Bar Crest Research Articles

Effects of basin bottom slope on jet hydrodynamics and river mouth bar formation

River mouth bars are strategic morphological units primarily responsible for the development of entire deltaic systems. This paper addresses the role of receiving basin slope in the hydrodynamics of an exiting sediment-laden turbulent jet and in resulting mouth bar morphodynamics. We use Delft3D, a coupled hydrodynamic and morphodynamic numerical model, along with a theoretical formulation to reproduce the physics of the problem, characterized by a fluvially dominated inlet free of waves and tides. We propose an updated theoretical model with a slope-dependent entrainment coefficient, showing that the rate at which ambient fluid is incorporated into a jet increases with higher basin slopes. Transient results reveal that the magnitude of a basin slope can alter the stability of a jet, favoring the formation of an unstable meandering jet. While a stable jet gives rise to “middle-ground” bars accompanied by diverging channels, a “lunate” mouth bar results from unstable jets. Additional morphodynamic simulations demonstrate that the time required for mouth bar stagnation in its final position increases linearly with the basin slope. In contrast, the distance at which the mouth bar eventually forms decreases until reaching an asymptotic value for slopes higher than 2%. Moreover, the basin slope highly influences sedimentary processes responsible for bar formation: for milder slopes, progradation processes prevail, while in steeper basins aggradation is more relevant. Finally, the minimum relative water depth over a bar crest that forces the flow to bifurcate around a fully developed bar decreases with the basin slope.

Video assessment of nearshore and beach evolution following the deployment of a submerged geotextile wave breaker

Balouin, Y.; Longueville, F., and Colombet, Y., 2016. Video assessment of nearshore and beach evolution following the deployment of a geotextile wave breaker. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 617–621. Coconut Creek (Florida), ISSN 0749-0208.The feedback on the use of submerged structures for beach protection is still scarce, and the shoreline response to structures is still poorly understood at present. Most of the studies presents structures close to the shoreline without significant morphologies on the shoreface that could interact with it. On the Lido of Sète, a narrow coastal barrier facing the Mediterranean Sea, an experimentation of beach protection using a submerged geotextile structure is undertaken. The structure was deployed in 2013, and to analyse nearshore dynamics, an ARGUS video system was installed in 2011. Video-derived shoreline, bar crest position, alongshore currents, swash processes, were used to assess the natural dynamics and to evaluate the evolution after the installation of the 850 m long structure.The monitoring permitted to describe a rapid dynamics of crescentic nearshore bars. Processes of longshore migration, linearization, as well as a strong coupling between bar dynamics and shoreline sinuosities were described. After the deployment of the geotextile, a progressive linearization and rotation of the nearshore bar (parallel to the tube) was observed. This process induced a progressive rotation of the shoreline and an important beach enlargement. The 3D patterns, both in the bar and emerged beach progressively disappeared. With this new beach morphology, storm run-up is more regular alongshore, and only major events can reach the dune front and generate impacts. This field experiment provide an interesting feedback on morphological response following the deployment of a submerged structure on a shoreface characterized by an important nearshore bar dynamics.

On the Intersite Variability in Inter-Annual Nearshore Sandbar Cycles

Inter-annual bar dynamics may vary considerably across sites with very similar environmental settings. In particular, the variability of the bar cycle return period (Tr) may differ by a factor of 3 to 4. To date, data studies are only partially successful in explaining differences in Tr, establishing at best weak correlations to local environmental characteristics. Here, we use a process-based forward model to investigate the non-linear interactions between the hydrodynamic forcing and the morphodynamic profile response for two sites along the Dutch coast (Noordwijk and Egmond) that despite strong similarity in environmental conditions exhibit distinctly different Tr values. Our exploratory modeling enables a consistent investigation of the role of specific parameters at a level of detail that cannot be achieved from observations alone, and provides insights into the mechanisms that govern Tr. The results reveal that the bed slope in the barred zone is the most important parameter governing Tr. As a bar migrates further offshore, a steeper slope results in a stronger relative increase in the water depth above the bar crest which reduces wave breaking and in turn reduces the offshore migration rate. The deceleration of the offshore migration rate as the bar moves to deeper water—the morphodynamic feedback loop—contrasts with the initial enhanced offshore migration behavior of the bar. The initial behavior is determined by the intense wave breaking associated with the steeper profile slope. This explains the counter-intuitive observations at Egmond where Tr is significantly longer than at Noordwijk despite Egmond having the more energetic wave climate which typically reduces Tr.

