Cross-shore Profile Research Articles

On the Predictability of Mid-term Cross-shore Profile Evolution

Fernàndez-Mora, À., Calvete, D., Falqués, A., Ribas, F., and Idier, D., 2013. On the Predictability of Mid-term Cross-shore Profile EvolutionThe aim of the present study is to analyse the mid-term beach profile evolution, considering the hypothesis that the alongshore processes can be neglected for the prediction of the mean profile evolution. To this end, a process-based model for the evolution of the cross-shore profile has been used. The model describes feedbacks between waves, rollers, depth-averaged currents and bed evolution, accounting for the effects of wave skewness and asymmetry on sediment transport. Offshore waves and tides conditions and bathymetric profiles measured at the FRF-USACE Duck are used to simulate a mid-term (72 days) onshore sandbar migration event. The model results agree with observed onshore movement and growth of the sandbar due to the inclusion of the intra-wave oscillatory flow with the skewness and asymmetry effects. The best predictions belongs to the averaging of the modelled evolution of individual cross-shore profiles that is better than the evolution of the mean cross-shore profile since. it takes into account the alongshore variability of the cross-shore profiles. These two methods result on better predictions than the individual profiles during the entire event.

A Centurial Record of Beach Rotation

ABSTRACT Thomas, T., Phillips, M. R., and Williams A. T., 2013. A Centurial Record of Beach Rotation. Beach rotations are reliant on a bi-directional wave climate and headlands to impede alongshore sediment transport. This manifests itself in localised shoreline retreat or advance but does not lead to long term sediment loss or gain, as beaches often return to initial conditions in response to wave direction shifts and these changes are often seasonal. This paper assesses morphological changes of a headland embayed beach (Tenby, West Wales) over a 180 year period using GIS, cross shore profiles, and wave modelling. Within GIS maps, aerial photographs and direct field measurements identified two significant changes in beach orientation between the periods 1830–1919 and 1919–2009. Analysis of more recent data (1941–2009) showed that a statistically significant (R2 = 64%) negative phase relationship existed between the beach extremities and correlation changes revealed central region rotation. Results were c...

Modelling morphological changes of beach and dune induced by storm on the Southern Baltic coast using XBeach (case study: Dziwnow Spit)

Bugajny, N., Furmańczyk, K., Dudzińska-Nowak, J., Paplińska-Swerpel, B., 2013. Modelling morphological changes of beach and dune induced by storm on the Southern Baltic coast using XBeach (case study: Dziwnow Spit)The purpose of this paper is to demonstrate the usefulness of XBeach model (1D) in modelling changes of beach and dune morphology in terms of significant storm influence at sandy Dziwnow Spit area located in the non-tidal Baltic Sea, on the western part of the Polish coast. Research were carried out in the framework of the MICORE project (7th FP) the main goal of which was to develop an operational early warning system for coastal hazard based on the XBeach model. For model calibration, one significant storm event registered in 2009 was simulated and compared against pre- and post-storm morphological data consisting of geodetic measurement and airborne laser scanning data. XBeach model has been run for 8 cross-shore profiles with different configuration of model parameters. The model skill were tested on the base of statistical descriptor BSS for the terrestrial part of the profiles. The highest BSS values varied from 0.54 to 0.90 depending on the profile. The best performance of XBeach model for each profile was obtained by deactivating longwave stirring lws and keeping hmin parameter value to 0.05 in all simulations. Values of wetslp equalled 0.3 or 0.4 while dryslp were 1 or 1.5. The facua parameter values oscillated between 0.1 and 0.5. The results show that XBeach is reasonably modelling morphological changes like dune and beach erosion induced by storm event in Southern Baltic conditions and can be successfully applied to create an effective tool for hazard prediction.

On the occurrence of non-reflecting cross-shore profiles along Estonian coasts of the Baltic Sea

Cross-shore beach profiles along Estonian coasts of the Baltic Sea are analysed from the viewpoint of the frequency of occurrence of convex sections that may support non-reflecting wave propagation and unexpectedly high run-up events. In total 194 beach profiles, measured in 2006- 2011 at 16 locations, are examined by means of their approximation with the power function () . b hx Ax = About half of the profiles can be adequately approximated using a single power function. These profiles are almost all concave. The relevant exponents are clustered around b = 2/3 that is characteristic to the Dean's Equilibrium Profile. The rest of the profiles can be divided into two sections, each of which is approximated by a power function. The underwater sections of such profiles predominantly match the Dean's Equilibrium Profile. About 10% of the subaerial sections (about 7% of all examples) have the exponent close to b = 4/3, for which high run-up events are likely.

Process-Based Morphodynamic Modeling of a Schematized Mudflat Dominated by a Long-Shore Tidal Current at the Central Jiangsu Coast, China

Gong, Z.; Wang, Z.B.; Stive, M.J.F.; Zhang, C.K., and Chu, A., 2012. Process-based morphodynamic modeling of a schematized mudflat dominated by a long-shore tidal current at the central Jiangsu coast, China.This paper focuses on the process-based morphodynamic modeling of a mudflat dominated by a long-shore tidal current. The model is schematized as a shore-parallel tidal basin with dimensions similar to the tidal flats near the Wanggang Mouth at the central Jiangsu coast, China. A long-shore tidal current and associated sediment transport are dominant in comparison with their cross-shore components. The mudflat evolution is reproduced for both the case with abundant sediment supply and that with insufficient sediment supply. The simulated flow patterns agree qualitatively with field observations from literature, i.e., involving tidal asymmetry, current directions, and tidal wave features. The analysis of sediment fluxes depicts that deposition occurs from spring tide to neap tide and erosion from neap tide to spring tide. The results show that siltation on intertidal flats is favored by flood dominance, as well as settling and scour lag effects, with less dependence on sediment supply. However, subtidal flats are strongly affected by sediment supply. The simulated double-convex cross-shore profile associated with an abundant sediment supply is qualitatively consistent with field observations. An insufficient sediment supply causes erosion and consequent widening of the subtidal flats. Finally, the practical case with an initial sand seabed is reproduced. The long-shore sediment transport contributes to long-shore variations of cross-shore profiles. Deposition rates increase clearly southward in the area between the lower intertidal and middle subtidal flats when sediment supply is abundant. In contrast, deposition rates increase southward only slightly when sediment supply is insufficient. In addition, stronger sediment supply generates wider intertidal flats. Most importantly, the creek patterns, i.e., the incised creeks on mudflats, are roughly reproduced.

Towards a process-based model to predict dune erosion along the Dutch Wadden coast

AbstractAn equilibrium dune-erosion model is used every six years to assess the capability of the most seaward dune row on the Dutch Wadden islands to withstand a storm with a 1 in 10,000 probability for a given year. The present-day model is the culmination of numerous laboratory experiments with an initial cross-shore profile based on the central Netherlands coast. Large parts of the dune coast of the Wadden islands have substantially different dune and cross-shore profile characteristics than found along this central coast, related to the presence of tidal channels, ebb-tidal deltas, beach-plains and strong coastal curvature. This complicated coastal setting implies that the predictions of the dune-erosion model are sometimes doubtful; accordingly, a shift towards a process-based dune-erosion model has been proposed. A number of research findings based on recent laboratory and field studies highlight only few of the many challenges that need to be faced in order to develop and test such a model. Observations of turbulence beneath breaking waves indicate the need to include breaking-wave effects in sand transport equations, while current knowledge of infragravity waves, one of the main sand transporting mechanisms during severe storm conditions, is strongly challenged by laboratory and field observations on gently sloping beaches that are so typical of the Wadden islands. We argue that in-situ and remote-sensing field observations, laboratory experiments and numerical models need to be the pillars of Earth Scientific research in the Wadden Sea area to construct a meaningful process-based dune-erosion tool.

