Modelling intensive near-bed sand transport under wave–current flow versus laboratory and field data

Abstract

The present study is focused on sand transport in combined wave–current flow (at any angle between them) in the intensive flow regime. The present model is based on the 1DV approximation. The mathematical method for the continuous description of processes from the immobile bed to the formation of sheet flow and suspended load is presented. The way of modelling of sediment–flow interaction is proposed, in which the near-bed flow is divided into three layers, as done previously by Kaczmarek and Ostrowski [Kaczmarek, L.M., Ostrowski, R., 1998. Modelling of a three-layer sediment transport system in oscillatory flow. Proc. 26th ICCE, ASCE, 2559–2572] for pure oscillatory flow. Measured by Katopodi et al. [Katopodi, I., Ribberink, J.S., Ruol, P., Lodahl, C., 1994. Sediment transport measurements in combined wave–current flows. Proc. Coastal Dynamics '94, ASCE, 837–851] and Dohmen-Janssen [Dohmen-Janssen, M., 1999. Grain size influence on sediment transport in oscillatory flow. FEBODRUK BV, ISBN 90-9012929-4, Enschede, The Netherlands], the time-averaged velocity profiles are compared with results provided by the present model for the respective conditions. The laboratory data of combined wave–current flow were further used to compare the model results for instantaneous and time-averaged concentration in the bedload (pick-up) layer and the contact load (upper sheet flow) layer, as well as for net sediment transport rate in the case of unidirectional ( γ=0) wave–current interaction. The model cases for non-zero wave–current angle were tested in the context of longshore sediment transport rate against field radio-isotopic tracer data of Lubiatowo (Poland) by Pruszak and Zeidler [Pruszak, Z., Zeidler, R.B., 1994. Sediment transport in various time scales. Proc. 24th ICCE, ASCE, 2513–2526] and field luminescent tracer data of Ancao Peninsula (Portugal), by Balouin and Howa [Balouin, Y., Howa, H., 2000. Correspondence contact with University of Bordeaux]. Generally, the agreement between the model results and the experimental data was found satisfactory in the context of the apparent roughness and time-averaged velocity profiles above the bed boundary layer. Further, the anti-phase behaviour of measured bedload concentration with respect to sediment concentration in the contact load layer and the asymmetric effects resulting from wave–current interaction are well represented by the model. Consequently, the present theoretical approach provides a useful tool for predictions of sand transport under wave–current flow within an accuracy factor of 1.5 either way.