Prediction of net bedload transport rates obtained in oscillating water tunnels and applicability to real surf zone waves

Abstract

Experimental studies of sediment transport rates due to near shore waves are often conducted in oscillating water tunnels (OWTs). In an OWT, the oscillatory motion produced by the piston propagates almost instantaneously along the entire tunnel. Consequently, unlike the wave motion in the sea or in a wave flume, flow in an OWT is uniform along the tunnel, and second - order wave propagation effects (such as Longuet - Higgins's streaming) are absent. The effect of these hydrodynamic differences between OWT and sea waves on sediment transport rates has generally been neglected. In this paper we present a simple, practical formulation to evaluate bed shear stresses and bed load transport rates due to asymmetric and skewed waves plus a current in an OWT, based on fitting the exact results of a rigorous, analytical model of the OWT wave - current boundary layer. By then accounting for real wave effects we find that wave propagation significantly affects the predicted period - averaged net sediment transport rates. Such real wave effects can therefore not be neglected when comparing near shore transport models with OWT data.