HR: 17:00h
AN: OS24A-05    [Abstracts]
TI: Wave Induced Sediment Transport over Flat Beds
AU: * Foster, D L
EM: foster.316@osu.edu
AF: Ohio State University, 470 Hitchcock Hall 2070 Neil Avenue, Columbus, OH 43210-1275
AU: Bowen, A
EM: tony.bowen@dal.ca
AF: Dalhousie University, Deptartment of Oceanography, Halifax, NS B3H4J1
AU: Stanton, T
EM: stanton@oc.nps.navy.mil
AF: Naval Post-graduate School, Depatment of Oceanography, Monterey, CA 99999
AU: Fredsoe, J
EM: jf@isva.dtu.dk
AF: Danish Technical University, ISVA, Lyngby, 99999 Denmark
AB: 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 applied to individual grains 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 alternative theory proposes that under certain free surface gravity waves, the horizontal pressure gradient may induce an instantaneous bed dilation and transport (Sleath, 1999). According to laboratory observations, plug flow occurs for Sleath parameters above 0.29 and can be several centimeters thick. In this presentation, we examine two sets of field observations of flow and suspended sediment over a flat bed. The first set of observations was obtained during the cooperative Duck94 Experiment. The orbital velocity, peak period, and water depth during the sampling period was 60 cm/s, 5 s, and 2.0 m, respectively. The Duck94 observations show that during large accelerations present under large wave crests, several centimeters of sediment are mobilized and transported onshore. The second set of observations was obtained during the cooperative SandyDuck Experiment. The orbital velocity, peak period, and water depth during the sampling period was 67 cm/s, 10 s, and 2.7 m, respectively. The SandyDuck observations also show suspension events associated with large wave crests, but show significantly smaller variations in the nearbed concentration. Numerical simulations of flow and sediment transport were performed with a 2-D k-$\omega$ bottom boundary layer model. The model assumes a no-slip bottom boundary condition and is forced with the observed free stream flow. The Duck94 observations exhibited lower bed stress and did not satisfy the no-slip bottom boundary condition required by the model. These observations were not consistent with sheet flow but were consistent with plug flow. The SandyDuck observations compare favorably with the wave bottom boundary layer model and are consistent with sheet flow theory. Although predictions of the Shields parameter are similar in both data sets, the larger accelerations present in the Duck94 observations yield significantly larger estimates of the instantaneous Sleath parameter and likely result in pressure gradient induced transport.
DE: 4546 Nearshore processes
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting