Post-storm Recovery Research Articles

Modelling the Influence of Vegetation on the Stochastic Dynamics of Coastal Dunes

ABSTRACTCoastal dunes are the highest natural features on a barrier island, where they protect beach communities, infrastructure and low‐energy back‐barrier ecosystems from flooding and erosion during storms and other high‐water events. Their formation, and poststorm recovery, is a result of a subtle competition between the physical and biological processes controlling the initial stages of dune growth. Vegetation colonises a barren back‐beach and traps wind‐driven (aeolian) sand to form dunes, but at low enough elevation, plants can be eroded by water‐driven transport during random flooding events, which slows down or prevents dune formation. This competition has been previously investigated using both process‐based and analytical models. However, the effect of finite vegetation recovery times together with the precise stochastic nature of flooding events has not been taken into account before. A recent stochastic dune model assumed that vegetation grows and recovers instantaneously, whereas an existing process‐based dune model, the Coastal Dune Model (CDM), did not properly resolve the stochastic flooding events. Here, we address this knowledge gap by adding a much more realistic description of high‐water events of the stochastic model to CDM and investigate the role of vegetation growth and recovery times in dune formation. We first replicate the stochastic model predictions assuming instantaneous vegetation growth. We then define the vegetation colonisation time and relate it to the initial dune formation time. Since dune formation requires the presence of vegetation, a finite colonisation time leads to an expected lag in dune formation and recovery. Depending on the competition between vegetation growth and aeolian erosion, we find that dune dynamics can be divided into two regimes: one with a stable (static) vegetated dune and another one with a mobile, partially vegetated, dune propagating landward. Within the stable dune regime, the influence of vegetation on dune recovery is solely controlled by the relation between the vegetation colonisation time, the dune growth time after plant colonisation and the return period of high‐water events flooding the back‐beach. We introduce two control parameters based on these times and use them to describe a simplified phase space of the dune state. We then find a simple analytical expression for the transition from a ‘high’ state with mature dunes to a ‘low’ state devoid of dunes based on the competition between dune recovery time controlled by vegetation and the flooding frequency. Finally, we use the transition threshold to propose a vulnerability indicator for dune recovery as the minimum elevation after an overwash required for vegetation to recover.

The impact of a storm on the microtidal flat in the Yellow River Delta

Strong hydrodynamic forces generated by storms are key in shaping coastal tidal flats. Most tidal flats achieve equilibrium by adapting to hydrodynamic conditions and sediment inputs. However, high-energy wave activity during storms disrupts this equilibrium, causing rapid and significant changes, particularly in tidal flats, especially in microtidal flats, which are characterized by low tidal ranges. In this study, we conducted an 11-d field campaign on a microtidal flat in the Yellow River Delta (YRD), capturing data during both stormy and calm weather conditions. We measured tidal currents, wave activity, suspended sediment concentrations and sediment grain sizes. The results demonstrated that the tidal flat maintained equilibrium under calm conditions, with minimal fluctuations in bed level (within ±2 mm). Contrastingly, severe erosion and sediment removal during the storm significantly altered the equilibrium of the area. The storm-induced high shear stresses, ranging from 1.02 to 1.48 N/m2, along with alongshore sediment transport, resulted in an elevation change of −10 mm. Furthermore, the subsequent bed level recovery was minimal and insufficient to offset the erosion. Compared to that of the mesotidal and macrotidal flats, post-storm recovery on microtidal flats was limited due to shorter inundation periods and weaker hydrodynamic forces. Therefore, frequent storms may lead to continuous shoreline retreat on microtidal coasts. Conclusively, the present findings underscore the significant impact of storm-induced erosion on the evolutionary processes of microtidal flats and suggest that greater attention should be given to protecting these areas during storms in the Yellow River Delta. The insights can guide the development of more effective coastal protection strategies, highlighting the need for enhanced measures to mitigate erosion and promote resilience in microtidal regions.