The Vertical Structure of Low-Frequency Motions in the Nearshore. Part I: Observations

AbstractField observations of oscillating currents in the surfzone of a natural beach show significant vertical structure in energy, phase, and rotation at low frequencies around 0.005 Hz, where most of the energy is associated with vorticity motions. Energy levels in the cross-shore component of the flow seaward of the sandbar decay near the bottom. Shoreward of the bar crest, the flow decays nearly linearly over the water column. Conversely, a weaker alongshore component of the flow increases near the bottom seaward of the sandbar and is roughly depth-uniform inside the bar crest. Near this 0.005-Hz frequency band, the coherence between the uppermost and successive vertically separated sensors drops off quickly, with as much as a 70%–80% coherence drop over the water column (ranging from 2.5 to 4 m). The phase relative to the uppermost sensor shifts approximately linearly over depth, with as much as 50° phase lag at the bottom that can lag or lead the surface. Rotary coefficients also vary across the surfzone and are generally nonzero with rotational directions (cyclonic or anticyclonic) and orientation that depend on sensor position relative to the sandbar and alongshore current profile. The rotary coefficients are generally not uniform with depth and can change sign in the vertical. The observed behavior is qualitatively predicted by boundary layer theory (discussed in the companion paper by Lippmann and Bowen). The nonuniform vertical structure has implications to the interpretation of field data and horizontal nearshore mixing.

Parameterization of Time-Averaged Suspended Sediment Concentration in the Nearshore

To quantify the effect of wave breaking turbulence on sediment transport in the nearshore, the vertical distribution of time-averaged suspended sediment concentration (SSC) in the surf zone was parameterized in terms of the turbulent kinetic energy (TKE) at different cross-shore locations, including the bar crest, bar trough, and inner surf zone. Using data from a large-scale laboratory experiment, a simple relationship was developed between the time-averaged SSC and the time-averaged TKE. The vertical variation of the time-averaged SSC was fitted to an equation analogous to the turbulent dissipation rate term. At the bar crest, the proposed equation was slightly modified to incorporate the effect of near-bed sediment processes and yielded reasonable agreement. This parameterization yielded the best agreement at the bar trough, with a coefficient of determination R2 ≥ 0.72 above the bottom boundary layer. The time-averaged SSC in the inner surf zone showed good agreement near the bed but poor agreement near the water surface, suggesting that there is a different sedimentation mechanism that controls the SSC in the inner surf zone.

Bar response to tides under regular waves

The morphological time scale for the response of outer bar crest level to tidal water-level changes is found to be remarkably short, namely around 15min for near-prototype wave heights and depths and regular waves. This corresponds to the bar crest level range being 0.77 times the tidal range with a time-lag of only 15min. Only one near-prototype scale experiment with tide and sufficient time resolution is known to the writers, namely Kraus & Larson (1988) Case 911, which had hmax=4.4m, d50=0.4mm, a semidiurnal tide range of ca 1m and regular waves with H=1.34m, T=7.9s. However, the fast crest-level response is also seen in profile development studies without tide, i.e., the bar crest level generally gets close to its final value in an hour or so while the overall profile adjustment (e.g. seaward bar migration) continues for tens of hours.

