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Beach loss is a growing global challenge that threatens the safety of coastal communities, the health of coastal ecosystems, recreational amenities, and regional economies dependent on tourism. Spatial gradients in longshore sediment transport, or divergence of drift (DoD), is a primary driver of beach width change over multi-annual time scales, but the response of any particular beach can be challenging to characterize and predict. Here we present a new method to characterize DoD using a non-uniform segmentation of the coastline informed by the spatial distribution of longshore transport potential, including the location of physical barriers identified using satellite imagery, and both maxima and reversals in longshore transport potential derived from nearshore wave data. The method demonstrates improved capacity to predict beach width trends at sandy beaches compared to methods that rely on a uniform segmentation of the coastline. In an application to southern California where satellite data documents two decades of beach width change, the non-uniform segmentation method correctly predicts the sign of beach trends at 93% of transects within a littoral cell and achieves a linear correlation between DoD and beach width trends exceeding 0.8. Moreover, we find that a minimum of five years of data are required to establish consistently strong correlations between DoD and beach width changes. Conversely, use of a uniform segmentation is shown to be unreliable for estimating beach width trends due to strong sensitivities to shoreline segment size. This work shows the potential to leverage nearshore wave data and satellite-based beach width data for improved characterization of shoreline dynamics relevant to beach erosion management. • A new shoreline modeling method is presented for estimating divergence of drift. • Segment boundaries are identified using longshore transport characteristics. • Strong correlations were shown between divergence of drift and beach width trends. • A littoral Peclet number characterizes the dominant longshore transport mechanism. [ABSTRACT FROM AUTHOR]

This paper presents the use of an Eulerian–Eulerian multi-phase model to examine sediment transport over a gravel beach under regular waves. A previously published full-scale experiment is reproduced numerically; the simulated wave deformation, flow velocity, pore pressure, and morphological evolution are consistent with measurements. The multi-phase model can produce berm formation for smaller waves and beach erosion for larger waves as observation in previous field studies. The sensitivity analysis suggests that the nonlinear component of drag has significant effect on the berm formation through infiltration and exfiltration. The two simulated forces applied on the sediment (drag and buoyancy) are examined. The drag is the main force directly for sediment transport; therefore the sediment transport rate has high correlation with depth-averaged flow velocity. The buoyancy that is caused by plunging flow impacting beach face can push the sediment landward in the bore front, where the flow acceleration has its peak. The flow acceleration is considered in some previous sediment transport models empirically for simulating morphological evolution of gravel beach under waves. The buoyancy links the flow acceleration and sediment transport rate in the bore front. • Sediment transport in a gravel beach is simulated using a multiphase model. • The drag is the main force directly for sediment transport. • The buoyancy links the flow acceleration and sediment transport rate in the bore front. • The nonlinear drag has the effect on berm formation. [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]

In this study the numerical model Delft3D is implemented on high-resolution grids to evaluate beach morphological response to groins and submerged breakwaters under sea-breeze conditions. The numerical model is compared with field measurements from three sites on the northern Yucatan coast of Mexico. At these sites, relatively low energy waves with significant wave heights arrive at a steep oblique angle to the coast that drives high sediment transport rates. After calibration of the model, by adjustment of model free parameters, the model results show accurate representations of the diurnal variation of hydrodynamic conditions (wave height and current velocity) driven by the daily sea-breezes. Moreover, the morphodynamic model shows a good representation of the rapid morphological changes, particularly of the accretion pattern caused by a temporary groin and the subsequent recovery following the structure removal. However, erosion patterns at the downdrift areas of structures were not reproduced, owing to the numerical model limitations of simulating swash morphodynamics. Model validation with field observations of the beach response behind submerged breakwaters enabled the model to be used for a parametric study. Following previous work, new equations are developed in the present study that show an improvement for predicting shoreline changes near submerged breakwaters under sea-breeze conditions. Despite the limitations in reproducing downdrift erosion, overall the high-resolution modelling approach satisfactorily simulates the hydrodynamic and the updrift accretion responses to coastal structures on sandy beaches with oblique waves. • Simulations of beach change near groins and submerged breakwaters are performed. • Results are compared to measurements at three sites with high sediment transport rates. • New equations show improved prediction of shoreline change near submerged structures. [ABSTRACT FROM AUTHOR]

Coastal dunes provide many ecosystem services including protection of infrastructure from wave overtopping and habitat for native species. Foredunes grow at different rates and assume different forms (i.e., short and wide to tall and narrow) depending on a range of factors including pre-existing beach and dune morphology, wind, wave, and water levels, sediment grain size, and vegetation characteristics, yet the relative importance of these factors on foredune growth is understudied. Here, we quantify foredune evolution (2016–2019) and explore the relative influence of a suite of metocean, sedimentary, and ecological factors for dune growth on three barrier islands in the U.S. North Carolina Outer Banks (Cape Lookout National Seashore). We incorporate observed and hindcast wind, wave, and water level data into the process-based, coupled beach-dune evolution model, Windsurf, to explore the relative contribution of factors likely to influence foredune growth at the annual timescale (2016–2017). Our cross-shore topographic profile observations show varied interannual foredune change rates and characteristics, including horizontal retreat and progradation at the dune toe and vertical erosion and accretion at the dune crest. Model results indicate that, of the factors explored, pre-existing morphology had the greatest influence on the type of foredune growth that occurred (i.e., incipient dune development, widening of the dune, and/or vertical accretion), which dramatically altered the final shape of the dune. Variations in wind and wave climates were associated with the relative contribution of marine- and aeolian-driven bed elevation changes and were particularly influential during storms. In addition, increases in the minimum elevation of vegetation on the dune profile (analogous to the cross-shore distance between the shoreline and established vegetation line, i.e., the vegetation limit, as coined by Duran and Moore, 2013) increased dune crest height and dune volume. Moreover, variations in the minimum vegetation elevation resulted in a larger range of dune crest elevations and dune volumes than differences in the median sand grain size. We suggest that insights into the relative influence of metocean, sedimentary, and ecological factors to dune growth can assist in the development of practical coastal management strategies. • Pre-existing beach-dune morphology had the largest influence on dune growth type. • Exposure to the dominant wave direction led to increased dune volume. • Higher minimum vegetation elevation and finer sand resulted in taller dunes. • Minimum vegetation elevation influenced dune growth more than sand grain size. [ABSTRACT FROM AUTHOR]

Experimental results involving shoreface nourishment scenarios are presented. The experiments are performed at small scale and the effects of nourishment placement and timing in relation to long-term cross-shore profile development are investigated. Four different nourishment scenarios are tested experimentally, with nourishment positioned both in the trough of a pre-developed profile as well as along the bar of the profile. The results demonstrate that under all scenarios the erosion of the shoreline slows relative to the case without nourishment. The two cases where the nourishment was placed along the bar reduced erosion of the shoreline more compared to the two cases with nourishment in the trough onshore of the bar. Compared to past experiments on nourishment presented in the literature, the present experiments were run for a longer duration, and the concepts of equilibrium profile and the development towards such an equilibrium were investigated. Curiously, the experiments showed a reversal of the sediment transport rate from being primarily offshore directed across the entire profile to onshore directed in the inner surf zone region closest to the shoreline. This reversal of transport rate occurred without changing the incoming wave conditions and resulted in deposition at the shoreline. This either questions the existence of equilibrium beach profiles or at the very least shows that the development towards such an equilibrium will not always be monotonic in time. • Shoreface nourishments are performed at model scale investigating the influence of timing and placement on long term profile development. • Placement of nourishment can disturb the cross-shore profile shape over long time. • After long time running with the same wave climate the transport rate changed from offshore directed to onshore directed in the inner surf zone. • The change in sign was related to cessation of wave-breaking and resulted in sand returning to an otherwise eroding beach. [ABSTRACT FROM AUTHOR]

Beach accretion is the natural mechanism that allows dry beaches to recover. Human response to climate change produces hardening of the coasts and the incident marine climate, which increases beach erosion produced during winter and prevents a full recovery during summer. Beach nourishments are performed all around the world to fight coastal erosion with a nature-friendly philosophy. Softer techniques as beach scraping are also widely applied. A kind of beach scraping is proposed here as an innovative nature-assisted beach enhancement technique, that aims to accelerate naturally produced sand accretion on the beach profile. It consists of mechanically ploughing the intertidal area of a beach to create ridges and furrows. Prototype-scale laboratory experiments were performed to analyse its effectiveness under controlled conditions. Seven 1 h tests having different water levels and the same initial bottom geometry and sea state conditions (H s = 0.3 m, T p = 7 s) were performed. Sea state, sand, and initial slope characteristics assured accretive conditions. Ploughed and flatbed initial slope geometries were simultaneously tested into two sub-channels of the wave flume. The results indicate that the bottom roughness is 3.57 times larger for the ploughed geometry. Consequently, wave energy dissipation is larger for the ploughed geometry, and therefore, the significant wave height at the shoreward end of the study area is 11% smaller for the ploughed geometry. This smaller wave height under the ploughed morphology leads to more accretive conditions, as observed from the measured onshore sediment transport, which is 2.9 times larger than the natural transport, and the larger accreted sediment volume in all seven test cases. These results demonstrate the effectiveness of ploughing to enhance natural beach accretion under highly probable accretion conditions. Further research is required to define the thresholds of marine conditions in which the potential of beach ploughing can be exploded. • Working with nature. • Ploughing the intertidal area of a beach enhances natural accretion. • Real scale laboratory experiments measuring sediment transport. • Mechanically ploughing increases bed roughness. • Extra wave energy dissipation is produced by ploughing. [ABSTRACT FROM AUTHOR]

