Bed Roughness Mapping in the Surf Zone Using Sidescan Sonar

Pierson, D.M., Gallagher, E.L. and Drake, T.G., 2000, Bed roughness mapping in the surf zone using sidescan sonar: EOS Trans. AGU, 79 (45), Fall Meeting Suppl., F416 (Special Session: Nearshore Processes)
Sidescan sonar images and sonic altimeter measurements obtained during the SandyDuck97 nearshore field experiment at Duck, NC are used to construct maps of geometrical bed roughness and bedform orientation for the entire 1 km by 0.5 km experimental area on a daily basis. Such maps, though difficult to obtain, have the potential to provide valuable bed-roughness information for models of nearshore circulation and bathymetric evolution. A dual-frequency (100 MHz and 500MHz) digital sidescan sonar was mounted on the three-wheeled CRAB a fixed distance of about 1 m above the seafloor. Sonar data were collected along 20 cross-shore profile lines about 25 m apart which extended from the beach to 6 m water depth. Sidescan images having nearly 100% overlap and having about 10 cm horizontal resolution were obtained seaward of a sand bar at 1 m depth during periods of relatively low waves; under large waves sonar performance degraded significantly due to bubbles and suspended sediment in the water column. Sonar data were integrated with bottom-profile information acquired using seven 1 MHz sonic altimeters mounted on the CRAB. The altimeters sampled seafloor elevations at 48 Hz and have a vertical resolution of about 5 cm. Frequency-domain analysis of sonar images and bathymetric data is used to establish a correspondence between variations in sidescan image intensity and variations in bathymetry profiles. Spectral peaks in the sonar images and bathymetric data are highly correlated across a wide range of significant nearshore length scales, in particular, for wavenumbers corresponding to megaripples. The standard deviation of pixel intensities in the sonar image is calculated in a small region around a point, and is directly related to a measure of roughness from the altimeter data. Bedform orientation is easily extracted from the sonar imagery, and can be used to generate maps of orientation-dependent roughness instead of the usual description of bed roughness as a scalar field.
Supported by the Coastal Dynamics Program of the Office of Naval Research.