Beach nourishment has a high cost but offers large economic benefits. Therefore, extending the nourishment lifespan using dredged sediment from navigation channels could have a significant economic impact. This numerical modeling study develops and compares two approaches incorporating nearshore sediment placements into an existing one-dimensional numerical modeling procedure to predict the lifespans of subaerial beach nourishment strategies. Both approaches build directly on the stochastic lifecycle simulation methodology and results used in the Coastal Texas Protection and Restoration Feasibility Study. Simulations for western Galveston Island were modified to include annually recurring nearshore nourishment. Cross-shore beach transects were forced with 50 years of tropical cyclones and nontropical storms in a cross-shore morphological evolution model, and rebuilt when the dune eroded to half of its initial height. One group of simulations applied this previously developed model forcing to cross-shore profiles that were updated with recurring nearshore nourishments. A second group of simulations included a simplified representation of sediment deposition from the alongshore transport gradient created by the nearshore nourishments. In both sets of simulations, a large sediment feature was incrementally constructed at depths between 2.5 and 6 m. Over 30 life cycles, the number of times the beach was rebuilt was tracked for each 50-year simulation. Comparing the number of predicted beach renourishments indicates that this particular nearshore nourishment strategy did not substantially impact the subaerial beach morphology unless alongshore transport gradients were also included. Simulations that did include this alongshore transport gradient predicted 23% longer lifespans. This work was not able to incorporate validation against measured data, but future testing of this approach should be pursued before widespread or high-impact application. Modeling results indicate that alongshore processes are an important part of quantifying the positive impacts of nearshore nourishment.

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.

Coastal regions are susceptible to increasing flood risks amid climate change. Coastal wetlands play an important role in mitigating coastal hazards. Vegetation exerts a drag force to the flow and dampens storm surges and wind waves. The prediction of wave attenuation by vegetation typically relies on a pre-determined drag coefficient CD. Existing CD formulas are subject to vegetation biomechanical properties, especially the flexibility. Accounting for vegetation flexibility through the effective plant height (EPH), we propose and validate a species-independent relationship between CD and the Reynolds number Re based on three independent datasets that cover a wide range of hydrodynamic conditions and vegetation traits. The proposed CD−Re relationship, used together with EPH, allows for predicting wave attenuation in salt marshes with high accuracy. Furthermore, a total of 308,000 numerical experiments with diverse wave conditions are conducted using the proposed CD−Re relationship and EPH to quantify the wave attenuation capacity of two typical salt mash species: Elymus athericus (highly flexible) and Spartina alterniflora (relatively rigid). It is found that wave attenuation is controlled by wave height to water depth ratio and EPH to water depth ratio. When swaying in large waves in shallow to intermediate water depth, a 50-m-long Elymus athericus field may lose up to 30% capacity for wave attenuation. As wave height increases, highly flexible vegetation causes reduced wave attenuation, whereas relatively rigid vegetation induces increased wave attenuation. The leaf contribution to wave attenuation is highly dependent on the leaf rigidity. It is recommended that leaf properties, especially its Young’s modulus be collected in future field experiments.

Abstract Sandbars are ubiquitous morphologic features found in the nearshore environment throughout the world, yet predictive capabilities of their evolution remain limited. In order to provide new insights on the relevant processes controlling sandbar morphodynamics, this study uses a 41‐year record of 637 monthly cross‐shore profiles from Duck, North Carolina, USA, to derive complex empirical orthogonal functions representative of the two dominant modes of sandbar migrations: offshore and onshore propagation. Interference of these two modes produces commonly observed sandbar states. While mild wave energies are traditionally assumed to drive onshore sandbar migration, the offshore mode is repeatedly seen to dominate sandbar migration in mild wave seasons following anomalously high late‐winter wave energy and when the inherited morphology is composed of a single bar or terrace, as opposed to a more common two‐bar state. A data‐derived conceptual model is presented synthesizing the effect of antecedent morphology and wave climate chronology on interannual trends in net offshore sandbar migration.

