Bar Crest Research Articles

Wave and Roller Transformation Over Barred Bathymetry

AbstractThe cross‐shore transformation of breaking‐wave roller momentum and energy on observed barred surfzone bathymetry is investigated with a two‐phase Reynolds Averaged Navier Stokes model driven with measured incident waves. Modeled wave spectra, wave heights, and wave‐driven increases in the mean water level (setup) agree well with field observations along transects extending from 5‐m water depth to the shoreline. Consistent with prior results the roller forcing contributes 50%–60% to the setup, whereas the advective terms contribute ∼20%, with the contribution of bottom stress largest (up to 20%) for shallow sandbar crest depths. The model simulations suggest that an energy‐flux balance between wave dissipation, roller energy, and roller dissipation is accurate. However, as little as 70% of the modeled wave energy ultimately dissipated by breaking was first transferred from the wave to the roller. Furthermore, of the energy transferred to the roller, 15%–25% is dissipated by turbulence in the water column below the roller, with the majority of energy dissipated in the aerated region or near the roller‐surface interface. The contributions of turbulence to the momentum balance are sensitive to the parameterized turbulent anisotropy, which observations suggest increases with increasing turbulence intensity. Here, modeled turbulent kinetic energy dissipation decreases with increasing depth of the sandbar crest, possibly reflecting a change from plunging (on the steeper offshore slope of the bar) to spilling breakers (over the flatter bar crest and trough). Thus, using a variable roller front slope in the roller‐wave energy flux balance may account for these variations in breaker type.

Experimental Analysis of the Changes in Coral Sand Beach Profiles under Regular Wave Conditions

This study utilized 50 laboratory experiments to document the evolution of coral beaches under varying regular wave conditions, including five distinct wave periods and ten wave heights. Both the type of equilibrium beach and the shape of sand bars were used to represent beach evolution. The evolution of coral sand beaches was then compared to quartz sand beaches. The experimental results show that the predicted (modeled) equilibrium profile of a quartz sand beach was not applicable to coral sand beaches. Compared to sand bars on quartz sand beaches, the distance from bar crests to the beach berm in coral sand beaches was greater, whereas the erosional depth of sand troughs was deeper. However, the grain size distribution of sand associated with the coral sand beach under wave action was consistent with Celikoglu’s law. Both an equilibrium beach profile classification model and a sand bar shape prediction model for coral sand beaches were developed based on the experimental data.

Roles of breaking and reflection in wave energy attenuation on the shoreface-nourished beach

Shoreface nourishments with the artificial sandbar are effective strategies to mitigate coastal erosions mainly by wave breaking and reflection. Thus, a better understanding of the contributions of breaking and reflection in wave energy loss is important for the prediction and description of the performance of the shoreface nourishment. Experiments are conducted in a wave flume, and data analysis is complemented with numerical simulations performed with a phase-resolving model. Both incident and reflected wave heights are well reproduced by the numerical model. The ratio of reflection to reflection-plus-breaking energy loss ranges from 4% to 11%, implying the dominant role of breaking-induced dissipation over reflection in present cases. The wave transmission coefficient decreases with the decrease in the bar crest depth and onshore implementation of the artificial sandbar. The trapezoidal artificial sandbar promotes an intense wave breaking over the berm width and is more effective in eliminating waves than the triangular artificial sandbar. Due to the erodible nature of the artificial sandbar, future works will consider the use of a hydro-morphological coupled approach to study the time-varying characteristics of wave breaking and reflection during morphological evolution.

