Date of Award
Doctor of Philosophy in Oceanography
Flow structures in the bottom boundary layer associated with uniform, periodic, and random distributions of bed roughness and sedimentary furrows are investigated to determine the processes reponsible for the formation and maintenance of the furrowed bedform. Furrows are streamwise lineations mostly found in cohesive sediments with common dimensions of 5 m width, 1 m depth, and spacing of 20 m to 60 m. The existing conceptual model relates furrow formation and maintenance to stream wise, counter-rotating, helical vortices in the bottom boundary layer.
Results from a two-dimensional planetary boundary layer model with second order turbulence closure confirm portions of the conceptual model and indicate that deposition patterns arising from roll vortices could account for furrow initiation. Over a uniform bed, instabilities in streamwise vorticity are responsible for vertical residual velocities Wr ≈ ± 10-5 cm s-1 with wavelengths of 28 to 60 m six hours after model initialization. Wavelength spectra show that both roll energy and the wavelength of maximum energy increase as geostrophic flow speed Ug increases. Strongest vertical velocities are at the top of the turbulent boundary layer. The circulations are most stable for Ug = 15 to 20 cm s-1, and affect the settling paths of clay-size particles.
Once cross-stream distributions in roughness exist, the roll circulations are oriented so downward flow is found over the increased roughness, with λ set by the roughness wavelength. Vertical velocities Wr = ± 5 x 10-5 cm s-1 are found at height above the bed z = 2 m within two hours of initialization.
Streamwise roll vortices associated with existing furrows are strongest at geostrophic flow speeds Ug = 1 cm s-1. Circulation with vertical velocities Wr = ±0.02 cm s-1 arises within an existing furrow inducing weaker circulations above the interfurrow area with residual velocities V1T ≈ 10-3 cm s-1. Stratification above the boundary layer increases vertical velocities associated with the rolls. Results suggest that shears arising from sub-inertial-scale dynamics, such as internal waves propagating into the boundary layer, are important in the formation of roll vortices in realistic bottom boundary layers.
Viekman, Bruce Eric, "STEAMWISE VORTICES IN THE OCEANIC BOTTOM BOUNDARY LAYER" (1994). Open Access Dissertations. Paper 1590.