Subaerial Beach Research Articles

The influence of seasonal to interannual nearshore profile variability on extreme water levels: Modeling wave runup on dissipative beaches

Wave runup, an important contributor to storm-induced extreme water levels, is commonly predicted via empirical formulations that parameterize coastal morphology using simple metrics such as the foreshore beach slope. However, spatially and temporally complex nearshore morphology, such as subtidal sandbars, have the potential to alter surf zone wave dissipation patterns and therefore influence setup, swash, and runup levels observed at the shoreline. In this study, a suite of numerical experiments using XBeach demonstrate reasonable skill in reproducing wave runup observations in dissipative settings, explore the relative influence of seasonal to interannual variability in nearshore morphology on runup and its constitutive components, and illustrate differences between empirical and numerically modeled estimates of runup. The numerical model results show that interannual variability in sandbar configuration, associated with net offshore sandbar migration, has a larger influence on wave runup than does seasonal sandbar variability. Although the particular configuration of sandbars was estimated to influence runup by as much as 0.18m during storm conditions, natural variability in subaerial beach topography has a stronger influence on runup than subtidal morphology. XBeach demonstrates that both wave setup and infragravity swash have morphologic controls. In experiments simulating storm conditions in which both nearshore and beach morphology was varied, natural interannual variability in beach topography explained about 80% of the variance in runup and its constituents. While XBeach predictions of setup, swash, and runup compare favorably with empirical predictors for low wave conditions, the numerical model predicts higher runup levels for storm-conditions on dissipative beaches raising potential concerns about coastal hazards assessments that use these empirical models to estimate extreme total water levels.

Shoreface storm morphodynamics and mega-rip evolution at an embayed beach: Bondi Beach, NSW, Australia

Embayed beach dynamics differ from open beaches due to the nature of headland control. Their resultant morphologies and morphodynamic behaviour are poorly understood due in part to a critical lack of surfzone and nearshore bathymetry observations. This study describes the morphodynamic storm response of a high-energy intermediate, 850m long embayed beach over a three week period spanning a cluster of storms. A headland and subaqueous ridge protects the northern end of the beach, resulting in an alongshore wave height gradient. Contrary to existing beach state conceptual models, under energetic forcing the beach did not ‘reset’ or enter a ‘cellular mega-rip’ beach state. The protected northern end persisted in a low energy state, while the wave exposed southern section transitioned from transverse-bar-and-rip to a complex double-bar system, a process previously undescribed in the literature. Bar-rip morphology at the exposed end of the beach migrated offshore to greater depths, leaving an inner-reflective beach and longshore trough, while a mega-rip channel with 3m relief developed at the exposed headland. The number of rip channels remained near constant over multiple storm events. Offshore sediment flux was 350m3/m at the exposed headland and 20m3/m at the protected end. Alongshore bathymetric non-uniformity decreased over the sub-aerial beach and inner surfzone, but increased in the outer surfzone and beyond. Suggested mechanisms for the persistence of 3D morphology during the cluster of storms include: (i) wave refraction to shore normal within the embayment; (ii) alongshore energy gradients; and (iii) pre-existing bar-rip morphology. Formation of the complex multi-bar state may be related to antecedent morphology, headland geometry, substrate gradient and localised hydrodynamic interactions near the headland. A new conceptual embayed beach state model is proposed for asymmetric, transitional embayed beaches. The model describes a pre-storm embayment where beach state changes gradually alongshore, while the post-storm embayment exhibits an extreme alongshore morphological gradient, from low-energy intermediate to a “complex multi-bar and mega-rip” state at the exposed end of the beach. Further observations are required to determine the prevalence of this high-energy state and to provide inputs to future numerical models designed to examine the dominant forcing controls involved in its formation.