Probabilistic Detection of Morphologic Indicators for Beach Segmentation With Multitemporal LiDAR Measurements

Airborne light detection and ranging (LiDAR) surveys provide a rich data source of topographic information. However, beach monitoring with LiDAR data has been mostly limited to visualization and first-order measures derived from digital elevation models (DEMs). To exploit more information from multitemporal LiDAR data acquired over a beach, we extract surface features to detect morphologies that are indicative of shoreline change patterns. First, through cross-shore profile sampling of LiDAR-derived DEMs, the continuous 3-D beach surface is parameterized into several 1-D morphologic features progressing alongshore. Profiles are subsequently partitioned into binary erosion or accretion classes dependent on measured shoreline change between surveys. Then, a feature's class separability is quantified using information divergence measures nonparametrically constructed via Parzen windowing. The more interclass separation provided by a feature, the greater its discriminative potential, and the higher its ranking as a morphologic indicator. Rankings are computed across the survey epochs to evaluate performance stability of the features. Finally, the top-ranked features are implemented within a naive Bayes classifier to assess their ability to segment terrain more likely to erode. Results demonstrate the utility of the developed framework to systematically extract and incorporate useful morphologic information from LiDAR data for discerning patterns in beach change.

SHOREWARD SAND TRANSPORT OUTSIDE THE SURFZONE, NORTHERN GOLD COAST, AUSTRALIA

To date, no suitable theoretical basis has been derived to predict with reliable accuracy the shoreward sand transport under waves in the deeper water outside the surf zone. This is important for understanding the rate of recovery of beaches after major storm erosion and, in some circumstances, to quantify net shoreward supply of sand to the shoreline from the active lower shore-face below the depth of storm erosion bar development. Even a relatively low rate of long term shoreward net supply may contribute to shoreline stability where it offsets a gradient in the longshore sand transport that would otherwise lead to recession. This paper outlines the results of analysis of a 41 year dataset of beach and nearshore profile surveys to quantify annual average rates of shoreward net sand transport in 6-20m water in an area where the profiles are not in equilibrium due to the existence of a residual river mouth ebb delta bar lobe. Additionally, an empirical adaptation of the sheet flow relationship of Ribberink and Al-Salem (1990) to provide for the effects of ripples has been derived from large wave flume data and correlates well with the measured Gold Coast transport rates. These have been applied to a new coastline modelling system developed as part of research into the long term evolution of Australia’s central east coast region in response to sea level change and longshore sand transport processes, which combines the one-line concept of shoreline profile translation within the zone of littoral sand transport with cross-shore profile evolution across the deeper shore-face profile below that zone. It demonstrates the importance of providing for both the shoreward supply from the continental shelf and the varying profile response time-scale across the shore-face in predicting shoreline evolution.

IMPACT ASSESSMENT OF EXTREME STORM EVENTS USING A BAYESIAN NETWORK

This paper describes an investigation on the usefulness of Bayesian Networks in the safety assessment of dune coasts. A network has been created that predicts the erosion volume based on hydraulic boundary conditions and a number of cross-shore profile indicators. Field measurement data along a large part of the Dutch coast has been used to train the network. Corresponding storm impact on the dunes was calculated with an empirical dune erosion model named duros+. Comparison between the Bayesian Network predictions and the original duros+ results, here considered as observations, results in a skill up to 0.88, provided that the training data covers the range of predictions. Hence, the predictions from a deterministic model (duros+) can be captured in a probabilistic model (Bayesian Network) such that both the process knowledge and uncertainties can be included in impact and vulnerability assessments.

