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Open-File Report 2015-1149


Sea-Floor Morphology and Sedimentary Environments in Southern Narragansett Bay, Rhode Island


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Click on figures for larger images
Thumbnail image of figure 7 and link to larger figure. Map of the bathymetry in the study area.
Figure 7. Hill-shaded multibeam bathymetry image of National Oceanic and Atmospheric Administration survey H12324.
Thumbnail image of figure 8 and link to larger figure. Map showing features interpreted in the study area.
Figure 8. Interpretation of the sea floor in the National Oceanic and Atmospheric Administration survey H12324 study area. Features include rocky areas, scour depressions, sand waves and megaripples, and shipwrecks.
Thumbnail image of figure 9 and link to larger figure. Map showing figure locations in study area.
Figure 9. Map of the study area showing locations of the figures with detailed bathymetric imagery.
Thumbnail image of figure 10 and link to larger figure. Bathymetric image of rocky areas in the study area
Figure 10. Detailed bathymetric image of a rocky area of the sea floor south of Aquidneck Island.
Thumbnail image of figure 11 and link to larger figure. Image of scour depressions in the study area.
Figure 11. Detailed bathymetric image of scour depressions south of Conanicut Island and exposed bedrock near the coast.
Thumbnail image of figure 12 and link to larger figure. An image showing scour depressions on the sea floor.
Figure 12. Detailed bathymetric image of scour depressions in West Passage.
Thumbnail image of figure 13 and link to larger figure. An image of sand waves on the sea floor.
Figure 13. Detailed bathymetric image of sand waves located in the deep channel west of Aquidneck Island.
Thumbnail image of figure 14 and link to larger figure. Image of a shipwreck on the sea floor.
Figure 14. Detailed bathymetric image of a shipwreck south of Aquidneck Island.

Bathymetry

Water depths in the study area range from about 1 m along shorelines to 62 m in a channel off Ragged Point on the southwestern part of Aquidneck Island (fig. 7). Several features are present on the sea floor in the study area including rocky areas, scour depressions, sand waves and megaripples, and shipwrecks (fig. 8). Much of the sea floor lacks these geologic features and has a smooth appearance within the resolution of the bathymetry data. These featureless areas are predominantly in the middle of West Passage and in moderate depths of East Passage (approximately 10 to 30 m water depth).

Rocky areas of the sea floor tend to be along the coast and in the eastern part of the study area where bedrock and boulders cover much of the sea floor (figs. 9 and 10). Rocky patches were also found south of Conanicut Island in the middle of the approach to East Passage. Bedding planes in the bedrock are visible in the bathymetric imagery (fig. 11) and have strikes similar to those mapped onshore (Jamestown Visitor Information, 2003), indicating the rocky areas are likely underwater exposures of bedrock.

Several areas of scoured sea floor are visible in the bathymetric data. The deeper parts of East Passage have been scoured by strong tidal currents that flow into and out of Narragansett Bay (Kincaid and others, 2003; White and White, 2008). Patches of depressions are visible near exposed bedrock and boulders along some coastlines, especially in the eastern part of the study area, south of Aquidneck Island (fig. 10). These depressions are about 0.5 m deep, and have flat floors and steep sides. They are similar to other scour depressions observed in Block Island and Rhode Island Sounds formed from the interaction with the sea floor of wave setup and return flow, wave and tidal currents flowing through constricted areas, and storm-wave-induced currents (McMullen, Poppe, Twomey, and others, 2007; Oakley and others, 2009; McMullen and others, 2011; Poppe and others, 2011; McMullen and others, 2012; Poppe and others, 2012; McMullen and others, 2013; McMullen and others, 2015). The scour depressions adjacent to the southeastern coast of Conanicut Island trend normal to the shore (fig. 11) and are likely similar to those discussed by Oakley and others (2009), which are thought to be formed and maintained by downwelling of return flow from storm waves. Most of the scour depressions south of Aquidneck Island form larger patches without a dominant alignment (fig. 10). They likely formed from, and continue to be maintained by, complex wave and tidal currents flowing around isolated rocky bathymetric highs. Several of the scour depressions in West Passage (fig. 12), located away from the shoreline, may have been formed by storm-wave-induced currents scouring the sea floor, a process discussed in McMullen and others (2015).

Patches of sand waves and megaripples can be found mostly in East Passage but also near the coast in West Passage and south of Aquidneck Island. Sand waves demonstrating the most substantial relief (up to 5 m in height) are located in the deepest part of the study area, where tidal currents are strong (fig. 13). Many of the other bedforms in the study area are megaripples, which are in areas with more moderate wave and tidal energy.

In addition to geologic features, anthropogenic features can also be seen in the bathymetric data. Two shipwrecks are visible in the study area, one south of Aquidneck Island where an 18-m stern trawler lies (fig. 14), and one in West Passage where another wreck occurred (National Oceanic and Atmospheric Administration, 2011b).

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