Observations and modeling of sand transport in a wave dominated environment

Abstract

Observations of bedforms, suspended sediment and water velocities were used to examine sediment transport processes at the sandy LE0-15 site located off the New Jersey coast. The bedforms were observed during storms using a rotary sidescan sonar and were found to be wave orbital scale ripples. The onshore migration of these ripples was forced by non-linear wave velocities, and could be related to a simple bedload model. Observations of suspended sand flux were calculated from acoustic backscattering profiles and water velocity profiles. Suspended sand transport forced by wave velocities was found to occur primarily during the weaker offshore phase of wave motion, as part of a vortex ejection mechanism. This net offshore suspended sediment flux was an order of magnitude less than the flux associated with onshore ripple migration. Thus it is hypothesized that ripple migration was forced by unobserved bedload or near bottom suspended flux. The net suspended sediment flux due to mean currents was a factor of five less than the waveforced offshore suspended flux. These wave dominated events at LE0-15 represent a contradiction of the conceptual idea that waves are primarily responsible for suspending sediment and mean currents provide the transport mechanism.