Facies Configuration Changes and Sediment Transport on the Charlestown/Green Hill Barrier Shoreface

Abstract

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). These upper shoreface surveys were reinterpreted where necessary and combined with lower shoreface (16-21 m water depth) surveys mapped in the present study. This resulted in the completion of a comprehensive facies map for the entire Charlestown/Green Hill shoreface surveyed in 1995, 1996, and 1997 (3- 21 m water depth). In addition to Csd, GLc, and GLb the present study identified three new facies on the lower shoreface surveyed in 1997. These include: 1) a fine sand facies (Fs) that is similar to Ss but does not occur as a sheet, 2) cobble pavement with a thin veneer of mud (GLcM), and 3) Coarse sand (Cs) with no small dunes. The multi-year side-scan sonar data provided the spatial resolution needed to identify small but significant facies configuration changes of the sand sheet (Ss) and the coarse sand with small dunes (Csd) facies. It was found that these facies are highly affected by fairweather (southwest sea breeze) and storm events. To further investigate these configuration changes and sediment transport, the study hindcast waves and currents for storm events that could have caused the observed configuration changes to the shoreface. Wave and current conditions during historical storm events (1938-1999) were also hindcast in order 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 are concentrated and steered by topographic highs on the shoreface. Fine sand in the sand sheet (Ss) is transported onshore by asymmetrical wave orbital velocities during fairweather (short period southwest wind waves) and swell conditions (long period offshore storm waves). The study primarily focused on the transport of sediment offshore during storm-induced combined flow. It was found that there were 22 storms during the multi-year sonar record (1995-1997) that may have affected facies configuration. Twenty-one of these storms were capable of transporting fine sand at a depth of 6 m offshore and 19 were able to transport coarse