AN ANALYSIS OF CROSS-SHORE PROFILE EVOLUTION OF A SAND AND A COMPOSITE SAND-GRAVEL BEACHES

In this paper, cross-shore morphodynamic evolution of a sand and sand-gravel composite beach, are compared and contrasted used historic surveys of beach profiles. The differential behavior of cross-shore morphodynamics at a range of time scales covering event-scale dynamics up to intra-annual scale variability are investigated and discussed. The application of equilibrium concept to both beach types is explored through Dean’s equilibrium profile and Vellinga’s beach erosion profile. Variability of cross-shore profile dynamics are quantified and discussed through bulk statistical analysis. Multi-scale morphodynamic trends are determined using Empirical Orthogonal Eigenfunction (EOF) analysis. The above analysis enables to recognise failure mechanism of each beach type during a storm.

Hybrid morphological modelling of shoreline response to a detached breakwater

We present a new type of model for calculating morphological changes induced by the presence of breakwaters. The model combines a process based area model, used to calculate the sediment transport field in the two horizontal dimensions, with a simplified morphological updating scheme where the evolving cross-shore profile is described by a limited number of parameters. The hybrid morphological model is a strong tool for medium and long term modelling because it is cost effective while containing important features of the sediment transport description.Two versions of the model are developed in order to study the evolution of beach morphology: one suited for offshore breakwaters (1D model) and one mainly dedicated to coastal breakwaters (“1.5D” model).The version for offshore breakwaters is first presented and tested against field observations of salient evolution. The model is then applied to a model study of the principle correlations between evolving salients (spatial and temporal scales), the characteristic dimensions of the breakwater (distance to shore and alongshore length) and wave climate (wave height, normal and oblique wave incidence).The second version is applied to investigate in more detail the evolving morphology behind coastal breakwaters. It is demonstrated how the model is able to calculate the evolution of either salient or tombolo planforms, and furthermore it is shown that the results are in reasonable agreement with existing rules.

The role of flow asymmetry and mud properties on tidal flat sedimentation

It is well known that landward transport of fine sediments on tidal flats is caused by lag effects, of which the scour lag and the settling lag are the best known. These lag effects result from a combination of sediment properties and hydrodynamic asymmetries. However, it is not well-understood how, in a quantitative way, these lag effects depend on the sediment properties and the hydrodynamic forcing, and what the relative importance of these sediment properties and hydrodynamics is. As a result, it is not known which lag effect is more important under which conditions. We therefore set out to explore the relative importance of hydrodynamics and sediment properties on tidal-flat sedimentation using a schematized 2DV cross-shore profile model with a tidal range of 6m and an intertidal width of ∼5km. The effect of hydrodynamics is parameterized through a varying offshore tidal asymmetry and four different cross-shore profiles (a horizontal, convex, linear and concave profile). The effect of sediment properties is examined by evaluating a range of settling velocities ws and critical bed shear stresses for erosion τcr. The main findings of this work are that (1) conditions of maximum sediment deposition rates exist for ws ∼0.5mm/s and τcr ∼0.1Pa, (2) deposition rates due to slack tide asymmetry are comparable to symmetric tides, while peak flow asymmetry produces the greatest deposition rates, (3) tidal flat deposition rates are greatest for concave profiles and least for convex profiles mainly due to the horizontal velocity gradient, and (4) the type of lag effect dominating landward transport varies spatially.

Short-term observation of beach dynamics using cross-shore profiles and foreshore sediment

Cross-shore beach profiles and textural characteristics of foreshore sediment were analyzed for understanding an annual cycle of intertidal beach dynamics at Devbag, an Island sheltered estuarine coast. This study is carried out as response to the shoreline management project proposed by Ministry of Earth Science, India. Cross-shore transects were monitored in a monthly interval during February 2008 to February 2010 along with collection of beach surface sediment samples. Change in beach volume over an annual cycle showed a slow rate of sediment accretion at the Devbag beach where the maximum accretion was found at dissipative zone. During the year 2008–09, the volume of sediment accretion was 24.23 m3/m with an average beach width and slope of 91 m and 3.3° respectively. During 2009–2010, the accreted beach volume was 33.54 m3/m with an average beach width and slope of 132 m and 2.6° respectively. The beach along the coast was varying from extremely gentle to a slightly steeper slope which showed dissipative to reflective nature. The Beach was dominated by both fine and medium sands. More quantity of unimodal and bimodal sediment was found with moderately well sorted and moderately sorted nature. No significant correlation in beach morpho-sedimentary characteristics was found due to the non-uniform trend in beach morphodynamics and textural characteristics. Present study emphasizes the need to observe the beach changes over inter-annual seasons before planning coastal protection measures. The results of the study will be helpful for formulating an effective coastal zone management plan at Devbag, Kali estuary, west coast of India.

