Tidal Inlet Migration "Cycles" and Tidal Hydrodynamics - a Cautionary Tale

Under natural conditions, tidal inlets in barrier beach systems frequently exhibit a sequence of change characterized by (1) downdrift migration of the inlet, (2) decreasing hydraulic efficiency of the system, and (3) new inlet formation updrift of its predecessor. Such a quasi-cyclic pattern, with a time scale of approximately a century-and-a-half, has long been recognized as characteristic of the southern (Chatham) portion of Nauset Beach in Cape Cod National Seashore. When a major new inlet formed in 1987, approximately 140 years after the previous (1846) iteration, it was widely believed to mark the beginning of another long-term cycle of inlet migration. However, the next major inlet formed just 20 years later, in 2007, leading us to seek a better understanding of the conditions associated with initiation of the historic long-term cycles.

Historical maps indicated that, like the 2007 inlet (but not that of 1987), past long-term cycles were initiated near the extreme northern limit of the historical inlet migration zone. This limit is east of the Pleasant Bay basin and immediately south of the tidally distinct Little Pleasant Bay basin. Field observations revealed that strong tidal currents at the north side of the 2007 inlet were hydraulically associated with Little Pleasant Bay, and were the primary cause of a half-kilometer post-breach retreat of the barrier beach north of the inlet with loss of resident beach cottages.

These observations, together with tidal changes recorded in Little Pleasant Bay, are consistent with the hypothesis that initiation of long-term Nauset Beach tidal inlet cycles requires a high degree of hydraulic connectivity with Little Pleasant Bay – a connection that was lacking in the case of the previous (1987) inlet.