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Vortex Ripple Morphology Using DUNE2D Model

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Master's thesis

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The DUNE2D boundary layer model for small-scale morphology Andersen, 1999 is compared with bedform morphology measured on the inner shelf in 11 m water depth during the SHOWEX experiment at Duck. N.C. The model consists of inter-linked modules for flow. sediment transport and morphology. The flow module is based on solving the Reynolds averaged Navier-Stokes equations in the vertical plane with k-omega turbulence closure. The model has been extended to accept a general but periodic bottom boundary to be able to compare with field data. Boundary layer velocity profiles were measured using a Bistatic Coherent Doppler Velocity profiler BCDV. A two-axis scanning sonar altimeter measured small-scale morphology over a 1 by 1.5 m area with 4 cm horizontal and 0.23 cm vertical resolutions. Bottom maps of small-scale morphology were obtained continuously every 20 minutes. A relatively simple data sequence was selected for model comparison, during which time the wave forcing evolved abruptly from Hsub sig 0.3 m to Hsub sig 3.0 m bed velocity 1 ms, and the bed evolved from no motion relic to actively migrating vortex ripples. SHOWEX bedform changes under low wave plus collinear current conditions resulted in minor changes of the vortex ripple fields. Bedform migration rates of the model were similar to the field migration rates. Like the field data, the modeled data under strong forcing removed smaller scale vortex ripples and redistributed the sediment into a larger scale ripple with a large portion of sediments in suspension above the bed. Limitations of the model owing to the 2-D assumption, periodic boundaries and monochromatic wave forcing constraints are discussed.

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  • Physical and Dynamic Oceanography
  • Fluid Mechanics

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