Large-eddy simulation of free-surface turbulent channel flow over square bars

The results of large-eddy simulations of free-surface turbulent channel flow over spanwise-aligned square bars are used to investigate the effects of bed roughness and water surface deformations on the root-mean-squared velocity fluctuations, dispersive shear stress, double-averaged Reynolds shear stress, wake kinetic energy and double-averaged turbulent kinetic energy. Two bar spacings, corresponding to transitional and k-type roughness, at similar Reynolds and Froude numbers are considered. The main peak of all statistical quantities occurs at the bar crest height. The effects of a standing wave at the water surface in flow over k-type roughness is marked by a local peak under the water surface for all statistical quantities considered here except wall-normal and spanwise velocity fluctuations. Quadrant analysis shows that sweeps and ejections are the strongest events contributing to both dispersive and double-averaged Reynolds shear stress but their contributions are different for the two bar spacings. Examining the budgets of dispersive and double-averaged Reynolds shear stress reveals that the dominant terms of these stresses are pressure–strain correlation and pressure transport and the contribution of wake production is similar for both of these stresses but with opposite sign. In addition to the main role of the bars in consuming or producing wake kinetic energy through production and transport and convection, the standing wave at the water surface in flow over k-type roughness induces large convection in the bulk flow too. The dominant terms in the double-averaged turbulent kinetic energy budget are similarly production, transport and convection. Large shear production renders large temporal fluctuations than spatial fluctuations of flow variables. The interaction of bars, bed and water surface is seen in the convection term in flow over k-type roughness.

Enhanced extreme wave statistics of irregular waves due to accelerating following current over a submerged bar

We present experimental results of irregular long-crested waves propagating over a submerged trapezoidal bar with the presence of a background current in a wave flume. We investigate the non-equilibrium phenomenon (NEP) induced by significant changes of water depth and mean horizontal flow velocity as wave trains pass over the bar. Using skewness and kurtosis as proxies, we show evidence that an accelerating following current could increase the sea-state non-Gaussianity and enhance both the magnitude and spatial extent of the NEP. We also find that below a ‘saturation relative water depth’ $k_p h_2 \approx 0.5$ ( $k_p$ being the peak wavenumber in the shallow area of depth $h_2$ ), although the NEP manifests, the decrease of the relative water depth does not further enhance the maximum skewness and kurtosis over the bar crest. This work highlights the nonlinear physics according to which a following current could provoke higher freak wave risk in coastal areas where modulation instability plays an insignificant role.

A new process-based, wave-resolving, 2DH circulation model for the evolution of natural sand bars: The role of nearbed dynamics and suspended sediment transport

We study the migration of natural sand bars that evolve in the nearshore by means of a new process-based wave-resolving, 2DH circulation model. In order to perform reliable and accurate computations, the robust Nonlinear Shallow Water Equations (NSWEs) hydro-morphodynamic solver of Brocchini et al. (2001) and Postacchini et al. (2012) implements a detailed description of the Bottom Boundary Layer (BBL) dynamics and a new predictor for the Suspended Sediment Transport (SST) based on the solution of a Depth-Averaged Advection-Diffusion Equation (DAADE) for the sediment concentration. The robustness and accuracy of the enhanced model are validated against literature theoretical, experimental, and numerical results, all comparisons highlighting good performances and clarifying the role of both BBL and SST contributions, the former one having a larger positive influence than the latter one on the results. Both original and enhanced models are, then, used to predict the evolution of the sand bar system that characterizes the nearshore of Senigallia (AN). The analysis leverages the field observations collected at such a site by means of the Sena Gallica Speculator video-monitoring system. Modeling of the storm-forced sand bar migration patterns reveals that: 1) the enhanced model can adequately reproduce the seaward migration of the sand bars of the system; 2) the process of shoreline retreat in coincidence with the generation of a new-born bar is well described; 3) inclusion of the BBL improves quantitative prediction of the bar crest migration; 4) the SST, beyond improving the prediction of the bar crest location, induces some smoothing of the bar profile, in line with the literature findings of SST being a stabilizing factor for the bar emergence. • Sand bar evolution in the nearshore zone is modeled using a process-based 2DH wave resolving NSWE model. • The model includes BBL dynamics and predicts the SST based on the depth-averaged advection-diffusion equation. • The model is validated against theoretical, experimental, and numerical results. • The model is used to predict the sandbar migration in the nearshore of Senigallia (AN) during a storm. • The developed model can accurately reproduce the sandbar evolution in both field and laboratory conditions.

Beach profile changes induced by surrogate Posidonia Oceanica: Laboratory experiments

New flume experiments with surrogate seagrass meadows are presented. The experiments included the creation of a full-scale and realistic Posidonia Oceanica model to evaluate the effect over wave attenuation, sediment transport and shoreline erosion. A hydrodynamic and morphodynamic comparison between cases with seagrass and without seagrass for two wave energy conditions was performed. Meadow density and submergence ratio were constant for tests under irregular waves. The wave height reduction, bar crest location, total sediment transport and shoreline position were used to evaluate the coastal protection efficiency of the meadow. The measured wave heights suffered a reduction due to the presence of the seagrass. These reductions were persistent in the area located between the onshore edge of the meadow and the depth of closure, being of greater magnitude in the more energetic case. All tests showed the development of a bar and the migration of the crest offshore over time. However, the dissipation of the incoming wave energy on the meadow made the bar crest stay closer to the shoreline and consequently generated a lower freeboard. In addition, the bar migration rate was reduced by the simulated meadow effect, in particular in the lower energetic case. A sediment volume significantly smaller was transported offshore when the seagrass meadow was present in both wave conditions. Additionally, in the higher energy case, a smaller shoreline retreat was observed when the meadow was present. • Flume experiments were carried out with a realistic Posidonia Oceanica model to assess the effect on beach sediment transport. • Two wave condition were analysed comparing the test with and without seagrass meadows. • Wave heights were reduced due to the effect of the Posidonia Oceanica model in areas after the meadow. • The total volume of sediment transported in the seagrass meadows cases was considerably less than without seagrass meadows. • Bar crest morphological parameters and shoreline position were analysed to verify the meadow effect on the coastal protection.

Seasonal morphodynamics of multiple intertidal bars (MITBs) on a meso‐ to macrotidal beach

AbstractMultiple intertidal bar (MITB) beach systems comprise a succession of subdued, shore‐parallel sandbars, developed under low‐energy conditions in meso‐ to macrotidal settings. Their relatively stable morphologies over long timescales are commonly attributed to a dynamic equilibrium, driven primarily by seasonal morphodynamics. The seasonal behaviour is, however, poorly understood. The relationship between temporal and spatial hydrodynamic forcing and morphological changes was investigated through monthly differential GPS surveys in Dundrum Bay (Northern Ireland) from April 2019 to March 2020, associated with variations in nearshore wave conditions, simulated using the SWAN wave model. During low‐energy summer conditions, southeast waves helped promote MITB stability and a slight increase in bar crest elevation, with the seaward‐most bar buffering and preserving the inner bar system through wave dissipation. In the winter, a progressive increase in wave energy, and a switch in wave direction (southwest to south), initiated the highest rates of cross‐shore bar migration and a lowering of bar crests. While the seaward‐most bar remained largely submerged, the shoreward‐most bar played a key role in protecting the upper‐beach and/or foredune. Winter conditions also forced a newly observed mesoscale longshore drift involving offshore sediment transport in the southwest and onshore transport in the northeast. Cross‐shore variability in MITB behaviour at seasonal timescales was, however, primarily driven by local hydrodynamic conditions, including variations in wave energy and direction. Conversely, alongshore variability was largely influenced by the complex nearshore bathymetry, headlands and an active ebb delta, all interacting with changing hydrodynamic forcing. These results challenge the seasonal equilibrium model for MITB systems and highlight longer‐term patterns of sediment movement.

