Nauset Spit was breached during a moderate northeast storm on 2 January 1987, forming New Inlet. Factors leading to the breach included a long-term narrowing of the barrier; a 0.5-m storm surge superimposed on near perigean, perihelion, and spring tide conditions; restricted flow through existing inlets, and large differences in tidal range and tidal phase between the ocean and Pleasant Bay. Ultimately, destruction of the foredune ridge and a large hydraulic slope across the barrier (> 1.2 m) facilitated the development of an overwash channel, which led to tidal exchange and inlet formation. As the channel captured an increasingly larger portion of the bay tidal prism, the inlet grew in size from 0.5 km after two months to almost 2.0 km wide by early 1988. Opening of New Inlet increased the tidal range from 1.2 to 1.5 m, which drastically changed the hydraulic character and sediment transport patterns in Pleasant Bay.

The opening of New Inlet washed much of the sand from the eroding barrier into the bay. As the dimensions of the inlet increased, the ebb-tidal delta grew in volume reducing the amount of sediment bypassing the inlet. Sand continues to enter the backbarrier building sand shoals and bedforms. The decrease in sand supply to the downdrift barrier caused 100 m to 300 m of shoreline recession immediately south of the inlet from 1990 to 2000. During the past six years the rate of erosion has decreased slightly due to more active transport of sediment past the inlet. The major mechanism of inlet sediment bypassing is accomplished through the breaching of hydraulically more efficient channels through the outer portion of the ebb delta. This process results in the formation of swash bars along the downdrift portion of the delta. These bars are 100 m to 300 m long and attach to the landward beach every 1 to 3 years. Permanent breaching of the spit platform has not occurred at New Inlet due to rapid accretion of Nauset Spit and backbarrier shoals directing ebb flow away from the spit platform.

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