The self-regulation process and its mechanism of channels’ bed changes in the Changjiang (Yangtze) Estuary in China

Recent bathymetric changes in the Changjiang Estuary under the influence of artificial regulation engineerings and basin reservoirs have been analyzed based on the maritime charts since 1997 and recent fieldworks. The results indicate a slight erosion of the channels in the upper and middle estuary, continuing deposition and seaward move of the mouth bar crest and intensifying erosion at the nearshore seabed. It is noteworthy that the morphological evolution caused by intensive human activities dominates over the changes from nature process. First, the riverbes are eroded overall in the South Branch (SB), the South Channel (SC) and the upper and middle reaches of the North Channel (NC). The nearshore seabed outside the river mouth is being eroded slightly, which is attributed to the declining sediment supply from the Changjiang Basin due to the construction of the Three Gorges Dam upstream. The sediment above the seabed is very active and coarsened, meanwhile, sand waves are becoming more distinct. Second, a deposition occurs in the North Brach (NB), the mouth of the NC, the mouth bars of the North Passage (NP) and the South Passage (SP) and especially the main channel of the NP, where it shows a massive siltation after the deep waterway project. The reasons for the recent changes are not only the dynamic structure in estuarine mouth bars, but also the supply of sediment resuspension in a local and offshore area. Meanwhile, the severe erosion and siltation in some reaches is related to the construction of estuarine engineerings. It is indicated that the Changjiang Estuary is gradually self-adjusted and adapting to the varying natural factors and intensive human activities. The study on the mechanism of self-regulation of the recent bathymetric changes in the Changjiang Estuary has important and practical significance.

Influences of wave forcing and morphological variability on the evolution of a double-sandbar system

Double-sandbar systems are common along sandy, wave-dominated coastlines. The evolution of a double-sandbar system is a complex process and is affected not only by the time-varying wave forcing but also by the local seabed morphology itself. The influences and relative importance of both wave condition and morphological variability on the evolution of a double-sandbar system still remain unclear and are investigated in this study using a well-established operational quasi-three-dimensional coastal evolution model. Results show that these two factors contribute to the final double-sandbar morphology in different ways. For energetic waves with high angles of wave incidence, waves determine the final sandbar morphology, regardless of the antecedent bathymetry. For moderate waves with small angles of wave incidence, however, the pronounced morphological variability (crescentic pattern of sandbars) will dominate the evolution, which remarkably enhances the existing morphological pattern regardless of the changed wave condition. A nondimensional index is proposed to quantitatively evaluate the relative importance of these two mechanisms. Further quantitative analysis reveals that the relative importance of wave condition and morphological variability depends on the alongshore current velocities over the bar crests. There also appears a critical value for these alongshore current velocities, below which morphological variability tends to be dominant and wave forcing is of less importance.

Regional scale sandbar variability: Observations from the U.S. Pacific Northwest

Understanding sandbar dynamics and variability is integral to developing a predictive capacity for nearshore flows, sediment transport, morphological change, and ultimately for determining coastline exposure to damaging storm waves. Here we report on a nearshore bathymetric data set from the U.S. Pacific Northwest (PNW) that stretches from Point Grenville, Washington to Cascade Head, Oregon, over approximately 260km in the alongshore and includes 8 distinct littoral cells. We describe and quantify the morphological variability of sandbars on a regional scale, using 560 individual cross-shore transects, as well as attempt to explain the inter-littoral cell variability via relationships to various environmental parameters. The cross-shore extent of the bar zone extends over 1km from the shoreline in the northern part of the study area, but only to about 600m from the shoreline in the southern part. Maximum bar crest depths are typically 7m below MLLW. Bar heights range from a step in the cross-shore profile to over 3m from crest to trough. The northernmost littoral cells typically have two or more subtidal sandbars per cross-shore profile whereas the littoral cells in the southern part of our study area have only one bar. The mean depths of the bars, however, are much more consistent across littoral cells even while the upper shoreface slope significantly increases from north to south, requiring that the maximum bar distance from the shoreline decreases from north to south. Results from a limited study of the temporal variability suggest that while data collected over large spatial scales captures significant amounts of overall sandbar variability, it does not completely characterize the variability over the entirety of the net offshore migration cycle.

