INVESTIGATIONS OF BLUFF EROSION AND SEDIMENT TRANSPORT IN BOSTON HARBOR, MA

The concentration of small islands in Boston Harbor is the result of submergence of a drumlin field approximately 5 ka during the Holocene transgression. Coastal processes have subsequently altered the shoreline of these of the islands. Studies of drumlin erosion, redistribution of this sediment, and morphological evolution of the islands have been undertaken though mapping, GIS analyses, and wave modeling. Beach composition and morphology, bluffs, salients, and coastal protection were mapped using differential GPS.

The outer islands are dominated by bedrock, thin covers of till, and boulder and cobble beaches. The lack of drumlin sediment is explained by the islands' exposure to northeast storm wave attack or the absence of drumlin formation in this region. Great Brewster Island (GBI), located just landward of the outer islands is an exception to this trend and contains the tallest drumlin in the Harbor Islands. The islands seaward of GBI form a V-shaped alignment that protects it from direct storm waves. A large elongate spit has developed on the sheltered southwest shore of GBI, derived from sediment eroded from the nearby drumlin as well as sediment derived from the seaward islands. These sediments have been transferred from the north by wave driven transport.

The inner harbor is a relatively low energy environment where drumlin erosion and sediment redistribution are controlled by local wind-generated waves and tidal currents. Constructional features include salients, spits, gravel beaches, and a dune system on Lovells Island. The size of the bluffs and the rate of erosion control the supply of sediment to the inner islands and their intertidal regions. Salients are most common on the sheltered south or southwest shore of the islands. Bluff heights correlate with exposure and amount bluff retreat and northeast exposures experience the greatest rate of retreat; on Peddocks Island 81% are actively eroding. In contrast bluffs oriented NW have a low rate of erosion; only 15% of the total length. Locally generated wind waves impact the NW bluffs, whereas more powerful storm generated wave s affect NE bluffs.