Subaerial Beach Research Articles

Integrating Hurricane Ian-induced morphology change and GIS-based channelization modeling for barrier island vulnerability

Beaches are vital for storm protection to coastal communities. The nearshore environment, subaerial beach, and dune system provide a first line of defense against hurricane impacts. Hurricane Ian made landfall in Lee County in 2022 and caused devastating damage to coastal communities, estimated at more than $112 billion U.S.. This study aims to create an integrated approach to analyze Hurricane Ian’s impacts on Estero Island, in Lee County, Florida, by examining changes in elevation, shore- line morphology, beach volume, and modeled channelization linked with estimated property damage to the households’ demographic and socio-economic characteristics. The modeled channelization resulting from receding flood waters and ebb surge is compared to impacts on the barrier island’s morphology and estimated damages. The study found that Estero Island sustained volume loss to the subaerial beach after Hurricane Ian with models indicating channelization of water across the barrier island and a greater number of buildings with increased damages were associated within 30 m of the modeled channels. Census data indicated that over 50% of the population on the island is 65 years of age and older, with the majority of households in income brackets below $100K, and approximately 17% living in poverty. The findings from this study present the importance of an integrated approach for extreme events for vulnerable populations, such as those on Estero Island, with a particular emphasis on addressing the needs of elderly residents, lower-income households, and structures built near modeled channels for future management efforts.

Interactions between Hydrodynamic Forcing, Suspended Sediment Transport, and Morphology in a Microtidal Intermediate-Dissipative Beach

This study aims to investigate the hydrodynamic-morphological interactions on a microtidal intermediate-dissipative beach under low to moderate wave energy conditions using field measurements during two climatic seasons. The separate contributions of currents, sea-swell waves, and infragravity waves to high- and low-frequency sediment fluxes were analyzed. The infragravity wave energy was more relevant near the swash zone than in other areas. Although the currents are the primary suspended sediment transport mechanism, the results suggest that the waves are an important driver of sediment suspension from the seabed. The results indicate that Sea-Swell (SS) waves and cross-shore currents are the prevailing hydrodynamic factors in nearshore sediment transport, and the cross-shore suspended sediment transport rates are higher than those in alongshore transport. The submerged bar intensified during the wet season (1–4 November 2018) when the wave height intensities were lower, contrary to the dry season (24–25 March 2018). Significant accretion nearshore was identified (in the subaerial beach) during the wet season when the suspended sediments were greater, the SS-wave heights nearshore were lower, and sediment flux was directed onshore. A notorious erosion was distinguished during the dry season. The most representative volume changes occurred during the dry season (with high erosion), which is attributed to the high SS-wave energy.

Insights into nearshore sandbar dynamics through process-based numerical and logistic regression modeling

Process-based nearshore morphodynamic models are commonly used tools by coastal engineers and planners to predict the nearshore morphology change of sandy beaches across various spatiotemporal scales. Accurate modeling of the morphological response on medium and long time scales is imperative for quantitative assessments of coastal infrastructure over a project’s intended life-span. However, most previous modeling applications have focused on single/sub-seasonal storm events and are often limited to an assessment of the subaerial beach (i.e. berm and dune). This not only leaves uncertainty concerning the quality of morphology predictions on extended (> weeks) time scales, but also the capacity of process-based models to emulate realistic nearshore sandbar dynamics and the corresponding exchange of sediment between the nearshore-beach system. To shed light on these meso-scale dynamics, CSHORE, a 1D phase-averaged, process-based nearshore morphodynamic model, was applied on an annual scale to a multi-barred, dissipative beach in Oysterville, WA, USA. Thousands of unique sediment transport and hydrodynamic parameter combinations were executed during model calibration. A large portion of these simulations displayed physically realistic sandbar dynamics, including the growth, decay, and migration of intertidal and subtidal sandbars. To explore the model mechanisms enabling realistic bar behavior, the binary and multinomial logistic regression model were used to quantify the relationship between model parameter selection and the probability of various categorical bar configurations occurring in the final predicted profile. The results indicate the most sensitive parameters associated with barred morphology, in this study, and support the use of separate sediment transport parameters for low and high wave energy conditions. The co-utilization of numerical and statistical modeling outlined in this publication is generalizable to future exploratory modeling and/or calibration routines concerned with categorical outcomes.

