Bar Crest Research Articles

Numerical Investigation on the Influence of Tide on Cyclic Seaward Bar Migration

The influences of tide and water level rise on the cyclic medium-term seaward bar migration were examined using a one-dimensional numerical model of beach profile change, in which the cross-shore sediment transport rate was assumed to consist of four contributions due to sediment suspension and undertow, velocity skewness, velocity atiltness and beach slope. The model was calibrated with a year-long dataset of beach profiles obtained at the Hasaki coast of Japan in 1989, when the duration of medium-term seaward bar migration was approximately 1 year. The comparison between bar crest positions predicted with and without the tide at Hasaki during a two-year period including the calibration period of 1989 and the following year shows that the bar migrated further seaward without the tide than with the tide. The comparison between bar crest positions during the two-year period predicted with the tide and the water levels 0.5 m higher indicates that the bar crest positions were located further seaward with the higher water levels than with the tide in 1989. The difference between cyclic medium-term seaward bar migrations with and without the tide, and that with the tide and the higher water levels, are attributed to wave breaking on and seaward of the bar crests.

QUASI-2D TRANSPORT MODEL OF SUSPENDED SEDIMENT IN A WAVE FLUME

A quasi-2D model of hydrodynamics and sediment transport has been developed in this study. An eddy viscosity model with a function of artificial viscosity has been applied to the Boussinesq-type equations to produce wave decay as well as sediment transport due to breaking. Numerical results are then compared with laboratory experimental data in order to verify the applicability of the numerical model. The results demonstrate that the proposed eddy viscosity model can be used to simulate wave propagation in the surf zone as well as suspended concentration distribution. Erosion before the breaking point can be predicted fairly well. However, the bar crest and erosion in the surf zone can not be predicted accurately.

Dynamics of a nearshore bar system in the northern Adriatic: A video-based morphological classification

The aim of this paper is to define a simplified morphodynamic classification suitable for low energy beaches exposed to microtidal conditions. The study site is located in the northern Adriatic (in Italy), it is an almost 2 km-long rectilinear beach bordered at the northern edge by coastal structures and at the southern end by a small river inlet. The mechanisms related to the evolution of the submerged part of the beach were derived from video-monitoring using Argus technology. The morphodynamic evolution of the system was studied using an automatic procedure on images for the characterisation of nearshore bars that showed good correspondence with hand-based (visual) interpretation. To apply this automatic procedure, the bar's plan crest shape was mapped using cross-shore pixel luminosity transects traced on time-averaged video images. A careful sensitivity analysis was undertaken to determine the best spacing between transects for the correct tracing of the shape of the bar crest. The error associated to a transect spacing every 25 m resulted in being comparable with the pixel resolution in the area and with the error found comparing the video interpretation with bathymetric surveys. From the study of a four and a half year dataset (February 2003–May 2007), the submerged beach was found to be characterised by the presence of a single bar in the area next to coastal protection structures. However, moving southwards of these, inner and outer bars were present. The morphodynamics of the outer bar and its plan shape modifications were dominated by rhythmic forms. Occasionally, after high energy events, the bar became rectilinear but during the following lower energy periods rhythmicity was re-established, supporting the hypothesis of self-organization mechanisms. The cross-shore position of the bar's crests only showed limited cross-shore mobility through time.

Large‐scale laboratory observations of wave breaking turbulence over an evolving beach

Wave breaking turbulence over an evolving beach was observed in a large‐scale laboratory flume, as part of the CROss‐Shore Sediment Transport EXperiment (CROSSTEX). The data set included comprehensive measurements of water surface elevation, fluid velocity, and morphology for irregular waves under erosive and accretive wave conditions. For the both conditions, the beach reached a quasi‐equilibrium state, defined as when the bar shape was stable. Wave breaking characteristics, such as wave heights, average rate of energy dissipation by bores, and surf similarity parameter, were investigated in response to morphodynamics of the bar. Turbulent kinetic energy (TKE) was estimated using the method by Shaw and Trowbridge (2001). As the beach evolved, a less amount of TKE was observed at the trough for the erosive case, while more TKE was observed at the trough for the accretive case. It was also found that the temporal variation of the time‐averaged TKE were closely associated with the average rate of energy dissipation by bores. Comparing with the bar trough, the vertical distribution of nondimensionalized time‐averaged TKE and turbulence dissipation rate at the bar crest showed a large increase near the bottom, probably due to a strong cross‐shore RMS velocity. Finally, in the quasi‐equilibrium state, time‐averaged TKE, and turbulence dissipation rate at the bar trough were smaller than those inside the surf zone. Inside the surf zone, significant turbulence intensities were observed due to a second breaking on a shallow water depth.