Accuracy evaluation of a smartphone-based technology for coastal monitoring

This paper introduces a smartphone-based technique for coastal monitoring and evaluates the accuracy of data such as images, three-dimensional coordinates, and attitude that can be acquired by such a technique. First, to determine intrinsic orientation (IO) parameters of a smartphone camera, a camera calibration was performed. The results were similar or slightly better than previous studies using a non-metric camera. 3-D coordinates provided by the assisted GPS (A-GPS) embedded in the smartphone showed lower accuracy. Attitudes calculated using an accelerometer and magnetometer showed 0.33–2.04° standard deviation, compared with ω, φ, and κ of extrinsic orientation (EO) parameters. Additionally, accuracy (RMSE 0.681pixels) of smartphone image triangulation using ground control points (GCPs) was about equal to the RMSE of 0.404pixels of a metric camera. Finally, ortho-rectified images of a test field were generated using DEM from terrestrial laser scanning and acquired images. Additionally, mapping of a shoreline was performed using the ortho-rectified images, and a profile of a representative cross-shore was composed. The results described the actual intertidal zone well. Given the results of the cross-shore profile and the horizontal and vertical accuracy evaluation of the extracted shoreline, this smartphone-based technique is considered appropriate for applications in coastal monitoring.

Stability of Wide-Graded Rubble Mounds

The reshaping of temporary rubble mounds like the core of breakwaters or reclamation bunds is often a concern for contractorsi n the construction stages of marine structures. The formulas found in literature for the prediction of such behavior are few, and they do not provide clear insight on the influence of relevant parameters, in particular the small dimensions and wide stone-size gradation of the material involved, usually consisting of quarry run or resulting from dredging. The previous research in the field of dynamic stability focused on berm breakwaters and gravel beaches. These two typologies of structures define the range to which the rubble mounds considered in this study generally belong. An overview on the design tools provided by the technical literature shows that, whenever the grading was included as a governing parameter, some influence was recognized in the characteristics of the structure (e.g. the permeability) and in the dynamism of the different fractions of stone sizes. However, very wide ranges of the parameter grading were never investigated and a specific analysis in this direction constitutes the main significance of this study. The Delft University of Technology provided the laboratory facilities to carry out physical model tests on a wide graded rubble mound structure representative of the core of a breakwater. The parameter D85/D15, describing the stone-size gradation of the construction material, was varied between the values 2.71 and 17.7, and two different seaward slopes of the model structure were also tested. The reshaped cross-shore profiles measured during the tests showed how if the grading increases the stability of the structure is reduced. This is not always in accordance with the findings of previous researchers, showing how the extrapolation of existing empirical formulas to structures with high values of the ratio D85/D15 do not give reliable results. Instead, the formulas given by van de Meer (1992) to estimate the whole reshaped profile of a dynamic slope predict with good agreement the shape of the measured profiles, although the physical model shows a larger horizontal extension of the displacements. This difference is governed by the grading, being more noticeable as this parameter increases. This result leads to the definition of new formulas, some of them being modifications of the ones given by van der Meer, to describe the geometry of a reshaped profile. The formulas, all including the parameter grading, are derived through curve fitting of the measured data. Also a formula for the direct estimation of the crest recession is given. As a final step, a simple numerical model is proposed in which the new formulas are implemented, constituting a quick way to assess the shape of a slope after a wave attack. As a suggestion for further utilization of the results of physical modeling, a brief comparison is also carried out between the output of the tests and the prediction of the numerical model XBeach (developed mainly at UNESCO-IHE). In conclusion, this research points out how the formulas provided by the technical literature are not reliable in representing the effects of a very wide stone-size gradation in the stability of a rubble mound structure. Physical model tests proved to be a suited way to investigate these effects, as the nature of the phenomena who play a role in the stability does not allow a simple analytical representation. The tests carried out within the present study lead to the implementation of a numerical model of practical use for engineers and contractors: further investigations through laboratory tests are recommended to validate and extend the findings of this study. Another proposed direction for further research is the comparison between the results of physical model tests and the output of numerical models.

