leading to more extreme morphological changes and landward barrier retreat. Furthermore, while high, steep barriers can be highly effective at preventing landward flooding, in cases where overwashing does occur, the resultant landward overtopping volume is typically higher than would be the case for a relatively lower crested barrier with a lower angled seaward slope. There is a growing preference within coastal risk management for less interventionist management regimes, incorporating natural processes. However, restoring natural processes does not immediately or inevitably result in a reduction in coastal risk. This paper contributes practical insights regarding the time taken for a previously managed barrier to relax to a more natural state, intermediary morphological states, and associated landward water flows during extreme events, all of which should be considered if gravel barriers are to be usefully integrated into broader risk management strategies.