Assessing the Suitability of Video Imaging for Studying the Dynamics of Nearshore Sandbars in Tideless Beaches

Nearshore sandbars, an important natural defense mechanism of the beaches, can be monitored using shore-based video systems. Before studying bar dynamics with video images, we must establish the relationship between the real bar positions and the videoed bar positions (detected by the preferential wave breaking on the shallows). This analysis becomes essential in the two studied tideless beaches of Barcelona due to the critical differences with respect to the sites studied previously. Bogatell beach is terraced (without a trough) in more than 50% of the profiles. There, the videoed barlines are a good proxy of the terrace edge position. In La Barceloneta beach, with dominance of barred profiles, the videoed barlines better represent the bar crest position. On average, the obtained distances between real and videoed bar positions ¿ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</i> are 10-15 m, with the videoed barlines located shoreward. Changes in the bathymetric profile shape and the root-mean-squared wave height <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rms</sub> induce a variability of ¿ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</i> of 16 m in La Barceloneta and 13 m in Bogatell. This apparent variability masks the real changes in bar position and should preferably be reduced before further analysis. As a highly significant correlation between ¿ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</i> and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rms</sub> is detected in the two beaches, the proposed reduction method consists of sampling at a specific range of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rms</sub> . This diminishes the variability by 10% to 14 m in La Barceloneta and 11 m in Bogatell. This paper confirms the suitability of using video systems for monitoring bars and terraces in the Barcelona beaches.

Temporal observations of rip current circulation on a macro-tidal beach

A field experiment was conducted on a high energy macro-tidal beach (Perranporth, UK) to examine rip current dynamics over a low-tide transverse bar/rip system in response to changing tide and wave conditions. Hydrodynamic data were collected using an array of in situ acoustic doppler current meters and pressure transducers, as well as 12 GPS-tracked Lagrangian surf zone drifters. Inter-tidal and sub-tidal morphology were measured through RTK-GPS and echo-sounder surveys. Data were collected for eight consecutive days (15 tides) over a spring-neap tidal cycle with tidal ranges of 4–6.5 m and offshore significant wave heights of 1–2 m and peak periods of 5–12 s. The hypothesis that rip current dynamics in a macro-tidal setting are controlled by the combination of variations in wave dissipation and morphological flow constriction, modulated by changes in tidal elevation was tested. During the measurement period, rip circulation was characterised by a large rotational surf zone eddy O(200 m) extending offshore from the inner-surf zone to the seaward face of the inter-tidal transverse bar. During high- and mid-tide, water depth over the bars was too deep to allow wave breaking, and a strong longshore current dominated the surf zone. As the water depth decreased towards low-tide, wave breaking was concentrated over the bar crests initiating the rotational rip current eddy. Peak rip flow speeds of 1.3 m s −1 were recorded around low-tide when the joint effects of dissipation and morphological constriction were maximised. At low tide, dissipation over the bar crests was reduced by partial bar-emergence and observations suggested that rip flows were maintained by morphological constriction and the side-drainage of water from the transverse bars.

