VOLUMETRICS AND SPATIAL DISTRIBUTION OF WASHOVER EVENTS AT BALLSTON BEACH AND THE MOUTH OF THE PAMET RIVER, TRURO, CAPE COD, USA

The Pamet River flows east to west and to Cape Cod Bay at Wellfleet. The headwaters are just landward of a frontal dune at Ballston Beach, Truro. The river is in a potential spring-sapped valley within the Wellfleet Outwash Plain. Elevations above sea level to the immediate north are 21 m and 17 m to the south. Presently, the river does not have an outlet due to a frontal dune 1 to 3 m high and a beach system that blocks exchange between the river and ocean. Ballston Beach faces east-northeast and is susceptible to storm surge created by northeast winter storms, southeast tropical storms and hurricanes. Past storm events have overtopped or breached the frontal dune by wave runup, or inundation at high tides coincident with storm surge.

Two large extratropical storms occurred less than a month apart in 2013. Winter storms Nemo (Feb. 8-9) and Saturn (Mar. 6-8) created high storm surge and large waves that breached the frontal dune and resulted in deposition of a large sand body that has buried existing river channels and marshes in the back dune environment. Since the breach, an artificial dune has been constructed to protect the area.

In 2013 and early 2014, the area was mapped to calculate the volume of sediment moved from beach and dune into the back dune marsh and river. We used a multi-proxy spatial and temporal framework to elucidate individual washover events and link the events to discrete storm events in the past 70 years and identify the linkages between fluvial and marine processes. Air photos that date to the 1940s, and high-resolution surficial mapping were used to construct a temporal and spatial understanding of recent events. Our surficial mapping was coupled with ground penetrating radar surveys to identify washover events and periods of enhanced fluvial activity that predate 1940.

At the time of submission, GPR surveys are planned for January, spatial analyses of air photos associated with the 2013 and 1991 storms are complete and results from earlier air photo sequences pending. Initial results suggest large storms of 1991 and 2013 created expansive washover events that extend in excess of 80 m landward of pre-storm high tide line. The breach point of the 1991 and 2013 events are not coincident with the 2013 breach occurring to the south and the 1991 breach to the north.