INSIGHTS INTO INNER CONTINENTAL SHELF SEDIMENTARY PROCESSES THROUGH REPEATED HIGH-RESOLUTION MULTIBEAM SURVEYS

An EM-3000 high-resolution multibeam echosounder has been used as part of investigations of sediment transport and benthic habitat in a number of inner-shelf and estuarine settings. High-resolution multibeam topographic data provides important information on the nature and distribution of sedimentary bed forms while co-registered backscatter data provides insights on sediment type and the presence of smaller-scale features. Ground truth for many studies has come from sediment samples as well as from penetrometers, and stereo photography. In several of these studies, there have been opportunities for repeat surveys over time intervals of a few months to several years that allow the identification of natural and man-made changes in sea-floor morphology. Repeated surveys have been conducted on the inner continental shelf at Shinnecock Inlet and ebb-tidal delta (NY), off Rockaway (NY), at LEO-15 (NJ), and at Humboldt (CA) and in estuarine environments at the Hudson River, Jamaica Bay, Long Island Sound and Port Jefferson Harbor (NY).

Observed anthropogenic changes include anchor drag marks, surf clam dredge grooves, fish net drag marks and other features. Observed natural changes include sand-wave migration, migration of coarse-grained depressions, the development of biogenic mounds, and both large and small water-depth changes. Depth changes up to several meters are observed at Shinnecock Inlet, occurring over intervals as short as three months. These depth changes are often correlated with changes in backscatter suggesting that there are changes in either surface morphology (such as bed forms) or sediment grain size. Studies off Rockaway suggest more active sediment transport in January than in June with well-defined sand waves and apparent localized scour in January. The orientation of sand waves and scour lineations suggest on-shore sediment transport.

These observations demonstrate that large-scale temporal variability can be an important characteristic of many near-shore environments, and that repeated high-resolution surveys can help to characterize this change and help understand sediment processes and sedimentary activity over large areas.