The Feedback from a Beach Berm during Post-Storm Recovery and How to Improve the Berm’s Restorative Efficiency

The efficiency of beach recovery during a time of moderate waves following storm waves is closely related to the interaction between dynamics, sediment, and the landform. The existing studies mainly focus on the description of erosion and accretion characteristics, while the response and feedback mechanism of beach berm sediment have not been elucidated. The main controlling factors of recovery efficiency are not clear. In this paper, field observation and the XBeach numerical model are utilized on the sandy beach in Puqian Bay, China, to capture high-frequency cross-shore data during the post-storm recovery period. The variation characteristics and rules of berm elements, including berm ridge height and slope on two sides of the berm ridge, are analyzed. It is observed that the berm constantly changes to adapt to dynamic conditions. Additionally, a correlation between volume change and certain landform parameters is proposed, leading to the identification of a new relationship in wave run-up. The new forum reflects berm influence and considers the berm ridge and berm width.

A Soil Moisture Profile Conceptual Framework to Identify Water Availability and Recovery in Green Stormwater Infrastructure

The recovery of soil void space through infiltration and evapotranspiration processes within green stormwater infrastructure (GSI) is key to continued hydrologic function. As such, soil void space recovery must be well understood to improve the design and modeling and to provide realistic expectations of GSI performance. A novel conceptual framework of soil moisture behavior was developed to define the soil moisture availability at pre-, during, and post-storm conditions. It uses soil moisture measurements and provides seven critical soil moisture points (A, B, C, D, E, F, F″) that describe the soil–water void space recovery after a storm passes through a GSI. The framework outputs a quantification of a GSI subsurface hydrology, including average soil moisture, the duration of saturation, soil moisture recession, desaturation time, infiltration rates, and evapotranspiration (ET) rates. The outputs the framework provide were compared to the values that were obtained through more traditional measurements of infiltration (through spot field infiltration testing), ET (through a variety of methods to quantify GSI ET), soil moisture measurements (through the soil water characteristics curve), and the duration of saturation/desaturation time (through a simulated runoff test), all which provided a strong justification to the framework. This conceptual framework has several applications, including providing an understanding of a system’s ability to hold water, the post-storm recovery process, GSI unit processes (ET and infiltration), important water contents that define the soil–water relationship (such as field capacity and saturation), and a way to quantify long-term changes in performance all through minimal monitoring with one or more soil moisture sensors. The application of this framework to GSI design promotes a deeper understanding of the subsurface hydrology and site-specific soil conditions, which is a key advancement in the understanding of long-term performance and informing GSI design and maintenance.

COASTAL DUNES CHANGES ALONG THE WESTERN COAST OF EUROPE

Coastal dunes are natural barriers buffering storm waves, protecting coastal communities from flooding and rising sea level, and providing a valuable source of biodiversity for the surrounding environment. Significant dune erosion caused by storm waves and high water levels generally takes place over hours or days, while post-storm recovery can take years or decades (Houser et al., 2015). Although coastal dunes have received quite a lot of attention over the last decades, knowledge gaps remain, and our understanding and predicting capacity of long-term (years to decades) coastal dune evolution remain limited. The large diversity of coastal dunes along the Atlantic coast of Europe and the sequence of extreme storms observed during the 2013/14 winter, considered as the most energetic storms since at least 1948 (Masselink, 2016), represent a unique opportunity to study the spectrum of coastal dune response and recovery from an extreme winter.

OBSERVATIONS AND MODELING OF EROSION AND RECOVERY OF A COUPLED BEACH-DUNE SYSTEM

The geomorphic and ecological vulnerability of barrier islands is influenced by the way they respond to oceanographic and anthropogenic forcing over a broad range of temporal scales. Integrated models capable of simulating these processes are increasingly necessary to understand barrier island trajectories under future conditions and to aid in management decisions by evaluating the impact of potential restoration activities. While there are numerical models capable of simulating some of the dominant barrier processes, the role of beach berm evolution is rarely included despite the important function of berms in sustaining beaches, enabling dune growth via Aeolian transport, and mitigating backshore and dune erosion during storm events. This is primarily due to a lack of available data that resolves details of beach profile evolution at necessary temporal scales (e.g., seasonal, post-storm recovery, etc.) to test and develop models and the inherent difficulty of including these intra-annual processes into decadal scale models of barrier island evolution. Here we describe a unique and growing dataset of environmental forcing and barrier island topographic change. We also use the dataset to develop and test a model for barrier island evolution and quantify the role of storms, moderate wave conditions, and wind-driven transport in dictating coastal change on various timescales.