Implications for the resilience of modern coastal systems derived from mesoscale barrier dynamics at Fire Island, New York

Abstract. Understanding the response of coastal barriers to future changes in rates of sea level rise, sediment availability, and storm intensity/frequency is essential for coastal planning, including socioeconomic and ecological management. Identifying drivers of past changes in barrier morphology, as well as barrier sensitivity to these forces, is necessary to accomplish this. Using remote sensing, field, and laboratory analyses, we reconstruct the mesoscale (decades–centuries) evolution of central Fire Island, a portion of a 50 km barrier island fronting Long Island, New York, USA. We find that the configuration of the modern beach and foredune at Fire Island is radically different from the system's relict morphostratigraphy. Central Fire Island is comprised of at least three formerly inlet-divided rotational barriers with distinct subaerial beach and dune–ridge systems that were active prior to the mid-19th century. Varying morphologic states reflected in the relict barriers (e.g., progradational and transgressive) contrast with the modern barrier, which is dominated by a tall and nearly continuous foredune and is relatively static, except for erosion and drowning of its fringing marsh. We suggest that this state shift indicates a transition from a regime dominated by inlet-mediated gradients in alongshore sediment availability to one where human impacts exerted greater influence on island evolution from the late 19th century onward. The retention of some geomorphic capital in Fire Island's relict subaerial features combined with its static nature renders the barrier increasingly susceptible to narrowing and passive submergence. This may lead to an abrupt geomorphic state shift in the future, a veiled vulnerability that may also exist in other stabilized barriers.

Morphodynamic Response of Open and Embayed Beaches to Winter Conditions: Two Case Studies from the North Atlantic Iberian Coast

The morphological responses of two mesotidal beaches located in different coastal settings (embayed and open sandy beaches) on the northwestern Iberian coast were monitored during the winter of 2018/19. The offshore wave time series analysis is related to high-resolution topo-bathymetric measurements to explore spatial-temporal morphological variability at monthly to seasonal scales. Both locations are subjected to the North Atlantic wave climate which exhibits a pronounced seasonality. Throughout the last decade (2010–2020), significant wave heights reached values of up to Hs~9 m during winters and up to Hs~6 m during summers. On average, approximately 12 storms occurred annually in this region. The results clearly reveal divergent morphological responses and sediment transport behaviors at the upper beach and the intertidal zone during the winter for each location. In the embayed beach (Patos), sediment transport in the nearshore is governed by cross-shore processes between the beach berm and a submerged sandbar. In contrast, the open beach (Mira) showed dynamic sediment exchanges and three-dimensional morphologies alternating between accumulation and erosion zones. Overall, both beaches exhibited an erosional trend after the winter, particularly concerning berm erosion and the subaerial beach volume/shoreline retreat. This study highlights the contrasting morphodynamic response on open and embayed beaches to winter conditions, integrating both the subaerial and submerged zones. Local geological and environmental factors, as well as the coastal management strategies applied, will influence how the beach responds to winter wave events. Monitoring and understanding these responses are essential for effective coastal management and adaptation to changing climate.

LIFE-CYCLE ANALYSES OF SUBAERIAL BEACH NOURISHMENTS WITH CONCURRENT NEARSHORE PLACEMENT OF DREDGED SEDIMENT

The practice of placing sediment dredged from navigation channels in the downdrift nearshore is common in the US. These nearshore placements of dredged sediment, or nearshore nourishments, often correspond to a variety of positive nearshore morphology and shoreline stability benefits. They are often able to beneficially use sediment not directly suitable for dry beach placement, which increases the volume available to nourish the full beach profile and keeps sediment in the system that would otherwise be removed. Concentrating dredged sediment placement in nearshore berms may dissipate wave energy farther offshore and reduce the sediment needs of the co-located shoreline. This strategy may be able to extend subaerial beach nourishment lifespan, which typically cost substantially more. Co-located nearshore and subaerial beach nourishments could lead to large cost savings, but the potential to increase subaerial beach fill lifespan has not previously been quantified.

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.