Observations and modeling of surf zone transverse finger bars at the Gold Coast, Australia

The occurrence and characteristics of transverse finger bars at Surfers Paradise (Gold Coast, Australia) have been quantified with 4 years of time-exposure video images. These bars are attached to the inner terrace and have an oblique orientation with respect to the coastline. They are observed during 24 % of the study period, in patches up to 15 bars, with an average lifetime of 5 days and a mean wavelength of 32 m. The bars are observed during obliquely incident waves of intermediate heights. Bar crests typically point toward the incoming wave direction, i.e., they are up-current oriented. The most frequent beach state when bars are present (43 % of the time) is a rhythmic low-tide terrace and an undulating outer bar. A morphodynamic model, which describes the feedback between waves, currents, and bed evolution, has been applied to study the mechanisms for finger bar formation. Realistic positive feedback leading to the formation of the observed bars only occurs if the sediment resuspension due to roller-induced turbulence is included. This causes the depth-averaged sediment concentration to decrease in the seaward direction, enhancing the convergence of sediment transport in the offshore-directed flow perturbations that occur over the up-current bars. The longshore current strength also plays an important role; the offshore root-mean-square wave height and angle must be larger than some critical values (0.5 m and 20∘, respectively, at 18-m depth). Model-data comparison indicates that the modeled bar shape characteristics (up-current orientation) and the wave conditions leading to the bar formation agree with data, while the modeled wavelengths and migration rates are larger than the observed ones. The discrepancies might be because in the model we neglect the influence of the large-scale beach configuration.

Application of higher-level GN theory to some wave transformation problems

Recently derived (Webster et al., 2011), simplified higher-level Green–Naghdi equations (GN-3, GN-5 and GN-7) are used in this work to simulate the transformation of two-dimensional, shallow-water wave problems. The spatial derivatives are discretized through a five-point difference scheme. A new algorithm is developed to solve the resulting block-pentadiagonal matrix. These high-level GN equations are then utilized to develop a numerical wave tank. A wave-maker is placed at the forcing boundary of the tank that uses the stream-function theory to generate nonlinear incident waves. The numerical wave tank is used to analyze the effects of large-amplitude waves passing over a submerged bar. A damping zone is placed near the wave-maker (up-wave side) to absorb the reflected waves from the front side of the submerged bar. Another damping zone is placed at the down-wave side of the computational domain to absorb the radiated waves. In the first test case, the front and back slopes of the bar are both mild (Luth et al., 1994). The waves that evolved over the bar are simulated by using the GN-3, GN-5 and GN-7 equations. The GN-3 equations provide time histories that compare well with the experimental data at different wave gauges, except at the ones behind the bar. The results of the GN-5 and GN-7 equations compare very well with all the experimental data considered here. In the second test case, the front and back slopes of the bar are both steep (Ohyama et al., 1995). The GN-5 equations predict the wave elevation well. In the third test case, the front and back slopes of the bar alternate, one of them being mild and the other one being steep (Zou et al., 2010). Again, the predictions of the GN-5 equations agree with the experimental data well. In all the test cases considered in this work, there are some differences between the GN-3 and GN-5 results after the crest of the bar. Numerical results obtained by the GN-5 and GN-7 equations are almost the same along the wave flume, but the GN-7 equations require more computational time. Therefore, the GN-5 results are accepted here as the converged GN theory results. The numerical validations show that the GN-5 equations can simulate the strongly nonlinear and dispersive waves observed behind the submerged bar crest satisfactorily.

Bathymetry correction using an adjoint component of a coupled nearshore wave‐circulation model: Tests with synthetic velocity data

The impact of assimilation of wave‐averaged flow velocities on the bathymetric correction is studied in tests with synthetic (model‐generated) data using tangent‐linear and adjoint components of a one‐way coupled nearshore wave‐circulation model. Weakly and strongly nonlinear regimes are considered, featuring energetic unsteady along‐beach flows responding to time‐independent wave‐averaged forcing due to breaking waves. It is found that assimilation of time‐averaged velocities on a regular grid (mimicking an array of remotely sensed data) provides sensible corrections to bathymetry. Even though the wave data are not assimilated, flow velocity assimilation utilizes adjoint components of both the circulation and wave models. The representer formalism allows separating contributions of these two components to the bathymetric correction. In a test case considered, involving a beach with an alongshore varying bar, the adjoint wave model contribution was mainly to determine the cross‐shore position of the bar crest. The adjoint circulation model provided an additional contribution, mostly adding to alongshore variability in the shape of the bar. The array mode analysis reveals that there are very few modes that can be effectively corrected, given the assumed data error level. Bathymetry perturbations associated with these modes are a mixture of near‐coast intensified modes as well as modes extending their influence to deep water (along the background wave characteristics). Additional tests show the utility of different observational arrays in providing the bathymetric correction.

