Net Sand Transport Rates Research Articles

Erosion mechanism of the landward Oostgat tidal channel slope, The Netherlands

The Oostgat tidal channel, located in the mouth of the Scheldt estuary in The Netherlands, migrates in the landward direction by about 0.2 m/year as a result of erosion of the landward channel slope. This erosion threatens beach stability and the effectiveness of beach nourishments. The physical mechanisms controlling this channel slope erosion are yet unknown. We have studied this erosion problem with a high-resolution Delft3D coupled wave-flow-sand transport model, which we have validated against available hydrodynamic data. Initial sedimentation and erosion patterns were inferred from the computed net sand transport rates. The model results show that channel slope erosion is controlled by the tide; effects of waves and wind are relatively small. The net sand transport is mainly controlled by the interaction between the residual current and the tidal velocity variation. Previous studies attributed a prominent role to the sand ridge Bankje van Zoutelande offshore from the Oostgat channel. This has not been confirmed by our model results: erosion of the landward channel slope was still present if the ridge was removed, albeit smaller. Instead, the erosion seems to be related to a sharp channel bend just north of the erosional area. The bend drives cross-channel differences in tide-dominance resulting in an increasing ebb-directed/northward net sand transport away from the erosional area.

APPLICATION OF A NEW SAND TRANSPORT FORMULA WITHIN THE CROSS-SHORE MORPHODYNAMIC MODEL UNIBEST-TC

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.

BOUNDARY LAYER FLOW AND SAND TRANSPORT UNDER FULL SCALE SURFACE WAVES

Existing models for wave-related (cross-shore) sand transport are primarily based on data from oscillatory flow tunnel experiments. However, theory and former experiments indicate that flow differences between full scale surface waves and oscillatory flow tunnels may have a substantial effect on the net sand transport. In this paper, high resolution measurements of boundary layer flow characteristics, sheet-flow layer sediment concentrations and net sand transport rates under full scale surface waves are presented. These experiments were performed in a large wave flume (GWK) for different wave conditions with medium (D50 = 0.25 mm) and fine (D50 = 0.14 mm) sand. It is shown that, especially under sheet-flow conditions, small wave induced net currents are of large importance for the total sand transport rates under these conditions.

Modelling of sand transport under wave-generated sheet flows with a RANS diffusion model

A 1DV-RANS diffusion model is used to study sand transport processes in oscillatory flat-bed/sheet flow conditions. The central aim is the verification of the model with laboratory data and to identify processes controlling the magnitude and direction (‘onshore’/‘offshore’) of the net time-averaged sand transport. The model is verified with a large series of measured net sand transport rates, as collected in different wave tunnels for a range of wave-current conditions and grain sizes. Although not all sheet flow details are represented in the 1DV-model, it is shown that the model is able to give a correct representation of the observed trends in the data with respect to the influence of the velocity, wave period and grain diameter. Also detailed mean sediment flux profiles in the sheet flow layer are well reproduced by the model, including the direction change from ‘onshore’ to ‘offshore’ due to a difference in grain size from 0.34 mm (medium sand) to 0.13 mm (fine sand). A model sensitivity study with a selected series of net transport data shows that the stirring height of the suspended sediment ε s /w s strongly controls the magnitude and direction of the net sediment transport. Inclusion of both hindered settling and density stratification appears to be necessary to correctly represent the sand fluxes for waves alone and for waves + a superimposed current. The best agreement with a large dataset of net transport measurements is obtained with the 1DV-RANS model in its original settings using a Prandtl–Schmidt number σ ρ = 0.5.

Database of full-scale laboratory experiments on wave-driven sand transport processes

A new database of laboratory experiments involving sand transport processes over horizontal, mobile sand beds under full-scale non-breaking wave and non-breaking wave-plus-current conditions is described. The database contains details of the flow and bed conditions, information on which quantities were measured and the value of the measured net sand transport rate for 298 experiments conducted in 7 large-scale laboratory facilities. Analysis of the coverage of the experiments and the measured net sand transport rates identified the following gaps in the range of test conditions and/or the type of measurements: (i) graded sand experiments, (ii) wave-plus-current experiments and (iii) intra-wave velocity and concentration measurements in the ripple regime. Furthermore, it highlights two areas requiring further research: (i) the differences in sand transport processes and sand transport rates between real waves and tunnel flows with nominally similar near-bed oscillatory flow conditions and (ii) the effects of acceleration skewness on transport rates. The database is a useful resource for the development and validation of sand transport models for coastal applications.