The Application of CNN-Based Image Segmentation for Tracking Coastal Erosion and Post-Storm Recovery

Coastal erosion due to extreme events can cause significant damage to coastal communities and deplete beaches. Post-storm beach recovery is a crucial natural process that rebuilds coastal morphology and reintroduces eroded sediment to the subaerial beach. However, monitoring the beach recovery, which occurs at various spatiotemporal scales, presents a significant challenge. This is due to, firstly, the complex interplay between factors such as storm-induced erosion, sediment availability, local topography, and wave and wind-driven sand transport; secondly, the complex morphology of coastal areas, where water, sand, debris and vegetation co-exists dynamically; and, finally, the challenging weather conditions affecting the long-term small-scale data acquisition needed to monitor the recovery process. This complexity hinders our understanding and effective management of coastal vulnerability and resilience. In this study, we apply Convolutional Neural Networks (CNN)-based semantic segmentation to high-resolution complex beach imagery. This model efficiently distinguishes between various features indicative of coastal processes, including sand texture, water content, debris, and vegetation with a mean precision of 95.1% and mean Intersection of Union (IOU) of 86.7%. Furthermore, we propose a new method to quantify false positives and negatives that allows a reliable estimation of the model’s uncertainty in the absence of a ground truth to validate the model predictions. This method is particularly effective in scenarios where the boundaries between classes are not clearly defined. We also discuss how to identify blurry beach images in advance of semantic segmentation prediction, as our model is less effective at predicting this type of image. By examining how different beach regions evolve over time through time series analysis, we discovered that rare events of wind-driven (aeolian) sand transport seem to play a crucial role in promoting the vertical growth of beaches and thus driving the beach recovery process.

Seasonal Morphological Variability on Estuarine Beaches and Implications for Low-Energy Beach Management

Mercaldi, B.; Brideau, L.E.; Tait, J.F., and Griggs, G., 2022. Seasonal morphological variability on estuarine beaches and implications for low-energy beach management. Journal of Coastal Research, 38(6), 1135–1147. Coconut Creek (Florida), ISSN 0749-0208. Estuarine beaches and other low-energy shorelines are critical for habitats and coastal infrastructure protection. Many different species inhabit estuaries because of the water bodies' unique characteristics, such as their mixed seawater and freshwater and variable tidal ranges. Estuarine beaches also have the important role of buffering storm wave energy. Yet, their natural physical processes are understudied. This research focuses on seasonal morphological variability of estuarine, low-energy beaches and evaluates the implications for beach management practices. Five sandy beaches were studied along the northern shoreline of Long Island Sound in the northeastern region of the United States. The studied beaches vary in location, width, length, and orientation within Long Island Sound to represent different exposures to wave energy. Three seasonal profiles were measured at each beach approximately every four months from late 2015 to early 2019 to document topographic changes. The results revealed minimal sediment movement throughout the study period and there was no evidence of significant cyclic cross-shore sediment transport to indicate patterns of seasonal profile changes. These findings reveal important implications for beach management practices on low-energy shorelines. Specifically, estuarine and other low-energy beaches should be studied on decadal time scales to document any long-term variability and post-storm recovery rather than the monthly or yearly time scales on which open-ocean beaches are typically evaluated. Additionally, estuarine beaches and other low-energy shorelines require specialized management practices that cater to their stagnancy to ensure adequate protection of coastal structures and infrastructure in the face of intensifying hurricanes and sea level rise.

Wrack enhancement of post-hurricane vegetation and geomorphological recovery in a coastal dune

Coastal ecosystems such as sand dunes, mangrove forests, and salt marshes provide natural storm protection for vulnerable shorelines. At the same time, storms erode and redistribute biological materials among coastal systems via wrack. Yet how such cross-ecosystem subsidies affect post-storm recovery is not well understood. Here, we report an experimental investigation into the effect of storm wrack on eco-geomorphological recovery of a coastal embryo dune in north-eastern Florida, USA, following hurricane Irma. We contrasted replicated 100-m2 wrack-removal and unmanipulated (control) plots, measuring vegetation and geomorphological responses over 21 months. Relative to controls, grass cover was reduced 4-fold where diverse storm wrack, including seagrass rhizomes, seaweed, and wood, was removed. Wrack removal was also associated with a reduction in mean elevation, which persisted until the end of the experiment when removal plots had a 14% lower mean elevation than control plots. These results suggest that subsides of wrack re-distributed from other ecosystem types (e.g. seagrasses, macroalgae, uplands): i) enhances the growth of certain dune-building grasses; and ii) boosts the geomorphological recovery of coastal dunes. Our study also indicates that the practice of post-storm beach cleaning to remove wrack–a practice widespread outside of protected areas–may undermine the resilience of coastal dunes and their services.

