GEOLOGICAL EVIDENCE FOR STORM BREACHING OF MICROTIDAL BAYMOUTH BARRIERS, UPPER CAPE COD, MASSACHUSETTS

The south-facing shoreline of Upper Cape Cod is characterized by a series of short mixed-sediment barriers fronting elongated salt ponds, which occupy proglacial spring-sapping valleys. This region experiences severe erosion from hurricanes and southerly extra-tropical storms, with episodes of extensive overwash occurring in the 1860s, 1930s, and mid-1950s. Shoreline change analyses indicate an eastward increase in the average annual retreat rates from 0.1 m/year at the Falmouth Heights headland to 1.4 m/year along the rapidly retrograding Eel Pond barrier (1846-1975 period; MCZM). Recent overwash events (e.g., Hurricane Bob, 1991) are represented by interbedded dune sands and coarse, gravelly washover sands, however, historically the most dramatic environmental changes are associated with barrier breaching.

Historical charts, aerial photography, geomorphic mapping and 8 km of ground-penetrating radar profiles were used to examine the patterns of breaching and inlet histories along five developed barrier systems between Oyster Pond and Bournes Pond. Inlet-related geomorphic features include localized areas of low elevation across the barriers and small flood-tidal deltas in the ponds. Subsurface images confirmed the historical inlet positions and revealed at least five former channels not present on the charts. The smaller channels (width: 15-30 m; depth: 2-3 m) were likely ephemeral storm breachways, whereas more permanent inlets had greater cross-sectional areas (width: 30-80 m; depth: 3-5 m). Although most inlets experienced little or no lateral migration, historical and geophysical data suggest that channel-fill sequences comprise 20-60% of the barrier lithosomes. Man-made modifications (e.g., road construction, emplacement of groins, channel relocation, and opening of new stabilized inlets) have dramatically affected the local coastal dynamics. These efforts, combined with small tidal prisms of the five salt ponds, have prevented the rapid segmentation of the barriers similar to that experienced at Eel Pond. This study provides a complementary field database on the former inlet locations that must be considered in future navigation dredging and shoreline management plans.