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Open-File Report 2014–1012

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### Combined Multibeam Bathymetry Data from Rhode Island Sound and Block Island Sound – A Regional Perspective 

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## Bathymetry

| Title PageList of FiguresConversion FactorsAbstractIntroductionMethods**Bathymetry**GIS Data CatalogSummaryAcknowledgmentsReferencesContactsIndex | Click on figures for larger images**Figure 1.** Locations of the surveys in Rhode Island and Block  Island Sounds with multibeam echosounder data that have been combined in this  report.**Figure 9.** Digital  terrain model of the sea floor produced from the combined multibeam  bathymetry collected during National Oceanic and Atmospheric Administration surveys H11922, H11995, H11996, H12009, H12010, H12011, H12015, H12023,  H12033, H12137, H12139, H12296, H12298, and H12299 in Rhode Island and Block  Island Sounds.**Figure 10.**Map, modified from Stone and Borns and  Gustavson and Boothroyd, showing locations of end moraines in southern New England and on Long Island, New York.**Figure 11.**Locations  of detailed planar views of the 4-meter digital terrain model  produced from the contiguous National Oceanographic and Atmospheric Administration multibeam surveys in Rhode Island and Block Island Sounds.**Figure 12.**Detailed  planar view of the sun-illuminated bathymetry from the combined National  Oceanic and Atmospheric Administration surveys in Rhode Island and Block Island  Sounds showing the bouldery sea floor near southwestern Block Island.**Figure 13.** Detailed  planar view of the sun-illuminated bathymetry from the combined National  Oceanic and Atmospheric Administration surveys showing the bouldery sea  floor south of Point Judith.**Figure 14.**Detailed  planar view of the sun-illuminated bathymetry from the combined National  Oceanic and Atmospheric Administration surveys in Rhode Island and Block Island  Sounds showing a deep isolated scour depression in a channel south of the  eastern end of Fishers Island.**Figure 15.**Detailed  planar view of the sun-illuminated bathymetry from the combined National  Oceanic and Atmospheric Administration surveys in Rhode Island and Block Island  Sounds showing obstacle marks in southwestern Block Island Sound.**Figure 16.**Detailed  planar view of the sun-illuminated bathymetry from the combined National  Oceanic and Atmospheric Administration surveys in Rhode Island and Block Island  Sounds showing storm-wave-induced scour depressions on a seaward-facing  slope in eastern Rhode Island Sound.**Figure 17.**Detailed  planar view of the sun-illuminated bathymetry from the combined National  Oceanic and Atmospheric Administration surveys in Rhode Island and Block Island  Sounds showing storm-wave-induced scour depressions on an isolated  bathymetric high in central Rhode Island Sound.**Figure 18.**Detailed  planar view of the sun-illuminated bathymetry from the combined National  Oceanic and Atmospheric Administration surveys in Rhode Island and Block Island  Sounds showing storm-wave-induced scour depressions on an isolated bathymetric  high in western Rhode Island Sound.**Figure 19.**Detailed  planar view of the sun-illuminated bathymetry from the combined National Oceanic  and Atmospheric Administration surveys in Rhode Island and Block Island Sounds  showing storm-wave-induced scour depressions on an isolated bathymetric high in  southwestern Rhode Island Sound.**Figure 20.**Detailed  planar view of the sun-illuminated bathymetry from the combined National  Oceanic and Atmospheric Administration surveys in Rhode Island and Block Island  Sounds showing storm-wave-induced scour depressions on an isolated bathymetric  high in eastern Rhode Island Sound.**Figure 21.**Detailed  planar view of the sun-illuminated bathymetry from the combined National  Oceanic and Atmospheric Administration surveys in Rhode Island and Block Island  Sounds showing dendritic channels in the sea floor beneath northern Block  Island Sound.**Figure 22.** Detailed  planar view of the sun-illuminated bathymetry from the combined National  Oceanic and Atmospheric Administration surveys in Rhode Island and Block Island  Sounds showing sand waves in the western part of the study area.**Figure 23.**Detailed planar view of the sun-illuminated bathymetry  from the combined National Oceanic and Atmospheric Administration surveys in Rhode  Island and Block Island Sounds showing large transverse sand waves on a shoal  west of Block Channel.**Figure 24.**Detailed planar view of the sun-illuminated bathymetry  from the combined National Oceanic and Atmospheric Administration surveys in Rhode  Island and Block Island Sounds showing the relatively flat, featureless  appearance of modern marine sediments, a shipwreck, and artifacts from  multibeam acquisition and processing. 