A punctuated equilibrium model for storm response of geologically controlled beaches: Application to western Portuguese beaches

Understanding time scales of beach response (erosion and recovery) to extreme storms is particularly relevant for management of coastal land and risk. Observations show that rock-bounded platform beaches are generally remarkably stable and only occasionally respond to extreme storm wave forcing with abrupt morphological changes. The present work aims to understand the conditions leading to the disruption of beach stability (storm thresholds for morphological changes) and to comprehend the conditions of the subsequent recovery.Results obtained at two rock-bounded beaches of the western Portuguese coast over five years provided the foundations of a new conceptual morphodynamic model. This model explains abrupt beach disruption as a consequence of full inundation of the beach profile during extreme storms, together with strong wave reflection at the rigid landward boundary, causing major erosion of the subaerial beach. We argue that the timescale of post-storm recovery depends essentially on the fate of the sediment transported offshore during the storm event. If sediments remain within the beach sediment cell (underwater beach section) beach recovery to pre-storm conditions is very fast, while recovery is much slower if sediments exit the beach sediment cell. In the latter case, beach recovery depends on external sand sources, which can lead to recovery time scales of several years.

Knowledge Gaps Update to the 2019 IPCC Special Report on the Ocean and Cryosphere: Prospects to Refine Coastal Flood Hazard Assessments and Adaptation Strategies With At-Risk Communities of Alaska

This article reviews the status of knowledge gaps and co-production process challenges that impede coastal flood hazard resilience planning in communities of northwestern Alaska, where threat levels are high. Discussion focuses on the state of knowledge arising after preparation of the 2019 IPCC Special Report on the Ocean and Cryosphere in a Changing Climate and highlights prospects to address urgent needs. The intent is to identify some key steps necessary to advance the integration of relevant multidisciplinary observations with flood modeling and infrastructure mapping to co-produce new online hazard and risk assessment tools that inform local community planning and improve science collaboration among Federal, state, and regional partners for enhanced pre-storm preparations and post-storm recovery, including partial or complete relocation. By focusing coastal data integration for delivery of priority geospatial hazard map products through a consistent yet customized approach to adaptation planning, the broad collaborative effort in Alaska may yield a path of stakeholder service delivery that can be applied to many Arctic communities and other vulnerable regions of the world.

DOONIES: A process-based ecogeomorphological functional community model for coastal dune vegetation and landscape dynamics

In ecogeomorphic systems, such as beach-dune habitats, complex couplings exist between geomorphology and ecology. Abiotic conditions influence vegetation growth and distribution while vegetation imposes a geomorphic feedback, impacting topography. Communities affect storm response by impacting pre-storm state, post-storm recovery, and landscape evolution. Despite their importance, beach-dune and other ecogeomorphic land-sea systems are deteriorating with increased anthropogenic modification and amplified natural disaster impact linked to climate change. A structured approach is needed to develop a more comprehensive understanding of coastal vegetation community interactions with the environment as these interactions underpin topographic change, strom response, and restoration and management efforts. Toward this goal, a spatially explicit process-based grid model, the DOONIES Model, encompassing biological, physiological, and geomorphological drivers of landscape change is presented. DOONIES simulates critical biotic and abiotic processes of vegetation growth, abundance, and spatial distribution dynamics impacting topography and storm response. Biological processes and ecogeomorphic responses are tailored to generalizable dune functional-communities with species-specific representatives. Estimates of the balance between photosynthesis and respiration dictate plant growth and morphology spatiotemporally which in turn impact sediment erosion and deposition. Relative sensitivity analyses indicate that the model is fundamentally driven by the photosynthesis formulation, where parameters such as maximum daily photosynthesis (grams of carbohydrate per day) and light intensity impact vegetation growth. These in turn, indirectly impact topographic change in modeled ecogeomorphic links. DOONIES is standalone and with a biological focus making it unique compared to more physical morphodynamic and hydrodynamic models with which this model is designed to couple. The model was evaluated by comparing simulation topography to actual across 6 years at Island Beach State Park, NJ while modeling Hurricane Sandy and daily wind conditions driving sediment input and output events. The predicted results were within the measurement error for the elevation datasets that the simulations were based on. This new model affords dynamic predictions of the response of naturally occurring and planted dune vegetation communities to typical abiotic conditions, as a tool for supporting and exploring restoration decisions.

