IN COOPERATION WITH THE NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION

Character, Distribution, and Ecological Significance of Storm-Wave Induced Scour in Sound, USA By L.J. Poppe and K.Y. McMullen DTM SHOWING STORM-WAVE INDUCED SCOUR DEPRESSIONS ON A LOWER SHOREFACE E Pleistocene ABSTRACT DTMS SHOWING STORM-WAVE INDUCED SCOUR ON ISOLATED BATHYMETRIC HIGHS outcrop Multibeam bathymetry from NOAA hydrographic surveys is coupled with USGS sampling and photographic A data to provide information on sediment transport and benthic habitat in Rhode Island Sound. Patchworks of N scour depressions control much of the benthic diversity on seaward-facing slopes and across bathymetric Boulders Multibeam highs. These depressions average 0.7 m deep, have steep well-defined sides, coarse-grained floors, and Modern marine A2 occur in various shapes and orientations to depths of 42 m. In places, individual scour depressions have artifacts Scour expanded, forming larger eroded areas that commonly contain outliers of original seafloor sediments. Where sediments cobbles and boulders floor the depressions, winnowed Pleistocene deposits are exposed. These depressions E1 depressions are formed and maintained under high-energy shelf conditions owing to repetitive hydraulic forces imposed 27m by storm-driven waves. These forces cause liquefaction and resuspension, exposing suspended sediments Outliers E2

to erosion by relatively weak wind-driven and tidal currents that alone are incapable of forming these features. Exposed 16m 38m Because sessile fauna dominate on gravel floors of the depressions and infauna are prevalent in the muddy 32m sand of Holocene deposits, we conclude that the resultant juxtaposition of sand- and gravel-dependent Pleistocene Modern communities promotes regional faunal complexity. surface Scour marine depressions 29m N 200 M sediments o 71 30'N Sakonnet CORRESPONDING BOTTOM PHOTOGRAPHS Rhode Point A1 Cuttyhunk 200 M 31m Island Island E1 Moon snail casing E2

Rhode Seismic Point Judith-Buzzards profileBay Martha's B Outliers Point moraine Vineyard Shell debris Judith concentrates Fig. A Modern in ripple troughs o marine 41 20'N Island 33m Scallops Fig. C sediments B1

Fig. D Nomans Study Land Area 30m

Block Island Sound Extent of Sound multibeam B2 Long 10 CM 10 CM data Island 33m Fig. E Fig. B Atlantic Ocean Block Terminal moraineAtlantic Ocean DISTRIBUTION Island o 71o 20'N 71o 0'N 41 10'N Scour Storm-wave induced scour depressions are prevalent on the tops and upper flanks of isolated bathymetric highs and shoreline-connected ridges, such as the submerged extension of the Point Judith moraine, on seaward INDEX MAP N depressions 200 M facing slopes, and on the lower shore face along shorelines, such as those of . They are scattered Map of Rhode Island Sound showing extent of available multibeam bathymetry data, distribution of observed CORRESPONDING BOTTOM PHOTOGRAPHS depressions attributed to storm-wave induced scour (gray polygons), local physiographic features, location of and discontinuous in the eastern and western parts of Rhode Island Sound, but become more pervasive in the seismic profile (green line), detailed bathymetric planar views A, B, C, D, and E (open red polygons), and A1 A2 central, less sheltered part of the sound. They occur in water depths as great as 42 m, but increase in extent approximate axes of submerged segments of the circa 18-21 ka and 20-24 ka end moraines. Ripples and abundance as depths shoal. Hydrozoans Worm CHARACTER NW SE tubes 0 0 The scour depressions have relatively steep, well-defined sides and average 0.7 m deep. Transitions from the AST75-6 Line 31 Sea level SEISMIC-REFLECTION DATA floors of the depressions to the surrounding seafloor occur within about 10 m. Individual depressions range Sea floor from less than 20 m to over 100 m in width and can extend to almost 1 km in length. Scour depressions exhibit Segment of a Boomer seismic- a variety of shapes, sizes, and configurations that range from elongate and narrow to broad and rounded. Some reflection profile collected across a are distinct and conspicuous; others are complex, patchy, and discontinuous. In places, individual depressions bathymetric high in the eastern part have expanded to combine with adjacent depressions, forming larger eroded areas that contain outliers of the Anemone Depth (M) of the study area (McMullen et al., original Holocene seafloor sediments. Although most of the elongate depressions are oriented across slope, 15 CM 10 CM 0.1 75 2009). The high is cored by coastal some of the depressions parallel bathymetric contours and others straddle bathymetric highs. Most of the scour Multiple plain sediments, but is mantled by Ripple crest Ripple trough depressions terminate by narrowing and pinching out. Asymmetry in the scour depressions is absent. Sediments T wo-W ay ravel ime (Sec.) B1 B2 Pleistocene glacial till, as evidenced flooring the scour depressions are invariably coarser grained than those on the surrounding seabed. Where by the presence of boulders in Mercenaria valve 0 0 coarse winnowed deposits of cobbles and boulders floor the scour depressions, the entire Holocene section has Boulders scoured areas on the crest and Marine Holocene been removed by erosion to reveal underlying lag deposits of Pleistocene glacial drift. The cobbles and boulders unconformity marine deposits upper flanks of the high. Boulders on are typically covered by sessile fauna (e.g. hydroids, hydrozoans, anemones, sponges) and, in shallow water, the floors of the storm-wave induced Moon snail flora (e.g. erect and encrusting algae). In deeper water (> 30 m), the lag deposits are also commonly covered by scour depressions suggests that the a thin veneer of silt. Where sand and gravelly sand floor the scour depressions, the Holocene section has been Undifferentiated deposits Holocene section has been removed Sea robin only partly removed. This sand is typically clean, moderately sorted, and covered by low, broad ripples. of glacial drift Coastal Plain

