RIPPLED SCOUR DEPRESSIONS: INSIGHTS FROM MULTIBEAM SONAR DATA

High-frequency multibeam sonar is used to rapidly acquire high-resolution bathymetry and co-registered backscatter intensity data that reveal the signatures of active processes in sedimentary environments. By studying patterns in seabed morphology and texture we gain new insights about the nature and significance of seabed features. Rippled Scour Depressions (RSDs) are common features have been identified in a variety of sedimentary environments and can be better understood through the use of multibeam sonar technology. RSDs are usually linear shore-normal depressions that are evident as distinct acoustic backscatter features associated with coarse sediments (typically gravel) formed into shore-parallel oscillation ripples.

Simrad EM 3000 multibeam sonar data were used to investigate a number of RSDs in a range of inner shelf environments. Since the estimated vertical accuracy of bathymetric soundings collected with this system is on the order of 5 to 10 cm RMS, we can resolve the detailed morphologic characteristics of RSDs associated with acoustic backscatter features. While the detailed information revealed in multibeam data are important in understanding the creation and evolution of RSDs, the large-scale bathymetric patterns associated with them also provide insight about their nature and non-local significance.

Many RSDs are asymmetrically superimposed on large-scale bathymetric undulations that indicate the importance of a process that is active over a larger area than is indicated by the RSDs alone. Based on bathymetric change and physical oceanographic data these undulations are related to longshore tidal currents that can be intensified by bathymetry and the presence of obstructions. Although some RSDs can be stable over time, others can be created and destroyed on time scales of less than 1 year. Multibeam data suggest that RSDs exist in areas in which sandy surface sediments are preferentially removed revealing underlying gravel. Once gravels are exposed, they become mobile and are formed into oscillatory ripples by wave orbital motion. Gravels can also be transported indicating that RSDs are not only windows in the sandy sediment that reveal underlying gravel, but can be active features that promote the redistribution of both sands and gravels in nearshore environments.