Nearshore sandbar migration

Field observations suggest that onshore sandbar migration, observed when br eaking-wave-driven mean flows are weak, may be related to the skewed fluid accelerations associated with the orbital velocities of nonlinear surface w aves. Large accelerations (both increases and decreases in velocity magnitu des), previously suggested to increase sediment suspension, occur under the steep wave faces that immediately precede the maximum onshore-directed orb ital velocities. Weaker accelerations occur under the gently sloping rear w ave faces that precede the maximum offshore-directed velocities. The timing of strong accelerations relative to onshore flow is hypothesized to produc e net onshore sediment transport. The observed acceleration skewness, a mea sure of the difference in the magnitudes of accelerations under the front a nd rear wave faces, is maximum near the sandbar crest. The corresponding cr oss-shore gradients of an acceleration-related onshore sediment transport w ould cause erosion offshore and accretion onshore of the bar crest, consist ent with the observed onshore migration of the bar crest. Furthermore, the observations and numerical simulations of nonlinear shallow water waves sho w that the region of strongly skewed accelerations moves shoreward with the bar, suggesting that feedback between waves and evolving morphology can re sult in continuing onshore bar migration.