Coastal Sediments 2023, pp. 1977-1987 (2023) No AccessPHASE-RESOLVING CROSS-SHORE SAND TRANSPORTBRADLEY D. JOHNSONBRADLEY D. JOHNSONU.S. Army Engineer Research and Development Center, Coastal and Hydraulics Laboratory, 3909 Halls Ferry Road, Vicksburg, MS 39180-6199, USAhttps://doi.org/10.1142/9789811275135_0181Cited by:0 (Source: Crossref) PreviousNext AboutSectionsPDF/EPUB ToolsAdd to favoritesDownload CitationsTrack CitationsRecommend to Library ShareShare onFacebookTwitterLinked InRedditEmail Abstract: Coastal morphodynamics models have idealized onshore and offshore transport that are regulated with arbitrary non-physical empirical coefficients. Even process-based models can be alternatively erosive or accretionary through parameter selection, and this ambiguity limits the value of the models. The mechanics of estimating a synthetic near-bed skewed asymmetric free stream time-series and computing the sediment transport are outlined herein. Initial model predictions are compared with the well-controlled and publicly-available BARSED flume data, as detailed in Mieras et al. (2017), including wave, hydrodynamics and transport measurements. Wave shape predictions agree well with data but have under-predicted extrema for the lower frequency waves included in the data suite. Sediment concentration fields are computed with phase-resolved, coupled bedload and suspended load models, where sediment pickup is estimated to be linear with excess shear, therefore naturally including the impact of wave skewness but not asymmetry. Onshore and offshore sand transport estimates computed under the wave crest and trough compare well with data, and error for most test cases is below 20%. FiguresReferencesRelatedDetails Recommended Coastal Sediments 2023Metrics History PDF download

This technical report presents the new numerical modeling capabilities for simulating wave attenuation and mean water level changes through flexible vegetation such as smooth cordgrass in coastal and marine wetlands. These capabilities were implemented into the Cross-SHORE (CSHORE) numerical model. The biomechanical properties of vegetation such as dimensions, flexibility, and bending strength are parameterized in terms of the scaling law. Correspondingly, a new formulation of the vegetation drag coefficient, CD, is developed using field data from a salt marsh in Terrebonne Bay, LA, by considering spatially varying effective stem and blade heights of species. This report also presents a general procedure for using the model to simulate hydrodynamic variables (i.e., waves, currents, mean water levels) at vegetated coasts, which are used to quantify the effects of wave attenuation and reduction of surge and runup due to vegetation. Preliminary model validation was conducted by simulating a set of laboratory experiments on synthetic vegetation, which mimicked the flexibility of Spartina alterniflora. The validation results indicate that the newly developed vegetation capabilities enable CSHORE to predict changes of wave heights and water levels through marshes by considering species-specific biomechanical features. The model is also applicable to assess vegetation effectiveness against waves and surges.

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 sys-tems 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 land-scape change is presented. DOONIES simulates critical biotic and abiotic processes of vegetation growth, abun-dance, and spatial distribution dynamics impacting topography and storm response. Biological processes and ecogeomorphic responses are tailored to generalizable dune functional-communities with species-specific repre-sentatives. Estimates of the balance between photosynthesis and respiration dictate plant growth and morphol-ogy 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 topog-raphy to actual across 6 years at Island Beach State Park, NJ while modeling Hurricane Sandy and daily wind con-ditions 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 re-sponse of naturally occurring and planted dune vegetation communities to typical abiotic conditions, as a tool for supporting and exploring restoration decisions.Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons. org/licenses/by-nc-nd/4.0/).

A large, low pressure Nor'easter storm and Hurricane Joaquin contributed to multiple weeks of sustained, elevated wave and water level conditions along the southeastern Atlantic coast of the United States in Fall 2015. Sea level anomalies in excess of 1 m and offshore wave heights of up to 4 m were recorded during these storms, as observed at the U.S. Army Corps of Engineers' Field Research Facility in Duck, NC, USA. In response to these energetic oceanographic conditions, there were highly variable morphologic changes to the dune over short spatial scales (<km) which included a range of responses from vertical dune scarping to no measureable response. The portion of the study area with the largest dune erosion occurred at a location fronted by an abnormally deep nearshore bathymetric feature, which altered surf-zone waves and hydrodynamics. The pre-storm beach and dune topography also varied throughout the study area, additionally influencing the frequency of dune collision and contributing to the spatially variable erosion patterns. This work uses field datasets and numerical modeling tools to investigate the causation of hotspot dune erosion at the Field Research Facility. Three different numerical models were tested against the available data in order to assess model skill at resolving complex spatial dune erosion patterns. The three models successfully reproduce the general spatial trends in alongshore variable responses, although not necessarily the details of profile response or net erosion magnitude. Analysis of the model outputs, in conjunction with the available field data, suggests that the observed hotspot dune erosion is related to a complex combination of both topographic and bathymetric controls on the processes driving dune erosion. Therefore, the most simplistic model tested, which only accounts for alongshore variations in topographic profile details, can only predict hotspot dune erosion in locations where steep beach and/or dune topography is the primary control on collisional dune impacts. The higher fidelity models, which account for feedback effects from subaqueous morphology, are similarly able to predict the locations of maximum hotspot erosion, but are sensitive to beach over-steepening and/or errors in wave runup calculations that can lead to over-prediction of simulated dune erosion. This work highlights that numerous existing tools are capable of identifying the foredune regions at most risk from hotspot erosion, as well as the need for continued research to improve representation of all relevant intra-storm morphodynamic processes.