Morphology and composition of beach-cast Posidonia oceanica litter on beaches with different exposures

Posidonia oceanica seagrass litter is commonly found along sandy shores in the Mediterranean region, forming structures called banquettes, which are often removed in order to allow the beach to be used for tourism. This paper evaluates the relationship between the morphology and composition of banquettes and beach exposure to dominant waves. A Real Time Kinematic Differential Global Positioning System was used to evaluate the variability of banquettes and beach morphology over a period of 1year. Banquette samples, collected at two different levels of the beach profile (i.e. foreshore and backshore), were used to evaluate the contribution of leaves, rhizomes and sediments to the total weight. Banquettes showed a higher volume, thickness and cross-shore length on exposed beaches, whereas narrower litter deposits were found on the sheltered beach. On exposed beaches, banquettes were deposited in beach zones characterized by changes in elevation. These changes in elevation were mainly due to the deposition and erosion of sediments and secondly to the deposition and or erosion of leaf litter. On sheltered beaches, the variability in beach morphology was low and was restricted to areas where the banquettes were located. The leaf/sediment ratio changed along the cross-shore profile. On the backshore, banquettes were a mixture of sediments and leaves, whereas leaves were the main component on the foreshore, independently of the beach exposure. The processes which control the morphodynamics in the swash zone could explain the variability of banquette composition along the cross-shore profile. Finally, this study highlighted that Posidonia oceanica seagrass litter plays an important role in the geomorphology of the beachface and its removal can have a harmful impact on the beaches.

Modelling the effects of macrofauna on sediment transport and bed elevation: Application over a cross-shore mudflat profile and model validation

The effects of 2 functional groups of bioturbators have been predicted in terms of long-term impact on erodability: (1) one superficial mobile deposit-feeder, the gastropod Hydrobia ulvae; and (2) one endobenthic deposit-feeder, the bivalve Scrobicularia plana. Different scenarios of morphodynamical cross-shore 1DH/1DV model were performed to simulate the equilibrium profile of an intertidal mudflat under tide and wave forcings. This process-based model for erosion is able to simulate multiphasic sequential resuspension, by discriminating various erosion behaviour like benthos-generated fluff-layer erosion (BGFL) and general bed loosening and burrowing activity in deep layers. The results were analysed and compared to examine the long-term effect of macrofauna after 14 years. It reveals that the impact of the bivalve S. plana is very significant after only 4 years of simulation while the effect of the gastropod H. ulvae is negligible in terms of sediment transport even after 14 years. More generally, this reveals the strong impact of stationary endobenthic bioturbators that induces a high downward shift of the upper shore while the effects of superficial motile bioturbators remain very low. This impact is mainly due to the effect of endobenthic species in deep layers associated to burrowing activities and their consequences on the bed erosion, but the production of a fluff layer by surface grazer like H. ulvae at the sediment surface can be neglected. The importance of macrofauna mediation of bed erodability is discussed in this study by comparing the activities of the two functional groups of bioturbation on the general functioning of intertidal mudflats. The model outcomes (transferred in a 1DV framework) were in close agreement with the measured results of flume data at 3 different bathymetric levels of the mudflat over the cross-shore profile. This validation step revealed that model of sediment transport under influence of biota effects does not need further refinements at the upper shore where S. plana dominates the species assemblage, whereas there is still a need to include further formulations of biota effects to simulate the erosion experimental results at the lower shores where other molluscs and Annelids significantly contribute to the species assemblage.

