Spatial Extent and Volume of the Shoreface Depositional Platform on the Upper Shoreface of the Glaciated Rhode Island South Shore

The extent and distribution of benthic geologic habitats on the upper shoreface (from the shoreline extending offshore to > 10-m water depth) was mapped along the 33-km-long Rhode Island south shore (RISS) in 2015–2016 using a combination of high-resolution side-scan sonar, seismic reflection profiles, surface sediment samples, and underwater video imagery. Modern (Holocene) sediment is contained largely in the shoreface depositional platform (SDP), the constructional, seaward sloping portion of the upper shoreface extending from the intertidal beach, offshore. The SDP varies in thickness and width along the RISS, with a mean thickness of 0.7 m and a mean width of 260 m. Maximum thickness (< 3 m) and width (750 m) was adjacent to a 1-km-long jetty system, supporting a net easterly longshore sediment transport along the RISS. The distribution and type of sedimentary environments on the uppermost shoreface here is largely related to the antecedent (glacial) geology. No clear correlation exists between decadal-scale shoreline change and the volume of sediment in the SDP, removing a potential metric for modelling shoreline change here and potentially for other glaciated shorefaces. The complex nature of the shoreface along the RISS, with abundant cobble to boulder gravel outcrop, makes the concept of an equilibrium profile here problematic at best.