deposits Depth (M) and winnowed Pleistocene deposits 0.1 Late Tertiary - early 75 exposed. Location of profile is shown Pleistocene Modified from Horizontal Wavelength S.A. Nelson unconformity 0 1 km on the index map. PROCESS

T wo-W ay ravel ime (Sec.) stress (2012) 10 CM 10 CM (WL) Long-period swell from distant storm systems to the Decreased pressure Increased pressure (vertical stress) on Seabed south and southeast and wind-generated waves from (vertical stress) on Seabed local storms moving up the coast can produce large Vertical DIGITAL TERRAIN MODELS (DTMS) SHOWING STORM-WAVE INDUCED SCOUR DEPRESSIONS ON SEAWARD FACING SLOPES changes in bottom pressure on the Rhode Island stress Sound seabed (Ullman and Codiga, 2010). Orbital 200 M CORRESPONDING BOTTOM PHOTOGRAPHS Depth to Wave Base C Ripple motion associated with these waves alone can (WL/2) C1 trough C2 effectively move water down to the wave base, a water depth equal to one half of the wavelength, and Sea floor unaffected Scour 20m by orbital motion Outliers effects of pressure forces can extend much deeper. of passing waves depressions Feeding pits As large, long-period storm waves repeatedly pass Sea floor affected from crabs over seabed that is shallower than the wave base, by orbital motion Seabed C1 Exposed cyclic vertical and horizontal shear stresses and of passing waves Pleistocene 25m pore-water pressure, compounded by gases from biologic activity, cause progressive deformation. Continued surface Ripple crest deformation, in turn, results in eventual liquefaction and, if the threshold velocity is exceeded, resuspension of Modern C2 the seabed sediments. Once resuspended, the sediment is advected by the weak tidal currents, perhaps marine supplemented by density- or wind-driven currents, and redeposited on the surrounding seafloor. sediments 10 CM 10 CM ECOLOGIC IMPORTANCE D1 Boulder D2 N 30m Boulders Burrows The storm-wave driven scour described here results in the juxtaposition of seafloor areas in Rhode Island Sound

with distinct boulder and gravel, coarse sand, and muddy sand textures. This textural heterogeneity reflects a N complex patchwork of contrasting sedimentary environments that in turn create different sessile- and infauna- D dominated habitats. The coarse lag deposits of Pleistocene glacial drift that floor some of the scour depressions reflect higher energy sedimentary environments characterized by processes associated with erosion and non- Cobbles deposition. These rocky deposits are ecologically important because they provide both shelter for finfish and a Starfish Outliers stable hard substrate for attached and encrusting fauna and flora that add to the overall benthic roughness. Boulders Coarse sand that floors the deeper scour depressions and the muddy sand of the surrounding Holocene deposits support extensive infaunal communities and reflect contrasting lower energy sedimentary environments characterized by processes associated with reworking, sorting, and deposition. 15 CM 10 CM This benthic diversity supports a commercial fishery that is an important part of the Rhode Island economy. Total Exposed D1 Modern marine employment directly connected to harvesting, processing, distributing and selling fish landed by Rhode Island Pleistocene sedimentsDTMs have a 4-m cell size, are sun illuminated from the north at 45 degrees above the horizon, and ported vessels exceeds 6,950. Estimated total dockside value of landings by these vessels exceeded $54 million surface vertically exaggerated at 5x. during 2010, a total of over 42,000 tons, and most of the trips by these vessels were to Rhode Island Sound. D2 Warmer colors (i.e., reds and oranges) reflect shallower depths; cooler colors (i.e., greens and blues)

33m 37m are deeper depths. Open circles show locations of individual depths. Take Home Messages: 35m 200 M Solid circles show locations of representative still photographs, which were collected with a SeaBOSS. Storm-wave induced scour controls much of the benthic geologic diversity in RI sound. Benthic geologic diversity promotes bentihic biological diversity. U.S. Geological Survey Department of the Interior February 2014