Sediment Transport In Swash Zone Research Articles

Beach and Dune Subsurface Hydrodynamics and Their Influence on the Formation of Dune Scarps

AbstractErosive beach scarps influence beach vulnerability, yet their formation remains challenging to predict. In this study, a 1:2.5 scale laboratory experiment was used to study the subsurface hydrodynamics of a beach dune during an erosive event. Pressure and moisture sensors buried within the dune were used both to monitor the water table and to examine vertical pressure gradients in the upper 0.3 m of sand as the slope of the upper beach developed into a scarp. Concurrently, a line‐scan lidar tracked swash bores and monitored erosion and accretion patterns along a single cross‐shore transect throughout the experiment. As wave conditions intensified, a discontinuity in the slope of the dune formed; the discontinuity grew steeper and progressed landward at the same rate as the R2% runup extent until it was a fully formed scarp with a vertical face. Within the upper 0.15 m of the partially saturated sand, upward pore pressure gradients were detected during backwash, influencing the effective weight of sand and potentially contributing to beachface erosion. The magnitude and frequency of the upward pressure gradients increased with deeper swash depths and with frequency of wave interaction, and decreased with depth into the sand. A simple conceptual model for scarp formation is proposed that incorporates observations of upward‐directed pressure gradients from this study while providing a reference for future studies seeking to integrate additional swash zone sediment transport processes that may impact scarp development.

Influence of Beach Slope on Morphological Changes and Sediment Transport under Irregular Waves

This paper presents new data from large-scale wave flume experiments. It shows the beach profile evolution and sediment transport for two different bed slopes (1:15 and 1:25), and three irregular high-energy erosive wave conditions and one low-energy accretive wave condition. The bulk cross-shore net sediment transport was investigated for the total active profile and for the surf and swash zone separately. It is shown that the steep slope is morphologically more active than the gentle slope, with faster and more pronounced morphological changes and larger sediment transport rates. For both slopes, the total and surf zone net sediment transport were offshore-directed for erosive waves and onshore-directed for the accretive wave condition. However, the net swash zone transport for the erosive wave conditions was offshore-directed for the steep slope and onshore-directed for the gentle slope. The direction and magnitude of the total and surf zone sediment transport correlate well with the slope-corrected Dean criterion with increasing offshore-directed sediment transport (erosion) observed for increasing wave energy and bed slope. This relation does not hold for the swash zone sediment transport along the gentle slope, suggesting that swash zone sediment transport processes are not well captured when using a simple predictor such as the (modified) Dean number. Differences in sediment transport in the swash for the different slopes are likely influenced by differences in incoming wave energy, wave–swash interactions and the relative importance of long- and short-waves.

Grain-scale investigation of swash zone sediment transport on a gravel beach using DEM-MPS coupled scheme

ABSTRACT Sediment transport in the swash zone directly affects beach changes such as shoreline recession; thus, detailed understandings of sediment transport mechanisms are necessary to accurately estimate the short-time scales sediment transport rate. However, these detailed mechanisms under runup waves have not been elucidated because of the complex solid-gas-liquid multiphase turbulence flow. In this study, we attempt to numerically investigate the sediment grain-scale mechanism to overcome the shortcomings of experimental measurements and the free surface treatment in many numerical simulations. The gravel transport process on a sloped beach under regular waves was simulated using a 2D coupled model of the discrete element method (DEM) and a modified moving particle semi-implicit (MPS) method; a sub-model was built into the DEM-MPS model to improve fluid volume conservation. After validating the simulated performance by comparing it to a previous experiment, the gravel motions were investigated for turbulence and inner beach structure. The Shields number, estimated using the drag force distribution, revealed that significant turbulence contributed to onshore gravel transport near the rundown limit. The inter-gravel contact structure inside the beach explained the decrease in offshore sediment transport during backwash as increased resistance to gravel motions resulting from beach compaction.