A two-phase flow model for asymmetric sheetflow conditions

A newly developed two-phase flow model was applied to simulate the sediment movement under 2nd-order Stokes wave sheetflow conditions with different sediment sizes and wave periods. As for the distribution of eddy viscosity and sediment diffusion coefficient, the difference between onshore and offshore phases was considered by using an equivalent sinusoidal velocity amplitude for the asymmetric velocity profile. Sophisticated comparisons between laboratory measurements [O'Donoghue, T., Wright, S., 2004b. Flow tunnel measurements of velocities and sand flux in oscillatory sheetflow for well-sorted and graded sands. Coast. Eng., 51 (11–12), 1163–1184.] and the present numerical simulation were performed for sediment concentration, sediment velocity, sand flux and net transport rate. Four existing engineering models, together with the present two-phase flow model, were introduced for net transport rate prediction. Taking both the net sand transport rate magnitude and direction into account, the present process-based two-phase flow model provided the best estimations, which can simulate both the onshore net transport for medium/coarse sand cases and offshore net transport for fine sand cases with the agreement by a factor of 2 for almost all the considered cases.

Modelling and measurement of sand transport processes over full-scale ripples in oscillatory flow

A new series of laboratory experiments was performed in the Aberdeen Oscillatory Flow Tunnel (AOFT) and the Large Oscillating Water Tunnel (LOWT) to investigate time-averaged suspended sand concentrations and transport rates over rippled beds in regular and irregular oscillatory flow. The wave-induced oscillatory near-bed flows were simulated at full-scale. Five series of experiments were carried out. During the two AOFT experimental series, ripple dimensions, ripple migration rates and net sand transport rates were measured under regular and irregular asymmetric flow for two different sand types. The three LOWT experimental series focussed on measurements of the ripple dimensions, ripple migration rates, time-averaged suspended sand concentrations and net sand transport rates under regular asymmetric and irregular weakly asymmetric flow for two different sand types. From analysis of new and other full-scale data, it is concluded that the lower part of the time- and bed-averaged concentration profile (up to two times the ripple height above the ripple crest level) has an exponential profile. A new reference concentration formula is proposed based on the formula of Bosman and Steetzel [Bosman, J.J., Steetzel, H.J., 1986. Time- and bed-averaged concentration under waves. Proc. 20th ICCE Taipei, ASCE, pp. 986–1000], which includes the grain-size influence. Furthermore, it is shown that the concentration decay length is strongly related to the ripple height and that the simple formula R c = 1.27 η gives good agreement with the data. A new transport model is proposed for the wave-related net transport over full-scale ripples based on a modified half wave cycle concept of Dibajnia and Watanabe [Dibajnia, M., Watanabe, A., 1992. Sheet flow under nonlinear waves and currents. Proc. 23rd ICCE Venice, ASCE, pp. 2015–2028; Dibajnia, M., Watanabe, A., 1996. A transport rate formula for mixed sands. Proc. 25th ICCE Orlando, ASCE, pp. 3791–3804]. The magnitudes of the half wave cycle transport contributions are related to the grain-related Shields parameter, the degree of wave asymmetry and a newly defined vortex suspension parameter P, which is the ratio between the ripple height and the median grain-size. The new model has been calibrated using transport data from the new regular flow experiments and has subsequently been validated using other data, including measurements from irregular flow experiments. The new model is seen to perform better overall than existing practical models for ripple regime net sand transport.

Transport processes of uniform and mixed sands in oscillatory sheet flow

New experiments were carried out in the Large Oscillating Water Tunnel of WL|Delft Hydraulics (scale 1:1) using asymmetric 2nd-order Stokes waves. The main aim was to gain a better understanding of size-selective sediment transport processes under oscillatory plane-bed/sheet-flow conditions. The new data show that for uniform sand sizes between 0.2 < D < 1.0 mm, measured net transport rates are hardly affected by the grain size and are proportional to the third-order velocity moment. However for finer grains ( D = 0.13 mm) net sand transport rates change from the ‘onshore’ direction into the ‘offshore’ direction in the high velocity range. A new measuring technique for sediment concentrations, based on the measurement of electro-resistance (see [McLean, S.R., Ribberink, J.S., Dohmen-Janssen, C.M. and Hassan, W.N.M., 2001. Sediment transport measurements within the sheet flow layer under waves and currents. J. Waterw., Port, Coast., Ocean Eng., ISSN 0733-950X]), was developed further for the improved measurement of sediment dynamics inside the sheet-flow layer. This technique enabled the measurements of particle velocities during the complete wave cycle. It is observed that for long period waves ( T = 12.0 s), time-dependent concentrations inside the sheet-flow layer are nearly in phase with the time-dependent flow velocities. As the wave period decreases, the sediment entrainment from the bed as well as the deposition process back to the bed lags behind the wave motion more and more. The new data show that size-gradation has almost no effect on the net total transport rates, provided the grain sizes of the sand mixture are in the range of 0.2 < D < 1.0 mm. However, if very fine grains ( D = 0.13 mm) are present in the mixture, net total transport rates of graded sand are generally reduced in comparison with uniform sand with the same D 50. The transport rates of individual size fractions of a mixture are strongly influenced by the presence of other fractions in a mixture. Fine particles in sand mixtures are relatively less transported than in that uniform sand case, while the opposite occurs for coarse fractions in a mixture. The relative contribution of the coarse grains to the net total transport is therefore larger than would be expected based on their volume proportion in the original sand mixture. This partial transport behaviour is opposite to what is generally observed in uni-directional (e.g. river) flows. This is caused by vertical sorting of grain sizes in the upper bed layer and in the sheet flow and suspension layers. Kinematic sorting is believed to be responsible for the development of a coarse surface layer on top of a relatively fine sub-layer, providing in this way a relatively large flow exposure for the coarser sizes. Furthermore fine grains are suspended more easily than coarse grains to higher elevations in the flow where they are subject to increasing phase-lag effects (settling lags). The latter also leads to reduced net transport rates of these finer sizes.

