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Figure 19. Digital terrain model of the sea floor produced from the multibeam bathymetry collected during National Oceanic and Atmospheric Administration survey H12013, gridded to 2 m and merged with the interpolated bathymetry data to give a more continuous perspective.

Figure 20. Interpretation of the digital terrain model from National Oceanic and Atmospheric Administration survey H12013.

Figure 21. Locations of detailed planar views of the digital terrain model and profiles of bedform symmetry.

Figure 22. Detailed planar view of the multibeam bathymetric data from National Oceanic and Atmospheric Administration survey H12013 off Hatchett Point.

Figure 23. Detailed planar view of the multibeam bathymetric data from National Oceanic and Atmospheric Administration survey H12013 showing bedrock outcrops south of Hatchett Reef.

Figure 24. Detailed planar view of the multibeam bathymetric data from National Oceanic and Atmospheric Administration survey H12013 showing Hatchett Reef.

Figure 25. Detailed planar view of the multibeam bathymetric data from National Oceanic and Atmospheric Administration survey H12013 showing part of the sand-wave field along the southern flank of Long Sand Shoal.

Figure 26. Cross-sectional views of sand waves produced from bathymetric data collected during National Oceanic and Atmospheric Administration survey H12013.

Figure 27. Detailed planar view of the multibeam bathymetric data from National Oceanic and Atmospheric Administration survey H12013 showing part of the sand-wave field along the southwestern edge of the study area.

Figure 28. Detailed planar view of the multibeam bathymetric data from National Oceanic and Atmospheric Administration survey H12013 showing the field of megaripples north of Long Sand Shoal.

Figure 29. Cross-sectional views of sand waves produced from bathymetric data collected during National Oceanic and Atmospheric Administration survey H12013.

Figure 30. Detailed planar view of the multibeam bathymetric data from National Oceanic and Atmospheric Administration survey H12013 showing the relatively flat character of the sea floor in the lower energy, more protected area northeast of Hatchett Point.

Surveyed depths within the study area range from about 1 to 38 m (fig. 19). The shallowest parts of the study area occur along the Connecticut shoreline off Hatchett Point, on Hatchett Reef, and on the eastern part of Long Sand Shoal near the mouth of the Connecticut River. The deepest parts of the study area occur along its southern, offshore edge. Also, an elongate, isolated depression parallels the shoreline, extending eastward and westward from the passage between the mainland and Hatchett Reef. Many sea-floor features visible in the DTM can be geologically interpreted and ongoing sedimentary processes can be identified because they are morphologically distinct (fig. 20).

Rocky Areas

Rocky areas within the study area are variably composed of bedrock outcrops and boulder fields (fig. 20). Bedrock outcrops are common off Hatchett Point (figs. 20, 21, and 22) and in the deeper areas southwest and southeast of Hatchett Reef (fig. 23). Most of the outcrops south of Hatchett Reef are slightly elongate, trending north-northwest to south-southeast. These trends are similar to those of glacially smoothed bedrock ridges onshore, which parallel the interpreted direction of ice movement as shown by grooves and striations (Goldsmith, 1962). Many small, individual, rounded bathymetric highs are also concentrated along the shoreline off Hatchett Point (fig. 22) and around and on top of Hatchett Reef (fig. 24). These features, which give the sea floor in these areas a rough appearance, are interpreted to be boulders. Although the boulders average 1 to 3 m in width, many exceed 10 m across. These bouldery areas are lag deposits remaining from winnowed Pleistocene deposits and, together with the bedrock trends, show that the effects of glaciation continue offshore under Long Island Sound. Elsewhere, rocky areas within the study area are also scattered along the Connecticut shoreline and in a band trending southeastward off Lynde Point.

Scour

Scour provides evidence of ongoing erosion by the strong, oscillating tidal currents. For example, swirling currents develop around bedrock outcrops and boulders that protrude through surficial sediments. Turbulence causes increased flow velocity sufficient to scour the sea floor around and downstream from these obstructions and to produce sharp-pointed depressions with coarse-sediment floors called obstacle marks, also known as comet marks (Werner and Newton, 1975). Good examples of these bedforms are present on the western side of the bedrock outcrops south of Hatchett Reef (fig. 23) and around boulders in the depression south of Hatchett Point (fig. 22). Asymmetry of the scour around these boulders reflects the stronger flood tide (White and White, 2012) and indicates that net transport is predominantly toward the west and farther into Long Island Sound (fig. 21; Werner and Newton, 1975; Reineck and Singh, 1980). Tidal currents, enhanced from constricted flow, have also produced the elongate east-west-trending scour depression between Hatchett Point and Hatchett Reef (fig. 22).

