Implications of Management Practices in Park-Adjacent Areas: New Inlet Formation in Cape Cod National Seashore

Two inlets have formed through the Nauset Beach barrier spit within the boundaries of Cape Cod National Seashore during extratropical storms in 1987 and 2007. The 1987 inlet was part of a 140 year cycle of spit elongation, increasing hydraulic inefficiency and inlet formation. The 2007 inlet did not form as a result of increases in hydraulic inefficiency associated with spit elongation. This study examines management practices outside of park boundaries and associated impacts on sedimentary processes within Cape Cod National Seashore; specifically, the construction of revetments on the mainland and the dredging and placement of dredged material in Chatham Harbor.

The main tidal channel in the central portion of the harbor was relatively stable for 54 years (1938-1992); after which a combination of a migrating flood-tidal delta and expanding intertidal areas restricted flow causing the channel to shift position. Migration of the flood-tidal delta was caused by increased sediment supply and tidal current velocities after the 1987 inlet formation. Expansion of the intertidal zone has two major components. First, is a 34% decrease (65,100 m²) in the surface area of Tern Island, in the central harbor from 1989 to 2007. Second, is the fate of a minimum of 139,270 m³ of dredged material placed on and around the island from 1987 to 1998. This volume of material was sufficient to transform a predominantly sub-tidal area into an intertidal area and redirect ebb-tidal flow.

The tidal channel migration increased the volume of water directly adjacent to the backbarrier shoreline. The expanding intertidal zone in the area caused a restriction which increased the hydraulic head during ebb-tidal flow. These are two of the conditions that increase the potential for new inlet formation through low-lying barrier islands and spits and contributed to the inlet formation of 2007.