BENTHIC GEOLOGIC HABITATS OF THE MATUNUCK-GREEN HILL SHOREFACE, RHODE ISLAND

Maintenance dredging of the Point Judith Harbor of Refuge, RI will remove approximately 90,000 m3 of sediment. Beach replenishment utilizing the dredged material from this project is viewed as a beneficial use and the goal will be to place the dredged material on the shoreface offshore of Matunuck, RI somewhere along a 3.7 km by 1 km stretch of shoreface in approximately 5 m water depth. The sediment hopefully will be transported shoreward to the beach during fair-weather conditions, predominantly by wave-orbital motion generated by southwest sea-breeze or moderate southerly swell. Sediment is transported offshore by combined flows during storm events; sediment transported seaward of 12 m water depth cannot be returned to the intertidal beach. Longshore transport is predominantly to the east, as indicated by records of past dredging in Point Judith Harbor of Refuge. Knowledge about the location and extent of essential fish habitat on the shoreface was needed to address concerns regarding those habitats for commercial and recreational fisheries. High resolution side-scan sonar, underwater video-imaging and surface sediment samples were used to map the distribution and characteristics of the shoreface. Four geologic habitats were identified within the area of interest. They include: a sand sheet immediately seaward of the intertidal beach comprised of medium to fine sand; coarse sand swaths aligned perpendicular to the trend of the shoreline, containing small 2D dunes, extending seaward beyond the study area; boulder-gravel concentrations containing clasts up to 3 m diameter; and a cobble gravel pavement seaward of the sand sheet that is areally the most extensive habitat. Two areas, 1 km long by 60 m wide, were identified for dredged material placement. These areas are located on the western sides of the widest portions of the sand sheet to minimize the possibility that longshore transport would move sand eastward before it could be transported to the beach. The addition of 90,000 m3 of dredged material to the existing inner shoreface sand sheet would result in a 15% increase in volume of sediment for that environment. Placement sites for dredged material were determined in to minimize impact to cobble and boulder habitats important to recreational and commercial fisheries, particularly habitats favored by lobsters.