Sediment pathways and morphodynamic response to a multi-purpose artificial reef – New insights

Multi-purpose Artificial Reefs (MPARs) are structures that may provide aesthetically acceptable coastal protection and improve recreational outcomes. Twenty years after construction of the first MPAR, Narrowneck Reef on the Gold Coast of Australia, most of the available literature is still focused on the planning, design and construction of such structures and peer-reviewed publications on their post-construction monitoring, interaction with sediment transport and impacts on coastal morphology are lacking. The aim of this paper is to evaluate how does Narrowneck Reef influence the sediment transport, and morphological changes around the anthropogenic structure, two decades after construction. To do so, a combination of ten high spatial resolution topo-bathymetric surveys from the top of the dune to the 10 m depth captured over 21 months and a series of 60 simulations using a calibrated numerical model were used. Our results demonstrate that: although not expected during design or reported in similar structures, sand can bypass the MPAR around its offshore end; under oblique waves, the longshore currents are deflected as they pass the reef, resulting in a shadow zone on the downdrift side where sand deposits; the bar crest tends to be higher on the reef's updrift side compared to downdrift, indicating that the MPAR can act as a store for sediments, as initially designed. Furthermore, the MPAR can act to stabilise the bar as it moves onshore with a downdrift offset of the inner bar as a result of low oblique wave incidence. The results presented here demonstrate that the short-term response to the MPAR twenty years after construction is more closely related to the updrift build up of sand and the deflection of longshore currents as they encounter the reef than to the dissipation of wave energy. This is because MPARs are designed to dissipate just enough wave energy so that they can achieve their recreational goal (surfing).

River Jets Versus Wave-Driven Longshore Currents at River Mouths

At river mouths, fluvial jets and longshore currents (LSCs) generated by waves interact hydrodynamically. This idealized numerical modeling study simulates a large number of hydro-morphodynamic conditions (650) to explore the emergent hydrodynamics determined by different mouth bar volumes and geometries, river discharge, wave heights, and directions and their potential stress on river-mouth development. We find that in the absence of a river-mouth bar (RMB), interactions are driven by momentum balances, expressed either as the balance of wave momentum flux (Mw) and jet momentum flux (Mj), or the balance of river jet discharge (QJet) and longshore current discharge (QLSC). When a RMB is present, the topography modifies the structure of the jet by spreading it, and we quantify this mechanism through the lateral jet transfer rate (LJT). Secondly, topography generates complex longshore wave-driven circulation as a result of the protruding shoreface which serves as a platform on which counter LSCs develop. The balance in QJet/QLSC may be used as an indication of the type of circulation. High and oblique waves favor longshore circulation and RMB bypass, whereas low waves and normal-to-coast angles generate diverging LSCs on the mouth bar crest which interrupts the longshore circulation. A quantification of the dynamic diversion is proposed in the form of the non-dimensional Dynamic diversion index (DyD), which scales with the product of Mj and Mw, and can account for the absolute strength of hydrodynamic interactions occurring at river mouths. RMB morphology can affect DyD in multiple ways by strengthening or by weakening the interactions. The DyD effect seems to increase with increasing RMB size, indicating that the RMB scale regulates the interplay of the wave-driven circulation and the river jet which further controls the adjacent topography changes.

Dynamics of Barred Shore Profile on the Temporal Scale of a Storm Cycle

A model is developed to simulate the morphodynamics of a barred shore profile on the scale of a storm cycle, comprising the phases of high and low wave activity. In the CROSS-PB model, the main mechanisms governing sediment transport in wave shoaling and surf zones are taken into consideration, in particular, skewness and asymmetry of the wave orbital velocities, undertow, and infragravity oscillations associated with wave groups. By comparing calculations with observed data, it is concluded that the model correctly reproduces seaward displacement of bars during a storm and shoreward under moderate or weak wave conditions. The first process is shown to be provided by balance of transport mechanisms when the peak offshore transport is located at the front slope of bar. The second process is conditioned by dominant shoreward transport and its sharp decay over the bar crest. Testing of the model yields estimates for the calibration coefficients, and it is concluded that shoreward sediment flux is mainly controlled by wave asymmetry.

