New Practical Model for Sand Transport Induced by Non-Breaking Waves and Currents (original) (raw)
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Practical sand transport formula for non-breaking waves and currents
Coastal Engineering, 2013
Many existing practical sand transport formulae for the coastal marine environment are restricted to a limited range of hydrodynamic and sand conditions. This paper presents a new practical formula for net sand transport induced by non-breaking waves and currents. The formula is especially developed for cross-shore sand transport under wave-dominated conditions and is based on the semi-unsteady, half wave-cycle concept, with bed shear stress as the main forcing parameter. Unsteady phase-lag effects between velocities and concentrations, which are especially important for rippled bed and fine sand sheet-flow conditions, are accounted for through parameterisations. Recently-recognized effects on the net transport rate related to flow acceleration skewness and progressive surface waves are also included. To account for the latter, the formula includes the effects of boundary layer streaming and advection effects which occur under real waves, but not in oscillatory tunnel flows. The formula
Prediction of sand transport rates by waves and currents in the coastal zone
Continental Shelf Research, 2002
The predictions of a sand transport research model and Bijker's (J. Waterways, Harbours Coastal Eng. Div. ASCE 97 (WW4) (1971) 687) engineering model are compared with data obtained in wave-current conditions at three field sites. A key element in the present study is that the bed roughness at the three sites has been estimated from predictions of the sand ripple dimensions. The comparisons between suspended sand concentrations and transport rates show that a considerable amount of uncertainty (factor 75 or more) arises when individual predictions are compared with the measurements. However, the overall bias in each set of comparisons is smaller than this, with overall agreement being within a factor of 72 in most cases. While the results demonstrate that research models, adapted for field application, may be used to make practical sand transport computations with as much accuracy as engineering formulations, the true benefit of research models lies in the improved understanding of transport processes that they provide. This is illustrated with reference to the mechanism of grain size sorting caused by oblique incidence of waves on a current.
Laboratory Experiments for Wave-Driven Sand Transport Prediction
NCK-days 2012 : Crossing borders in coastal research : jubilee conference proceedings, 2012
Since the first NCK-Days 20 years ago, a significant body of large-scale laboratory experiments on wave-driven sand transport has been conducted in oscillatory flow tunnels and large wave flumes. The experiments have yielded measures of net sand transport rates for a wide range of flow and sand conditions and have provided insights that have informed the development of practical predictors for sand transport in oscillatory flows and under waves. An overview of these experiments and a commentary on some of the important insights and quantitative results is presented. Particular attention is given to unsteady aspects of the sand flux for ripple regime and fine sand sheet-flow conditions, the role of flow acceleration on bed shear stress and net transport, and the differences in net transport and transport processes occurring in tunnel oscillatory flows and occurring under progressive surface waves.
Sand transport under combined current and wave conditions: A semi-unsteady, practical model
Coastal Engineering, 2006
For the general purposes of morphodynamic computations in coastal zones, simple formula-based models are usually employed to evaluate sediment transport. Sediment transport rates are computed as a function of the bottom shear stress or the near bed flow velocity and it is generally assumed that the sediment particles react immediately to changes in flow conditions. It has been recognized, through recent laboratory experiments in both rippled and plane bed sheet flow conditions that sediment reacts to the flow in a complex manner, involving non-steady processes resulting from memory and settling/entrainment delay effects. These processes may be important in the cross-shore direction, where sediment transport is mainly caused by the oscillatory motions induced by surface short gravity waves. The aim of the present work is to develop a semi-unsteady, practical model, to predict the total (bed load and suspended load) sediment transport rates in wave or combined wave-current flow conditions that are characteristic of the coastal zone. The unsteady effects are reproduced indirectly by taking into account the delayed settling of sediment particles. The net sediment transport rates are computed from the total bottom shear stress and the model takes into account the velocity and acceleration asymmetries of the waves as they propagate towards the shore. A comparison has been carried out between the computed net sediment transport rates with a large data set of experimental results for different flow conditions (wave-current flows, purely oscillatory flow, skewed waves and steady currents) in different regimes (plane bed and rippled bed) with fine, medium and coarse uniform sand. The numerical results obtained are reasonably accurate within a factor of 2. Based on this analysis, the limits and validity of the present formulation are discussed.
Marine Geology, 1985
Various formulae are used to predict the transport of sand under the combined influence of waves and currents. These approaches include the use of the unidirectional formulae of Einstein (1950), Frijlink (1952), Yalin (1963), Engelund and Hansen (1967), Sternberg (1972), Ackers and White (1973) and Gadd et al. (1978) modified, for the presence of waves, in accordance with the techniques described by Bijker (1967) and Swart (1976) and those of Bagnold (1963), Madsen and Grant (1976) and Vincent et al. (1981).Predicted rates are compared with measurements, based upon fluorescent sand tracer studies, in an area of high tidal current and wave energy (northern Bristol Channel, U.K.). In general, predicted rates are lower than those measured and only the formulae of Madsen and Grant (1976) and the modified formula of Sternberg (1972) appear to provide realistic estimates of transport under wave- and tidally induced currents, in comparison with the field measurements.
