Intrawave observations of sediment entrainment processes above sand ripples under irregular waves

Abstract

Measurements of intrawave sediment entrainment processes are reported above ripples under irregular waves generated in a large-scale flume facility. The data consist of substantive observations of small-scale processes collected at high spatial and temporal resolution under irregular wave forcing, typical of coastal environments. Acoustic measurements were made of water velocities, bed forms, and suspended sediment concentration over plane and rippled beds. During each rippled bed experiment, measurements were taken over the crests of steep sided ripples, above which flow separation was considered likely to occur. Above the ripple crests, a strong intrawave variation in the near-bed suspended sediment, indicative of vortex formation and shedding, was observed to dominate wave half cycles where the orbital diameter was greater than 1.2 times the ripple wavelength λ. The irregular waves were parameterized with a significant wave height Hs and a significant orbital diameter d 0s. Thus, during irregular wave conditions and over steep ripples where d0s/λ > 1.2, vortex shedding was deemed as the dominant sediment entrainment process. Furthermore, the percentage of wave half cycles where vortex shedding was observed, was found to scale as 26 d 0s/λ. These results demonstrate, for under irregular waves, the existence of a clear suspended sediment structure consistent with vortex shedding, previously observed under regular waves. © 2011 by the American Geophysical Union.