Edward B. Thornton, John R. Dingler,
Office of Naval Research, Coastal Dynamics Program
United States Geological Survey
US Army Corps of Engineers, Coastal Research and Development Program
Predict small-scale bedforms (O(0.1-10m), ripples and megaripples) and their relation to larger bedforms (O(0.1-km), bars and cusps), in the nearshore due to waves and currents.
Hypotheses will be tested on the growth and migration of the small-scale bedforms and how they contribute to the processes of erosion and progradation of the nearshore bar system. The small-scale morphology will be measured using an acoustic altimeter and side-scan sonar (collaborative with Dingler) attached to the CRAB for repeated large area surveys in the nearshore. A B&W video camera mounted on the CRAB will provide a visual record of the bottom during times of clear water (with Birkemier).
Time series of the bathymetry at specific locations will be measured using two scanning acoustic altimeters. The narrow beam acoustic scanning altimeter rotates on two axes to obtain a raster scan of the bottom elevation in an approximate 5m diameter area. One scanner will be at a fixed location (trough or outer flank of bar) and the other will be mounted on a moveable sled to obtain detailed bathymetry at various locations across the surf zone.
The detailed spatial bathymetry will be compared with the array of sonic altimeter point measurement time series (collaborative with Elgar). The space/time series of mega-ripples will be correlated with wave/current measurements (collaborative with Guza) to obtain a quantitative understanding of their formation and propagation. Some specific hypotheses to be tested are:
a. Bar movement is due to migration of two-dimensional megaripples propagating perpendicular to shore in response to sea-swell waves.
b. The small-scale morphology in the trough of a barred profile system is due to strong longshore currents and aligned in the direction of the dominant flow.
c. The current magnitude is a function of the bed form, or bottom roughness, as related through the bottom stress, and the bedforms are highly dependent on the wave and current regime.
d. The small scale morphology is an indicator, or precursor, of changes in the bottom. The perturbations on the flow due to these bumps acts as feed back to induce even greater changes dependent on flow intensity.
Collaborations: Elgar, Guza