Sheet flow and large wave ripples under combined waves and currents: field observations, model predictions and effects on boundary layer dynamics

Large wave ripples and upper-plane bed sheet flow were observed under combined waves and currents during storms at 39 m water depth on Scotian Shelf These data show that the critical wave mobility number and Shields parameter for sheet flow are not constant, but rather their values decrease with an increase in grain size. The critical sheet-flow Shields parameter under combined flows is found to be about 50% smaller than that for pure waves. Though the bed shear stress based on bedload roughness or wave parameterization using the one-tenth largest waves both offer adequate resolution to this difference, a definitive explanation does not exist at present. The large wave ripples (LWR) observed under combined flows are generally symmetrical, rounded and 3-dimensional, with small current ripples superimposed. The wavelengths of these LWR are approximately half of the wave orbital diameter and their average steepness is about 0.1. Analyses of storm processes on continental shelves show that as a storm builds up, ripples generally change directly into upper-plane beds. LWR generally form following the peaks of storms due to sediment fall out from suspension and moulding of the seabed by the long wave oscillation. If the spin up of a storm is gradual and strongly wave dominant, however, LWR can also develop from small ripples before sheet-flow conditions are reached. We interpret the three-dimensional, mound-like, low-relief LWR as combined-flow hummocky megaripples. This suggests that hummocky megaripples and hummocky cross-stratification (HCS) on continental shelves are formed under wave-dominant combined flows, and that they can form in medium sand as well as in silt and fine sand. The application of the combined-flow boundary layer model of Grant and Madsen (1986) and a modified Rouse suspension equation shows that the incorrect prediction of the presence of ripples in place of upper-plane bed causes over-estimation of shear velocities and suspension concentration and a 300% over-prediction of the suspended sediment transport rate. By contrast, failing to predict correctly the formation of LWR causes an under-estimation of shear velocities and sand resuspension, and will result in under-prediction of the suspended sediment transport rate by nearly one order of magnitude. (C) 1999 Elsevier Science Ltd. All rights reserved.