SEDIMENT DEPOSITION IN A BREAKWATER HARBOR; TOWARDS A SEDIMENT BUDGET FOR THE SOUTHERN RHODE ISLAND SHORELINE

The 30 km Rhode Island south shore (RISS) is a wave dominated micro-tidal system that is essentially a single coastal compartment consisting of a series of mainland segmented barrier spits. The total volume of sediment within the berm (1 x 10⁶ m³) and upper shoreface (3 x 10⁶ m³) making it relatively sediment starved system. Volumes of sediment transported both along and across shore remain poorly constrained. Four jettied inlets trap sediment in flood-tidal deltas, and previous studies quantified the depositional rates for these at 5,000 to 10,000 m³ yr^-1. Recent work on one of these inlets (Ninigret Pond) show that deposition is highly episodic, and previous studies may underestimate the deposition. Net longshore transport along the RISS is towards the east, and a breakwater harbor constructed in the late 19^th century located at the eastern end of the compartment acts as a sediment sink. A multi-faceted project using seismic reflection profiles, bathymetric mapping, and sediment cores quantified the volume of sediment within the harbor. Seismic reflection profiles show a district seismic facies up to 4 m thick above the ravinement surface, particularly in the western and central portion of the harbor. This is markedly different than the adjacent RISS outside the harbor. Century-scale bathymetric changes revealed shoaling of a similar magnitude, and isotopic data support the deposition of this sediment package within the 20^th century. The total volume of sediment within the harbor exceeds 5 x 10⁶ m³, with an estimated sand volume of 3.6 x 10⁶ m³. The results show that the harbor is a substantial sediment sink for the RISS and provide the basis for future studies of the sediment budget for this coastline.