CGRG Bibliography of Canadian Geomorphology
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Author : Foster, D.L.; and Bowen, A.J.
Date : 2005.
Title : The incipient motion of sediment in response to waves.
Publication : Proceedings of the 12th Canadian Coastal Conference. Dartmouth, Nova Scotia. November 6-9, 2005.
Issue :
Page(s) :
Abstract
Traditional sediment transport models assume the incipient motion of sediment is a function of the shear stress applied to the bed. In these models, transport occurs when the vertical stress gradient distributed over a single grain thickness applied to grains resting on a bed exceeds the horizontal resistive force of the grains resting on a fixed bed (Shields, 1936). When the Shields parameter exceeds a value of 0.8, a 10-100 grain diameter thick mobile sediment layer called sheet flow may occur. An additional theory proposes that under certain free surface gravity waves, the horizontal pressure gradient may also induce an incipient motion. Sleath (1999) showed that the incipient motion of sediment is dependent on both the shear stresses and normal stresses (ie. pressure) applied to a thickness of sediment. In a force balance applied to the thickness, the sediment motion results from the net effect of the shear distributed over the thickness and the pressure gradient induced by free surface gravity waves. In this presentation, we examine role of the shear stress, pressure gradient, and grain size in the incipient motion processes over flat and rippled beds. The shear stress gradient is calculated over thicknesses ranging from a single grain size (ie. the Shields parameter) to several millimeters. The pressure gradient is approximated with the local free stream acceleration. The effect of the two forcing mechanisms is evaluated over a range of grain sizes observed on natural beaches (0.1 mm < d < 1. mm). Simulations are compared with observations flow velocity and sediment velocity obtained during the collaborative CROSSTEX experiment. Planar velocity fields were measured with a submersible Particle Image Velocimetry system over a 25 cm x 25 cm window. The observations were made offshore of the break point in 1.5 m water depth. Offshore significant wave heights ranged from 20 to 60 cm and wave periods ranged from 4 to 7 seconds. Velocity correlations were determined at 15 Hz intervals with 1 mm resolution. Images were collected in both downward looking mode to resolve the planar evolution of the bedform over scales ranging from 10-1-103 seconds and also sideward looking mode to resolve the flow field over the bed form. In this presentation, we address the following question: Could the horizontal pressure gradient enhance the sub-period migrations of ripple crests?
Bibliography of Canadian Geomorphology