Morphology and evolution of an anastomosed channel network where saline underflow enters the Black Sea

AbstractThe Bosphorus Strait accommodates two‐way flow between the Aegean and Black Seas. The Aegean (Mediterranean) inflow has speeds of 5 to 15 cm sec−1 in the strait and a salinity contrast of ∼12‰ to 16‰ with the Black Sea surface waters on the shelf. An anastomosed channel network crosses the shelf and in water deeper than 70 m is characterized by first‐order channels 5 to 10 m deep, local lateral accretion bedding, muddy in‐channel barforms, and a variety of sediment waves both on channel floors and bar crests, crevasse channels entering the overbank area and levée/overbank deposits which are radiocarbon‐dated in cores to be younger than ∼7·5 to 8·0 ka. This channel network accommodates the saline density current formed by the Mediterranean inflow. The density contrast between the density underflow and the ambient water mass is ∼0·01 g cm−3, similar to the density contrast ascribed to low‐concentration turbidity currents in the deep sea. Channel‐floor deposits are sandy to gravelly with local shell concentrations. Low‐relief bedforms on the channel floor have relatively straight crests, upflow‐dipping cross‐stratification, heights 1 to 1·5 m and wavelengths 85 to 155 m. Bankfull flows are subcritical, so these probably are not antidunes. Bar tops are ornamented locally with mudwaves having heights 1 to 2 m and wavelengths ∼20 to 100 m; these are potentially antidunes formed under shallow overbank flows. Towards the shelf edge, the degree of channel bifurcation increases dramatically and bar tops are dissected locally by secondary channels, some of which terminate in hanging valleys. Conical mounds on the shelf (possibly mud volcanoes or sites of fluid seepage) interact with the channel network by promoting accretion of muddy streamlined macroforms in their lee. This channel network may be one of the largest and most accessible natural laboratories on Earth for the study of continuously flowing density currents. Although the driver is salinity contrast, the underflow transports sufficient sediment to form levée wedges and large streamlined barforms, and presumably transports sediment into deep water.

Short term morphodynamics of an intertidal bar on megatidal ebb delta

The morphodynamics of an intertidal bar located on an ebb delta of a megatidal inlet system in Normandy, France, was examined during four short experiments under low to high energy wave conditions and spring or neap tide contexts. Although there have been numerous studies on ebb-tidal bar morphology and on the processes affecting such bars, these concern only microtidal or mesotidal settings, and are mainly based on observations. No detailed work involving hydrodynamic conditions embracing the whole tidal cycle has been carried out so far on the mechanisms of onshore migration of these coastal accumulation features, especially along macro and megatidal coasts. In this study, the morphological response of the bar was coupled with the residence times and intensities of each hydrodynamic process (swash, surf, shoaling waves) over a representative bar cross-shore profile, and sediment transport patterns deduced from fluorescent tracer. The results show that the bar migrated exclusively shoreward during moderate to storm conditions (Hs > 0.7 m at the bottom of the bar seaward slope). During low energy wave conditions, no bar movement was observed. Swash action was not the dominant process, mainly due to its duration, which did not exceed 8 min in the course of a semi-diurnal tidal cycle. The movement of sediments and the bar morphology is induced mainly by surf processes and their associated currents. The increase in the significant wave height disturbed the general mean current behaviour, which was parallel to the bar crest during low energy conditions. Under surf conditions, mean flows are directed onshore, with an absence of bed return flow; this allows sediment to be transported towards the bar crest causing a landward bar migration. An offshore-directed cross-shore current (not favourable for bar migration) is recorded with larger water depths and no breaking waves (shoaling conditions). This study also highlights the influence of the tidal water level fluctuations in such a large tidal range setting, which induce a rapid shift in processes (such as bed shear and sediment re-working) across the shore-normal profile. These conditions explain the relative low rate of migration of the bar over the delta platform (about 33 m/year). This rate is much less than those reported from micro-mesotidal environments. Calculations of bed shear stress also show that the mean currents alone are not responsible for onshore bar movement, notwithstanding the megatidal character of the field site.

