Field evidence of pressure gradient induced incipient motion

[1] Recently, models for the onshore migration of nearshore sandbars were improved by including an empirical acceleration term in the sediment transport formulation (Hoefel and Elgar, 2003). Here, field observations of the fluid-sediment interface suggest that the success of this empirical parameter may result from unsteady forcing of sediment beds by nearshore waves. The velocity and sediment observations show that, under large pressure gradients present during the strengthening of onshore-directed flow, several centimeters of previously still sediment can be mobilized and transported onshore. The onshore-directed mobile-bed layer rapidly settles as the wave velocity weakens. These observations are inconsistent with traditional sediment transport theories (Shields, 1936; Bagnold, 1966) that assume incipient motion is solely based on the force applied by bed stress but are consistent with theories that include an additional force induced by the horizontal pressure gradient (Madsen, 1974; Sleath, 1999). In this paper, we provide the first field evidence of incipient motion that is significantly influenced by the unsteady forcing introduced by the horizontal pressure gradient due to surface gravity waves.