NUMERICAL SIMULATION OF CROSS-SHORE PROFILE EVOLUTION USING OPENFOAM

In the present work, an innovative numerical approach was developed coupling two models in order to simulate the wave propagation over a sloping beach and the sediment transport in surf and swash zones. The first model, synthesized on the basis of OpenFOAM (version 2.4.0) is used to describe the hydrodynamic characteristics of the flow and the wave propagation while the second one is applied for the sediment transport and erosion/deposition prediction using the results of the first model. The method above constitutes an iterative procedure which is tested hereby and seems to yield satisfactory numerical results in comparison with experimental data (Dette 1998).

Laboratory Measurements of Sediment Flux and Bed Level Evolution in the Swash Zone

A detailed laboratory study of swash zone sediment transport under dam-break waves on a fine sandy slope was carried out. A newly developed sediment transport measurement system was applied for simultaneous measurement of sediment concentration and transport velocity with a vertical resolution of 0.53 mm based on particle image velocimetry (PIV) and a light extinction method, respectively. Concentration data were combined with velocity data to produce sediment flux and were used to investigate the characteristics of sediment transport in the swash zone under dam-break waves. It was found that sand particles followed the flow well in the experiment. Phase difference between the maximum velocity and maximum concentration in the initial uprush was obvious. The maximum concentration varied significantly between different elevations. The onshore flux was much larger than the offshore flux, and the flux near the bed was much larger than that at the higher elevations. The net transport was offshore close to the bed, onshore in the middle layer, and almost zero in the upper layer. The measurement error between the mass change calculated from bed form evolution and the net sediment transport obtained from the measurement system was smaller than 20%.

Bed level motions and sheet flow processes in the swash zone: Observations with a new conductivity-based concentration measuring technique (CCM+)

Detailed measurements of bed level motions and sheet flow processes in the lower swash are presented. The measurements are obtained during a large-scale wave flume experiment focusing on swash zone sediment transport induced by bichromatic waves. A new instrument (CCM+) provides detailed phase-averaged measurements of sheet flow concentrations, particle velocities, and bed level evolution during a complete swash cycle. The bed at the lower swash location shows a clear pattern of rapid erosion during the early uprush and progressive accretion during the middle backwash phase. Sheet flow occurs during the early uprush and mid and late backwash phases. Sheet flow sediment fluxes during these instances are highest in the pick-up layer. Sediment entrainment from the pick-up layer occurs not only during instances of high horizontal shear velocities but also in occurrence of wave–backwash interactions. As opposed to oscillatory sheet flow, the pivot point elevation of the sheet flow layer is time-varying during a swash event. Moreover, the upper sheet flow layer concentrations do not mirror the concentrations in the pick-up layer. Both differences suggest that in the lower swash zone the dynamics of the upper sheet flow layer are not only controlled by vertical sediment exchange (such as in oscillatory sheet flows) but are strongly affected by horizontal advection processes induced by the non-uniformity of the flow.

CHARACTERISTICS OF SEDIMENT TRANSPORT IN SWASH ZONE DUE TO SATURATED-UNSATURATED SLOPED BEACH

The influence of saturated-unsaturated sloped beach on sediment transport in swash zone is investigated experimentally and numerically. The movement of fluorescent sand on seabed and the orbit of fluorescent paint injected into sandy beach are measured in bore wave experiments. As the result, the uplift seepage flow leads to the activation of sediment transport in the direction of run-up and backwash under the saturated slope. The numerical modelling simulates the interacting swash wave motion and saturated-unsaturated coastal seepage flow. The tractive force under the influence of seepage flow is quantified through a permeable Shields parameter. The results of prediction are in good qualitative agreement with those of experiments.

