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Quantifying How Often Sediments Move on the U.S. Continental Shelf—the U.S. Geological Survey Sea Floor Stress and Sediment Mobility Database



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Tidal currents, wind-driven currents, and ocean waves all create a force at the seabed called bottom shear stress. This stress, if sufficiently strong, can cause sediment to be resuspended from the seafloor (see “How Often Do Sediments on the Seafloor Move?” in Sound Waves, March/April 2012). Information on the strength and variability of bottom shear stress, and the consequent frequency and intensity of sediment mobility events, is of interest to geologists and others because of implications for seafloor geology, benthic habitats, and human use, such as installation of offshore wind turbines where near-bottom forces may induce scour around structures and cables.

The U.S. Geological Survey Sea Floor Stress and Sediment Mobility Database has been established (http://pubs.usgs.gov/of/2012/1137/), and data for the Middle Atlantic Bight—the continental-shelf area between Cape Hatteras and Cape Cod—are now online (http://woodshole.er.usgs.gov/project-pages/mobility/).

The database provides spatially and seasonally resolved statistics on bottom shear stress estimated at an approximately 5-kilometer scale using numerical models. Statistics on sediment mobility, reported as the percentage of time that bottom shear stress is sufficient to move sediments, are also provided where data on the grain size of the seafloor sediment are available.

The statistical representations of bottom shear stress and sediment mobility are available for download as ArcGIS polygon and point shapefiles. Statistics for the South Atlantic Bight, the Gulf of Mexico, and the Gulf of Maine will be added to the database as analysis is completed.

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Above Left: Map showing location of the four study areas included in the U.S. Geological Survey Sea Floor Stress and Sediment Mobility Database (http://woodshole.er.usgs.gov/project-pages/mobility/). Statistics for the Middle Atlantic Bight are currently available; data for the other regions will be added in late 2012 and early 2013 as analysis is completed. Users can download zip files containing ArcGIS shapefiles and associated metadata in polygon format (for characterizations of bottom shear stress) and point format (for characterizations of sediment mobility). Statistics for bottom shear stress include spatially variant, yearly and seasonal estimates of the median, representing the midpoint in the range of values; the interpercentile range, characterizing variability; and the 95th percentile, representing the shear-stress value that is exceeded 5 percent of the time. Statistics for sediment mobility include the percentage of time bottom sediment is subject to being mobilized and the recurrence interval of mobility events by season and year. [larger version]

Above Right: Sample of the data available for download in ArcGIS format from the U.S. Geological Survey Sea Floor Stress and Sediment Mobility Database. Shown is the 95th percentile (an estimate of the most extreme values, exceeded only 5 percent of the time) of combined wave- and current-induced bottom shear stress in pascals (Pa) for the Middle Atlantic Bight. The analysis delineates regions of high stress, shown in reds and yellows and commonly found in coastal areas owing to the stronger influence of waves at shallower depths; and more quiescent regions, shown in blue and found at greater depths extending to the edge of the continental shelf. [larger version]


Related Sound Waves Stories
How Often Do Sediments on the Seafloor Move?
Mar. / Apr. 2012

Related Web Sites
U.S. Geological Survey Sea Floor Stress and Sediment Mobility Database
USGS
Documentation of the U.S. Geological Survey Sea Floor Stress and Sediment Mobility Database
USGS

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Fieldwork
cover story:
USGS Scientists Exploring Mars

Topographic Maps Help Curiosity Navigate Mars

Methane Seep off San Diego, California

Research
Sea-Level Rise Accelerating on U.S. Atlantic Coast

Hawaiian Seabirds Vulnerable to Sea-Level Rise

Corals Damaged by Deepwater Horizon Oil Spill

Gulf Coast Vulnerable to Erosion During Category 1 Hurricanes

Outreach
Sanctuary Exploration Center Opens in Santa Cruz, California

Meetings
U.S. Extended Continental Shelf Project Holds Workshop

Biannual Meeting of the Monterey Bay Marine GIS User Group

Staff Coastal and Marine Geology Program Participates in Federal Food Drive

Publications Sea Floor Stress and Sediment Mobility Database

Sept. / Oct. 2012 Publications

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