Temporal and spatial behaviour of rip channels in a multiple‐barred coastal system

AbstractA 15‐month data set of daily time‐averaged video images (Argus) has been analyzed to describe the spatial and temporal variability of the rip channels on a multiple‐barred coast at Noordwijk aan Zee, The Netherlands. The landward boundary of the intertidal bars and a proxy of the subtidal bar crest, defined as the intertidal and subtidal bar lines respectively, were derived from the Argus images. Local seaward‐directed deviations of the bar lines represent the cross‐shore and alongshore locations of the rip channels. The average intertidal rip spacing ($\bar \lambda $) was 243 m, but the rips were not spaced regularly (σλ/$\bar \lambda $= 0.47). Some intertidal rips were observed to fill up during falling tide, but the majority remained open. The filled intertidal rip channels had more landward positions and migrated more slowly (2.4 versus 4.6 m/day) in the alongshore direction than the open intertidal rip channels. The number and the alongshore migration rate of open intertidal rip channels increased with the preceding wave heights (r = 0.26, p &lt; 0.01) and alongshore component of the offshore wave power (r = 0.25, p &lt; 0.01), respectively. The shape of the intertidal bar lines was similar to the subtidal bar line shape, suggesting that the intertidal morphology is coupled to the subtidal alongshore variability. The phase of two bar lines could vary from in phase (0°) to out of phase (180°). The phase changes gradually, due to different alongshore migration rates of the intertidal and subtidal bar lines. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Seasonal Changes of Sandbar Behavior in Nanwan Beach, South China

Frequent leveling data of beach profiles is collected from Nanwan Beach, which is characterized by a large seasonal variation in wave climate. Variability of the bar crest position obtained from the data of measured profiles is examined to quantify seasonal changes of the sandbar by empirical orthogonal function (EOF) analysis. The first eigenfunction showed that a rhythmic bar is a common feature of the beach all year round. The second eigenfunction effectively indicated that the two-dimensional variability of the bar behavior is of seasonal changes. The cross-shore bar movement ranged −30∼40 and −2∼60 m in summer and in winter, respectively. The next two eigenfunctions represented three-dimensional components of the bar behavior with a range of −21∼21 and −30∼21 m in summer and in winter, respectively.

The morphological response of a nearshore double sandbar system to constant wave forcing

Results of 2DH morphodynamic computations are presented to quantify the temporal evolution of the crescentic patterns emerging in a double nearshore bar system in response to constant wave boundary forcing. Sixteen different conditions varying both offshore wave height and angle of wave incidence were applied. The mean length scales of the emerging irregular crescentic patterns are linearly proportional to the local longshore velocity over the inner and outer bars. For similar longshore velocities, the length scales of the outer bar are larger than of the inner bar. This is explained by accounting for the difference in water depth above the bar crest. The variable morphological response times can be explained by including additional bathymetrical parameters. The active volume of the bar, defined by the breaker index, plays an important role in this response time. With larger active volumes the bar responds more rapidly to identical boundary conditions. Also, bars with a smaller total volume respond more quickly. This faster response is due to the steeper active volume of the bars. Different initial perturbations resulted in different locations of the emerging features, showing that their location is sensitive to the initial bathymetry. However, the range in length scales and response times due to the different perturbations was significantly smaller than those obtained for the different hydrodynamic conditions. Based on the present findings we hypothesize that morphological length scales in the field are rarely in equilibrium with the concurrent offshore wave height and angle of incidence owing to the slow response of the sandbars under constant conditions relative to the stochastic nature of natural wave forcing.

Infragravity wave contribution to surf zone sediment transport — The role of advection

Field measurements reported in the literature demonstrate that suspended sediment transport due to infragravity wave motions can sometimes be very large and dominate the net sediment transport at a given measurement location within the surf zone. At other times, however, this transport component is insignificant. Moreover, infragravity transport directions are inconsistent across the shoreface. This study tests the hypothesis that net sediment transport caused by infragravity waves may be due to advection (at infragravity frequencies) of sediment that has been suspended by breaking wind/swell waves. It is shown that infragravity sediment transports are onshore directed at the landward side of relative (incident) wave height maxima, and offshore directed at the seaward side of such maxima. If a longshore infragravity wave structure exists, such as in the case of standing edge waves, the advection process can create a perturbation of the bar crest and/or generate a crescentic bar. These results provide support for the template model for crescentic bar formation, first proposed by Bowen and Inman [Bowen, A.J. and Inman, D.L., 1971. Edge waves and crescentic bars. J.Geophys.Res., 76, 8662–8670] although the template mechanism is distinctly different from the template originally suggested and morphodynamic feedbacks between the evolving bar perturbations and the forcing mechanisms are important.