Development of an automated mobile grain size mapping of a mixed sediment beach

AbstractGrain sizes influence beach morphologies, processes, and states (e.g., dissipative, reflective and intermediate), however previous studies analysing beach grain size are often spatially and temporarily sparse. In this study, an automated mobile grain size estimation method using digital photographs (mobile digital grain size, MDGS) was developed and tested at a mixed sediment beach consisting of sand and gravels. The data collection system consisted of a downward‐looking camera, Real Time Kinematic Global Navigation Satellite System, and a camera‐triggering single‐board computer, attached to an all‐terrain vehicle. Digital photographs were collected monthly from January 2021 to March 2022 over a section spanning ~700 m alongshore and processed automatically using an existing (non‐mobile) DGS estimation method. The MDGS results were generally consistent with manually estimated grain sizes (n = 962, r2 = 0.74, RMSE = 5.9 mm) and UAV orthomosaic imagery, although the MDGS sometimes overestimated manually estimated grain sizes particularly for photographs with relatively small sediments. Modelled nearshore waves and subaerial beach survey data from mobile terrestrial LiDAR were used to explore using MDGS in morphological studies. Seasonal grain size trends, observed relationships between nearshore wave height and spatially averaged beach grain size, and the general increasing trends of grain sizes with both beach elevation and local roughness/slope were consistent with previous studies, highlighting the use of MDGS for morpho‐sedimentary mixed sediment beach research. The MDGS provides a unique method to conduct spatially and temporarily high‐resolution mapping of gravel size sediments over large areas (>102 m alongshore). Potential improvements of the MDGS include additional low elevation cameras for improved fine (e.g., sand) grain size estimates, decreased time between photo triggering, and automated removal of erroneous photographs containing surface irregularities (e.g., tire tracks) and non‐sediment features (e.g., seaweed). Overall, the MDGS provides improved surface grain size mapping and investigations of complex grain size–morphology relationships of mixed sediment beaches.

Measuring drivers of shoreline and subaerial beach change using limited datasets in a temperate, wave-dominated sandy system: Inverloch, Australia

Understanding the drivers of coastal change is important for establishing how a system behaves and responds under different conditions, which is then used for informing management interventions. At seasonal to decadal scales this ideally requires long periods of repeat and frequent monitoring to detect changes in, for instance, shoreline position and beach volume. In this study, we analysed just two years of unmanned aerial vehicle (UAV) surveys over a single eroding beach-dune system (Victoria, Australia), in order to assess its utility in understanding the drivers of change. This morphological data was combined with a WAVEWATCH III (CAWCR) model hindcast, that was assessed against a newly installed, portable, wave buoy. Between August 2018–2020, net shoreline recession, at a rate of 3.1 m/year and net erosion volume change of 1736 m3 occurred along Main Beach in Inverloch. Negative correlations were found between coastal change metrics and wave height, wave direction and rougher sea states. Stronger correlations were found between oceanic conditions and volumetric change than with shoreline change. Beachface changes at this scale have a more instantaneous response to driving conditions, whereas the shoreline reflects longer-term patterns and the impact of extreme events. Spatial and temporal variations were evident, resulting from increasing wave exposure from west to east along the coast, geological controls, human interventions, and variable wave and storm patterns. Strong seasonal patterns were identified in rates of coastal change; however, these changes were offset by the most energetic wave and storm seasons due to morphological hysteresis and the relative change in magnitude of waves or frequency of storms between seasons. The data prove that studies over short period and spatial scales can be highly effective in understanding the drivers of beach change. These findings allow managers to have confidence that small datasets can be highly useful for sound planning strategies.

Short-term dynamics of a high energy embayed beach: Stanwell Park, NSW, Australia

The three-dimensional variability of the subaerial beach is examined for Stanwell Park Beach, New South Wales (NSW), Australia. This embayed environment has previously been studied over long time scales (decades), but not over shorter time scales (days, weeks). Embayed beaches experience rotation events during which opposite accretion and erosion patterns are observed at the extremities of the beach. To analyse this phenomenon, the beach is mapped with 10 Real-Time Kinematic Global Positioning System (RTK-GPS) surveys collected from 10th February 2016 to 14th May 2016. This study aims to examine short-term changes (days, weeks) to the morphology of the subaerial beach of Stanwell Park Beach and any factors that may influence its behaviours. We show that a short-time beach rotation event occurs over one month, caused by a large eastern swell. However, swell with a southern direction generates rip current channels on the subaerial beach face. During this particular event, the amount of sediment transported is lower than during beach rotation events. Furthermore, Stanwell Park Beach has two lagoons that can open to discharge water accumulated in these lagoons. We show that during heavy rainfall, the lagoons open, transporting sediment in surf zone and thus causing erosion. This study provides a clear demonstration of the sensitivity of embayed beaches to short-term variability in wave climate.

