Date of Award

Spring 2015

Project Type

Thesis

Program or Major

Ocean Engineering

Degree Name

Master of Science

First Advisor

Diane Foster

Second Advisor

Thomas Lippmann

Third Advisor

Anne Lightbody

Abstract

A laboratory study was conducted in a field scale flume to further examine the very near-bed dynamics in the near-shore environment using natural, non-cohesive sediments with d50 = 0.22 mm. A Particle Imaging Velocimetry (PIV) system was used to capture the dynamics of the movable ripple bed and the 2D flow field in the vertical (z) and along-tank (x) direction in 1.6 m still water depth. Two wave types were analyzed: regular waves with periods ranging from 4 s to 8 s and bimodal wave pairs with periods of 3.7 and 4.3 s. Orbital ripples developed in all wave environments with ripple wavelengths ranging from 5 to 15 cm.

Following Roth and Katz (1995), instantaneous ripple migration rates were calculated by spatially shifting subsequent bed profiles until a minimum difference was achieved. The ripple migration signature is cyclical but asymmetric, with higher amplitudes during onshore directed movement. This asymmetry leads to a net onshore migration, ranging from 0.1 to 0.6 cm/min in the wave conditions tested. The cyclic motion of the ripple field was compared to potential dominant forcing mechanisms: bed shear stress, coherent structure generation, and free-stream velocity.

Peak ripple migration rates occurred during strengthening onshore flow, which coincides with peak bed shear stresses and the onset of coherent structure formation identified with the swirling strength criteria of Zhou et al. (1999). Two estimates of bed shear stress were similar in magnitude and phase, with direction changes coincident with the bed migration. The combined sediment mobilization parameter of Foster et al. (2006) was found to be consistent with the bed migration signature. Oscillations associated with the wave groups caused periods of high suspension which were coincident with an increase in onshore migration rates. Low energy waves (period of 6 s and height of 10 cm) did not result in ripple motion.

The bed shear stress analysis from this study provides further evidence for the reliable application of the DANS bed shear stress approach taken by Rodriguez-Abudo and Foster (2014). Paired with the ripple migration rate, this analysis can be applied to sediment transport models to more accurately represent the sediment and flow dynamics along sandy beaches that experience wave dominated forcing.

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