SPATIAL EXTENT AND VOLUME OF SAND ON THE UPPER SHOREFACE OF THE GLACIATED RHODE ISLAND SOUTH SHORE; RELATIONSHIP TO DECADAL SCALE SHORELINE CHANGE AND LONGSHORE SEDIMENT TRANSPORT

The Rhode Island south shore (RISS) is largely sediment starved, with an estimated berm volume of ~1 x 10⁶ m³ over the entire 30 km shoreline. Previous work mapping the upper shoreface worked on the type and extent of the key facies on the shoreface. Along most of the shoreline, seaward of the intertidal beach a deposit of sand extends offshore to a water depth of 5 to 10 m below MSL. Sub-bottom profiles show this unit unconformably overlies the underlying deposits. This unit has a variety of names (beyond the generic name of upper shoreface deposits) in the literature, i.e. shoreface prism, nearshore zone, sand veneer, beach wedge, shoreface attached sands, shoreface ramp etc. We refer to this unit as the shoreface depositional platform (SDP); defined as the constructional, seaward sloping portion of the upper shoreface, extending from the step or base of the beach, offshore.

The SDP along the Rhode Island south shore is not limited to fronting barrier spits, and the same feature was mapped offshore of headlands, particularly those composed of glacially deposited sand and gravel. While the SDP extent was previously mapped for portions of the RISS, little if no research had been done to quantify the volume of sediment in the nearshore. This represents important baseline information regarding the local sediment budget and shoreface dynamics as a potential source and sink for the shoreline. To quantify the thickness and volume of the SDP, the spatial extent was mapped using high frequency (300/900 kHz) side-scan sonar (24 km²) and the thickness determined with ~200 km of 2-10 kHz and 2-7kHz ‘chirp’ sub-bottom seismic reflection profiles.

The width of the SDP varies from near zero to < 700 m, the thickness ranges from 0 to 3 m (0.6 m mean), although portions thicker than 2 m are rare and are related to anthropogenic features. Total SDP volume is < 3 x 10⁶ m³ and represents a substantial portion of the overall sediment budget. Comparing SDP volume to annualized shoreline change rates (1939-2014) shows an inverse relationship (more sand offshore, higher rates of erosion), at least for portions of the RISS. The pattern of thickness and extent of SDP supports previous interpretations that net longshore transport of sediment is to the east along the RISS.