Can beach dune ridges of the Texas Gulf Coast preserve climate signals?

A study of the evolution of North Padre Island (southern Texas Gulf Coast) dunes was carried out using LIDAR topographic data, dune vibracores through the center of the dunes, and grab samples of shoreface sand at four locations along a cross-shore profile. Grain-size analyses of the vibracores show vertical variations in shoreface sand deposition over decimeter depth intervals. A dune ridge growth model is introduced that describes the dune vertical accretion rate as a function of island progradation and freshwater lens expansion. This model allows indirect dating of the dune core samples based on a known island progradation rate (1 m/year), and height and spacing of the dunes calculated from the topographic data. A sand provenance model is also proposed that links the sand deposition in the dunes with sand sourced from various depths along the shoreface profile, depending on storm activity. We present evidence linking the changes in storm-sand deposition in the dune cores with yearly climatic fluctuations in the Gulf of Mexico associated with landfalling tropical storm activity in the period from 1942- 1965. This record of storm-induced sand variability is negatively correlated with El Nino-Southern Oscillation (Pacific) sea surface temperature variability, and positively correlated with North Atlantic decadal sea surface temper- ature variability.

Short-term sandbar variability based on video imagery: Comparison between Time–Average and Time–Variance techniques

Time–exposure intensity (averaged) images are commonly used to locate the nearshore sandbar position ( x b ), based on the cross-shore locations of maximum pixel intensity ( x i ) of the bright bands in the images. It is not known, however, how the breaking patterns seen in Variance images (i.e. those created through standard deviation of pixel intensity over time) are related to the sandbar locations. We investigated the suitability of both Time–exposure and Variance images for sandbar detection within a multiple bar system on the southern coast of Brazil, and verified the relation between wave breaking patterns, observed as bands of high intensity in these images and cross-shore profiles of modeled wave energy dissipation ( x D ). Not only is Time–exposure maximum pixel intensity location ( x i-Ti ) well related to x b , but also to the maximum pixel intensity location of Variance images ( x i-Va ), although the latter was typically located 15 m offshore of the former. In addition, x i-Va was observed to be better associated with x D even though x i-Ti is commonly assumed as maximum wave energy dissipation. Significant wave height (Hs) and water level (η) were observed to affect the two types of images in a similar way, with an increase in both Hs and η resulting in x i shifting offshore. This η-induced x i variability has an opposite behavior to what is described in the literature, and is likely an indirect effect of higher waves breaking farther offshore during periods of storm surges. Multiple regression models performed on x i , Hs and η allowed the reduction of the residual errors between x b and x i , yielding accurate estimates with most residuals less than 10 m. Additionally, it was found that the sandbar position was best estimated using x i-Ti ( x i-Va ) when x b was located shoreward (seaward) of its mean position, for both the first and the second bar. Although it is unknown whether this is an indirect hydrodynamic effect or is indeed related to the morphology, we found that this behavior can be explored to optimize sandbar estimation using video imagery, even in the absence of hydrodynamic data.

Formation of two-section cross-shore profile under joint influence of random short waves and groups of long waves

We show that beach profiles may develop a two-section almost-equilibrium structure under joint impact of short wind waves and groups of long waves with duration of a few minutes and properties that are independent of wind waves. The upper section of the profile is convex and follows the power law h ~ x 4/3 at small depths and in the swash zone. Wind waves may cause either erosion or accretion of this section but groups of long waves effectively redistribute the sediment over the convex section and practically do not alter the sediment volume. The ability of short groups of highly energetic waves to regularly build up such profiles from those created by random sea waves is a principally new feature of wave-coastal interaction that may have extensive consequences on the estimates of the wave-induced coastal hazards.