Nature-Based Engineering: A Review on Reducing Coastal Flood Risk With Mangroves

Integration of mangroves in projects to reduce coastal flood risk is increasingly being recognised as a sustainable and cost-effective alternative. In addition to the construction of conventional hard flood protection infrastructure, mangroves not only contribute to attenuating flood events (functionality), they also recover in, and adapt to, a changing climate (persistence). The implementation of mangroves in flood risk reduction, however, remains complex. This is because the innate functionality and persistence of mangroves depend on a range of environmental conditions. Importantly, mangroves may collapse when environmental impacts or climatic changes exceed key system thresholds, bringing uncertainty into a situation where failure could endanger lives and livelihoods. The uncertainties in mangrove functionality and persistence can be dealt with by (1) improving insights in how ecological and physical processes affect mangrove functionality and persistence across scales, (2) advancing tools to accurately assess and predict mangrove functionality and persistence, and (3) adopting an adaptive management approach combined with appropriate engineering interventions to enhance mangrove functionality and persistence. Here, we review existing evidence, monitoring techniques and modelling approaches from the viewpoint of mangrove functionality and persistence. Inspired by existing guidelines for Nature-based Solutions (NbS) to reduce flood risk, we provide an operationalization for this new approach. In addition, we identify where further research efforts are required for the practical application of mangroves in coastal flood risk management. Key aspects in the variability and uncertainty of the functionality and persistence of mangroves are their failure and recovery mechanisms, which are greatly site- and storm-specific. We propose five characteristic damage regimes that result in increasing reductions of mangrove functionality as well as post-storm recovery periods. Further research on the quantification of these regimes and their thresholds is required for the successful integration of mangroves in coastal flood risk management. Ultimately, the key challenge is the development of adaptive management strategies to optimise long-term mangrove functionality and persistence, or their resilience. Such adaptive strategies should be informed by continued mangrove functionality and persistence assessments, based on continued monitoring and modelling of key mangrove thresholds, and supported through well-established guidelines.

The extreme 2013/14 winter storms: Regional patterns in multi-annual beach recovery

Understanding the mechanisms and timescales required for beaches to recover from extreme storm events is fundamental for coastal management worldwide. Yet the post-storm recovery characteristics of different beach types have rarely been investigated over multi-annual timescales. Previous work along the southwest coast of England has suggested that the magnitude and alongshore variability of beach response to storms can be grouped into four key response types controlled by the level of exposure, angle of storm wave approach and degree of embaymentisation. This study aims to enhance our understanding of the spatial and temporal patterns of post-storm beach recovery within this storm response classification framework. Analysis was based on morphological survey data from 23 sites along the southwest coast of England collected between 2012 and 2017. We found that beaches that responded similarly to the unprecedented storm sequence of 2013/14, recovered in a similar manner too, and that spatio-temporal patterns of post-storm recovery can, for the most part, also be described by four coherent classes. In terms of complete recovery to pre-2013/14 volumes, 7 of the 23 beaches we studied recovered >90% of their sediment within 3 years or less, including some of the most affected sites. The magnitude of intertidal beach volume recovered (in the order of 1-100m3m-1) was well correlated with the storm erosion volume (R = -0.81, p = 0.00) and, importantly, was similar for beaches within the same response class. Fully exposed, cross-shore dominated beaches experienced the highest gross erosion and recovery volumes, but showed the lowest net recovery after 3 years (median: 74% volume recovered), while semi-exposed cross-shore dominated beaches showed lower gross change, but the highest net recovery (median: 93% volume recovered).In most cases, the spatial pattern of recovery mirrored that of the storm impact, regardless of whether the beach was cross-shore or alongshore dominant. The observed coherency within each of the four studied beach response classes indicates that regional monitoring programmes could make considerable cost savings by strategically targeting monitoring at representative sites within each class, rather than monitoring all beaches within a region.