A moderate-size (344,000 m3) 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. • A simple drift divergence model that captures refraction over an ebb shoal describes the alongshore variable sand volume trends. [ABSTRACT FROM AUTHOR]

Beaches are dynamic systems characterized by frequent morphological changes due to storm events, gradients in longshore sediment transport, seasonal variations in wave climate and more. The complexity of variations in beach morphology can increase significantly after a beach nourishment, particularly when coastal structures are involved. This study investigates the impacts of a groin field, consisting of both notched and traditional groins, on the morphological evolution of a beach in Deal, New Jersey, USA. More specifically, an evaluation of the performance and specifications of the notched groins is undertaken. In 2016, the beach in Deal was nourished and three of the eight groins comprising the groin field were concurrently notched in order to promote increased movement of longshore sediment transport while maintaining a stable shoreline. The study site was monitored nine times over a period of two years (2018–2019) using Unmanned Aerial Systems Structure from Motion (UAS-SfM) photogrammetry. The near-term morphological response of the study site was evaluated by analyzing high-resolution digital elevation models (DEMs) and longshore sediment transport estimates based on measured wave data. The elevation change maps established that in general, the groin notching was partially successful in reducing shoreline asymmetry in consequence of sediment transport bypassing through the notches. Differences in behavior and patterns of elevation change adjacent to each individual notched groin was observed even though they were all notched according to the same specifications. While all three notched groins were effective in stabilizing the adjacent shoreline, they varied in their effectiveness at promoting longshore sediment transport through the notch. This is likely attributable to the relative location of the notched groins within the groin field and the characteristics of the post-nourishment morphological evolution; the direction of net annual longshore sediment transport at Deal beach is northward, which caused the southern areas of the study site to narrow and the northern areas of the study site to widen. Consequently, the notched groins in the wider sections of beach did not function as intended, as the notches filled in with sediment causing them to mimic the behavior of a traditional groin, including restricted longshore sediment transport and significant erosion/accretion offsets. In the narrower section of beach, the notched groin functioned as designed, such that the notch was located in the swash zone and therefore allowed bi-directional sediment transport and maintained a stable and relatively symmetrical shoreline. Thus, the notched groins performed most effectively when the notch was consistently located toward the seaward end of the swash zone. • Groin notches function optimally when the notch is located in the swash zone. • Improperly constructed notches can fill-in with sediment and behave like traditional groins. • Notch specifications can vary to account for position and uncertainty in the equilibrated beach. • Notch designs should reflect the direction and magnitude of net longshore sediment transport. [ABSTRACT FROM AUTHOR]

Increasing pressures on coastal environments induced by sea level rise and coastal squeeze has meant that tracking the morphological evolution of sedimentary coasts from the last known survey, pre-empting storm impacts and forecasting potential beach recovery following extreme events is of substantial and increasing societal importance. Equilibrium models for forecasting coastal evolution have figured prominently in the literature in the past two decades and show a strong potential for fulfilling this societal need. In particular some very skilful shoreline evolution models have been proposed based on equilibrium concepts. These models are stable, simple and permit long-term (O(10) years) predictions of coastal change. However, equilibrium models are typically highly empirical and in many cases do not consider explicitly the impact of dynamic sea level, which is modulated by tides, surge and global sea level rise. Equilibrium-based models of shoreline evolution have shown particular promise, but these models generally do not consider the role of the sub- and supra-tidal morphology on coastal evolution (e.g. the importance of coastal dune systems). This contribution presents a new model for For ecasting C oastal E volution (ForCE), which addresses these issues. The model algorithm adopts a reduced complexity but fundamentally physics-based approach, whilst maintaining equilibrium principles. Unlike, most prior models, the sub- and supra-tidal areas are represented explicitly in the model, as are sea level variations. Sediment transport is equated directly with the disequilibrium in wave energy dissipation flux, leading to a sediment transport formulation that negates the normal intermediate step of computing surfzone currents, generally required in process models. Two components of sediment transport are considered: The first is forced by the turbulent kinetic energy associated with wave breaking and the second diffusive term, is related to a sea bed-slope disequilibrium. The first component perturbs the equilibrium profile and dominates in the surfzone, whilst that latter component plays an important role in beach recovery. Equations are developed for a depth-averaged, beach profile model, assuming longshore uniformity. These computational efficient and stable equations facilitate long forecasts (>decade) and easy comparisons with a field data at cross-shore transport dominated field sites. At the test field site, the model is capable of reproducing qualitative observations of nearshore sand-bar dynamics and quantitative comparisons with measured coastal state indicators including both the shoreline displacement (r = 0.90, N.M.S.E. = 0.145) and intertidal beach volume (r = 0.87). • Days to decades prediction of coastal evolution • Accurate predictions of storm erosion & post-storm beach recovery • Skilful long-term prediction of beach volume and shoreline change & bar-dynamics • Explicit consideration sub- and supra-tidal topography & water level • Stable, efficient, equilibrium model suitable for longterm probabilistic forecasts [ABSTRACT FROM AUTHOR]