Relations between geomorphic form and sedimentologic-stratigraphic variability: Holocene ooid sand shoal, Lily Bank, Bahamas

Although the linkages among surface sediments, geomorphic forms, and hydrodynamics in Holocene ooid tidal sand shoals have been evaluated recently, how these factors are reflected in the geomorphic evolution and stratigraphic record of shoals is less constrained. Yet, such understanding is essential to developing meaningful predictive conceptual models of three-dimensional architecture of ancient reservoir analogs. Integrating remote-sensing imagery, high-frequency seismic data, and core characterization from Lily Bank, a modern tidally dominated Bahamian ooid shoal in which sedimentologic processes are well documented, reveals the stratigraphic record of geomorphic change. An irregular, gently dipping rocky surface (interpreted as the top Pleistocene) with no pronounced topographic high underlies the Holocene oolitic succession. A 6-m (20-ft)–thick poorly sorted, gravelly muddy sand with few ooids overlies this basal surface. This lower interval is overlain by sand with an upward increase in proportion of ooids, sorting, and grain size. The uppermost unit, present only under active bar forms, is well-sorted oolitic medium sand with accretionary foresets. Sediments vary stratigraphically and geomorphically; the lower unit is finer and less well sorted than the upper units, and in the oolitic upper unit, sediment size and sorting on bar crests are distinct from bar flanks. Collectively, these results suggest that a marked antecedent bump is not necessary for occurrence of ooid shoals and that the stratigraphic record of analogous ooid shoal systems may preserve clues of geomorphic position, as well as geobody size and orientation.

MULTIPLE REGRESSION ANALYSIS OF EFFECTS OF BAR AND TIDE ON SHORELINE CHANGE

The effects of bars and tides on shoreline change were investigated by a multiple regression analysis. The shoreline change rates used for the analysis were estimated from the beach profiles measured every workday during a 22-year period from 1986 to 2007 on the Hasaki coast in Japan. The examined parameters which had the potential to affect shoreline change rates were offshore wave energy fluxes, previous shoreline positions, maximum, minimum and average tides, and inner and outer bar crest elevations. In the multiple regression analysis, parameters which affected the shoreline change rate were selected by comparing the multiple regression models developed by combining the parameters on the basis of the Akaike Information Criterion (AIC) value, and the effects were also estimated by using the coefficients of the best model which had the smallest AIC. The shoreline change was affected by not only the offshore wave energy fluxes and the previous shoreline positions but also the maximum and minimum tides and the inner and outer bar crest elevations. The largest effect was the offshore wave energy fluxes, followed in order by the maximum tides, the previous shoreline positions, the minimum tides, the outer bar crest elevations and the inner bar crest elevations.

Prediction of time-dependent sediment suspension in the surf zone using artificial neural network

A predictive model of time-dependent sediment suspension was developed with artificial neural network, using velocity and integrated sediment concentration (i.e., suspended load) data observed from a large-scale laboratory experiment under erosive and accretive beach conditions. Low-frequency motion, wave-induced motion, and turbulent kinetic energy per unit mass at lower (1cm above the bed) and upper (50cm above the bed) ADV were used as inputs and suspended load near the bed was used as output. To account for the history prior to sediment suspension, five time steps (corresponding to 2s) of data prior to sediment suspension were included in the inputs. The artificial neural network model shows a predictive capability of the time-dependent sediment suspension in the surf zone for both erosive and accretive conditions. The correlation coefficient (r) between the measured and predicted suspended load reached up to r=0.79 at the bar crest in the accretive beach condition, and typical average correlation coefficients over all subsets (r̄) were 0.53 over the bar crest. The results from various input cases showed that turbulent kinetic energy near the bed is most important to predict the sediment suspension in the surf zone for both erosive and accretive conditions. Also, the best predictive performance was shown at the bar crest, supporting the hypothesis that turbulence induced by intense wave breaking at the bar crest is a dominant mechanism for sediment suspension. Outside the surf zone, the wave-induced motion was the highest contributing factor for the sediment suspension. Furthermore, when the information from the upper ADV was included as inputs, the model improved the prediction capability by 40% at the bar trough in the erosive beach condition. The increased prediction at this location may possibly be attributed to the effect of vertical shear motion in the low-frequency range due to strong undertow from wave breaking.

