Nantucket Island's Shifting Shoals and Moving Shores: Near-shore Bathymetry Controls Beach Deposition and Erosion

Historical records and our observations show that the near-shore shoal complex of Pochick Rip and Old Man Shoal has slowly but continuously built to the north along Nantucket Island's eastern shore. This shoal system has served to moderate the strength and direction of waves breaking on the shore, specifically on Low Beach running along much of Nantucket's eastern coast, thus slowing the velocity of long-shore currents, and promoting sand deposition on the beach. Low Beach is not stable. Both the beach and the coastal bank backing and flanking it have experienced recorded episodes of erosion, retreat, flooding, and redeposition from 1814 to the present. We conclude that these episodes result from breaches or changes in the protective shoals, resulting in deeper water lying closer inshore, and permitting larger waves to break on and erode the beach and bank. Shoals may also concentrate or diffuse wave attacks on beaches, resulting in erosional or depositional events.