VARIABLE RATES OF SHORELINE CHANGE ALONG THE RHODE ISLAND SOUTH SHORE: A CLOSER LOOK AT PROCESSES AND METHODS

Shoreline change trends vary depending on the timescale and method of observation, along the 33 km, roughly east to west trending Rhode Island south shore is a microtidal, wave dominated coastline, consisting of alternating barriers (67% of the shoreline) and headlands (33%). The headlands are composed of glacial till (26%), glacial stratified material (sand/gravel deposits capped by a sandy silt eolian mantle) (6%) or limited bedrock outcrops (1%). The position of the last high tide swash (LHTS) between 1939 and 2014 was digitized to quantify shoreline change rates along the Rhode Island south shore. When historic rates of shoreline change are filtered by shoreline type, in general, the barriers experienced the highest rate of long-term erosion, with the lowest rates on the glacial till and bedrock shoreline segments. One section of glacial stratified deposits show rates similar to barriers, showing shoreline type alone is not the determining factor for predicting erosion.

A portion of the Matunuck headland, specifically South Kingstown Town Beach (SK-TB), experienced the highest rates of long-term shoreline change for the entire RI south coast (-1.44 m yr^-1). This low elevation bluff (~4.5 m above MLLW), is impacted by waves even from smaller storm events (8 m during Sandy), causing a variable, but incremental retreat over time (29 m since 1996). After each episode of bluff erosion, the beach and subsequently, the position of LHTS shifts landward. This is a different response than observed along the barriers. These smaller storms, which do not completely overwash the barriers, cause frontal erosion and narrowing of the barrier but do not systematically migrate the average position of LHTS. Following Sandy, many segments of barrier shoreline the position of LHTS moved seaward as the berm widened when sediment packages removed from the foredune zone during the storm were deposited back onshore.

The erosion at SKTB is exacerbated by multiple factors; cohesive glacial sediment scarps in even small storms, reflecting wave energy offshore. The absence of an erosional terrace present at other glacial headlands allows breaking waves to act break on the beach. Southeasterly storm swells refract around an adjacent shallow terrace and boulder revetment to the east amplifying wave focusing.