Alongshore variability of a southern California beach, before and after nourishment

A moderate-size (344,000 m 3 ) subaerial beach nourishment was placed in 2012 in front of flood-prone homes at Imperial Beach, California. The subaerially persistent coarser-than-native nourishment sand spread alongshore over a ∼ 4 km span. By winter 2018/19 subaerial sand volumes in front of the flood-prone homes were eroded to near pre-nourishment levels, while sand accumulated near a river mouth ∼ 1.5 km to the south. The observed alongshore patterns of post-nourishment erosion/accretion are consistent with the modeled convergence/divergence of alongshore currents driven by wave radiation stress S x y gradients associated with refraction over a relic cobble ebb shoal. The same alongshore patterns of subaerial erosion/accretion were observed in the 6 years preceding the nourishment, indicating a wave-driven erosion hotspot in front of the flood-prone homes, and an accretion hotspot near the region of river mouth migration. Over the 13-yr observation period, the river mouth migrated 600 m southward and closed in 2016, possibly owing to nourishment sand and El Niño intensified local drift convergence. The computationally efficient drift (radiation stress) gradient model uses simplified wave and surfzone dynamics, assumes proportionality between alongshore sediment transport and the radiation stress component S x y , but nevertheless captures the overall patterns of alongshore nourishment evolution and provides a framework to inform sand management. • The subaerial influence of relatively coarse beach nourishment sand spans 4 km of coast 6 years after placement. • Accretion and erosion hotspots are persistent, 6 years before and after nourishment. • Patterns in the divergence of the drift, controlled by an ebb shoal, cause the hotspots.

Morphodynamic Behaviour of a Mediterranean Intermittent Estuary with Opening Phases Primarily Dominated by Offshore Winds

This study focuses on the dynamics of an intermittent estuary in a wave-dominated (microtidal) area, with low fluvial discharges and strong dominant offshore wind regimes. The aims are to understand the effect of these particular environmental factors in the dynamics of such estuaries. The results allow us to propose a synthetic morphodynamic model of evolution whereby opening phases are predominantly controlled by offshore winds, which have a significant influence in the northern Mediterranean. Inputs from rainfall/karst discharge and the overtopping of storm waves cause the lagoon to fill. Closing phases are controlled by the slight easterly swell which forms a berm at the inlet entrance. On occasion, major storms can also contribute to barrier opening. Nevertheless, offshore wind remains the main controlling factor allowing the surge of lagoon waters behind the beach barrier and the lowering of the berm by wind deflation. This leads to opening of the barrier due to the overflow of lagoon waters at the beach megacusp horns, thus connecting the sub-aerial beach with the inner bar system that is developed on topographically low sectors of the barrier. To the best of the authors’ knowledge, this type of estuary is not described in the literature.

Quantitative regional observations of gravel and bedrock influence on beach morphologies

Geological features such as underlying bedrock and coarse sediment layers/deposits are known to influence sandy beach morphology, however existing observations are often spatially limited, qualitative, and/or focused on limited geological feature types. Using mobile terrestrial LiDAR data collected twice yearly from 2017 to 2021, this study quantitatively examines the influence of intermittently and/or semi-permanently exposed gravel deposits and bedrock on seasonal subaerial beach elevations along ~40 km of coast in northern San Diego County, California. Both seasonal beach elevations, surface gravel/bedrock exposure, and modeled nearshore wave conditions varied considerably in space and time. Sandy beaches with limited bedrock/gravel exposure generally had relatively large typical winter-erosion and summer-accretion elevation changes, while beaches with frequent bedrock exposure often had relatively small elevation changes and occasionally exhibited localized seasonal elevation change reversals (e.g. winter-accretion). Beaches with surface gravel often had relatively large foreshore elevation changes compared to the upper beach, and exhibited upper beach elevation change reversals (e.g. winter-accretion). The results also suggest gravel and bedrock influences can be interrelated. Localized seasonal elevation change reversals were also observed on beaches with large alongshore variation in radiation stress near submarine canyons. This study highlights the utility of spatially large-scale and concurrent quantitative mapping of beach elevation and multiple geologic feature types.