Sand Transport Process Measurements Under Large-Scale Breaking Waves
The Proceedings of the Coastal Sediments 2015, 2015
The effects of wave breaking on sediment transport are studied through a new series of mobile-bed experiments in a large-scale wave flume. During the campaign, one experiment involving detailed sand transport process measurements was repeated at 12 different cross-shore location. This procedure allows studying of the cross-shore variation of sand transport processes along the breaking zone. Starting from an initially 1:10 slope followed by a horizontal test section, a breaker bar developed in the breaking region as a result of onshore transport pre-breaking and offshore transport post-breaking. Near-bed suspended sediment fluxes were directed offshore along the complete test section, suggesting that the onshore transport pre-breaking is mainly attributed to bedload. The offshore suspended flux was the sum of an onshore wave-driven component and an offshore current-driven component. The wave-driven contribution to total suspended transport rates seems significant mainly before the breaking point where they account for ~30% of total suspended transport fluxes.
Application of a New Sand Transport Formula Within the Cross-Shore Morphodynamic Model Unibest-TC
Coastal Engineering Proceedings, 2012
In this paper, we have implemented and tested the new SANTOSS sand transport formula with the cross-shore morphodynamic model UNIBEST-TC using data from the LIP and Grasso wave flume experiments. It is shown that the total net sand transport is a delicate balance between wave- and current-related transport in the wave boundary layer (which can be on- or offshore-directed) and offshore-directed current-related suspended load above it. The change from onshore to offshore net transport for the two Grasso cases was reproduced by the SANTOSS model and seems to be due to the increasing importance of phase-lags between intra-wave velocities and sand concentrations. More generally, measured net sand transport rates are reasonably well reproduced by the SANTOSS formula outside the surf zone if orbital velocities and ripple heights are predicted correctly and phase-lags between velocities and suspended sand concentrations are accounted for.
Wave influence on coastal sand transport paths in a tidally dominated environment
Ocean and Shoreline Management, 1988
Detailed physical oceanographic and sedimentological data, collected from a coastal embayment and adjoining inner continental shelf, are used to exemplify the changes in sediment transport vectors which take place when wave action is superimposed upon that of tidal currents. The area selected for study is one which is subjected to large tidal ranges (10.2 m on spring tides) and occasional storm-induced swell waves (with a predicted 50-year maximum wave height of 15 m).Bedload transport vectors are determined, on the basis of self-recording current meter observations combined with transport formulae and fluorescent or radioactive sand tracer studies, for: (i) an ‘open’ area of the inner shelf, with regular bathymetry running parallel to the coastline;(ii) a ‘near-coastal’ location, where current flow is constricted by the coastline itself; and(iii) in the vicinity of a linear sandbank, which rises some 10m above the surrounding sea bed.On the basis of analyses carried out for various tidal and wave conditions, the outer (‘open’) area is considered to be one through which sedimentary material passes from the offshore deep water areas to adjacent beaches. Here, the net transport paths are susceptible to wave action and can reverse during storms, in comparison to tidal currents alone. Inshore, the sandbank is at the ‘distal’ end of the transport system: sand is concentrated onto the bank by recirculating tidal currents, but is removed from its crestline by wave action. Wave action also intensifies sand transport rates around such an effectively ‘closed’ system, enhancing the hydrodynamically controlled maintenance process.
Sand Transport by Surface Waves: Can Streaming Explain the Onshore Transport?
Coastal Engineering Proceedings, 2011
In wave flumes an onshore boundary layer current is present that is not present in oscillating flow tunnels. We investigate numerically the hypothesis that this streaming explains the measured increase of onshore directed sediment transport in flumes over tunnels. In the formulation and validation of the model special attention has been given to the wave-generated net current profile. From model experiments we conclude that the additional current indeed contributes to onshore transport, but can not be the full explanation of the measured differences in transport rates. Other contributing mechanisms are the amplification/reduction of the fall velocity by vertical sediment advection (only relevant for fine grains) and the amplification/reduction of the concentration at maximum onshore/offshore velocity by intra-wave gradients in horizontal sediment flux. The latter contributes, for the investigated cases, to onshore transport with comparable order as the boundary layer current. These ...
Marine Geology, 1999
Seabed video images and S4 wave-current meter data, collected during the build-up of a moderate storm on the Scotian Shelf, are analysed for bedform development and sediment transport threshold of fine sand under combined waves and currents. As the storm built up, the following sequence of bedforms was observed: (1) relict wave-dominant ripples with worm tubes and animal tracks during the preceding fairweather period; (2) irregular, sinuous, asymmetrical current-dominant and intermediate wave-current ripples under bedload transport; (3) regular, nearly straight or sinuous asymmetrical to slightly asymmetrical wave-dominant ripples under saltation=suspension; (4) upper-plane bed under sheet-flow conditions; (5) small, crest-reversing, transitory ripples at the peak of the storm; and (6) large-scale lunate megaripples which developed when the storm decayed. These data also show that only single sets of asymmetrical intermediate wave-current ripples will form when waves and currents are co-linear. The development of the crest-reversing transitory ripples indicates a high-energy transition stage under quasi-sheet-flow conditions. A direct comparison of the skin-friction combined shear velocity and the critical shear velocities for bedload, suspension and sheet-flow transport underestimated the onset of these sediment transport modes. As the presence of ripples causes the shear stress to increase from ripple trough to ripple crest, the ripple-enhanced skin-friction shear velocity must be used to determine properly the initiation of bedload transport. At high transport stages, the boundary layer dynamics is controlled mainly by the thickness of the bedload transport layer. Thus a transport-related bedload shear velocity, predicted based upon the sum of the grain roughness and bedload roughness, has to be compared against the conventional threshold criterion to properly define the onset of suspension and sheet-flow transport modes.