COMPREHENSIVE STUDY OF SWASH-ZONE HYDRODYNAMICS AND SEDIMENT TRANSPORT

A comprehensive study of swash-zone sediment transport was conducted on a macro-tidal beach in Perranporth, UK. The unique study is the first to simultaneously measure suspended sediment and sheet flow sediment concentrations and near bed velocity on a natural beach. Data collected during the study will be used to address the importance of sheet flow processes on swash zone sediment transport and morphology, the variability in the near bed velocity profile and the importance of alongshore and vertical flows, cross-shore advection of turbulence and the inference of swash zone forcing and momentum transfer from remotely sensed data.

Coarse sediment particle motion under highly asymmetrical waves with implications for swash zone sediment transport

Hydrodynamics and sediment transport in the swash zone are unique and very complex phenomena. Acquiring a robust model for predicting sediment motion in the swash zone requires in-depth insight about the behavior of sediment particles in highly asymmetric and turbulent waves that continuingly uprush the beach face and retrieve in backwashes. In order to consider a physical system of the coarse particle motion in the swash zone with a reduced number of external parameters and capture some of the realism of the situation, the experiments are conducted in rather simple conditions to provide some information about the particle responses in terms of jump length against asymmetric waves to be implicated for the swash zone. The results show that despite reduction of many external parameters, jump length is still a highly skewed stochastic parameter. The probability distribution of jump length is quite wide (large variance) and skewed, different from that of saltation length in steady unidirectional flow, in which the distribution is close to normal. Considering integral of excess energy of an arbitrary single wave over its duration as a rational and meaningful representative of flow condition, the average jump length JL¯ incremental trends reduces in the higher energy condition, resulting in a linear trend in logarithmic scale. The JL¯s due to higher turbulent kinetic energy follow similar trend of less turbulent flow conditions.

Large-scale laboratory investigation into the effect of varying back-barrier lagoon water levels on gravel beach morphology and swash zone sediment transport

It is generally accepted that the effect on beach morphology of the interactions between swash motion and the beach groundwater table depends on the elevation of the latter relative to that of the sea. Specifically, a low beach groundwater table is considered to enhance onshore sediment transport and beach accretion, while the reverse is thought to hold for a high beach groundwater table. Here, we use the results of a carefully designed series of tests conducted in the Delta Flume, the Netherlands, as part of the Barrier Dynamics Experiment (BARDEX), to unequivocally demonstrate the validity of this assertion. During the experiment, a 4-m high and 50-m long gravel barrier (D50=11mm) was constructed within the central region of the 250-m long and 7-m deep wave flume, enabling a back-barrier ‘lagoon’ to be located at its landward side. The groundwater table in the beach was manipulated by lowering and raising the lagoon level, and the gravel beach was subjected to variable wave conditions and sea/lagoon levels. Using as the initial beach profile the morphology resulting from 90min of wave forcing with the sea and lagoon at the same level, the identical wave forcing with a low lagoon level consistently resulted in onshore sediment transport in the swash zone and beach accretion, whereas the identical wave forcing with a high lagoon level consistently caused offshore sediment transport in the swash zone and beach erosion. The divergent beach morphological responses for the low and high lagoon levels could not be attributed to changes in the ‘bulk’ swash hydrodynamics (i.e., based on single-point velocity measurements) arising from a difference in the amount of swash infiltration into the unsaturated beach. Instead, application of a sediment transport model that accounts for the effects of in/exfiltration suggested that the onshore sediment transport and beach accretion for the low lagoon tests mainly resulted from enhanced bed shear stresses during the wave uprush due to infiltration into the saturated part of the beachface. Due to the large hydraulic conductivity of gravel (K=O(0.1ms−1)), the effect of altered shear stresses due to in/exfiltration may be an important factor in sediment transport in the swash zone of gravel beaches.

不飽和特性を考慮した前浜浸透流が波打ち帯の漂砂移動に及ぼす影響

A numerical simulation is performed to evaluate the influence of wave-driven coastal seepage on sediment transport in swash zone. The tractive velocity of swash motion and infiltration-exfiltration rate are estimated using a Navier-Stokes model coupled with a porous flow model incorporated the unsaturated region. The tractive force under the influence of seepage is quantified through a permeable Shields parameter, which is concerned with both the effective weight of sediment and swash boundary layer thickness. The comparison between model results and previous experimental data shows that the direction of sediment transport changed from onshore to offshore can be evaluated by the infiltration-exfiltration process.