Sand Waves and Megaripples

Alternating narrow, elongate bathymetric highs and lows reveal the crests and troughs of adjacent sand waves and megaripples (fig. 20). The sand waves, which cover approximately 9 percent of the study area, occur in three fields. One field covers the southern flank and eastern end of Long Sand Shoal; two fields lie along the southwestern edge of the study area. Where sediment supply is abundant, most of the sand waves exhibit transverse morphologies, such as those associated with Long Sand Shoal (fig. 25). The largest of these transverse bedforms exceed 3 m in crest to trough relief and 120 m in wavelength (fig. 26). Bedforms characterized by barchanoid morphologies are less common but occur in the two fields along the southwestern edge of the study area where sediment supply is more limited, as evidenced by nearby gravel pavements and scour features (fig. 27). The largest of the barchanoid waves exceeds 10 m in crest to trough relief and 200 m in wavelength (fig. 26).

Megaripples, which cover approximately 44 percent of the study area, occur in two east-northeast-trending swaths across its central and southern parts (fig. 20). As with the sand waves, megaripples with transverse morphologies are prevalent where sediment supply is abundant, such as in the field north of Long Sand Shoal and on the shoal's northern flank (fig. 28); barchanoid morphologies dominate where the sediment supply is limited. The presence of megaripples on the stoss slopes of the sand waves (fig. 26) suggests that this transport is active and that the sand waves are propagating under the present hydraulic regime (Dalrymple and others, 1978; Reineck and Singh, 1980; Allen, 1982).

Asymmetry of the sand waves and megaripples can be used to interpret directions of net sediment transport (figs. 21, 26, and 29). For example, the stoss slopes of sand waves are oriented upstream, and the slip faces downstream in the transport regime (Allen, 1968; Ludwick, 1972; Reineck and Singh, 1980). Similarly, the horns and concave sides of barchanoid sand waves and megaripples are oriented downstream in the transport regime (McKee, 1966). Where sand waves, megaripples, and obstacle marks are present in the study area, orientation and asymmetry of these bedforms indicate that net sediment transport is toward the southwest throughout most of the study area. Exceptions to this observation include the area north of Long Sand Shoal, where bathymetric profiles show that the megaripples tend to be symmetrical, and on the shoal itself, where bathymetric profiles show that megaripples are migrating up the flanks of the shoal and perhaps providing a mechanism for the maintenance of the shoal (fig. 29).

The numerous sand waves and obstacle marks reflect the action of strong bottom currents in this part of the sound. Average maximum near-bottom tidal currents near the eastern end of Long Sand Shoal are flood-dominated and can exceed 40 cm/s (Signell and others, 2000). These current velocities, which correspond to the traction threshold velocities for very coarse sand (Nevin, 1946), are strong enough to initiate entrainment and transport and explain the observed sizes, morphologies, and distributions of the bedforms observed in the study area (Belderson and others, 1982).

Although current ripples may reverse their orientation during semidiurnal tidal cycles, because of the small volume of the ripples, no reversal in asymmetry of either the sand-wave or megaripple morphology was observed in the multibeam data of adjacent lines collected at different tidal stages. Whether this tidal independence of large bedform asymmetry is due to the spatial distribution of residual currents, or to an asymmetry of current velocities is also uncertain, but it does suggest that the larger bedforms are more stable and that they move over longer time scales. While the sand-wave and obstacle-mark asymmetry in the bathymetric data reveal direction of transport, no information on the rate of advance is supplied.

Areas of Relatively Flat Sea Floor

Some of the sea floor within the study area has a relatively smooth featureless appearance at the 2-m grid size of the DTM. Places with this appearance include (1) areas protected from the strong tidal currents, such as along the Connecticut shoreline, in the northeastern corner of the study area, and in elongate bands extending east and west from Hatchett Reef; and (2) the southeastern corner of the study area where the supply of sand-sized sediment is limited and the sea floor is armored by gravel (figs. 21 and 30). Features visible on the DTM within these relatively flat areas are generally artifacts produced by the equipment used and oceanographic conditions prevalent during multibeam acquisition (figs. 22, 24, and 30). These acquisition-related artifacts are made more conspicuous by the sun-illumination and fivefold vertical exaggeration of the imagery.