New insights into embayed beach rotation: The importance of wave exposure and cross‐shore processes

AbstractAlthough embayed beach rotation has been viewed and modeled as an alongshore sediment transport process acting on a uniform beach profile, recent research suggests a more complex response whereby alongshore variability in cross‐shore sediment fluxes may be more significant. This study utilizes 5 years of fully three‐dimensional beach surveys at Narrabeen‐Collaroy Beach (SE Australia) to quantify the control of alongshore nonuniform wave exposure and cross‐shore processes on embayed beach rotation. Empirical orthogonal function analysis of the alongshore variability in subaerial beach volume/width and berm slope confirms that the dominant mode of subaerial beach variability is an onshore/offshore sediment exchange that is strongly controlled (R > 0.8) by the alongshore gradient in breaker wave height and coincides with a uniform flattening/steepening of the berm slope. A secondary rotation‐like signal is observed in both the subaerial beach volume/width data and, significantly, the berm slope. This inverse flattening/steepening of the berm slope between beach extremities is most likely a proxy for differing cross‐shore processes within the surf zone between the exposed and sheltered ends of the embayment, particularly with regards to dissipation of storm wave energy by offshore sandbars and beach recovery following storms. Analysis of the corresponding wave data reveals two distinct time scales of wave forcing characteristic of short‐term erosion and longer‐term recovery processes. A new conceptual model is presented of three differing modes of embayed beach rotation, with the newly identified beach rotation mode controlled by offshore sandbars considered of particular importance at embayments where headland sheltering of oblique waves is pronounced.

An analysis of subaerial beach rotation and influences of environmental forcing adjacent to the proposed Swansea Bay Tidal Lagoon

Beach Profiles surveys and gale climate data were utilised to assess medium timescale beach rotation at four beaches located along the shores of a crenulated embayment within Swansea Bay, Southwest Wales. The proposed Tidal Lagoon is located within this Bay. Results identified a 7 year (1998–2005) record of cyclic summer/winter rotation and a 14 year (1999–2013) record of annual rotation within the subaerial zone on all four assessed beaches. In the absence of headlands to trap sediment it is asserted that the driving force for beach rotation is the presence of Swansea Dockland/Tawe dredged channel complex, Port Talbot Harbour and the Neath dredged channel which form surrogate headlands essentially creating four separate beach systems through restricting sediment by-pass. Seasonal averaged wind and wave variables showed differing correlation with volume changes and cross-correlation results showed that volume variation lagged behind forcing variables by up to six months (i.e. the resolution of the data). This was confirmed by the annually averaged results which showed only subtle correlation. Here volume change in most cases lagged forcing variables by less than one year. Based on correlations, wind direction variability follows closely with volume changes but wind speed, wave period and height are generally opposite. Initial results suggest that the proposed Swansea Bay Tidal Lagoon, located between sediment cells and surrogate headlands, would have little negative effect on subaerial coastal processes. These datasets will be used as a benchmark for monitoring prior to, during and post construction, with results being used to update and inform subsequent strategies.

Variabilidade do sistema praia-dunas frontais para o litoral norte do Rio Grande do Sul (Palmares do Sul a Torres, Brasil) com o auxílio do Light Detection and Ranging – Lidar

Parâmetros morfométricos e geológicos, como orientação da linha de costa, potencial de deriva de vento, declividade e largura da praia subaérea, percentual de areia média e tamanho mé- dio do grão, são determinantes para a geomorfologia do sistema praia-duna frontal. Levantamentos feitos no litoral norte do Rio Grande do Sul (RS) através do sistema laser scanner permitiram a obtenção contínua dos principais parâmetros morfométricos, de forma rápida e precisa, sem a necessidade de levantamentos em campo. A variabilidade lateral desses parâmetros e sua associa- ção com as características geológicas foram consideradas para um setor de praia de 123 km entre Palmares do Sul e Torres através de análises cluster e escalonamento multidimensional, as quais definiram quatro grupos de afinidade. As variáveis que mais contribuíram para a diferenciação dos grupos foram: largura da praia subaérea, potencial de deriva de vento, azimute da linha de costa e percentual de areia média. Os grupos 1 e 4, situados mais ao sul da área de estudo, possuem balneá- rios com maior largura da praia subaérea, potencial de deriva de vento e variação de volume acima do datum, e apresentam os menores azimutes da linha de costa. O grupo 1 mostra o maior volume de perfil e altura das dunas frontais. O menor volume de perfil pertence ao grupo 3, consequência do maior azimute da linha de costa, responsável pela entrada quase paralela do vento dominante no sistema praia-duna e menor potencial de deriva de vento. O grupo 2 é caracterizado por valores de largura da praia subaérea semelhante ao grupo 3, porém possui maior volume de perfil e altura das dunas frontais. Essas diferenças podem ser explicadas pelo menor azimute da linha de costa do grupo 2. As variáveis largura da praia subaérea, potencial de deriva de vento e azimute da linha de costa e as características granulométricas da praia são fundamentais para explicar a variabilidade das dunas frontais no litoral norte do RS.

