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Figure 12. Hill-shaded multibeam bathymetry from National Oceanic and Atmospheric Administration survey H11446.

Figure 13. Interpretation of sea-floor features in the multibeam bathymetry and sidescan-sonar imagery from National Oceanic and Atmospheric Administration survey H11446.

Figure 14. Detailed multibeam bathymetry in the western part of the study area showing sand waves.

Figure 15. Detailed multibeam bathymetry image from the north central part of the study area showing sand waves with branching crests and boulders with scour marks.

Figure 16. Sand-wave profiles of waves in the study area.

The bathymetry in the study area consists of smooth sea floor that deepens gradually offshore from 2 m to more than 100 m and contains areas of sand waves and boulders (figs. 12 and 13). Although most of the study area is less than 40 m deep, a large scour depression with the study area's maximum depth of 111 m is located in the northeastern corner of the study area, north of Orient Point, at the northern entrance to Plum Gut. This scour depression was eroded by the strong, oscillating tidal currents that flow through the gap between Plum Island and Orient Point.

A large area of sand waves is located in the eastern half of the study area and patches of sand waves are located in the western section. Most sand waves in the study area have wavelengths of about 10 to 20 m and north-south-oriented crests (fig. 14). A bank of sand waves on the northern edge of the study area has branching crests and wavelengths of 20 m to more than 100 m (fig. 15). This sand-wave bank extends into the adjacent survey area to the north in eastern Long Island Sound (Poppe and others, 2011). Asymmetry in the sand-wave profiles in the western and northern parts of the study area indicates sediment transport to the west, and in the eastern part of the study area sediment transport is to the east (fig. 16).

Bouldery areas in the study area are located in a band reaching from the northwestern to the southeastern part of the study area, along much of the shoreline, and in the large scour depression at the mouth of Plum Gut (fig. 13). The sea floor in most of these areas has a hummocky appearance (fig. 14). The boulders, reaching more than 5 m wide, are lag deposits, remnants of the Roanoke Point-Orient Point-Fishers Island moraine system that are found in nearby study areas offshore of Plum Island and Rocky Point (Sirkin, 1980; McMullen and others, 2010; Poppe and others, 2010). Asymmetry of scour marks around boulders indicates net easterly transport at the eastern end of the study area and westerly net transport in the central region (fig. 15).