Subaqueous and Subaerial Beach Changes after Implementation of a Mega Nourishment in Front of a Sea Dike

Sandy nourishments can provide additional sediment to the coastal system to maintain its recreational or safety function under rising sea levels. These nourishments can be implemented at sandy beach systems, but can also be used to reinforce gray coastal infrastructure (e.g., dams, dikes, seawalls). The Hondsbossche Dunes project is a combined shoreface, beach, and dune nourishment of 35 million m3 sand. The nourishment was built to replace the flood protection function of an old sea-dike while creating additional space for nature and recreation. This paper presents the evolution of this newly created sandy beach system in the first 5 years after implementation based on bathymetric and topographic surveys, acquired every three to six months. A significant coastline curvature is created by the nourishment leading to erosion in the central 7 km bordered by zones with accretion. However, over the five-year period, net volume losses from the project area were less than 5% of the initial nourished sand volume. The man-made cross-shore beach profile rapidly mimics the characteristics of adjacent beaches. The slope of the surfzone is adjusted within two winters to a similar slope. The initially wide beaches (i.e., up to 225 m) are reduced to about 100 m-wide. Simultaneously, the dune volume has increased and the dune foot migrated seaward at the entire nourished site, regardless of whether the subaqueous profile gained or lost sediment. Our results show that the Hondsbossche Dunes nourishment, built with a natural slope and wide beach, created a positive sediment balance in the dune for a prolonged period after placement. As such, natural forces in the years after implementation provided a significant contribution to the growth in dune volume and related safety against flooding.

Emulator For Eroded Beach And Dune Profiles Due To Storms

AbstractDunes and beaches are vulnerable to erosion during storm events. Numerical models can predict beach response to storms with fidelity, but their computational costs, the domain‐specific knowledge necessary to use them, and the wide range of potential future storm and beach conditions can hinder their use in forecasting storm erosion for short‐ and long‐term horizons. We develop an emulator, which is an efficient predictive model that behaves like a numerical model, to predict the morphologic response of the subaerial beach to storms. Specific emphasis is placed on providing antecedent beach states as an input to the emulator and predicting the post‐storm profile shape. Training data include beach profiles at multiple stages in a nourishment life cycle to assess if such a framework can be applied in locations that nourish as a coastal defense policy. Development and application of the emulator is focused on Nags Head, North Carolina, which nourishes its beaches to mitigate hazards of storm waves, flooding, and erosion. A high‐fidelity, process‐based morphodynamic model is used to train the emulator with 1250 scenarios of sea‐storms and beach profiles. The post‐storm beach state is emulated with a parameterized power‐law function fit to the eroded portion of the subaerial profile. When the emulator was tested for a sequence of real storms from 2019, the eroded beach profiles were predicted with a skill score of 0.66. This emulator is promising for future efforts to predict storm‐induced beach erosion in hazard warnings or adaptation studies.

Sandy beach evolution in the low-energy microtidal Baltic Sea: Attribution of changes to hydrometeorological forcing

We analyse the response of small embayed sandy beaches in marginal seas to various hydrometeorological drivers to isolate a specific mechanism that may stabilise beaches under bi-directional wind patterns. The focus is on the evolution of two sandy beaches (Pirita and Russalka) in Tallinn Bay, northeastern Baltic Sea, that are about 3 km apart and oriented at an angle of 35° with respect to each other. A large proportion of annual wave energy flux is packed into a very few days in the year. The wind direction, and thereby the direction of the wave fields, of the strongest storms in the year determines which coastal areas are most affected. We quantify the response of the beaches to hydrometeorological conditions over 12 years, 2008–2020 based on annual high-resolution measurements using airborne and terrestrial laser scanning technology. The fastest sediment removal from the subaerial beach occurs during time periods with elevated water level and strong wave energy flux. Even though annual variations in the sand volume of the subaerial beach are significant, both beaches are stable during the decade studied but respond differently. We outline specific mechanisms that maintain the beaches, best illustrated at Pirita beach where there is strong longshore sediment transport component driven by waves that often arrive the beach at a large angle. Sand that is removed from the higher subaerial segments of the northern part of Pirita beach and moved to the south by north-west storms during periods of elevated water levels, is deposited in the shallow nearshore (above about −0.5 m of the mean sea level). This sediment is then able to be transported back to the north during periods of lower water levels where even relatively small waves generated by moderate south-westerly winds can entrain and move the sediment. This mechanism, different from the classic cut and fill cycle, may implicitly stabilise beaches where there is a bi-directional wind regime and a small tidal range. Another, less explicit cross-shore mechanism, is found for the differently oriented Russalka beach where waves usually approach at a small angle with respect to the shore normal. This beach is maintained by a synchronisation of intense waves and water level that limits the depth of deposited sand with respect to mean sea level. The two mechanisms are thus fairly similar, however Pirita beach has extensive alongshore excursions of sand.