Quantification of changes in the beach volume by the application of an inverse of the Bruun Rule and laser scanning technology

We address the possibilities of combining terrestrial (TLS) and airborne laser scanning (ALS) techniques with the classical concept of equilibrium beach profile to quantify the changes in the total sand volume of slowly evolving sandy beaches. The changes in the subaerial beach are determined from a succession of ALS surveys that were reduced to the same absolute height using a TLS survey of a large horizontal surface of constant elevation. The changes in the underwater sand volume from the waterline down to the closure depth are evaluated using an inverse of the Bruun Rule. The relocation of the waterline is extracted from the ALS scanning of elevation isolines of 0.4-0.7 m. The method is applied to an about 200 m long test area in the central part of Pirita Beach (Tallinn Bay, north-eastern Baltic Sea). The sand volume in this area exhibits extensive interannual variations. The annual gain of sand in the entire beach was about 2000 m 3 /y in 2008-2010 and the annual loss was about 1100 m 3 /y in 2010-2014. The changes in the underwater part of the beach are by a factor of 2-2.5 larger than the changes in the

TYPHOONS DRIVEN MORPHODYNAMICS OF THE WAN-TZU-LIAO SAND BARRIER (TAÏWAN)

This study focuses on the dynamics of the Wan-Tzu-Liao sand barrier when it is struck by one typhoon, a group of typhoons or the whole season of typhoons and monsoon. In the framework of the KUN-SHEN project, 7 months of monitoring (2011-2012) provided 20 topobathymetric surveys (within a 200 m long segment of the barrier from the subtidal zone to the back-barrier) and acquisitions of offshore, nearshore and shallow water hydrodynamics including velocity profiling, free surface measurement and absolute pressure. Offshore waves were extracted at Cigu buoy (18 m of water depth). Nearshore waves were acquired from the current profiler deployed 400 m off the coast in 4 m of water depth and water level on the subaerial beach were acquired from two pressure sensors deployed in the lower part of the subtidal zone and the dune crest. Morphologic changes of the emerged beach were monitored using D-GPS each week during winter monsoon season and just before and after each event during summer typhoons season. Amongst the eight studied typhoons, TALIM is the most energetic event observed. Offshore wave height reached H s = 10.34 m (T p = 14.6 s) at Cigu buoy and H s = 2.3 m (T p = 13.4 s) at the profiler. Surface waves dynamics throughout surf and swash zones are progressively dominated by infragravity motions. A part of the dune is surged at the storm apex. Morphological changes include 6.7 m of dunefoot retreat and a sand transfer from a dune breach to wash-over deposits in the lagoon. More surprisingly, the foreshore was nourished (2261 m3 +/- 268 m3 ) as well as the whole sand barrier (+1920 m3 +/- 1071 m3 ). The sand input is concomitant to the redistribution of sand from the intertidal sandbars interpreted as the ultimate stage of the subtidal bars landward migration. Although winter are erosive season (-4995 m3 +/- 1071 m3 ), the summer results in an accretion period (3556 m3 +/-1071 m3 ) with a shoreline seaward shift about 10.4 m. Over the annual time period, the sand barrier recorded 18.4 m of retreat coupled with a small sand loss (-1439 m3 +/- 1071 m3 ) without any significant abrasion of the dune-top.