Dinamika podvodnogo vala peschanogo berega pod vozdeystviyem shtormovogo volneniya po dannym monitoringovykh nablyudeniy

Purpose. Morphodynamic system of the accumulative sandy coast can include one or more underwater bars. Position and shape of the underwater bar can reflect both seasonal changes of the coastal profile and its unidirectional movements landward and seaward. Determination of the character of the underwater bar movement under the influence of various wave conditions permits to reveal common factors of the coastal deposit multidirectional transport along the coast profile. Methods and Results . The results of field observations of morphodynamics of a section of the Baltic Spit sandy coast (600 m length) were analyzed. From May to November 2019, a series of measurements of the coastal zone relief were conducted. The obtained data were analyzed along with the wave regime parameters (reanalysis ERA5 data was used). The coastal profile of the area under study is complicated by the external underwater bar with its crest located at the depth 2.65 m, and by the internal one of a crescent shape. Conclusions . Analysis of displacement of the external underwater bar from May to November showed that this form was of a morphodynamics two-dimensional character, i.e. it possessed the same morphometric characteristics along the coast. It was revealed that the underwater bar crest was located at the depths close to those of wave breaking during the most recent relatively strong and sustainable storm. Based on this concept as well as on the available literature data on the relationship between a wave height and dynamics of an underwater bar crest, described is the landward displacement (recorded during the observation period) of the external underwater bar. Due to the field data, it was shown that the underwater bar morphodynamics was effected both by duration of individual waves and by difference between the wave parameters of a sequence of storm events.

Dynamics of Underwater Bar of Sandy Coast under the Influence of Wave Action According to the Monitoring Observations

Purpose. Morphodynamic system of the accumulative sandy coast can include one or more underwater bars. Position and shape of the underwater bar can reflect both seasonal changes of the coastal profile and its unidirectional movements landward and seaward. Determination of the character of the underwater bar movement under the influence of various wave conditions permits to reveal common factors of the coastal deposit multidirectional transport along the coast profile. Methods and Results. The results of field observations of morphodynamics of a section of the Baltic Spit sandy coast (600 m length) were analyzed. From May to November 2019, a series of measurements of the coastal zone relief were conducted. The obtained data were analyzed along with the wave regime parameters (reanalysis ERA5 data was used). The coastal profile of the area under study is complicated by the external underwater bar with its crest located at the depth 2.65 m, and by the internal one of a crescent shape. Conclusions. Analysis of displacement of the external underwater bar from May to November showed that this form was of a morphodynamics two-dimensional character, i.e. it possessed the same morphometric characteristics along the coast. It was revealed that the underwater bar crest was located at the depths close to those of wave breaking during the most recent relatively strong and sustainable storm. Based on this concept as well as on the available literature data on the relationship between a wave height and dynamics of an underwater bar crest, described is the landward displacement (recorded during the observation period) of the external underwater bar. Due to the field data, it was shown that the underwater bar morphodynamics was effected both by duration of individual waves and by difference between the wave parameters of a sequence of storm events.

Bound and free infragravity wave energy over a bar

The propagation of bound and free infragravity wave energy under short-wave breaking conditions induced by a longshore bar is investigated through field observations. Near-bottom pressure records deployed in a cross-shore array at S. Jacinto barred beach (Portugal) were analysed. In these observations, short-wave breaking induced by the longshore bar occurred mainly in intermediate water depths.The ratio between bound and total (bound plus free) infragravity wave energy was estimated with the methodology of Herbers et al. (1994). This ratio had a maximum value of 0.6 at the breaking onset, which is consistent with values presented elsewhere.A decrease of the bound infragravity wave energy was observed after short-wave breaking near the bar crest. This is in part due to nonlinear energy transfers from infragravity to short-wave frequencies as detected on the imaginary part of the bispectrum, in a weakly dispersive Boussinesq approximation. The other part is due to the process of release/transform bound into free infragravity wave energy that was assessed with a simplified energy balance equation.A conceptual model for the exchange of bound infragravity wave energy during short-wave breaking in intermediate water depths is proposed. This conceptual model involves both the kh and the ga/C2 generalized nonlinearity parameter.

