FACIES CONFIGURATION CHANGES AND SEDIMENT TRANSPORT ON THE SOUTHERN RHODE ISLAND SHOREFACE

Four facies have been previously interpreted and mapped (Boothroyd and Klinger 1998; Klinger 1996; Brenner 1998) on the Charlestown/Green Hill barrier upper shoreface using side-scan sonar surveys obtained in 1995, 1996, and 1997. These include: 1) a sand sheet (Ss) composed of fine to very fine sand, 2) coarse sand covered with small dune bedforms (Csd), 3) cobble pavement (GLc), and 4) glacial boulder outcrop (GLb). The present study identified three new facies on the lower shoreface survey of 1997. These include: 1) a fine sand facies (Fs), 2) cobble pavement with a thin veneer of mud (GLcM), and 3) Coarse sand (Cs) with no small dunes.

The multi-year sonar data provided the spatial resolution needed to identify small but significant facies configuration changes of the sand sheet (Ss) and the coarse sand/small dune (Csd) facies. These facies are highly affected by both fairweather systems and storm events. Waves and currents were hindcast for storm events (1995-1997) that could have caused the observed configuration changes to the shoreface. Wave and current conditions also were hindcast for historical storm events (1938-1999) to provide insight into the relative magnitude of sediment transport during extreme events along the south shore of Rhode Island. During storm events, sediment is transported offshore by strong combined flows. These combined flows follow topographic lows on a very complex "bumpy" shoreface. Fine sand in the Ss is transported onshore by asymmetrical wave orbital velocities during fairweather strong seabreeze and swell conditions.

From 1995 to 1997 storminess on the Rhode Island coast increased, this resulted in a decrease in beach profile volume on the Charlestown barrier. Sand eroded from the active berm during this period was transported offshore and alongshore. The sand eroded from the active berm was directly related to an observed increase in the extent of the 1996 Ss configuration. As storminess continued into 1997, sediment volumes of both the active berm and the upper shoreface decreased. We infer that this sediment was transported offshore by combined flow to the lower shoreface. We further infer that the sediment transported to the lower shoreface is not transported shoreward during fairweather conditions and thus is lost from the upper shoreface/berm system.