The study area  covers most of Block Island Sound and seaward portions of Rhode Island Sound within  water depths ranging from less than 1 to more than 103 m (figs. 1, 9). Other than near  the shorelines of Block Island and southwestern Rhode Island and Fishers  Island, New York, shallow areas are above isolated bathymetric highs, such as  submerged moraines and shoals. The deepest waters are in western Block Island  Sound near the entrance to Long Island Sound. Elsewhere the large-scale  bathymetry in the study area is controlled by numerous minor ridges, channels,  and knolls that are products of the study area’s Pleistocene glacial history  and the subsequent Holocene eustatic rise in sea level (Needell and Lewis,  1984). The 4-m resolution of the DTM also allows us to identify specific  morphological features of the sea-floor geology, sedimentary processes, and  anthropogenic effects. Examples of features that can be identified in this  dataset are reviewed in the following sections.
Rocky Outcrops
Over  the past 15,000 years, energetic tidal- and storm-driven currents have removed  or winnowed much of the glacial and younger sediment covering the study area’s  sea floor, producing lag deposits of coarse gravel and boulders (Needell and  Lewis, 1984; Gustavson and Boothroyd, 1987; Boothroyd,  2009; Poppe and others, 2011b; Poppe and others, 2012; McMullen and others,  2013). These lag deposits are coarsest where end moraines are exposed.  Bouldery areas of sea floor associated with the Montauk Point-Block Island segment  of the terminal moraine (figs. 10, 11, 12), which represents the maximum  advance of the Laurentide ice sheet about 20,000 to 24,000 years ago, and the  Point Judith moraine (figs. 11, 13),  which represents a retreated ice-sheet position from about 18,000 to 21,000  years ago, are both exposed in the study area (Uchupi and others, 1996; Balco  and others, 2011; Boothroyd, 2009; McMullen and others, 2012; Poppe and others,  2012; McMullen and others, 2013). Although seismic data suggests that bedrock  may also crop out offshore of southwestern Rhode Island (Needell and Lewis,  1984; McMullen and others, 2009c; McMullen and others, 2009b), verification is  difficult because rocky outcrops there are typically overgrown with sessile  fauna and flora (McMullen and others, 2014) and because the outcrops are too  small to exhibit features that have been used to infer exposed bedrock in  eastern Long Island Sound, such as glacially smoothed ridges and strike ridges  (Poppe and others, 2011a). Sedimentary environments within the rocky areas are  characterized by processes associated with erosion or nondeposition.
Scour Features
Tidal  currents enhanced by constricted flow are responsible for most of the  high-energy sedimentary environments and much of the topography in Block Island  Sound (Needell and Lewis, 1984; Boothroyd, 2009; Poppe and others, 2012;  McMullen and others, 2013). These currents have maintained the channels originally  formed by drainage patterns during the late Pleistocene, when the sea floor  beneath the sound was subaerially exposed (Needell and Lewis, 1984; Boothroyd,  2009), that extend east and southeastward across the sound to Block Channel  (Poppe and others, 2012). The tidal currents have also produced and maintained  deep scour depressions associated with the late Pleistocene drainage patterns  and located adjacent to The Race at the entrance to Long Island Sound, within the  channels south of the eastern end of Fishers Island (figs. 9, 11, 14),  and at the entrances to the passage between Block Island and the mainland. Two  of the depressions near The Race (Poppe and others, 2011a) and one south of the  eastern end of Fishers Island are greater than 100 m deep. 