Sedimentary in-filling of an urban Great Lakes waterfront embayment and implications for threshold-driven shoreline morphodynamics, Montrose Beach, SW Lake Michigan

This paper addresses subaqueous and subaerial patterns of geomorphic change across Montrose Beach, an urban embayment along Chicago’s engineered SW Lake Michigan coastline. Our goal was to better characterize the urban littoral zone, its sediment-transport processes, and associated shoreline morphodynamics (from the early 1950s to present). Succinct beach geomorphic responses to decadal base-level changes (i.e., regression during lake-level fall and transgression during lake-level rise) occurred once a morphologic threshold in the subaqueous portion of the system had been crossed. Despite continuous sand trapping, nearshore elevations were initially not conducive to promoting expansion of the subaerial beach environment, regardless of water-level condition. Rapid beach expansion after 1990 (by a factor of four in <25 years) was facilitated by prior decades of nearshore accretion. Shoreline morphodynamic trajectories and degree of coupling to nearshore sedimentary processes are important considerations for developing long-term beach-management strategies. Rapid cross-shore movements of the shoreline in response to oscillatory base levels are expected to persist at Montrose and other urban beaches of similar design (and nearshore conditions). This has important implications for managing urban lakefront ecosystems, including coastal dunes and shore-bird habitats. Few datasets have thus far quantified time-variant and threshold-driven patterns of beach geomorphic development along engineered coastlines. Such insights should help coastal managers better understand littoral sediment interconnectivity across the urban lakefront and anticipate future geomorphic trajectories of beach environments with anticipated decadal-scale oscillatory patterns in lake level.

Subaerial and upper-shoreface morphodynamics of a highly-dynamic enclosed beach in NW Baja California

Estimating sediment budget is a challenge that requires understanding the morphological relationship between the subaerial beach and the upper and lower shoreface. Based on a unique dataset of monthly measured topographic and bathymetric measurements, this study describes the morphodynamics of the subaerial and subtidal sections of a highly-dynamic beach over a 5-year period (2015–2020). Typically, the beach presents a longshore bar trough (LBT) state at the end of the winter, driven by net offshore sediment transport. The nearshore morphology evolves into more reflective states (rhythmic bar beach, RBB; transverse bar rip, TBR; and low tide terrace, LTT) driven by onshore sediment transport when the incident wave-energy decreases over the spring and summer. Seasonal variations of the incident wave-energy resulted in large subaerial-subtidal exchanges of sediment (200–300 m3 m−1), but these exchanges were an order of magnitude smaller in yearly scales. Subaerial and upper-shoreface morphological changes varied alongshore. Larger variations existed in the south, influenced by persistent rip channels. Instead, alongshore processes resulting from changes in wave direction may control the morphodynamics in the center and north. The subtidal section in north and center gained significant amount of sand (300 m3 m−1) during high-energy conditions, which contributed to a net annual gain of 60 m3 m−1. The origin of that sediment is currently unknown, it could have bypassed from an adjacent beach, shifted alongshore or inputted from the lower shoreface deeper than the depth of closure. This research shows the presence of very large monthly-to-seasonal total volume fluctuations relative to net annual or multi-annual changes, and highlights that comparatively small additions to the annual sediment budget could potentially contribute to the long-term stability of the beach.

Subaerial Beach Profiles: the Application of Erosion and Accretion Balanced Approach in Southwestern Maine, USA

Subaerial Beach Profiles: the Application of Erosion and Accretion Balanced Approach in Southwestern Maine, USA

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.