Proceedings of the U.S. Geological Survey (USGS) Sediment Workshop, February 4-7, 1997

HIGH-RESOLUTION GEOLOGIC MAPPING OF THE COASTAL OCEAN: APPLICATION TO COASTAL AND SEA FLOOR MANAGEMENT

Schwab, William, C.,
Butman, Bradford,
Buchholtz ten Brink, Marilyn, and
Danforth, William W.,
U.S. Geological Survey, Marine and Coastal Geology, 384 Woods Hole Road,
Woods Hole, MA 02543-1598

Improvements in remote sensing techniques (sidescan sonar, multibeam bathymetry, and high-resolution seismic-reflection profiling) and image processing now allow detailed, rapid mapping of relatively large areas of the sea floor. The 100% coverage digital sidescan and bathymetric images, when combined with subbottom profiles and ground-truth from sampling and visual observations, provide a new, unprecedented, highly detailed view of the sea floor and shallow subsurface that shows variability of sedimentary features and environments over a wide range of spatial scales. The surveys provide a framework for addressing science and management issues in the coastal ocean and can be used to: (1) provide a framework for understanding the distribution of contaminants in sediments, including sites of accumulation; (2) identify sites and extent of anthropogenic impact, such as from bottom trawling, ocean disposal, or dredging; (3) locate deposits of sand and gravel for potential beach replenishment and/or aggregate resources, (4) identify the distribution of sea-floor environments and associated biological habitats; and (5) document long-term change in sedimentary environments. Results from the mapping efforts can also be used to develop predictive models for sediment-related processes, which will guide habitat and resource management, monitoring strategies, and other research studies. The techniques can be used in oceans, rivers and lakes from water depths of about 5 m or greater to map and characterize the bottom.

The US Geological Survey and collaborators are presently undertaking a national effort to map and characterize the sea-floor geology in selected areas of the inner shelf and coastal zone of the United States (Fig. 1). Areas surveyed or presently being surveyed along the U.S. Atlantic margin and the Gulf of Mexico include Massachusetts Bay, the Stellwagen Bank National Marine Sanctuary, Block Island Sound, Long Island Sound, the New York Bight, segments of the New York, New Jersey, North Carolina, and South Carolina coasts, the Grey's Reef National Marine Sanctuary (off Georgia), Oculina Bank off Florida, parts of the insular shelf around Puerto Rico, and the inner shelf off Sarasota, Florida. Selected areas in the Great Lakes and along the west coast are also being mapped as part of the Coastal and Marine Geology Program. Study sites are selected to provide geologic information around major metropolitan centers, to support process-oriented studies in the broad themes of Environment, Hazards, and Resources, and to respond to state and Federal agency information needs.

One example of the application of these techniques is the extensive mapping currently being conducted on the inner shelf and coastal areas off the New York-New Jersey Metropolitan area. Understanding the shallow geology and sediment transport pathways in this area is essential for resource and environmental management. The present investigation, conducted cooperatively with the US Army Corps of Engineers (USACOE), was motivated by the need to develop an environmentally acceptable solution for the disposal of dredged material from the ports of New York and New Jersey, by the need to identify potential sources of sand for nourishment of the southern shore of Long Island, and by the opportunity to develop a better understanding of the transport and long-term fate of modern sediments and contaminants.

Approximately 125 million cubic-meters of sewage sludge from the NY-NJ metropolitan area was disposed between 1923 and 1987 at a site 20 km offshore near the head of the Hudson Shelf Valley. Recent results have shown that some sewage sludge material has been transported more than 25 km south from the disposal site into the Hudson Shelf Valley where it is deposited in a series of bathymetric lows, reworked by biologic organisms, and occasionally resuspended during storm events. In addition, sewage sludge remaining in the original disposal site has been reworked and, in places partially buried by fine-grained sand.

At present, approximately 5 million cubic meters of material needs to be disposed of each year to maintain the port of New York and New Jersey which serves a regional market of about 15 million people. Currently, 14-33% of dredged material is classified as contaminated and therefore not suitable for unrestricted ocean disposal. The USACOE is developing a management plan for both uncontaminated and contaminated dredged materials. One option investigated is disposal offshore in new artificial islands or dredged pits. Developing this plan requires information on the surficial geology, shallow stratigraphy, and sediment transport patterns. Sediment transport is complex in this region and is affected by the change in shoreline orientation (which affects littoral transport), the relatively shallow water of most of the study area (25 m), and offshore bathymetric features such as the Hudson Shelf Valley and outcropping (Upper Cretaceous - early Tertiary) Coastal Plain strata which partially define the sources, transport paths, and sinks of sediment.

Erosion of the barrier islands along the south shore of Long Island, New York, has led the USACOE to begin long-term planning for potential nourishment of selected regions of the shoreline. Such activities require an understanding of sources and the transport of sand on the shoreface and inner shelf region. In the nearshore area off Long Beach, Long Island, and off Sandy Hook, New Jersey, there are sharply defined, linear, features that are oriented oblique to the shoreline and are interpreted to be scour depressions. These features are < 1 m deep and are 40 to 250 m wide and are floored with straight crested, symmetrical rippled, sandy gravel and gravelly sand. Inter-depression areas are blanketed by fine sand. Exactly how these features form is unknown, but they appear to be an erosional features indicative of cross-shelf sediment-transport processes from the shoreface. Such transport offshore could significantly influence predictions used in developing strategies for management of coastal areas. Farther east, along the southern shore of Long island, mapping activities on the shoreface and inner shelf reveal an extensive series of shore face-connected ridges composed of fine- to medium-grained sand which are a potential source for beach nourishment.

High-resolution sea-floor mapping is providing a fundamentally new understanding of shelf and coastal environments. The mapping provides a regional overview of geologic conditions and processes and detailed information on the spatial variability of sea-floor environments; the essential framework for environmental, hazard, and resources studies. Potential initiatives utilizing this new technology to map the sea floor include:

* Multi-disciplinary studies of critical habitats, waste disposal, sand and aggregate resources, coastal change, and other issues that require a regional understanding of the sea-floor environment and processes.

* A national effort to systematically map the shallow Exclusive Economic Zone and the adjacent coastal region, providing digital base maps, suitable for incorporation into GIS, to guide science and management activities of state, local, and other federal agencies.

* International efforts to manage coastal growth and change, and to address environmental hazard and resource issues.

Autobiography

Schwab, William C., U.S. Geological Survey, Woods Hole, MA: Currently serves in the Geologic Division, Marine and Coastal Geology Team, as a geophysicist and supervisor of the Marine Technology Group of the Woods Hole Center. Responsibilities include planning, direction, and supervision of all aspects of equipment development and technical support of sea-floor mapping investigations. Experience during 20 years with the USGS includes data-acquisition and research in marine geology, slope stability, sediment transport, marine geotechnical engineering, and instrumentation.
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