Benthic Species Distribution Linked to Morphological Features of a Barred Coast

The composition of benthic species communities in the nearshore zone is closely related to the hydrodynamic and morphodynamic conditions. Sustainable management of the coastal ecosystem requires knowledge about the natural dynamics as well as human-induced changes on the ecosystem. To improve our knowledge of the benthic species distribution along a dissipative sandy shore with multiple breaker bars, an extensive dataset was collected in the nearshore zone of the barrier islands Ameland and Schiermonnikoog in the Dutch North Sea. From 2010 to 2014, every year, approximately 180 grab samples along 18 cross-shore transects were collected and analyzed for sediment characteristics and macrobenthic species composition. Mixed-effect-models and partial redundancy analysis were used to analyze the importance of morphological features (i.e., slopes, bar crests, and troughs) as an explanatory variable for the benthic species distribution. The results indicate that the morphological features in themselves explain three times more variation than the environmental parameters used. This demonstrates the importance of morphological features as a factor in explaining the distribution of benthic species communities in the nearshore. Detailed information on morphological features is easy to obtain from bathymetry maps or visual inspection. Incorporating morphological features in species distribution models will therefore help to improve sustainable management of our valuable sandy coastal systems.

Flows over crescentic bars under control of wave energy

Crescentic bars with wavelengths of 250–450 m were observed on a micro-tidal, open coast (Anmok Beach, Korea) to unravel the flow patterns over them under various wave conditions. The horn and bay environments of the crescents were distinguished to measure respective near-bottom flow conditions in the nearshore 3–8 m deep. Offshore waves were measured at a station about 18 m deep. The measurement duration spanned approximately one winter month and two stormy spells of several days each in summer and another winter comprising three different field campaigns. In addition, bar morphologies were measured with single- and multi-beam echo-sounders before and after each hydrodynamic measurement. Winter-storm waves yielded discernable bar movements offshore accompanied by a merging of small bays into a larger one. During the field campaigns, the bars moved alongshore by far less than a half of their wavelength. Hydrodynamic measurements show a suite of the flow patterns including onshore and offshore uniform flows and rip cell circulations. The flow patterns depend exclusively on the offshore significant wave height (Hs), or a non-dimensional parameter $$ {\mathrm{H}}_{\mathrm{p}}={H}_s\sqrt{\tan \theta }/{h}_c $$, where hc is the water depth of the bar crest and tanθ the slope gradient. Therefore, the specific ranges of Hp could be given to individual flow patterns measured. These results are used to illustrate a coupled behavior of the bars and flows in a beach-state cycle forced by varying wave energies.

Characteristics of saw-tooth bars on the ebb-tidal deltas of the Wadden Islands

Saw-tooth bars are shore-oblique sand bars that are found along most ebb-tidal deltas of the Frisian Wadden Islands. Although they might significantly affect sediment transport pathways and volumes on the deltas, their general characteristics and dynamics are largely unknown. The main aim of this paper is to determine the typical saw-tooth bar heights, wave lengths, widths, orientations, migration rates and depths of occurrence. To this end, we analysed bathymetries from the Dutch and German Frisian Wadden Islands between 1970 and 2015. Bar heights range between 0.5 and 2 m, and wave lengths range between 300 and 900 m, with an average of 670 m, and the bar crests have a down contour orientation of approximately 25°. The bars are between 800 and 2200 m wide. Saw-tooth bars are located at depths from 3 to 12 m, depending on the slope of the area. Migration speeds of up to 112 m/year were found, using a spatial correlation method. Bar height and migration speed are positively linearly correlated, as well as bar wave length and bar width, bar height and the orientation of the depth contours and migration speed and the orientation of the area. The derived characteristics are compared to those of other bar types to evaluate hypotheses regarding the formation mechanism of the bars.