A Fundamental Study on the Long-Term and Large-Scale Evolution of Sandbars on Ishikawa Coast

Fundamental statistical analysis has been conducted on the long-term and large-scale evolutions of sandbars on Ishikawa Coast, Japan. The bar behavior is quantified in terms of bar crest position, crest/trough depth, bar height, and other parameters using a bathymetric data set with an alonghosre extent of 21.5 km and sampled for more than 40 years. The correlations for couples of parameters as well as the characteristics of temporal and spatial variation have been examined. The results revealed that the bar behavior along the Ishikawa Coast is characterized by the presence of cyclic cross-shore migration of outer bars with approximately 10-year periods. Superimposed on periodical migration, the outer bar position has a clear onshore trend in longer time scale of decades under the influence of overall erosion of the study area.

Characteristics and dynamics of surfzone transverse finger bars

Patches of transverse finger bars have been identified in the surf zone of Noordwijk beach (Netherlands). They consisted of three to nine elongated accumulations of sand attached to the low‐tide shoreline. The bars extended up to 50 m into the inner surf zone, had an oblique orientation with respect to the shore‐normal, and were quasiregularly spaced in the alongshore direction. We analyzed nearly 6 years of video data and observed a significant presence of finger bars (14% of the time with good data). Bars were visible on 193 days, gathered in 44 events that persisted from 2 days to 2 months. Obliquely incident waves of intermediate and approximately constant height were dominant during finger bar presence. Shore‐normal incident or more energetic wave fields destroyed the bar patches. The underlying bathymetry affected finger bar formation: inner surfzone troughs with cross‐shore areas of 100 m2 and inner surfzone slopes of 0.02 were more conducive to their growth. The mean alongshore wavelength of the finger bar patches was 39 m, ranging from 21 to 75 m. Bar crests deviated up to 40 degrees from the shore‐normal against the alongshore current direction (“up‐current orientation”) and bar patches migrated at rates up to 22 m/day in the direction of the alongshore current. We used these observations to test existing theoretical self‐organization mechanisms for transverse bar formation. The “bed‐flow mechanism” was the most viable explanation for the generation and persistence of Noordwijk finger bars. Our observations were consistent with most of the predictions of two models that included this interaction, but migration rates differed by 1 order of magnitude.

Storm-generated morphological change and longshore sand transport in the intertidal zone of a multi-barred macrotidal beach

The morphology, hydrodynamics and sediment transport processes of a multi-barred macrotidal beach were surveyed over 21 semi-diurnal tides, the first eight of which were dominated by storm conditions with offshore wave heights ranging from 1.2 to nearly 3.5 m. The hydrodynamic conditions prevailing over the intertidal beach showed a rapid response to these storm conditions, notably through strong longshore currents that attained peak velocities of 1.6 m s − 1 , i.e. over three times the peak spring tide velocities of tide-generated longshore currents during calm conditions. The bars showed no cross-shore migration during the storm events. Morphological mapping of the beach, tracer dispersal patterns, and calculations of potential sand transport rates using the energetics-based model of Bailard [Bailard, J.A., 1981. An energetics total load sediment transport model for a plane sloping beach. J. Geophys. Res., 86, 10938–10954.] showed significant longshore sand transport, as well as longshore migration of an intertidal drainage channel. These longshore movements were bi-directional, responded to changes in incident wind and wave directions, and involved wave stirring, coupled with longshore advection of sand along the bar crests in particular. Strong trough and cross-shore channel flows also led to the longshore migration of crescentic dunes. The longshore sediment movements were embedded in a cross-shore gradient associated with increasing overall profile mobility from the lower to the upper beach in response to wave shoaling and post-breaking processes, assessed through examination of the relative wave height Hs h − 1 , based on the relationship between significant wave height (Hs) and water depth ( h). Longshore sediment transport is a characteristic feature of the long-term evolution of Wissant Bay, which is dominated by a net northeastward residual sand transport within a bay sediment cell characterised by significant updrift erosion and downdrift accumulation. Longshore transport should be considered as an essential element of the morphodynamics of bar-trough beaches subject to large tidal ranges and significant wind stress, but there is a need for a better understanding of the role of wind stress on longshore currents in the shallow intertidal zone of these beaches.