Sub-centennially resolved behavior of an accreting sandy shoreline over the past ∼ 1000 years

ABSTRACT Coastal ridge plains represent a valuable record of past shoreline deposition. However, there remain questions regarding shoreline behavior on intermediate timescales (sub-centennial), the impact of storms, and process of ridge genesis. We address these questions through high-resolution reconstruction of the sandy-beach progradation at Boydtown Beach in Twofold Bay, southeastern Australia, over the past 1000 years using ground-penetrating radar (GPR) and optically stimulated luminescence (OSL) dating. GPR profiles are dominated by seaward-dipping reflections that result from beach and dune progradation. Prominent reflections with heavy-mineral concentrations are also preserved resulting from storm erosion. OSL ages reveal alternative phases of steady and episodic accretion, rather than a constant progradation. We hypothesize that steady phases may result from moderate storm events where each successive storm only partially erodes the recovery of the previous event. This results in incremental seaward accretion of the active beach. Phases of episodic accretion could be the result of larger storm events or storm clusters when large post-storm recovery rapidly shifts the active shoreline seaward. The two modes of shoreline progradation (steady and episodic) appear broadly associated with a change in ridge-and-swale morphology whereby subdued ridge swale topography is associated with steady or incremental progradation and higher, better-defined ridges with episodic accretion. These results suggest that a single coastal ridge plain experiences variable intermediate-scale shoreline behavior in response to storm events which then lead to multiple modes of ridge genesis.

Simulating Destructive and Constructive Morphodynamic Processes in Steep Beaches

Short-term beach morphodynamics are typically modelled solely through storm-induced erosion, disregarding post-storm recovery. Yet, the full cycle of beach profile response is critical to simulating and understanding morphodynamics over longer temporal scales. The XBeach model is calibrated using topographic profiles from a reflective beach (Faro Beach, in S. Portugal) during and after the incidence of a fierce storm (Emma) that impacted the area in early 2018. Recovery in all three profiles showed rapid steepening of the beachface and significant recovery of eroded volumes (68–92%) within 45 days after the storm, while berm heights reached 4.5–5 m. Two calibration parameters were used (facua and bermslope), considering two sets of values, one for erosive (Hm0 ≥ 3 m) and one for accretive (Hm0 < 3 m) conditions. A correction of the runup height underestimation by the model in surfbeat mode was necessary to reproduce the measured berm elevation and morphology during recovery. Simulated profiles effectively capture storm erosion, but also berm growth and gradual recovery of the profiles, showing good skill in all three profiles and recovery phases. These experiments will be the basis to formulate event-scale simulations using schematized wave forcing that will allow to calibrate the model for longer-term changes.

Storm impacts on a coupled human-natural coastal system: Resilience of developed coasts

Human occupation of and alteration of the world's coast has transformed large stretches of it into Coupled Human-Natural Systems (CHANS) in which humans both influence and are influenced by coastal evolution. In such systems, human activity is as critical on natural resilience as processes and sediment supply derived from the natural setting. Pre- and post-storm observations of these interactions on the intensively developed Atlantic coast of the Gulf of Cádiz, (Spain and Portugal) are examined to determine natural and engineering resilience. Three case studies are used in three CHANS, showing that human interventions interact in complex ways with the natural system influencing post-storm recovery. In natural coasts, storm impact is assessed in terms of geomorphological response; on developed coasts, it is quantified as damage to infrastructure or loss of amenity. Preparedness, availability of resources, choice of response and the speed at which human agencies respond affect resilience for post-storm beach behaviour. Results show in some sites natural resilience adjusting by post-storm sediment transfers and an equilibrium morphology that may differ from pre-storm morphology; engineering resilience ensured that CHANS regained their pre-storm human infrastructure and amenity. Their management requires a fundamentally different approach to that of natural coastlines. The current immature stage of understanding of CHANS (especially the human preparedness and response components) is illustrated by the case studies presented where short-term political decisions and reactions to storms play a strong role in post-storm response. The nature and extent of many developed coasts as CHANS is slowly becoming more widely acknowledged, but to increase natural resilience and decrease vulnerability in CHANS better planning is required so that future storms are anticipated and when they happen, pre-planned human response actions are activated. Storms are an integral and inevitable element in the behaviour of coastal CHANS, not a disaster or emergency.

