This thesis presents an examination of the nearshore wave bottom boundary layer
under conditions of significant sediment response. Using both field observations and
simple models, the response of the bottom boundary layer to random waves is shown to
have a complex behavior. First, the linearized wave bottom boundary layer governing
equation is solved with a transformation of the cross-shore velocity to a distorted spatial
domain, resulting in an analytic expression for the temporal and vertical structure of the
cross-shore velocity under an arbitrary wave field. Model predictions of the bed shear
velocity are in good agreement with laboratory measurements. The model is limited by
assuming zero velocity at a fixed bed and that turbulence generation is solely due to bottom
shear.
Next, a comprehensive set of near bed cross-shore velocity, sediment suspension,
and bed elevation observations, collected in 2 m water depth on the North Carolina coast,
are presented. The observations show a cross-shore velocity structure which decays with
increasing proximity to the bed as predicted by simple theory. Bottom shears based on
rms amplitude decay and time-averaged phase shifts are lower than model predictions and
may be indicative of more rapid mixing of momentum than assumed in the above model.
Also, frequency-dependent estimates of the phase and amplitude vertical structure show a
nonlinear response of the wave bottom boundary layer over the incident band. Through
most flow phases, estimates of turbulent kinetic energy increase linearly from the bed,
however under large wave crests, enhanced turbulence levels are observed and are well
correlated to active sediment suspension events. Estimates of dissipation rates are
significantly less than those observed in an actively breaking surf zone wave, and
significantly greater than those observed in ocean boundary layers, and continental shelf current boundary layers. Finally, an Oregon coast field experiment showed an intermittent high frequency
velocity variance structure which was correlated to suspended sediment events. A linear shear instability analysis determined that during the period of flow reversal there exists a potential for generating turbulence due to shear instabilities of the vertical structure of cross-shore velocity. / Graduation date: 1997
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/34380 |
| Date | 13 June 1996 |
| Creators | Foster, Diane Lyn |
| Contributors | Holman, Robert A. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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