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Research

How Often Do Sediments on the Seafloor Move?



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U.S. Geological Survey (USGS) scientists Soupy Dalyander and Brad Butman have developed a method for characterizing bottom shear stress—the force created at the seabed by currents and waves—to address the question “How often do seafloor sediments move?” Working out of the USGS Woods Hole Coastal and Marine Science Center in Woods Hole, Massachusetts, Dalyander (Mendenhall postdoctoral research fellow) and Butman (oceanographer) have applied the method to an initial study area of the U.S. east coast’s Middle Atlantic Bight. Their work is part of a USGS-wide effort to provide scientific data to inform coastal and marine spatial planning (http://www.whitehouse.gov/administration/eop/oceans/cmsp/).

Currents and waves cause stress on the seafloor that can affect sediments, structures, plants, and animals, as well as human use.
Above: Currents and waves cause stress on the seafloor that can affect sediments, structures, plants, and animals, as well as human use. Large waves, especially those with long periods, can affect the seafloor across the entire continental shelf; some of these waves may be caused by storms far offshore. Currents are caused primarily by tides and winds, especially in association with large storms. [larger version]

Shear stress is the force created at the seabed by tidal currents, wind-driven currents, and the orbital motion of surface waves (see diagram). If strong enough, this shear stress resuspends sediment and other particles from the seafloor. Shear stress affects the geology and habitat of the seafloor through its influence on sediment texture. For example, sandy sediments are typically found in areas of high stress and muddy sediments in areas of low stress. Shear stress also influences habitat in other ways. For example, filter feeders—such as scallops and oysters—must have an energetic environment for food to be in suspension, but the stress environment must also have periods of sufficiently low stress to allow the planktonic phase of these animals (a free-drifting stage they go through before attaching to the seafloor or some other surface) to settle for successful colonization. The shear stress environment also affects humans’ use of the seafloor. For example, excess shear stress can result in scour around the foundations of offshore structures, such as wind turbines; and shear stress may resuspend material disposed in the coastal ocean, including any associated contaminants.

In small enough areas, measurements of waves and currents can be used to calculate bottom shear stress. But Dalyander and Butman are looking at the entire continental shelf, across which long-term measurements of waves and currents are not available, and so they estimate bottom shear stress by using data from numerical models. In the Middle Atlantic Bight, the current data are provided by the Experimental System for Predicting Shelf and Slope Optics (ESPreSSO) Regional Ocean Modeling System (ROMS) nowcast/forecast circulation model developed by John Wilkin at Rutgers University and operated as part of the U.S. Integrated Ocean Observing System (IOOS). Wave data are provided by a Simulating WAves Nearshore (SWAN) model hindcast developed for this study.

Estimates of stress based on modeled waves and currents provide a “time series” (a sequence of data points at uniform time intervals, in this case every hour) of bottom shear stress at a spatial resolution of approximately one location per every 25-square-kilometer area over the continental shelf over a 1-year period. For this information to be easily used by planners and engineers in determining habitat types or implications for human use, it is displayed on maps of seasonal and spatial patterns that show areas of high and low stress (for example, see map below). Comparison of these patterns with data in the USGS East-Coast Sediment Texture Database reveals the influence of shear stress on sediment texture. Finer grained sediments (such as mud) are located in areas of weaker stress, and coarser sediments (sand to gravel) are located in areas of stronger stress, as a result of finer material being resuspended and swept away in areas of high stress.

Combined wave-current bottom stress, in Pascals (Pa, a unit of pressure), that was exceeded 5 percent of the time during winter (December 2010–February 2011)
Above: Combined wave-current bottom stress, in Pascals (Pa, a unit of pressure), that was exceeded 5 percent of the time during winter (December 2010–February 2011). The strongest stresses (red) are nearshore and over Nantucket Shoals. Weaker stresses occur over the outer shelf, in an area southwest of Cape Cod, and in the Hudson Shelf Valley. [larger version]

Determining the processes (waves versus different types of currents) that cause bottom shear stress provides insight into the frequency of stress events, to which organisms must adapt, and the ways in which an evolving climate might change the bottom shear stress distribution. Estimates of stress induced by tidal currents, storm-driven currents, and waves are compared to the total wave-plus-current stress to determine what processes dominate the generation of bottom shear stress on a regional basis. For example, over Nantucket Shoals, southeast of Cape Cod, Massachusetts, bottom shear stress is tidally dominated, resulting in a high-stress environment with frequent significant stress events. In contrast, waves created by storms—which are not as frequent as tides—dominate stress generation in a band along the coast throughout the Middle Atlantic Bight.

In order for sediment on the seafloor to move, a grain-size-specific stress value, called the “critical stress,” must be exceeded. Critical stress values are established at points where sediment texture observations are available from the USGS East Coast Sediment Texture Database. These critical stress values are then compared to modeled stress at the closest model point to determine when sediment will be mobilized (see map below). Sediment moves every tidal cycle over Nantucket Shoals, whereas mobility in the rest of the Middle Atlantic Bight occurs as a result of storm forcing and at frequencies governed by storm frequency and water depth, with sediment moving more frequently at shallower depths where the influence of surface waves is stronger.

Percentage of time the critical stress for each sediment observation (dots) is exceeded for winter (December 2010–February 2011).
Above: Percentage of time the critical stress for each sediment observation (dots) is exceeded for winter (December 2010–February 2011). Sediments move more than 20 percent of the time in a band along the coast and over Nantucket Shoals. Sediments are less mobile over the outer shelf, in an area of fine-grained sediments southwest of Cape Cod, and in the topographic low of the Hudson Shelf Valley. [larger version]

Dalyander and Butman’s work to date has produced a way to quantify the spatial and seasonal distribution of seafloor shear stress and sediment mobility and to determine the physical processes that generate stress and cause sediment movement. Areas of frequent sediment mobility have been identified. The next step in this study is to expand the analysis to cover the rest of the U.S. east coast and the Gulf of Mexico by using output from other IOOS regional models. The resulting regional stress characterizations will be incorporated into a USGS Seafloor Stress and Mobility Database for access by other researchers and the coastal and marine spatial-planning community.

 

Related Web Sites
USGS East-Coast Sediment Texture Database
USGS
Coastal and Marine Spatial Planning
National Oceans Council
U.S. Integrated Ocean Observing System (IOOS)
collaboration among federal and regional agencies

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Fieldwork
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Seabird and Mammal Surveys Off U.S. West Coast

Research
Maps Based on Satellite Telemetry Help Tanker Avoid Sea Ducks

Declines in Everglades Mammals Linked to Pythons

How Often Do Sediments on the Seafloor Move?

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Monterey Bay Marine GIS Users Meeting

Staff Ph.D. Student Researching Marine Mineral Deposits

Dutch Student Visiting USGS in California

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