Quantifying Seasonal‐to‐Interannual‐Scale Storm Impacts on Morphology Along a Cuspate Coast With a Hybrid Empirical Orthogonal Function Approach

AbstractThe direct impacts of storms on beaches have been well studied; less well understood are the controls of storms on their seasonal‐interannual evolution. We apply a novel empirical orthogonal function (EOF) analysis to a 5‐year data set of monthly subaerial beach topography at Kennedy Space Center, Florida, to extract dominant spatiotemporal topographic patterns. Our hybrid surface EOF (SEOF) approach applies 1‐D EOF techniques to a data set varying in two dimensions, thereby extracting topographic patterns in the longshore and cross‐shore simultaneously while preserving the analytical simplicity of 1‐D approaches. These topographic patterns are then linked to observations of storm impacts. Our approach comprehensively illustrates that storms influenced local topographic evolution over event‐to‐interannual‐timescales. The first mode of topographic variability captures an interannual‐scale change in behavior triggered by the impact of Hurricane Sandy (2012), whereby a cape feature began and sustained rapid aggradation following the event. This pattern is likely linked to storm‐response processes and pre‐existing morphodynamic feedbacks, illustrating that storm response can dictate poststorm recovery processes. This pattern overwhelmed a spatially uniform, seasonal topographic signal, which we attribute to seasonal variability in water level and storminess forcings. The third mode shows links to local framework geology and suggests that some storms can create and/or destroy subaerial berms at this site, where spatial variability of berm existence in discrete longshore compartments can remain stable until a future event. Our results illustrate mechanisms by which storms can create persistent topographic imprints that remain visible for months‐years following events, potentially influencing response to future storms.

Variations in storm-induced bed level dynamics across intertidal flats

Hydrodynamic forces on intertidal flats vary over a range of temporal and spatial scales. These spatiotemporal inhomogeneities have implications for intertidal flat morphodynamics and ecology. We determine whether storm events are capable of altering the long-term morphological evolution of intertidal flats, and unravel the contributions of tidal flow, wind-driven flow, waves, and water depth on inhomogeneities in bed level dynamics (bed level changes over ~days) across these areas. We complement decades of bed level measurements on eight intertidal flats in two estuaries in the Netherlands with an extensive 1-month field campaign on one of those flats. Across this intertidal flat, the hydrodynamics and morphodynamics of a storm event were captured, including the post-storm recovery. We show that individual events can persistently alter the morphological evolution of intertidal flats; magnitudes of some bed level changes are even comparable to years of continuous evolution. The morphological impacts of events are largely controlled by the relative timing of the forcing processes, and not solely by their magnitudes. Spatiotemporal variations in bed level dynamics of intertidal flats are driven by a combination of: (1) the inhomogeneous distributions of the hydrodynamic forcing processes (including the under-explored role of the wind); and (2) the linear proportionality between bed level dynamics and the local bed slope.

Evaluating the geomorphic response from sand fences on dunes impacted by hurricanes

The coastal zone is morphologically dynamic and a range of engineering practices are used to stabilize it. The dune system serves as the first line of defense against storms and rising sea levels. Sand fences on dunes are one popular and economical restorative engineering alternative. This study evaluates the geomorphic response of dunes with and without sand fences resulting from the impact of high energy events, specifically Hurricanes Florence and Michael, which impacted South Carolina, United States in 2018. Innovative cost-effective field methods were applied to calculate dune volumes on a mechanical dune. Measurements compared a site with eleven sand fences and an unmodified control site. Dune volume decreased after Hurricane Florence for the fenced and control sites and increased after Hurricane Michael for the fenced site. There was differential geomorphic change post-storm between the control and fenced sites. Results support that sand fences are resilient under high wind conditions and are an effective strategy to aid in dune recovery and growth, suggesting that they can be installed prior to the storm season rather than being reserved as a post-storm recovery technique.