Paper No. 139-2
Presentation Time: 8:20 AM
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 19th 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 20th 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.