SEDIMENT TRANSPORT ON THE WATCH HILL – MISQUAMICUT, RHODE ISLAND SHOREFACE BASED ON REPEAT SIDE-SCAN SONAR SURVEYS

As part of a project to locate a placement site for the beneficial use of dredged material on the upper shoreface offshore of Westerly, RI, an 8.5 km2 portion of the shoreface was mapped using high-resolution side-scan sonar, underwater video imagery and surface sediment grab samples. The eastern portion of this study area was previously mapped using side-scan sonar in the early 1990's, and this represents the first decadal scale, repeat survey of the Rhode Island shoreface. Similar side-scan sonar facies were identified in both the 1993/94 and 2008 surveys. These facies have been deposited in the following depositional environments: 1) Dp ss – upper shoreface depositional platform; a sand sheet immediately seaward of the intertidal beach comprised of medium to fine sand; 2) CSS sd - cross-shore swaths of medium-to-coarse sand and gravel that contain small two-dimensional bedforms with roughly shore parallel crests (large ripples to small dunes); 3) Sh ss – a mid-to-lower shoreface sand sheet of fine to medium sand with no bedforms visible in the side-scan record 4) DPv cg – an extensive depositional gravel pavement comprised primarily of cobbles (10 – 15 cm intermediate axis); and 5) GO bgc – outcrop of glacial sediment indicated by concentrations of 1-3+ m boulders.

A comparison of depositional environments indicates that the areal distribution has changed less than 15% in the intervening years, and the overall spatial distribution of environments has changed very little. Much of the areal change is attributed to lower positional accuracy and interpretation artifacts in the original dataset. Of particular interest is the lack of change in the spatial distribution of the cross-shore swaths, which have a variety of names in the literature. Regardless of origin, the swaths represent areas of significant offshore sediment transport during storms, as the result of combined asymmetric wave orbital motion and downwelling return flow. While the Rhode Island shoreline has not had a direct hurricane strike in the past 15 years, there have been several east-passing tropical and numerous intense extratropical cyclones, including the Patriots Day Sou'easter of 2007. The persistent extent and distribution of swaths indicates that the sediment transport pathways on the shoreface are consistent at scales of years to a decade of more.