Vortex Ripple Morphology Using DUNE2D Model

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 Reynold's 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 H(sub sig) = 0.3 m to H(sub sig) = 3.0 m (bed velocity < 1 m/s), 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.Naval Postgraduate School Monterey CA is the author of 'Vortex Ripple Morphology Using DUNE2D Model', published 2001 under ISBN 9781423530312 and ISBN 1423530314.