Carolyn Smyth

Coherent Doppler Profiler Measurements of Near-Bed Vertical Suspended Sediment Fluxes for Different Bedstates in the Nearshore Zone

Thesis Approved July, 2001

Remote acoustic field measurements of turbulence intensities and suspended sediment fluxes in the wave bottom boundary layer are investigated for a variety of bedstates. Comparable near-bed peak turbulence intensities are found for the 4 bedstates considered, despite the factor of 7-10 difference in wave energy. Changing physical roughness compensates for the change in wave energy as the mobile sediments adopt different bedstates, resulting in an enhancement in the bed friction factor for low-energy ripple beds and a reduction in the bed friction factor for high-energy flat beds. Model predictions of near-bed turbulence intensities from Tolman [1994] are found to be generally consistent for low-energy cases, while those based on monochromatic wave data [Grant and Madsen, 1982; Nielsen, 1992; Sleath, 1987; Swart, 1974] are generally inconsistent.

At heights greater than 50 cm, de-aliased vertical velocity power spectra exhibit a -5/3 power law dependence on frequency (between 0.7 and 4 Hz), and on wavenumber, after invoking Taylor's hypothesis. Spectral slopes become progressively less steep as the seafloor is approached, reaching values between -1/2 and -1 at the bed. These observations, combined with previous observations of a -5/3 slope in the horizontal power spectra [Foster, 1997; Conley and Inman, 1992] suggest that the turbulence is anisotropic. Enhanced turbulence anisotropy is inferred within the wave boundary layer for the high energy cases and is likely related to the generation scales of the turbulence.

Estimates of the vertical suspended sediment flux partitioned into mean, wave and turbulent components, show that in general, there is a balance between upward fluxes due to waves and turbulence, and downward settling; except immediately above the bed, and except for the case of a stationary ripple field. The suspended sediment flux coherence indicates enhancement at incident wave frequencies, with the largest coherence for flat bed conditions very near the bed.

Accuracy of velocity and concentration measurements is assessed through flux divergence measurements in a sediment- laden jet experiment and through comparison of vertical velocity spectra with those estimated with Acoustic Doppler Velocimeter field data.