3-D Architecture and Sequence Stratigraphic Evolution of a Forced Regressive Top-Truncated Mixed-Influenced Delta, Cretaceous Wall Creek Sandstone, Wyoming, U.S.A.

Abstract Ground-penetrating radar (GPR) is used to delineate the 3-D facies architectural elements and examine the evolution of a top-truncated, forced-regressive, mixed-influenced delta front in the Cretaceous Wall Creek Member of the Frontier Formation, Wyoming. The GPR data provide a bridge between outcrop facies architecture and recently published high-resolution 2-D seismic studies of Quaternary delta systems. The GPR data were integrated with outcrops, photomosaics, cores, and GPS data. Two orthogonal grids of 2-D GPR profiles provide information on the 3-D facies architecture and stratigraphy of the deltaic deposits. Three main GPR architectural elements are identified within the delta front: (1) mouth bars are characterized by seaward-dipping reflections (foreset beds) in dip view, and mounded bidirectionally downlapping reflections in strike view; (2) shallow delta-slope channels show truncation and onlap and also show low-angle landward (northwest) dipping reflections. The mouth bars are locally interrupted by (3) sub-horizontal radar reflections that are interpreted as tidally modulated bars. Within the mouth-bar radar facies, both top-preserved (proximal) and top-truncated (distal) examples are observed. Top-preserved proximal bars show evidence of mounded bar crests and landward accretion, as is observed in the upstream end of mouth bars on the modern Atchafalaya Delta in the Gulf of Mexico. Distal bar deposits primarily comprise the top-truncated, seaward-dipping foresets. The GPR interpretation shows two laterally overlapping delta lobes, interpreted to represent autocyclic delta switching. The older lower lobe forms a fan-shaped delta lobe composed of top-truncated coalesced mouth bars. The second phase of bars is largely top-preserved, building over the older bars. The presence of thicker bedsets and upstream accretion suggests that the younger lobe was quickly abandoned, as seen in the overlying transgressive ravinement. Although marine erosion during transgression was less effective on these later-stage bar deposits, ravinement increased landward. A comparison of the architectural elements at a more regional scale in two separate Wall Creek delta lobes (Murphy Creek and Raptor Ridge, ~ 30 km apart) indicates that the bar complexes were deposited on the delta fronts as accretionary forced-regressive deposits during a relative sea-level fall; the older river-dominated deltaic deposits at the Murphy Creek site prograded southeastward (~ 124°), whereas the younger and more tidally influenced Raptor Ridge bar deposits expanded southward (~ 171°) before final abandonment and regional transgression. The most distal lowstand lobe sediments within the offlapping parasequence at Raptor Ridge are more completely preserved and also show the greatest tidal influence, compared to the more severely top-truncated Murphy Creek, which is in an otherwise more landward location. This challenges the notion that tidal facies are predominantly associated with transgressive or highstand systems tracts. Greater preservation of deltaic facies in forced-regressive and lowstand deltas, as shown here, has also been documented in Quaternary shelf-edge deltas, such as the Lagniappe in the Gulf of Mexico and the Mahakham in Kalimantan.

Note on sediment sorting in a sandy bed under standing water waves

We present new quantitative data on the sorting of sediments on a sandy seabed under standing waves. Starting from a flat bed composed of a homogeneous mixture of a coarse and a fine sand with mean diameters 0.11 and 0.21 mm, we observed simultaneous ripple and sand bar formation and sand sorting on the seabed. Over days of wave action, sand bars formed with crests beneath the surface wave nodes and flat plateaus flanked by mounds beneath the antinodes. Bar crests were composed of sand coarser on average than 0.21 mm, while the flat plateaus were covered by sand finer on average than 0.11 mm. Comparison with two experiments involving sand beds of more homogeneous size distributions shows that the mounds are characteristic of the motion of fine suspensions.

Cross‐shore sediment transport on natural beaches and its relation to sandbar migration patterns: 2. Application of the field transport parameterization

A cross‐shore sediment transport parameterization (shape function) based on data from intensive field experiments carried out on five European beaches is incorporated into a simple time dependent model to explore its capability for reproducing sandbar generation and evolution in the medium term (O(months)). Model results are compared with bar‐crest migration patterns observed at Duck, North Carolina. The model comprises a simple wave transformation routine that accounts for linear shoaling and assumes a saturation law for wave decay inside the surf zone. Second‐order statistics derived from the cross‐shore distribution of wave heights are used to scale the sediment transport parameterizations which estimate the cross‐shore structure of the third and fourth velocity moments. Cross‐shore gradients in sediment fluxes, calculated with an energetics approach, produce bed elevation changes. The updated morphology affects the wave propagation in the next time step forming a fully coupled model for predicting sandbar evolution. Bar generation close to shore and the details of the sandbar evolution are successfully reproduced (R2 = 0.86) over the 77‐day period of the observations using default values for drag coefficients and efficiency factors. In accordance with the observations, the model shows rapid offshore sandbar migration in response to individual storms, and weak onshore bar movement for lower energy conditions. Owing to its simple nature, the model is not capable of reproducing the whole profile morphology especially in the trough (no trough generation), and it is only valid for replicating bar crest position. The results suggest that a convergence of sediment near the breakpoint (breakpoint hypothesis) combined with morphological feedback, can successfully explain the evolution of bar crest over periods of months from the details of forcing (incident waves and water levels).

Tidal influence on the intertidal bar morphology of two contrasting macrotidal beaches

Analyses of morphological and hydrodynamic data from two macrotidal beaches (mean spring ranges respectively 5.6 and 6.4 m) with intertidal bars and troughs were carried out in order to detect and explain the relationship between tides and bar morphological attributes (bar crest height and location) and morphological change. The following four aspects are treated in the paper: (1) tidal levels, vertical tidal translation rates and bar morphology, (2) tidal range and morphological relaxation, (3) the neap–spring tidal variation, and (4) tide-generated longshore currents. Bar distribution and bar heights across the profile are not related in a straightforward manner to tidal levels and expected vertical tidal translation rates at these levels, because the wave process domains involved in bar formation and bar morphological change are strongly modulated by wave energy, by tide-modulated water levels which vary with the neap–spring cycle, and by the beach morphology. The variability of these conditions may explain the large variability of bar–trough beach morphology. The relatively large tidal ranges and rapidly shifting wave domains across the profile imply long periods of non-exposure of the bars to wave processes during each semi-diurnal tidal cycle, and this tide-generated effect reduces the scope for prolonged wave reworking while contributing to enhancing the long relaxation times and morphological feedback which minimise change on these beaches. The effects of the neap–spring tidal range variation on wave activity on these beaches are subordinate to those due to the random wave energy regime, but this neap–spring variation appears to influence the morphologically important intertidal drainage channels associated with bar–trough topography. Although longshore tidal currents are relatively strong on these beaches, digital elevation models show no significant longshore bar and tidal channel migration over time, due to the absence of sustained strong wind conditions, which considerably reinforce such longshore tidal currents, and which therefore are the dominant control on longshore processes on these beaches. It is also tentatively suggested that there is no established relationship between tidal range and the number of intertidal bars on these beaches.

Onshore Migration of a Swash Bar During a Storm

Abstract The morphodynamics of a swash bar was monitored over two tidal cycles during a storm event at Skallingen, Denmark. Higher water levels during the first tidal cycle forced waves to break either landward of, or at, the bar crest. The bar was unable to significantly alter the pattern of wave breaking over the foreshore slope, creating a potential for bar erosion through an offshore transport of sediment. In response, the bar became more symmetric in cross-section, and its height decreased. Lower water levels during the second tidal cycle forced waves to break on the seaward slope, causing the bar to rebuild and its crest to migrate onshore. The onshore migration of the swash bar occurred despite little change in the offshore wave height from one tidal cycle to the next and in conditions that are typically associated with offshore transport and bar migration. The results further illustrate a strong tidal dependency in which water depth relative to bar height is an important control on the morphodynam...