Cross-shore suspended sediment transport in the surf zone: a field-based parameterization (original) (raw)
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Bedform contributions to cross-shore sediment transport on a dissipative beach
Coastal Engineering, 2015
Field measurements of hydrodynamics, suspended sediment transport rates and bedform sediment transport rates were made in the intertidal section of a dissipative sandy beach (D 50 = 0.26 mm, slope = 1/80) at Perranporth (UK). Pressure Transducers, Acoustic Doppler Velocimeters, Optical Backscatter Sensors and an acoustic Sand Ripple Profiler were deployed for 12 tides, measuring in a range of wave heights from 0.5 to 2.2 m, water depths from 1 to 6 m, and in current strengths up to 0.4 m/s. Data were analysed in terms of the distance to shore (x) normalised by the surf zone width (x s), and spanned the region 0.4 < x/x s < 3. Bedforms heights up to 30 cm and wavelengths 0.5 to 2.7 m were recorded. Maximum wavelengths were observed just shoreward of the breakpoint. Bedforms were classified as sub-orbital, vortex ripples. Bedform migration was mostly onshore directed, and correlated with positive (onshore) wave skewness. Migration rates increased through the shoaling zone to a maximum of 1.5 cm/min just shoreward of the breakpoint (x/x s = 0.8). The bedform component of sediment transport was generally onshore directed, and maximum just shoreward of the breakpoint (0.021 kg/m/s). Point measurements showed that the cross-shore suspended sediment transport 25 cm above the bed was dominated by the mean component, with an offshore directed maximum at x/x s = 0.5. Contributions to onshore transport were only made by the incident wave (gravity band) component. The total depth integrated suspended sediment transport was offshore directed and maximum in the mid surf zone (-0.16 kg/m/s). The depth integrated suspended sediment transport dominated over the bedform sediment transport in the inner to mid surf zone (x/x s < 0.5) and in the outer shoaling zone (x/x s > 1.5). The fractional contribution of the shoreward directed bedform transport to the total absolute transport was up to 100%, and occurred broadly in the region of the breakpoint (0.5 < x/x s < 1.5). However, spatial averaging in the cross-shore indicated that a more realistic bedform contribution was up to 15% of the transport, with a maximum at x/x s = 0.9. Results from this dissipative beach experiment generally agree with previous findings on intermediate beaches, steep beaches, and offshore sandbars.
Cross-shore suspended sand and bed load transport on beaches
Journal of Geophysical Research, 2008
1] Simple formulas are developed to predict the time-averaged rates of cross-shore suspended sand and bed load transport. The net suspended sand transport rate is expressed as the product of the depth-averaged current and the suspended sediment volume per unit bottom area with a reduction factor that accounts for the correlation between the timevarying fluid velocity and sediment concentration. The net bed load transport rate under nonlinear waves is assumed to be onshore and proportional to s U 3 where s U is the standard deviation of the horizontal velocity. The probabilities of sediment movement and suspension are introduced to account for the initiation of sediment movement and suspension. Simple functions are proposed to account for the effects of a steep bottom slope on the bed load and suspended sediment transport rates. The proposed formulas are found to be in agreement with three data sets within a factor of about 2. The proposed formulas are shown to be consistent with existing simple formulas. The formulas are incorporated into a time-averaged wave model and the continuity equation of bottom sediment to predict the beach profile evolution. The numerical model is compared with seven small-scale tests including berm erosion tests and seven large-scale tests including dune erosion tests. The numerical model predicts the overall beach profile evolution including the berm and dune erosion but does not always predict the fairly subtle profile changes including bar migration accurately. Citation: Kobayashi, N., A. Payo, and L. Schmied (2008), Cross-shore suspended sand and bed load transport on beaches,
A cross-shore suspended sediment transport shape function parameterisation for natural beaches
Continental Shelf Research, 2009
A new field-based parameterisation ('shape function') describing the distribution of cross-shore suspended sediment transport across a beach profile is presented. Time-averaged and depth-integrated suspended sediment fluxes were measured over 39 tides at Sennen Cove, Cornwall, UK, for a range of wave conditions (offshore significant wave heights 0.1-2.5 m). The suspended sediment flux data were heuristically separated into four transport components: (1) mean flux in the surf/shoaling zone; (2) oscillatory flux in the surf/shoaling zone; (3) onshore flux in the swash/inner surf zone and (4) offshore flux in the swash/inner surf zone. Each of these transport components was related to the local water depth (h) normalised by the breakpoint depth (h b) and the four resulting suspended transport shape functions were combined to form a total suspended load shape function. Each shape function component is scaled independently by the wave energy level through h b. The total suspended load shape function predicts onshore sediment transport under low-energy conditions, with peaks at the breakpoint and in the swash zone, in agreement with the field observations. Under high-energy conditions the total suspended load shape function predicts onshore transport in the shoaling zone, offshore transport in the surf zone and onshore transport in the inner swash zone.