Resolution and Accuracy of an Airborne Scanning Laser System for Beach Surveys

Abstract Airborne scanning laser technology provides an effective method to systematically survey surface topography and changes in that topography with time. In this paper, the authors describe the capability of a rapid-response lidar system in which airborne observations are utilized to describe results from a set of surveys of Narrabeen–Collaroy Beach, Sydney, New South Wales, Australia, over a short period of time during which significant erosion and deposition of the subaerial beach occurred. The airborne lidar data were obtained using a Riegl Q240i lidar coupled with a NovAtel SPAN-CPT integrated Global Navigation Satellite System (GNSS) and inertial unit and flown at various altitudes. A set of the airborne lidar data is compared with ground-truth data acquired from the beach using a GNSS/real-time kinematic (RTK) system mounted on an all-terrain vehicle. The comparison shows consistency between systems, with the airborne lidar data being less than 0.02 m different from the ground-truth data when four surveys are undertaken, provided a method of removing outliers—developed here and designated as “weaving”—is used. The combination of airborne lidar data with ground-truth data provides an excellent method of obtaining high-quality topographic data. Using the results from this analysis, it is shown that airborne lidar data alone produce results that can be used for ongoing large-scale surveys of beaches with reliable accuracy, and that the enhanced accuracy resulting from multiple airborne surveys can be assessed quantitatively.

Estimating Changes in Near-Shore Bathymetry with Subaerial Surveys

Abstract Surveys of the subaerial beach (e.g., landward of approximately the MSL depth contour) are widely used to evaluate temporal changes in sand levels over large alongshore reaches. Here, seasonal beach face volume changes based on full bathymetry beach profiles (to ~8 m in depth) are compared with estimates based on the subaerial section of the profile. The profiles span 15 years and 75 km of Southern California shoreline, where seasonal vertical fluctuations in near-shore sand levels of a few meters are common. In years with relatively low winter wave energy, most erosion occurs above the MSL contour, and subaerial surveys capture as much as 0.8 of the total (relatively small) seasonal beach face volume change. In response to more energetic winter waves, beach face erosion increases and occurs as deep as 3 m below MSL, and subaerial surveys capture as little as 0.2 of the total beach face volume change. Patchy, erosion-resistant rock and cobble layers contribute to alongshore variation of the subaerial fraction of beach face volume change.

Evaluating proxies for estimating subaerial beach volume change across increasing time scales and various morphologies

ABSTRACTProxies, such as changes in beach profiles and shoreline positions, are commonly used in management and research for estimating changes in subaerial beach volume; however, the accuracy of these proxies across increasing time scales and complex morphologies is unclear. Volume changes associated with along‐beach morphologic variability may not be captured well by changes in profiles, while volume changes associated with across‐beach morphologic variability may not be captured well by measuring shoreline change. This study assesses the impacts of morphologic variations, associated with beach cusps and nourishment material, on volume change estimates from profiles and shoreline change at 0.5 to 3.5 year time periods. Results indicate that profiles spaced ≥ 150 m apart and the shoreline‐change proxy will likely estimate volume change inaccurately over periods ≤ 1 year at beaches that are consistently eroding or accreting and contain cusps. However, over longer time periods (1–3.5 years), estimates of volume change from both proxies improved at those types of beaches. Volume changes at the edges of nourishment areas are not captured well by profiles. When the nourishment material is graded to a ramped morphology, which minimizes across‐beach morphologic variability, the shoreline‐change proxy does accurately estimate volume changes. Both proxies estimate volume changes inaccurately at beaches where volume changes oscillate between erosion and accretion on both short and long time scales because the magnitude of small‐scale changes in volume from the formation and erosion of morphologic features, such as cusps and berms, will always be similar to the longer‐term net volume change. This study suggests that decadal records of shoreline change, which are commonly developed using aerial photography, can be used to help identify the best proxy for estimating volume change; however, recent anthropogenic modifications that impact patterns of beach sedimentation, including nourishment, terminal groins, and inlet‐channel dredging, makes decadal records less useful. Copyright © 2013 John Wiley & Sons, Ltd.

The influence of limestone reefs on storm erosion and recovery of a perched beach