An investigation on the formation of submerged bar under surges in sandy coastal region

Cross-shore sediment transport rate exposed to waves is very important for coastal morphology, the design of marine structures such as seawalls, jetties, breakwaters etc, and the prevention of coastal erosion and accretion due to onoff shore sediment transportation. In the present study, the experiments on cross-shore sediment transport are carried out in a laboratory wave channel with initial beach slopes of 1/8, 1/10 and 1/15. By using the regular waves with different deep-water wave steepnesses generated by a pedal-type wave generator, the geometrical characteristics of beach profiles under storm conditions and the parameters affecting on-off shore sediment transport are investigated for the beach materials with medium diameters of d50=0.25, 0.32, 0.45, 0.62 and 0.80 mm. The offshore bar geometric characteristics are the horizontal distances from the shoreline to the bar beginning (Xb), crest (Xt), and ending (Xs) points, the depth from the bar crest to the still water level (ht), and the bar volume (Vbar). The experimental results have indicated that when the deep-water steepness (H0/L0) increased, the net movement to seaside increased. With the increasing wave steepness, the bars moved to widen herewith the vertical distances from still water level to the bar beginning (Xb), crest (Xt) and ending (Xs) points and the horizontal distances from the coast line to the bar beginning, crest and ending points increased. It was also shown from experimental results that the horizontal distances from the bar beginning and ending points to the coast line increased with the decrease of the beach slope. The experimental results obtained from this study are compared with previous experimental work and found to be of the same magnitude as the experimental measurements and followed the expected basic trend.

Cross-shore variation of intermittent sediment suspension and turbulence induced by depth-limited wave breaking

Intermittent features of turbulence and sediment suspension were investigated in a large-scale laboratory flume, under erosive and accretive conditions with measured free surface elevation, fluid velocity, and sediment concentration across the surf zone. Intermittent events of turbulence and sediment suspension occurred for a small portion of the time series but contained a significant amount of motions in these events. Comparison of intermittency statistics with previous studies conducted under different experimental conditions showed similar results, indicating that intermittency is a general aspect of turbulence and sediment suspension in the surf zone, including the bar crest, the bar trough, and the inner surf zone. The relationship of these intermittent turbulence and suspension events was examined with conditional probabilities. Here, it was found that only 20–35% of the turbulent events were associated with sediment suspension events, implying that much of the intermittent turbulent motion may act to dissipate wave energy rather than suspend sediments. On the other hand, 50–65% of the sediment suspension events were associated with turbulent events, implying that intermittent turbulent motion is one of the fundamental mechanisms for the initiation of sediment suspension in the surf zone. The sediment suspension events that were uncorrelated with turbulent events were mostly induced by strong offshore low-frequency motion, suggesting that advection plays a key role. The vertical structures of intermittency showed that the fraction of intermittent events for both turbulence and sediment concentration were vertically uniform and quantitatively similar each other, although their thresholds varied vertically. The motion contained in the intermittent suspension events increased toward the bed, while it was vertically uniform in the intermittent turbulent events. Finally, it was found that the intermittent sediment suspension events play an important role in each bathymetric change.

On bar growth and decay during interannual net offshore migration

Multiple bar systems often show a cyclic net offshore directed migration with return periods on the order of years. Generally, a bar is generated near the shoreline, grows in height and width, while migrating offshore before finally decaying at the seaward limit of the surf zone. Based on a three-year hindcast of a bar cycle at Noordwijk (Netherlands) and on additional synthetic runs using a wave-averaged cross-shore process model, this study identifies the dominant mechanisms that govern the bar amplitude growth and decay during net interannual offshore migration. The bar amplitude response is particularly sensitive to the water depth above the bar crest, hXb, and the angle of wave incidence, θ. These variables largely control the amount of waves breaking on the bar and the strength and cross-shore distribution of the associated longshore current. The longshore current has its maximum landward of the bar crest, inducing additional stirring of sediment on the landward bar slope and trough. The enhanced sediment concentration in the trough region shifts the cross-shore transports peak landward of the bar crest forcing bar amplitude growth during offshore migration. For increased hXb-values wave breaking becomes less frequent, consequently reducing the influence of the longshore current on sediment stirring. Therefore, the resulting dominance of the cross-shore current results in a sediment transport peak at the bar crest causing bar amplitude decay. All four types of bar response (viz. all combinations of onshore/offshore migration and bar amplitude growth/decay) can occur for a single wave height and wave period combination, depending on hXb and θ. Additional hindcast runs in which the wave direction was assumed time-invariant confirmed that hXb and θ largely control the transient bar amplitude response.

DCIV measurements of flow fields and turbulence in waves breaking over a bar

The experimental investigation of laboratory waves breaking over a bottom topography resembling a bar is presented in this paper. The experiments were conducted in a glass flume. The instantaneous velocity fields were measured using a video based digital correlation image velocimetry (DCIV). Measurements of velocities were restricted to positions shorewards of the bar crest. Measurements of time averaged velocities, turbulence intensities and turbulent kinetic energies that span the entire water column are provided. As the plunging-like wave at the crest of the bar turns into a spilling-like wave with increasing depth on-shore of the bar, it is shown that a limited transfer of TKE down to the bed is observed compared to spilling breakers on uniform slopes.

Anatomy of a laterally migrating tidal bar in front of a delta system: Esdolomada Member, Roda Formation, Tremp‐Graus Basin, Spain

AbstractThe first sandstone unit of the Esdolomada Member of the Roda Formation (hereafter referred to as ‘Esdolomada 1’) was formed by a laterally‐migrating, shelf tidal bar. This interpretation is based on detailed mapping of the bedding surfaces on the digital terrain model of the outcrop built from light detection and ranging data and outcrop photomosaics combined with vertical measured sections. The Esdolomada 1 sandbody migrated laterally (i.e. transverse to the tidal currents) towards the south‐west along slightly inclined (1.6° to 4.6°) master bedding surfaces. The locally dominant tidal current flowed to the north‐west. This current direction is indicated by the presence of stacked sets of high‐angle (average 21°) cross‐stratification formed by dunes that migrated in this direction, apparently in an approximately coast‐parallel direction. The tidal bar contains sets and cosets of medium‐grained cross‐stratified sandstone that stack to reach a thickness of about 5·5 m. Individual cross‐bed sets average about 50 cm thick (with a range of 10 to 70 cm) and have lengths of ca 130 to 250 m in a direction perpendicular to the palaeocurrent. Set thickness decreases in the direction of migration, towards the south‐west, and the degree of bioturbation increases, so that the cross‐bedded sandstones gradually change into highly bioturbated finer‐grained and thinner‐bedded sandstones lacking any cross‐stratification. The rate of thinning of individual dune sets as they are traced down any obliquely‐accreting master surface is some 40 cm per 100 m (0·004) for the older, thicker sandstones, whereas the younger, thinner beds thin at a rate of 15 cm over 100 m (0·0015). The tidal bar has a sharp base and top and is encased in finer‐grained bioturbated, marine sandstones. The Esdolomada bar crest was oriented north‐west to south‐east, parallel to the tidal palaeocurrents and to the nearby palaeoshoreline, but built by lateral accretion towards the south‐west. Lateral outbuilding generated a flat‐topped bar with a measured width of about 1700 m, and a preserved height of 5·5 m. The bar, disconnected from a genetically related south‐westward prograding delta some 2 km to the north‐east, developed during the transgressive phase of a sedimentary cycle. The tidal bar was most probably initiated as a delta‐attached bar at the toesets of the delta front and during transgression evolved into a detached tidal bar.