This study aims at improving the XBeach model in predicting dune erosion in the presence of belowground (land-based) biomass under varying hydrodynamic conditions. In this regard, the XBeach model was extended by a literature-derived root model, which increases the critical velocity for erosion in user-defined areas due to additional root cohesion. The model was validated by using the results of a small-scale wave flume experiment as a basis, where the presence of belowground biomass reduced the measured dune erosion. Control runs considering an exclusively sand-based dune, which served as a comparison in the physical experiment, were used to setup and calibrate a one-dimensional (1D) XBeach model. Due to the application to small-scale, default parameters related to sediment transport were scaled within the scope of calibration. Results showed that the XBeach model is not capable to reproduce the observed control dune profiles satisfactorily for all hydrodynamic conditions. Subsequently, the calibrated model was applied to the physical model runs with belowground biomass. Regardless of the hydrodynamic conditions, applying the root model led to a decrease in the mean sediment concentrations and, in turn, the model-predicted erosion in the vegetated area was mainly driven by avalanching. As a result, erosion volumes were reduced in the vegetated area and a higher agreement with the measurements was achieved. At this stage, the root model is highly simplified and only validated for collision at small-scale. In this regard, the translation of the model setup to field scale would eliminate the need for parameter scaling and could result in an overall better performance of the default model. Further research should address the influence of belowground biomass on the time evolution of dune failure and avalanching. • Novel approach to consider effect of belowground biomass on dune erosion volumes. • XBeach model extension and 1D-modelling of small-scale wave flume experiment. • Novel root model increases critical velocity for erosion in user-defined areas. • Accounting for belowground biomass improves erosion volume estimates considerably. • Large underprediction of overwash volumes due to general model-data mismatch. [ABSTRACT FROM AUTHOR]

In the present study, we addressed the potential impact of climate change on coastal erosion along the 28 beaches of the Basque Coast (150 km). A shoreline evolution model including cross-shore and alongshore sediment transport formulations, short- and long-term processes and hard structure treatment is developed. The historical analysis of the shoreline evolution is performed by forcing the model with waves and storm surges from reanalysis data, and with tides from in situ water level measurements. The model is calibrated and validated by using morphological information derived from the KostaSystem coastal video monitoring system. The analysis of the future impact of climate change on the sandy beaches is carried out using two sea level projections (RCP 4.5 and RCP 8.5 scenarios) and their confidence intervals, combined with synthetic wave series that allowed a probabilistic fashion outcome. The results show that the shoreline mean erosion due to both short- and long-term processes is in the range of 10–45 m under the RCP 4.5 scenario and 14–66 m under RCP 8.5 by the year 2100, which indicate the great vulnerability of the beaches of the Basque Coast. Still, the response of the studied systems is highly variable, owing to the intrinsic characteristic of each site. This result points to the difficulty of taking general management measures and highlights the need for management strategies at the local level. • A shoreline evolution model suitable for urban beaches is set up along the sandy systems of the Basque Coast. • The model outcome is validated against the data obtained from a video monitoring system and handled in a probabilistic way. • A new climate change beach impact indicator (beach recovery potential loss) is analysed. • Shoreline mean erosion is in the range of 10–45 m under the RCP 4.5 scenario and 14–66 m under RCP 8.5 by the year 2100. • Results point to the need of local management strategies. [ABSTRACT FROM AUTHOR]