Swash zone response under various wave regimes

The modelling of swash zone (SZ) sediment transport and the resulting morphodynamics have been areas of active research over the last decade. However, many details are still to be understood, whose knowledge will be greatly advanced by the collection of high-quality data under the controlled large-scale laboratory conditions. The research describes tests carried out in the large wave flume of the Maritime Engineering Laboratory at Catalonia University of Technology, to investigate the SZ under the storm conditions. Its main aim was to compare beach-profile responses for monochromatic waves, monochromatic waves plus free long waves, bi-chromatic waves and random waves. Both erosive and accretive conditions were considered. Results discussed here were derived from the analysis of only a part of the whole data set.

Large-scale experiments on beach profile evolution and surf and swash zone sediment transport induced by long waves, wave groups and random waves

New large-scale laboratory data are presented on the influence of long waves, bichromatic wave groups and random waves on sediment transport in the surf and swash zones. Physical model testing was performed in the large-scale CIEM wave flume at UPC, Barcelona, as part of the SUSCO (swash zone response under grouping storm conditions) experiment in the Hydralab III program (Vicinanza et al., 2010). Fourteen different wave conditions were used, encompassing monochromatic waves, bichromatic wave groups and random waves. The experiments were designed specifically to compare variations in beach profile evolution between monochromatic waves and unsteady waves with the same mean energy flux. Each test commenced with approximately the same initial profile. The monochromatic conditions were perturbed with free long waves, and then subsequently substituted with bichromatic wave groups with different bandwidth and with random waves with varying groupiness. Beach profile measurements were made at half-hourly and hourly intervals, from which net cross-shore transport rates were calculated for the different wave conditions. Pairs of experiments with slightly different bandwidth or wave grouping show very similar net cross-shore sediment transport patterns, giving high confidence to the data set. Consistent with recent small-scale experiments, the data clearly show that in comparison to monochromatic conditions the bichromatic wave groups reduce onshore transport during accretive conditions and increase offshore transport during erosive conditions. The random waves have a similar influence to the bichromatic wave groups, promoting offshore transport, in comparison to the monochromatic conditions. The data also indicate that the free long waves promote onshore transport, but the conclusions are more tentative as a result of a few errors in the test schedule and modifications to the setup which reduced testing time. The experiments suggest that the inclusion of long wave and wave group sediment transport is important for improved near-shore morphological modeling of cross-shore beach profile evolution, and they provide a very comprehensive and controlled series of tests for evaluating numerical models. It is suggested that the large change in the beach response between monochromatic conditions and wave group conditions is a result of the increased significant and maximum wave heights in the wave groups, as much as the presence of the forced and free long waves induced by the groupiness. The equilibrium state model concept can provide a heuristic explanation of the influence of the wave groups on the bulk beach profile response if their effective relative fall velocity is larger than that of monochromatic waves with the same incident energy flux.

Swash zone sediment transport, step dynamics and morphological response on a gravel beach

