Date of Award

Spring 2011

Project Type

Thesis

Program or Major

Ocean Engineering

Degree Name

Master of Science

First Advisor

Diane Foster

Abstract

Laboratory-scale observations of a two-dimensional near-bed velocity field were collected using a submersible PIV system. A quasi-steady approach was employed to characterize the observed wave-bottom boundary layer thickness and shear stress. Friction velocities, corresponding to Re * = 15 -- 40, were obtained from a nonlinear least-squares regression of the velocity field averaged over individual half-waves. The regressions show zero-velocity elevations located roughly 1 cm under the ripple crests. The resulting Shields parameter is O(1) greater than the reported threshold for sheet flow, even though bedforms persist. An unsteady analysis using the Double-Averaged Navier-Stokes equations show turbulent and wave-induced stresses dominating the total shear stress near the bed. Bedform-induced stresses are significant up to two ripple heights above the crests, while viscous stresses are negligible. The phase and magnitude of the total shear stress agrees with estimates inferred from a simple bedload transport model. Momentum imbalance below ripple crests is consistent with bedform acceleration.

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