SPECTRAL ANALYSIS OF ROTARY SONAR IMAGES FOR COMPARISON TO PREDICTIVE RIPPLE MODELS

A concurrent seabed morphology and hydrodynamic study was completed from August-September 2006 at Hen and Chickens Shoal located just outside the mouth of the Delaware Bay. An instrumented frame deployed at a depth of 7.4 meters was fitted with a 1 megahertz (mHz) Nortek AWAC and a 2.25 mHz Imagenex rotary sonar. Acoustic rotary images were recorded every hour along 8-meter radials. Current direction and velocity were recorded every 10 minutes. Wave height, direction, and period were sampled for a 17 minute burst once every hour. Matlab software was used to run a 2-dimensional Fast Fourier transform analysis on the acoustic images, resulting in output values for wavelength and ripple orientation. Comparison of these automated values to manually extracted values supports the automated values’ reliability. Daily tidal fluxes result in a frequent shift of the field site’s dominant forcing hydrodynamics. Consequently, the imaged wave orbital scale ripples tend to be in a state of non-equilibrium. Three ripple evolution models were tested and proved unable to account for the site’s influencing current conditions.