Modeling and measurement of sediment transport by waves in the vortex ripple regime

Above steep, wave‐induced sand ripples, which occur extensively in shallow sea areas, the momentum transfer and suspended sediment dynamics are dominated by the formation and shedding at flow reversal of lee wake vortices. Since two‐dimensional models of this process are unduly complex for practical application, a simple, one‐dimensional vertical (1DV), two‐layer, model is presented here for the flow and transport above such ripples. In the lower layer of thickness equal to two ripple heights, vortex shedding is represented by a time‐varying eddy viscosity with peak values at flow reversal while, in the upper layer, a standard turbulence‐closure formulation is used. Suspended sediment is introduced at the ripple crest by a time‐varying pick‐up function. The ripple dimensions and suspended grain size are also predicted. The model results are compared with data obtained beneath weakly asymmetrical waves in a large‐scale flume. Intrawave measurements of suspended concentration were obtained using an acoustic backscatter system, and sediment profiles obtained above different locations on a moving rippled‐bed profile are used to provide intraripple and ripple‐averaged descriptions of the intrawave concentration field for comparison with the model. The results of a harmonic analysis suggest that the mean component and second harmonic (two‐peak symmetry term) of the concentration are well predicted, particularly near the bed. The modeled wave‐related component dominates the net sand transport rate; near‐bed transport is in the onshore direction, while transport in the outer boundary layer is offshore. The new 1DV formulation provides a simple, but realistic, modeling approach for the rippled regime.

The development of large aeolian bedforms

Large aeolian bedforms (complex and compound varieties of dunes or draas) dominate over wide areas of many sand seas and are well represented in the rock record. Analyses of allometric relationships between dune height and spacing in several sand seas suggest that draas develop by vertical growth of existing dunes. In the Namib and other sand seas draa tend to occur in areas of low total and net sand transport rates. The distribution of such bedforms in a sand sea reflects long-term patterns of deposition which are related to regional-scale changes in wind regimes and sand transport rates. Superimposed bedforms are almost ubiquitous on draa and develop only after a minimum size of draa has been reached. Formation of superimposed dunes reflects spatial and temporal variations in secondary flow patterns developed on the draa. Measurements of winds and depositional patterns on complex linear dunes in the Namib Sand Sea suggest that an important mechanism for their growth is the migration of superimposed dunes from areas of high to low wind velocity on draa flanks, leading to the deposition of sediment by bedform climbing.

Geometry of Sand Ripples and Net Sand Transport Rate Due to Regular and Irregular Oscillatory Flows

Geometry of Sand Ripples and Net Sand Transport Rate Due to Regular and Irregular Oscillatory Flows

LABORATORY STUDY ON SAND TRANSPORT OVER RIPPLES DUE TO ASYMMETRIC OSCILLATORY FLOWS

Mechanism of sand movement due to asymmetric oscillatory flows was investigated through experiments. Measurements of bed forms, suspended sand concentration and net sand transport rate were carried out by using an oscillatory flow tunnel. The process of entrainment and suspension of sand above asymmetric ripples was quantitatively described. The geometry of ripples and the net sand transport rate in regular and irregular flows were expressed in terms of hydraulic parameters characterizing the oscillatory flow. Two-dimensionality of ripples was found to be an important factor in the estimation of the net sand transport rate.

EXPERIMENTS ON BEACH PROFILE CHANGE WITH A LARGE WAVE FLUME

Two-dimensional beach profile changes were investigated with a newly constructed prototype-scale wave flume. The flume is 205 m long, 3.4 m wide and 6 m deep. Sand of two grain sizes was used in the experiments. Analysis of the results was made through use of the parameter C, introduced by Sunamura and Horikawa (1974) to classify beaches as either erosional and accretionary. Beach profile changes obtained in the flume were similar to those in the prototype (field). Net sand transport rate distributions were classified into five types, two of which do not seem to have been observed in laboratory (smallscale) experiments. A simple model describing the five types was developed for evaluating two-dimensional beach profile changes.