The hazard imposed by storms may be worsened with an increase in frequency and magnitude of future hur ricanes. The C2SHORE numerical model is applied during this study to estimate the sediment transport pathways dur ing differing storm conditions, incorporating both high -energy wave-driven processes and current-dominated over-topping conditions. The predictive capability of C2SHORE is demonstrated through an application at Ship Island, Mississippi, USA for Hurricane Katrina. Finally, the performanc e of various restoration alternatives is assessed, including sensitivity to grain size for sediment placement. Three hypothetical storms are presented, with appro ximate return periods of 1, 10, and 500 years, and the transport environment is examined within the framework of these events for the proposed restoration scenarios.

Typical practice for a century has been to transport dredged sand to an offshore disposal site in deep water where the sediment is lost from the littoral system. The alternative of nearshore placement can retain the sand, but the fate of the material is poorly understood. A set of laboratory experiments were conducted, using tracer sand, with the intent of quantifying the migration of material with alternative dredged mound placements within the surf zone. Conventional depth-integrated tracer sand transport models can utilize a correction factor or a gradient diffusion mechanism to represent the effects of the depth variation. In the surf zone, however, an analytical correction factor is not available and a gradient diffusion coefficient is arbitrary with no physical basis. An alternative simple advective transport sand model is introduced herein that explicitly predicts both the advection associated with the return current and the wave-related onshore transport. With a simple framework based on a suspended layer and a bedload layer of arbitrary transport directions, the Taylor dispersion of tracer sand is explicitly computed without any dependence on a diffusion mechanism. Both the modeled and measured results indicate transport directed offshore by the undertow, onshore by the wave asymmetry, and down-drift as forced by the longshore current.

: The present recommendations for dune removal or dune retreat on the Great Lakes for Federal Emergency Management Agency (FEMA) flood mapping purposes are based on a simple geometric method as outlined in FEMA (2009). The simple procedure establishes a relationship between dune survival and storm intensity. The method was adopted several decades ago when numerical models were inadequate for predicting beach profile change and dune retreat. Herein, a robust and efficient model for predicting nearshore waves, circulation, water levels, sediment transport, and nearshore morphology is provided as an option in replacing the geometric model. The numerical model is compared with results from the existing FEMA guidelines for storm-induced beach change at three sites with different nearshore characteristics. A typical storm with detailed modeled hydrodynamics has been used to provide a basis for boundary conditions, and use is made of scaling to approximate a variation in intensity. A change in water levels is also included in the analysis through a reasonable fluctuation of the static lake level. In general, the methodology in FEMA guidelines computes much larger eroded volume than the numerical model predictions. Additionally, the dependence of the volumes on recurrence interval is determined to be much stronger utilizing the FEMA method. Generalizations are difficult, however, and the predicted volume from the numerical model was larger for some cases of moderate storm intensity. The numerically predicted results were shown to depend on the details of both the subaqueous profiles and dune configuration, and simple universal predictions may suffer gross error.

Earthen levees are designed for little wave overtopping during a design storm, but excessive overtopping and overflow can occur due to the combined effects of an extreme storm, sea level rise, and land subsidence. The transition from little wave overtopping to excessive wave overtopping and overflow on an impermeable smooth levee is examined in wave-flume experiments consisting of 107 tests. Existing empirical formulas are shown to be applicable to the cases of excessive wave overtopping and overflow in these tests. A numerical model based on time-averaged continuity, momentum, and wave action equations is connected to a new probabilistic model for the wet-and-dry zone, in order to predict the cross-shore variations of the mean and standard deviation of the free surface elevation and depth-averaged fluid velocity from outside the surf zone to the inner slope of the levee. The new model is calibrated to predict the measured overtopping and overflow rates within a factor of about two. The agreement is also shown to be similar for the water depths and velocities measured in the wet-and-dry zone on six different structures in 100 Dutch tests. The developed hydrodynamic model is coupled with new formulas for suspended-sand and bedload transport rates to predict dune erosion and overwash. The coupled model is compared with two small-scale tests on dune erosion with minor overwash, three large-scale tests on dune erosion, and field data on dune erosion and overwash due to severe storms lasting several days. The overwashed-dune profiles are predicted reasonably well, but the coupled model will need to be evaluated using measurements of wave overtopping and overwash rates.