TOWARD A SEDIMENT BUDGET FOR THE SOUTHERN RHODE ISLAND SHORELINE

The 30 km long southern Rhode Island shoreline, Watch Hill east to Pt. Judith, can be treated as a coastal compartment with a mostly closed sediment transport system. There is little to no sediment "leakage" alongshore; leakage offshore remains a complex issue. Sediment is transported shoreward by overwash across the barriers and low headlands and into coastal lagoons by tidal-current and storm-surge augmented flow. The alternating microtidal barrier and glacial headland shoreline has an instantaneous berm volume of 1 million m3, making it relatively sediment starved beach system. A long-term shoreline monitoring program (24 years of weekly beach profiles), compilation of dredging records, extensive vibracoring and mapping of flood-tidal deltas in the coastal lagoons, and side-scan sonar surveys of the upper and lower shoreface allow a semi-quantification of sediment transport rates and volumes.

The entire system is storm driven, thus size and intensity, forward speed, path, tidal phase, and time between storms (Hayes and Boothroyd, 1969) control the resulting changes. A rapid, short-term exchange (days to weeks) of sediment occurs between the berm and upper shoreface sand sheet on the order of 50-100,000 m3; longer-term exchanges (months) may be up to 100,000 m3. An average of 5,000 m3.yr-1 of sediment enters each of four tidal inlets, to be deposited as flood- tidal delta lobes. An excess of 5-20,000 m3.yr-1 of sand is transported eastward past any given point toward Pt. Judith. Another 25-100,000 m3 is transported landward by overwash processes during 10-year and greater storms.

One can consider onshore and alongshore transport volumes to be conserved, but sediment transported to the lower shoreface and not returned can be considered lost to the system and to have "leaked". The retreat of the high-water line (HWL) has averaged 0.4 m.yr-1 over a 60-year time span, a highly speculative number. It could be inferred that linear distance, recomputed to a berm volume, represents the sediment leaving the system via storm-driven combined flows to the lower shoreface.