Smaller  scale scour features also provide evidence of on-going erosion by the strong,  oscillating tidal currents in Block Island Sound. For example, swirling  currents develop around shipwrecks and boulders that protrude through surficial  sediments. The resulting turbulent flow scours the sea floor downstream from  obstructions and produces coarse-sediment-floored, sharp-pointed depressions  called obstacle marks (Werner and Newton, 1975). Good examples of these  bedforms are in the channels cutting across the sound (fig. 15)  and at the western entrance to the passage between Block Island and the  mainland. Asymmetry of the scour around these boulders is toward the direction  of net sediment transport, as indicated by the greater scour (Reineck and  Singh, 1980).

The  near absence of asymmetrical obstacle marks and deep scour depressions in the  eastern half of the study area reflects the weaker tidal currents in Rhode  Island Sound (Haight, 1942; Boehm and Quinn, 1978; White and White, 2011).  Patchworks of relatively shallow scour depressions, however, have formed on  seaward-facing slopes and on top of and adjacent to bathymetric highs in this  sound, controlling much of the benthic diversity (figs. 11, 16, 17; Poppe  and others, 2011b, 2012; McMullen and others, 2011, 2012). These depressions average  0.5–0.8  m deep, occur in water depths as great as 42 m, have relatively steep  well-defined sides and coarse-grained floors, and occur in a variety of shapes,  sizes, and configurations. Some are elongate and narrow; others are broad and  rounded. In places, individual scour depressions have expanded to combine with  adjacent depressions, forming larger eroded areas that commonly contain  outliers of the original sandy sea-floor sediments (figs. 18, 19). In other  places, cobbles and scattered boulders are present on the depression floors and  indicate that the muddy Holocene sands have been completely removed, exposing  winnowed relict Pleistocene deposits (figs.  16, 20). Because low tidal-current speeds and the lack of asymmetrical  scour features suggest that bi-directional tidal currents alone are not capable  of forming these features and because wave analysis has shown that waves can  significantly affect the sea floor in Rhode Island Sound (Spaulding, 2007;  Grilli and others, 2009; Ullman and Codiga, 2010), we contend that these  depressions are formed and maintained under high-energy shelf conditions resulting  from the repetitive hydraulic forces imposed by storm-driven waves that cause  liquefaction and resuspension. We also contend that once resuspended the sediments  are then exposed to erosion by wind-driven and tidal currents. Because sessile  fauna dominate on the gravel floors of the depressions and infauna are  prevalent in the finer grained Holocene deposits, we have concluded that the  resultant close juxtaposition of silty sand- and gravel-dependent communities  promotes regional faunal complexity (Poppe and others, 2011b, 2012; McMullen  and others, 2011, 2012, 2013). This faunal complexity has helped to support a  commercial fishery that is an important part of Rhode Island’s economy (Beutel  and others, 2010; Hasbrouck and others, 2011).

Although much less common than  in Rhode Island Sound, storm-wave induced scour also occurs in Block Island  Sound where scour depressions are present on the lower shoreface along Rhode  Island’s southwestern coast and adjacent to Block Island. A few of these  depressions, however, connect with single-channel and dendritic-channel systems  that extend downslope (fig. 21). Individual channels average 1 to 1.5 m  deep, continue upslope and over minor bathymetric highs along their courses,  and exceed 5 km in length. The mechanisms governing formation and maintenance  of these features are uncertain, but interpretations of seismic-reflection  profiles show no correlation between these channel systems and the underlying  channel systems cut during either glacial or post-glacial subaerial exposure  (Needell and Lewis, 1984). Because of the lack of subsurface structural control  and because scour depressions are commonly located at the upper ends of these  channels, we would argue that the channels are related to modern processes,  such as the return flow of bottom waters pushed up against the shoreline during  storms.
Sand Waves and Megaripples
For the purposes of this report, bedforms are defined by  morphology and amplitude—sand waves (that is, subaqueous dunes) are higher than  1 m and megaripples are 0.2 to 1 m high (Ashley, 1990). Sand waves and  megaripples in the study area are best developed where constrictions in the  tidal flow widen, energy levels drop, and processes that characterize the  sedimentary environments change from those dominated by erosion and  nondeposition to those associated with coarse-bedload transport. These areas  include the western part of Block Island Sound near The Race (fig. 22), southeastern Block Island  Sound near Block Channel (fig. 23),  and the entrances to the passage between Block Island and the mainland (Poppe  and others, 2012).

Transverse  bedforms dominate where the sand supply is abundant. Crest-to-trough amplitudes  of the largest transverse sand waves exceed 7 m, but these bedforms exhibit a  range of amplitudes and crestline lengths. Many transverse sand waves exceed 1  km in length, and several are longer than 2 km. Megaripples occur on the flanks  of sand waves, and smaller sand waves occur on the flanks of larger sand waves.  The presence of current ripples and megaripples on the sand waves and the  steepness of the slip faces suggest that sediment transport is active, and that  the sand waves are propagating under the present hydraulic regime (Dalrymple  and others, 1978; Reineck and Singh, 1980; Allen, 1980). Smoothly sinuous  crestline morphologies are most common, but transverse bedforms with smoothly  curved, bifurcating, and relatively straight crestlines occur. Borders of  sand-wave fields can be abrupt and are typically fringed with megaripples.