State-of-the-art morphodynamic field data were collected from Slapton Sands (UK), a fine-gravel beach ( D 50 = 2–10 mm), over a 10-day period under a range of wave–tide conditions, comprising a mixture of southerly swell and easterly wind waves ( H s = 0.5–1 m; T s = 4–8 s; tide range = 3–4 m). Over the field period, the upper beach experienced accretion and berm formation during tides with low steepness, swell-dominated wave conditions ( H/ L < 0.01), and underwent erosion during tides dominated by high steepness wind waves ( H/ L > 0.01). Surprisingly, the intertidal beach showed an opposite trend with sediment losses under swell conditions and an increase in sediment volume due to wind waves, and suggests that upper beach morphological change can be decoupled from the overall sediment budget. A submerged accretionary feature (the beach step) grew and migrated onshore on the rising tide, typically reaching a maximum relief of around 0.5 m, then gradually decreased in prominence while migrating offshore as the tide fell, resulting in little net morphological change. The step was better developed during the falling tide than the rising tide and, through its control on the swash hydro- and sediment dynamics, the step had a significant effect on the development of the berm. Swash flow velocities under the conditions measured were often in excess of 2 m s − 1 for both the uprush and the backwash. Onshore mean flows occurred in the upper swash region (< 40% inundation), while the lower swash region (> 40% inundation) was characterised by offshore mean flows. Swash infiltration losses in the swash zone increased from 40% in the lower swash region to 100% in the upper swash region. The most energetic swash flows and the strongest offshore mean flows occurred at the transition between swash zone and surf zone (90–100% inundation).

Suspended Sediment in the Swash Zone: Heuristic Analysis of Spatial and Temporal Variations in Concentration

Abstract Swash zone sediment transport is generally measured in an Eulerian reference frame, but this approach tends to give insufficient information on spatial transport characteristics. This paper utilizes a novel technique to map measured suspended sediment concentrations (SSC) onto the x–t plane, thus providing a visualization of SSC at all locations throughout a swash cycle. The widely adopted energetics or bed shear stress approaches to modeling suspended sediment transport in the swash predict that SSC varies directly with the horizontal velocity, u, to the power of either 2 or 3. A calibrated ballistic swash model was used to predict SSC(x,t) ∝ u2 and SSC(x,t) ∝ u3. The qualitative comparison between field measurements and transport model predictions in x–t space provides the opportunity to examine the transport model performance throughout the swash zone. The agreement between observed and modeled SSC patterns was generally poor. Similar to previous observations, SSC was larger during the uprush ...

Beachface morphology and surf beat sediment transport in laboratory scale surf and swash zones

Beachface morphology and surf beat sediment transport in laboratory scale surf and swash zones

External Forces of Sediment Transport in Surf and Swash Zones Induced by Wave Groups and Their Associated Long Waves

This study investigates the coupling field of grouped wind waves and their associated long waves in the surf and swash zones. Based on the calculated wave fields, the contributions of the wind waves and the long waves on the sediment transport efficiency axe discussed. Spatial variations of the incident grouped wind waves propagating over a plane slope are calculated based on time-dependent mild slope equation. Generation of the long waves is reproduced based on a time-varying breakpoint model proposed by Symonds et al. [1982]. In order to obtain the long wave solutions extending over the landward region from the still water shoreline, calculations using nonlinear shallow water equations are connected to the Symonds' model invoking a moving boundary treatment. The Shields parameters under composition of the grouped wind waves and the associated long waves are evaluated to assess the mobility of the bottom sediment. The results show that the long waves have greater sediment transport efficiency over the grouped wind waves in the swash zone.

Numerical study on wave-induced filtration flow across the beach face and its effects on swash zone sediment transport

A numerical model, coupling an analysis of beach groundwater flow with an analysis of swash wave motion over a uniform slope, is presented. Model calculations are performed to investigate the variations of swash-induced filtration flows across the beach face for different input parameters. Swash zone sediment transport under the influence of such filtration flow across the beach face is investigated through modification of effective weight of sediment particle and modification of swash boundary layer thickness. These effects are quantified based on a bed load transport model with a modified Shields parameter.

Image Analysis of Sediment Transport in the Swash Zone

The purpose of this study is to investigate sediment transport in swash zone by using an image processing technique. Although a number of researches have been carried out into sediment transport in swash zone, the sediment transport rate in that area has not been measured directly. It was found that the sediment transport rate in swash zone had spatial distributions, and the peak value appeared where the bottom elevationc hanging from accumulation to erosion.

Waves and Sediment Transport in Swash Zone during Storm

Waves and Sediment Transport in Swash Zone during Storm