Unit bar architecture in a highly‐variable fluvial discharge regime: Examples from the Burdekin River, Australia

AbstractUnit bars are relatively large bedforms that develop in rivers over a wide range of climatic regimes. Unit bars formed within the highly‐variable discharge Burdekin River in Queensland, Australia, were examined over three field campaigns between 2015 and 2017. These bars had complex internal structures, dominated by co‐sets of cross‐stratified and planar‐stratified sets. The cross‐stratified sets tended to down‐climb. The development of complex internal structures was primarily a result of three processes: (i) superimposed bedforms reworking the unit bar avalanche face; (ii) variable discharge triggering reactivation surfaces; and (iii) changes in bar growth direction induced by stage change. Internal structures varied along the length and across the width of unit bars. For the former, down‐climbing cross‐stratified sets tended to pass into single planar cross‐stratified deposits at the downstream end of emergent bars; such variation related to changes in fluvial conditions whilst bars were active. A hierarchy of six categories of fluvial unsteadiness is proposed, with these discussed in relation to their effects on unit bar (and dune) internal structure. Across‐deposit variation was caused by changes in superimposed bedform and bar character along bar crests; such changes related to the three‐dimensionality of the channel and bar geometry when bars were active. Variation in internal structure is likely to be more pronounced in unit bar deposits than in smaller bedform (for example, dune) deposits formed in the same river. This is because smaller bedforms are more easily washed out or modified by changing discharge conditions and their smaller dimensions restrict the variation in flow conditions that occur over their width. In regimes where unit bar deposits are well‐preserved, their architectural variability is a potential aid to their identification. This complex architecture also allows greater resolution in interpreting the conditions before and during bar initiation and development.

Modeling the dynamics of a sand beach governed by the wave and underwater bar interaction

Effect of bar position on underwater profile of sandy beach was studied at the timescale of one storm using the xBeach numerical model. Beach profiles were extracted from the bathymetry of the Shkorpilovtsy beach (the Bulgarian coast of the Black Sea). Computed results were verified by field measurements. The largest shoreline retreat occurred in the first hour of a storm. For the chosen wave regime (largest wave height 1.5 m, wave period 10.5 s), an equilibrium profile was formed after 6 hours. The resulting profile contained an underwater terrace with the slope close to that of the theoretical equilibrium profile. It was shown that the position of the underwater bar affects the shoreline retreat rate. The smallest and the largest shore retreat occur if bar crest is located at a distance about 0.7–0.8 and 0.5 of the deep water wavelength, correspondingly. It was found that the shoreline retreat depends on the height of infragravity waves and mean wave period: the smaller mean wave period and the higher infragravity waves near the coast, the smaller is the retreat of the coastal line. Distance of seaward sediment transfer is directly relates to the height of large waves near the shore.

Scroll bars are inner bank levees along meandering river bends

AbstractScroll bars across a 65‐km stretch of the Trinity River in Texas, USA were studied using LiDAR data as well as with a series of 11 trenches spread out across the survey area. We conclude that scroll bars are levees that are deposited along the inner banks of these meandering river bends. Scroll bar crests were found to have similar elevations to those of outer bank levee crests, implying that they are constructional features that create positive topographic relief above the elevation of the floodplain. Trenches reveal that scroll bars are built from reworked suspended sediment, with common ripple‐scale cross stratification, planar laminations and muddy bioturbated layers – characteristics often associated with levee sedimentation in other systems. LiDAR observation of the erosion of scroll bars by bed material transport during flood implies that scroll bar spacing is an imperfect proxy for estimating overall channel migration rates. In addition, interspersed lenses of coarser sediment with dune‐scale cross stratification represent the stratigraphic record of these erosional events and suggest that erosion of the channel‐ward edge of the scroll bar is not uncommon. Preservation of scroll bars is unlikely, given that they are responsible for an average of only the uppermost 12% of the total inner bank relief. We suggest that misidentification of point bar lateral accretion surfaces as scroll bars is common and can lead to issues with reconstructing channel properties due to systematic differences between point bar and scroll bar planform geometries. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.