Sand  waves and megaripples with crescent-shaped barchanoid morphologies are present,  but are much less common than transverse morphologies (fig. 22). They occur where sediment supply is limited, such as away  from shoals and on the edges of transverse bedform fields. Transverse sand  waves that are relatively straight crested, display no bifurcation, and are  symmetrical in profile occur near the western end of the study area (fig. 22). Megaripples are typically  absent from the stoss slopes of these waves, and their ends are marked by  distinct scour zones or moats. These sand waves, which closely resemble  bedforms in Great South Channel (Valentine and others, 2002) and eastern Long  Island Sound (Poppe and others, 2011a), are probably immobile.

The  sand waves reflect the strength of the bottom currents, and their asymmetry can  be used to indicate directions of net sediment transport (Allen, 1968; Ludwick,  1972; Allen, 1980). For example, most of the transverse sand waves in the  western part of the study area display pronounced asymmetry with slip faces that  are oriented toward the west and northwest and stoss slopes oriented to the  east and southeast (figs. 22, 23). This asymmetry suggests a  flow-transverse crest orientation and net sediment transport toward westernmost  Block Island Sound and into Long Island Sound. Barchanoid morphologies can also  be used to infer direction of net sediment transport because their horns migrate more rapidly than the wave's main body causing  their concave faces to be oriented downstream in the transport regime (McKee, 1966).
Lower Energy Sedimentary Environments
Holocene  marine deposits composed of fine-grained sand and silty sand are prevalent  where the sea floor is relatively smooth and featureless in the 4-m grid DTM (fig. 24). These  deposits occur in deeper, less  constricted parts of the study area where tidal-current speeds are low (less  than about 20 cm/s), such as in Rhode Island Sound (White and White, 2011;  Poppe and others, 2011b; McMullen and others, 2011, 2012), and in areas  protected from high-energy storm conditions, such as central Block Island  Sound. Sedimentary environments in these areas are characterized by processes  associated with sorting, reworking, and deposition (Poppe and others, 2012). 
Anthropogenic Artifacts
Anthropogenic  features in the DTM include dredged channels and shipwrecks. Dredged channels  are present near the entrances to the harbor at Point Judith and to the harbor on  Block Island. Shipwrecks, which are of interest to historians and the diving  community, are common. Examples of prominent shipwrecks include a previously  uncharted, unidentified wreck that lies in approximately 47 m of water near the  southern entrance to Block Island Sound (fig.  15), and that of the *U*–*853*, a World War II German submarine, that  lies in western Rhode Island Sound (fig.  24; Poppe and others, 2012). Pronounced scour around the wreck near the  southern entrance to Block Island Sound shows that the oscillating tidal  currents are stronger there than in western Rhode Island Sound where scour is  absent around the submarine.
Acquisition and Processing Artifacts
Some  of the features visible in the DTM are artifacts of environmental conditions,  time limitations, equipment capabilities, and methodology during data acquisition  and processing. Rougher seas during acquisition can result in minor noise and  loss of resolution in the data that appear as narrow intermittent bands of  noisy data (fig. 24). Artifacts due  to time limitations and equipment capabilities, however, are more conspicuous.  For example, gaps of no data (white polygons) owing to incomplete multibeam coverage  during surveys H12023, H12296, and H12298 are present along the shorelines of southwestern  of Rhode Island and Fishers Island, New York (fig. 9). In these  shallow areas, where obstructions on the sea floor create hazards to  navigation, these gaps have typically been filled by means of sidescan-sonar  data (National Oceanic and Atmospheric Administration, 2009g, 2011b, 2011c).  Processing artifacts are largely confined to the edges of survey areas that  overlap other surveys (fig. 22) and  to tie lines (figs. 16, 17).  Inasmuch as no attempt was made to eliminate these artifacts, offsets in the  datasets resulted where discrepancies were  present. Although minor in the original bathymetric data (0.2 to 0.5 m), the  artifacts are made more conspicuous by the 5-times vertical exaggeration  applied during production of the GeoTIFFs provided in this report. |
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U.S. Department of the Interior |            U.S. Geological Survey                                                URL: http://pubs.usgs.gov/of/2014/1012/html/bathy.html            Page Contact Information: GS Pubs Web Contact                        Page Last Modified: Friday, February 21, 2014, 11:55:49 AM