SUBTLE MODIFICATION OF GLACIALLY DERIVED MATERIALS ALONG MASSACHUSETTS’ SOUTHERN COAST BY PASSING SUMMER STORMS

Engineered resupply of sand to coastal environments, i.e. nourishment, offers an attractive short-term strategy to address beach erosion in Massachusetts. For efficient nourishment, site-specific knowledge of seasonal grain size and sediment volume variability at eroding beaches is essential. We have begun measuring grain size and profile at 22 eroding Massachusetts beaches, capturing summer and winter conditions at each site through four to nine representative transects perpendicular to the shore and spaced 100-500 meters apart. Our recently completed first summer field season (August/September 2014) visited eight beaches along Massachusetts’ south coast from Rhode Island to Nantucket. These environments should reflect regional glacial history and a summer interval of reduced storm activity. Where unstratified surficial materials characterize the coast, erosion of glacial till (Horseneck and East beaches, Westport) and end moraine (Barges Beach, Cuttyhunk Island; Town and Sylvia State beaches, Oak Bluffs/Edgartown) can yield cobble berms capping steep intertidal zones. We noted that increased wave activity during storms strips a thin (inches-thick) layer of intertidal sand to reveal gravel and cobble below, while leaving beach profile essentially unchanged. In contrast, where (cobble-free) glacial outwash intersects the coast (Surf Beach, Falmouth; Miacomet and Low beaches, Nantucket) sand and gravel are distributed more evenly across beach facies. Here passing summer storms modify beach profile but not grain size: high surf cuts sandy berms, shifting steepened intertidal zones landward. We will reoccupy south coast sites at the end of winter in 2015 to examine effects of seasonally related increase in storm (and wave) activity. Survey of Massachusetts’ east coast (Sandwich to New Hampshire) is planned for summer of 2015 and winter of 2016. Additionally, overwash sequences recovered through backbarrier basin coring at selected sites complement our beach survey by providing depositional records of particularly strong storms. Study results will allow identification of suitably matched nourishment sources onshore, or offshore, as described in Massachusetts’ Office of Coastal Zone Management’s extensive grain-size database.