EVIDENCE FOR TWO DIRECTIONS OF LONGSHORE SEDIMENT TRANSPORT ALONG THE RHODE ISLAND SOUTH SHORE?

A significant portion (24km²) of the upper shoreface along the Rhode Island south shore (RISS) was mapped in 2015-2016 using high-resolution side-scan sonar, seismic reflection profiles, underwater video imagery and surface sediment grab samples. The focus the mapping was to determine the extent and volume of the shoreface depositional platform (SDP). The SDP is the constructional, seaward sloping portion of the upper shoreface, extending from the step or base of the beach, offshore up to 700 m to a typical water depth of 7 - 8 m. Thickness of the SDP along the RISS ranges from 0 - 3 m, (mean = 0.6 m) although portions thicker than 2 m are rare and are related to anthropogenic structures. Total SDP volume is < 3 x 10⁶ m³ and represents a substantial portion of the overall RISS sediment budget.

Various rates and directions of longshore sediment transport along the RISS have been proposed over the last 5 decades, ranging from net transport to the east (Fischer, 1988; Boothroyd, 2002), transport to the west (Morton et al., 1984) or a multi-celled system with east or west net transport directions (McMaster, 1960). ADCP observations collected in 2010-2011 support net eastward transport, with wave approach from the south and southwest <70% of the time. The mapped distribution, thickness and volume (m³ m^-1) of the SDP largely supports previous interpretations that net longshore transport of sediment is to the east along most of the RISS. This includes increasing volume/width of the SDP on the western side of headlands/shoals, adjacent to the 1 km long western jetty of the Point Judith Harbor of Refuge (HOR) and deposition of sediment inside the HOR (>2,500,000 m³) since it was constructed in ~1900. Complicating the interpretation of net eastward transport, the mapping shows an increase in volume of the SDP towards the west at the far western end of the RISS. The shoreline here is a mixture of barriers, welded barriers and a headland consisting of moraine that is largely armored and likely does not contribute much sediment to the system. This is interpreted to suggest that either net longshore sediment transport at the western end of the RISS is towards the west OR sediment transported west during storms (easterly wave approach) is not transported out towards the east during subsequent periods of fair-weather.