Mechanisms through which naturally-occurring hard landforms, such as rock and coral reefs, influence coastal sediment transport are still poorly understood. Therefore, field investigations were undertaken during storm conditions on the sandy beaches of Yanchep Lagoon in southwestern Australia, which are perched on Quaternary limestone reefs. During two consecutive winter storms, the response of three subaerial beach profiles were quantified at: (a) an Exposed Profile which was fronted to seaward by a predominantly sandy substrate; (b) a Reef Profile that was fronted directly seaward by limestone outcrops submerged below mean sea level; and (c) a Bluff Profile where the dry beach was perched on a limestone bluff that reached above mean sea level and that contained a shallow coastal lagoon. The subaerial beach response to the storms had considerable spatial variation alongshore and was strongly dependent on the local rock topography. The Exposed Profile eroded most with a 2m-high scarp cut into the dune while the dunes at the Reef and Bluff Profiles were stable. The Bluff Profile also eroded considerably and the coastal lagoon widened and deepened. The Reef Profile was the most stable overall because erosion was balanced by short periods of accretion during the storm period which was partly due to sediment supplied by longshore transport through the coastal lagoon from the Bluff Profile. During the month after the storms wave energy was relatively low and the beach at the Exposed Profile accreted almost to the pre-storm volume, although the scarp in the dune was still present. The Reef Profile accreted most in the month after the storms while recovery at the Bluff Profile was low. It appeared that the bluff inhibited onshore sediment transport during and after the storms and in addition, strong currents in the lagoon transported sediment alongshore to supply the other beach profiles. These observations indicated that rock topography, especially elevation relative to sea level determined if beach erosion was reduced during storms and whether accretion was dampened in the post-storm recovery phase.

An experimental study on sediment transport and bed evolution under different swash zone morphological conditions

In this study, new large‐scale experimental data are presented showing evidences of a link between the swash zone dynamics and the surf zone morphodynamics. Two sets of large‐scale experiments are presented. The first set of experiments investigates the swash and surf dynamics under the same hydrodynamic forcing but with two different swash zone morphological conditions, one of which was created by manual reshaping of the sub-aerial beach face. It is shown that more dissipative swash zone conditions (a more mildly sloping beach face) significantly reduce the rate of seaward sandbar migration such that it almost ceases during the performed experimentation time. This behavior is compared to a second set of experiments which shows the natural sand bar offshore migration under almost identical hydrodynamic forcing. The changes in the sandbar migration are investigated in terms of the differences in the swash zone dynamics. The more reflective swash zone is characterized by more intense backwashes that, in turn, promote significant quantities of sediment suspension during the interaction of the backwash with the subsequent bore. The dissipative swash zone leads to a larger number of wave–swash interactions, reducing the backwash magnitudes and the magnitude of suspended sediment concentration events in the swash zone, with a corresponding reduction in the offshore suspended sediment transport. The total offshore sediment transport rates are also shown to reduce, leading to the observed modification of the sandbar migration.

Submarine bars on sandy coasts

A brief review of the studies concerning the problem of submarine bars over the past decade is presented. Various types of bars are distinguished, and the mechanisms responsible for the formation thereof are discussed. The short-term (days, weeks) behavior of the bars is described in relation to the changes in the local wave parameters. The bar systems are shown to demonstrate cyclic behavior of two different types on time scales of years and decades. In the former case, the bars arise near the shore, migrate towards the sea, and degrade in the external margin of the coastal zone. The other cycle’s type is characterized by the landward migration of the bars and their welding with a subaerial beach, which results in creating a conveyor delivering material to the beach and eventually to the foredune by Aeolian transport. In the former case, the bar zone behaves as a closed system, while, in the latter, as a transition zone. It is noted that the long-term evolution of a bar system is controlled by a feedback mechanism tending to return the system to the initial state.

Sub-weekly to interannual variability of a high-energy shoreline

Sixty-one Global Positioning System (GPS), sub-aerial beach surveys were completed at 7 km long Ocean Beach, San Francisco, CA (USA), between April 2004 and March 2009. The five-year time series contains over 1 million beach elevation measurements and documents detailed changes in beach morphology over a variety of spatial, temporal, and physical forcing scales. Results show that seasonal processes dominate at Ocean Beach, with the seasonal increase and decrease in wave height being the primary driver of shoreline change. Storm events, while capable of causing large short-term changes in the shoreline, did not singularly account for a large percentage of the overall observed change. Empirical orthogonal function (EOF) analysis shows that the first two modes account for approximately three-quarters of the variance in the data set and are represented by the seasonal onshore/offshore movement of sediment (60%) and the multi-year trend of shoreline rotation (14%). The longer-term trend of shoreline rotation appears to be related to larger-scale bathymetric change. An EOF-based decomposition technique is developed that is capable of estimating the shoreline position to within one standard deviation of the range of shoreline positions observed at most locations along the beach. The foundation of the model is the observed relationship between the temporal amplitudes of the first EOF mode and seasonally-averaged offshore wave height as well as the linear trend of shoreline rotation. This technique, while not truly predictive because of the requirement of real-time wave data, is useful because it can predict shoreline position to within reasonable confidence given the absence of field data once the model is developed at a particular site.