Review of Wave Transformation and Cross-Shore Sediment Transport Processes in Surf Zones
Journal of Coastal Research, 2012
An attempt is made to assemble and synthesize recent publications which may contribute to the improvement of our quantitative capabilities for predicting shoreline changes due to the cross-shore sediment transport in the surf and swash zones on beaches. This review is essentially limited to the cross-shore hydrodynamics of incident wind waves and surf beat motions as well as the cross-shore sediment transport and resulting beach profile evolution.
Wave-induced sediment transport and onshore sandbar migration
Coastal Engineering, 2006
The 25-m onshore migration of a nearshore sandbar observed over a 5-day period near Duck, NC is simulated with a simplified, computationally efficient, wave-resolving singlephase model. The modeled sediment transport is assumed to occur close to the seabed and to be in phase with the bottom stress. Neglected intergranular stresses and fluid-granular interactions, likely important in concentrated flow, are compensated for with an elevated (relative to that appropriate for a clear fluid) model roughness height that gives the best fit to the observed bar migration. Model results suggest that when mean-current-induced transport is small, wave-induced transport leads to the observed onshore bar migration. Based on the results from the simplified phase-resolving model, a wave-averaged, energetics-type model (e.g., only moments of the near-bottom velocity field are required) with different friction factors for oscillatory and mean flows is developed that also predicts the observed bar migration. Although the assumptions underlying the models differ, the similarity of model results precludes determination of the dominant mechanisms of sediment transport during onshore bar migration.
A review and assessment of longshore sediment transport equations for coarse-grained beaches
Coastal Engineering, 2000
Previous assessments of analytical longshore sediment transport formulae have been heavily biased towards sand-sized sediment. All have noted the shortage of high quality field data from coarse-grained beaches against which to test predictions of longshore transport rates. In this paper, 12 existing formulae were identified as being potentially applicable for coarse-grained sediments and predictions from these formulae are compared using a measured annual transport rate from a shingle beach and a concurrent hindcast wave climate. Two new empirical equations are also derived, one from a numerical model calibrated against the same data set, the other derived from field experiments on coarse grained beaches. Energetics-based equations are found to give reasonable predictions of the shingle transport, despite being derived for sand beaches. In contrast, those dimensional analysis type equations which had been validated using laboratory data, grossly over-predicted the measured transport rates. The most accurate predictions were from formulae previously validated at sites similar to that used for this comparison and therefore require further testing against field data from dissimilar sites. q E. Van Wellen , achadwick@plymouth.ac.uk Ž . A.J. Chadwick . 0378-3839r00r$ -see front matter q 2000 Elsevier Science B.V. All rights reserved.
Investigation of bar parameters occurred by cross-shore sediment transport
International Journal of Naval Architecture and Ocean Engineering, 2013
Cross-shore sediment transport is very important factor in the design of coastal structures, and the beach profile is mainly affected by a number of parameters, such as wave height and period, beach slope, and the material properties of the bed. In this study cross-shore sediment movement was investigated using a physical model and various offshore bar geometric parameters were determined by the resultant erosion profile. The experiments on cross-shore sediment transport carried out in a laboratory wave channel for initial base slopes of 1/8, 1/10 and 1/15. Using the regular waves with different deep-water wave steepness generated by a pedal-type wave generator, the geometrical of sediment transport rate and considerable characteristics of beach profiles under storm conditions and bar parameters affecting on-off shore sediment transport are investigated for the beach materials with the medium diameter of d 50 =0.25, 0.32, 0.45, were obtained by using linear and non-linear regression methods through the experimental data and were compared with previously developed equations in the literature. The results have shown that the experimental data fitted well to the proposed equations with respect to the previously developed equations.
Chapter 294 Modelling of 3D Sediment Transport in the Surf Zone
The three-dimensional sediment transport in the surf zone has been investigated using two different approaches for modelling of the flow pattern. The first approach is based on the integrated momentum concept for the turbulent wave-current boundary layer, see Fredsoe (1984), the second approach is based on the k-model, as described in Deigaard et al. (1991). The k-model allows for a more consistent description of the time and space varying eddy viscosity than the integrated momentum approach, but demands considerably more computation effort. The driving forces are calculated according to the formulations of Deigaard et al. (1991) and Deigaard (1993). The model based on the integrated momentum equation is able to reproduce the details of the flow satisfactorily. The presence of a longshore current increases the turbulence near the bed. This results in a decreased offshore directed flow velocity near the bed and an increase in the sediment concentration. Comparisons with field measure...
A phase-resolving cross shore sediment transport model for beach profile evolution
Coastal Engineering, 1997
A phase-resolving wave transformation module is combined with an intra-wave sediment transport module to calculate the on-/offshore sediment transport rates. The wave module is based on the Boussinesq equations extended into the surf zone. The vertical variation of the mean undertow and the intra-wave sediment concentrations are calculated. The net sediment transport rates are calculated, and the equation for conservation of sediment is solved to predict the beach profile evolution. The results of the present paper showed that the undertow contribution to the sediment transport rates is not dominating in all parts of the surf zone, even for eroding beaches, suggesting that other contributions should not be neglected. The present model also showed that for the same offshore wave energy the time series of the oscillatory motion is important and that the effect of wave groups cannot be disregarded.
Fuel and Energy Abstracts, 2011
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.