Sand transport under combined current and wave conditions: A semi-unsteady, practical model

Abstract

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.

Experimental study of the turbulent boundary layer in acceleration-skewed oscillatory flow

“…If the present results with acceleration-skewed flows were plotted against a/k s , the results would still be in good agreement with Swart, but with f wp and f wn lying either side of the Swart (1974) curve. These results confirm that the approaches of Silva et al (2006) and Gonzalez-Rodriguez & Madsen (2007), whereby the friction factor for each half-cycle is based on an equivalent sinusoidal orbital amplitude, is a promising, practical approach to determine bed shear stress for acceleration-skewed flow.…”

“…For the present conditions, the positive half-cycle friction factor, f wc , is plotted against a c /k s and similarly f wt is plotted against a t /k s where a t = a2T at /T t with the periods T at and T t as outlined in figure 2. In this case the calculation of a for each half-cycle takes account of the fact that for β increasing above 0.5 the time from zero velocity to peak positive velocity decreases and the time from zero to peak negative velocity increases (e.g Silva et al 2006; Gonzalez- Swart (1974) Bagnold ( …”

A semianalytical model for sheet flow layer thickness with application to the swash zone

“…A main outcome was the development of empirical formulas for the net sheet flow sediment transport aimed toward use in large-scale morphodynamic models [Drake and Calantoni, 2001;Hoefel and Elgar, 2003;Watanabe and Sato, 2004;Camenen and Larson, 2006;da Silva et al, 2006;Nielsen, 2006;Gonzalez-Rodriguez and Madsen, 2007;van der A et al, 2010. Formulas for other parameters such as the maximum sheet flow layer thickness, d s , and the maximum erosion depth, d e , were also proposed.…”

Sheetflow Sediment Transport Under Asymmetric Waves and Strong Currents

“…However, as shown in figure 11, sheetflow layer thickness measured in present study is about 4-6 times larger than predicted by Eq.(20). This is because sheetflow layer thickness could be limited by the turbulent kinetic energy dissipation due to high sand concentration inside sheetflow layer (Da Silva et al 2006). So far, it is somehow difficult to quantitatively estimate the turbulent part.…”