Perigos E Riscos Associados A Processos Costeiros No Litoral Sul Do Brasil (RS): Uma Sintese

Some processes related to hydrodynamics, geomorphology and sediment movement represent coastal hazards and risks along the littoral of Rio Grande do Sul (RS) state in southern Brazil. The hazards imply in coastal erosion, habitat loss and environmental change. The risks are related to serious accidents including fatalities to the coastal users. Shoreface morphology, storms and washouts induce erosion at specific locations of the coastline. Storms and washouts, together with beach morphodynamics and depositional processes both aeolian and hydrodynamic embody latent risks to the physical integrity of people that live in and use this environment. Wave refraction patterns from SE to SW cause erosion in two coastal sectors at the proximities of Conceicao Lighthouse and Hermenegildo Beach respectively located at the central and southernmost portion of the RS coastline. In both areas, the high number of washouts amplifies the erosion destroying foredunes and the subaerial beach. In addition, the lowlands of the barrier adjacent to small pocket lagoons in the northern littoral are more susceptible to washout erosion. Both, the refraction patterns and the washouts can be associated with two storms tracks and extra-tropical cyclones which magnify erosion and inundation of the coastal zone. The combination of higher population during summer seasons and intermediate beaches with considerable amounts of medium sand make beaches from the northern littoral and extreme south more risky for bathers. Coastline orientation in relation to the predominant NE wind causes wind-blow sand to invade the coastal plain in the form of transgressive dunes causing several hazards mainly in the northern littoral between Tramandai and Mostardas. Short-term effects associated with episodic events of mud deposition during heavy storms on Cassino Beach influence the morphodynamics behavior on the sectors affected by the mud deposits creating coastal risks relating to beach usage.

Dramatic beach and nearshore morphological changes due to extreme flooding at a wave-dominated river mouth

Record flooding on the Santa Clara River of California (USA) during January 2005 injected ∼ 5 million m 3 of littoral-grade sediment into the Santa Barbara Littoral Cell, approximately an order of magnitude more than both the average annual river loads and the average annual alongshore littoral transport in this portion of the cell. This event appears to be the largest sediment transport event on record for a Southern California river. Over 170 m of local shoreline (mean high water (MHW)) progradation was observed as a result of the flood, followed by 3 years of rapid local shoreline recession. During this post-flood stage, linear regression-determined shoreline change rates are up to −45 m a − 1 on the subaerial beach (MHW) and − 114 m a − 1 on the submarine delta (6 m isobath). Starting approximately 1 km downdrift of the river mouth, shoreline progradation persisted throughout the 3-year post-flood monitoring period, with rates up to + 19 m a − 1 . Post-flood bathymetric surveys show nearshore (0 to 12 m depth) erosion on the delta exceeding 400 m 3/m a − 1 , more than an order of magnitude higher than mean seasonal cross-shore sediment transport rates in the region. Changes were not constant with depth, however; sediment accumulation and subsequent erosion on the delta were greatest at − 5 to − 8 m, and accretion in downdrift areas was greatest above –2 m. Thus, this research shows that the topographic bulge (or “wave”) of sediment exhibited both advective and diffusive changes with time, although there were significant variations in the rates of change with depth. The advection and diffusion of the shoreline position was adequately reproduced with a simple “one line” model, although these modeling techniques miss the important cross-shore variations observed in this area. This study illustrates the importance of understanding low-frequency, high volume coastal discharge events for understanding short- and long-term sediment supply, littoral transport, and beach and nearshore evolution in coastal systems adjacent to river mouths.

