HR: 17:30h
AN: OS24A-07    [Abstracts]
TI: Multiple Frequencies of Waves on an Intertidal Mudflat, and Their Effect on Turbulence Statistics, Boundary Layer Structure, and Sediment Transport
AU: * Talke, S A
EM: stalke@berkeley.edu
AF: University of California, Berkeley, 412 O'Brien Hall, Berkeley, CA 94720 1715 United States
AU: Stacey, M T
EM: mstacey@ce.berkeley.edu
AF: University of California, Berkeley, 412 O'Brien Hall, Berkeley, CA 94720 1715 United States
AB: On an intertidal mudflat in the Central San Francisco Bay, we conducted an experiment to measure the dynamics of waves, currents, turbulence, and sediment transport in shallow water (0-1.5 m depth). We obtained vertical profiles of velocity, sediment concentration, and density by deploying six Acoustic Doppler Velocimeters (ADVs), five Optical Backscatter sensors (OBS), and 3 Conductivity-Temperature (CT) probes between 1 cm and 35 cm for a period of 4 days in April 2003, logging nearly continuously at frequencies of up to 16 Hz. Multiple frequencies of waves are evident at this site, including a seiche with a period of ~ 500 seconds, ocean swell with a period of 8-15 seconds, and locally driven wind waves with a period of 1-3 seconds. These waves are irregular and intermittent, and interact nonlinearly with currents and potentially each other. Estimating turbulent kinetic energy (TKE), dissipation rate, and stresses are complicated by the multiple frequencies of motion. We use the Soulsby (1983) and Trowbridge (1998) methods to separate waves and TKE, and suggest a hybrid approach that includes components of each method. In order to model the various boundary layers, we separate the raw velocity data into vertical profiles of tidal, seiche, ocean swell, and wind wave velocities using a simple bandpass filter. The tidal and seiche velocity profiles are best described by a log-linear model in which the roughness z0, the friction velocity u*, and the linear parameter (z-z0)/L are optimized in a least-squares sense. However, the length scale `L' is not adequately predicted by a combination of an acceleration length scale (Soulsby-Dyer, 1981) and a stratification length scale (Monin-Yaglom, 1971). Instead, we find that the curvature in the seiche boundary layer is best described by a two equation TKE numerical model (GOTM, Burchard et al, 1999). The boundary layers of higher frequency waves such as ocean swell are well described by an analytical model such as the Smith (1977) model, which assumes a linear eddy viscosity. The superposition of the seiche motions with the tidally-driven mean velocity profile creates a pattern of alternating high and low mean stress between 0.05 Pa and 0.5 Pa. Sediment concentration and salinity can fluctuate at the same seiche frequency at up to 150 g/L and 0.5 ppt. However, sediment concentration bursts can be ~180 degrees out of phase with mean bed stress and velocity during an ebbing tide. This non-intuitive result highlights the need for detailed measurements of local wave climate, sediment concentrations, and their phasing in order to accurately model hydrodynamic and transport processes. In fact, this asymmetric phasing of sediment during the ebb accounts for as much as 20 percent of the residual shoreward sediment flux near the bed for a particular tidal period.
DE: 4235 Estuarine processes
DE: 4546 Nearshore processes
DE: 4558 Sediment transport
DE: 4560 Surface waves and tides (1255)
DE: 4568 Turbulence, diffusion, and mixing processes
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting