UPDRIFT INLET MIGRATION OR SHIFTING NODAL POINT? RECENT AND ONGOING TIDAL INLET EVOLUTION AT NAUSET INLET, CAPE COD, MASSACHUSETTS USA

It was hypothesized by Aubrey and Speer (1984) that Nauset Inlet had been migrating updrift (northerly) since the early 1950s. The inlet has continued to migrate in a northerly direction, with episodic new inlet formations resulting from storms events. Those new inlet positions are typically to the south and in a more hydrodynamically efficient location, with northward migration resuming post-storm. A map from 1779 shows the inlet in the southernmost portion of the embayment adjacent to a headland, as do subsequent historical maps through to the early 20^th century. In these maps, no barrier spit extending from the headland, south to north was present. In aerial photographs from 1938 to the early 1950s the inlet is also in the southern portion of the embayment when the northward migration commences and spit creation and elongation from the southern headland can be seen.

Nauset Marsh is on a mixed-energy, wave-dominated coast. The tidal range seaward of Nauset Inlet is 2.0 m. The rate of longshore sediment transport is approximately 150,000 m³/yr. Since 1950 there were several episodes of inlet migration, new inlet formation, and subsequent northward migration. During inlet formation, the new inlet was invariably in a more hydrodynamically efficient position and several years later the older inlet closed. From 1950 to 1992 the inlet migrated a total of 3.5 km to the north (~83 m/yr). Given the hydraulics of the system it is unlikely it could migrate further northward. In 1993 a new inlet formed 1.5 km to the south, the northern inlet subsequently closed and the new inlet migrated back to pre-1993 location by 2009 (~94 m/yr) after which migration slowed considerably due to the hydraulically inefficient location.

We propose, that rather than an anomalous, consistent updrift migration from 1950 to the present, a shift to the south of the nodal point is an alternative hypothesis worthy of consideration. Studies documenting the changing orientation of the outer Cape Cod shoreline and subsequent alterations of sediment transport patterns during the Holocene and in the 19^th and 20^th centuries are consistent with the southern movement of the nodal point. Thus, the migration seen at Nauset Inlet is likely not an ongoing, anomalous phenomenon, but is a result of a shifting nodal point.