Short‐term to decadal‐scale onshore bar migration and shoreline changes in the vicinity of a megatidal ebb delta

Swash bar development has been well documented from coasts with low to moderate tidal ranges, while studies of the effects of these forms on the morphology and dynamics of the adjacent shore in large tide range environments are rare. The analysis of sequential vertical aerial photographs was combined with field work in order to highlight the effects of swash bar development on the adjacent shoreline in the vicinity of a megatidal inlet (mean spring tidal range of 11 m). Swash bars are observed to form on the ebb tidal delta and to migrate landward before welding onto the coast. A close relationship was noticed between the position of the swash bar and shoreline dynamics. The bar protects the shore against wave attack in a sedimentary system controlled by longshore transport. This protective role is, however, modulated by the large tidal range. As the bar migrates upward toward the high‐tide level and the subaerial beach, it develops morphologically into a transverse form that acts as a cross‐shore obstacle to longshore sediment transport, thus resulting in shoreline accretion updrift and in strong erosion downdrift. This disturbance may persist for years because of the relatively slow speed of movement of the bar at this stage, an aspect characteristic of large tide range environments. This pattern of behavior differs fundamentally from that documented in the literature where the perturbation of the longshore sediment transport occurs over shorter periods. An original conceptual model of swash bar morphodynamics and repercussions on the adjacent shoreline for this megatidal environment is proposed.

Analytical model of beach erosion and overwash during storms

During severe storms high waves and water levels may greatly impact the sub-aerial portion of the beach inducing significant morphological change at elevations that the waves can not reach under normal conditions. Morphological formations such as dunes and barrier islands may suffer from direct wave impact and erode. Overwash occurs if the wave run-up and/or the mean water level are sufficiently high allowing for water and sediment to pass over the beach crest, which in turn causes flooding and deposition of sediment shoreward of the crest. An analytical model of sub-aerial beach response to storms was developed based on impact theory, including overwash, and the evolution of schematised dunes was investigated. Furthermore, the analytical model was applied to the case of schematised barrier islands exposed to extensive overwash. After validation using field data, the analytical model was employed at two coastal sites, namely Ocean City on the United States east coast and the Ebro Delta on the Spanish Mediterranean coast, in order to calculate quantities for assessing the storm impact on beaches, such as eroded volume, overwash volume, beach crest reduction, and contour-line retreat. These quantities were subsequently analysed to derive empirical probability distribution functions to be utilised in different types of risk assessment concerning flooding and erosion in coastal areas.

Monitoring Beach Renourishment along the Sediment-Starved Shoreline of Grand Strand, South Carolina

In this paper, we examine temporal and spatial beach profile volume changes, sediment budget changes, and side-scan sonar images at nourished beaches of northeastern South Carolina. Results of bulk volume change indicate that most sands eroded from the subaerial beach section remain and circulate within the coastal system. The results also indicate more active sediment exchange between the nearshore and offshore zones than expected, indicating that the offshore can be both a sink and a source for the nearshore morphologic change. Our profile volume change results do not show a unidirectional net southerly transport pattern in the study area during the initial postnourishment period. Instead, results show that the net downdrift direction alternates along the shore and that longshore volume drift patterns are often disrupted by the seaward cross-shore transport events that occur at erosional hotspots. Prenourishment erosional patterns, particularly local areas of elevated erosion rates, were re-established after nourishment. Furthermore, those erosional locations often correspond to the offshore locations of paleoriver channel fill sands with low relief, indicating existence of the specific seaward transport pathways along the shore in the study area.

Contemporary morphodynamics of a high-energy headland-embayment shoreface

A 2-year investigation into shoreface morphodynamic behaviour off a high-energy headland-embayment coast in Northern Ireland reveals important process–response mechanisms that cannot be explained solely by existing conceptual models. Fourteen sequential bathymetric surveys, conducted every 1–2 months, show that morphologic (seabed) change is not directly related to oceanographic forcing—extensive nearshore and shoreface accretion and erosion occurs under fair-weather, modal and high-energy conditions. The main factors which seem to cause significant change are long-duration (swell) events coupled with onshore winds, availability of (recently) introduced sediment, surges and elapsed time between storms and the next scheduled survey. Several high-energy events over a short time period (<30 days) did not result in extensive seafloor changes, contrary to expectations. Net seabed change over 2 years shows an average to 0.6 m m −2 of shoreface accretion from the nearshore to 24 m depth. Net erosion was not observed anywhere, including the subaerial beach. Geologic evidence strongly suggests that the source of the significant volume (7.7×105 m 3) of sediment introduced into the study area must have been derived from the lower shoreface and/or inner shelf, beyond 24 m depth.