MEASURING ELEVATION CHANGES FROM OVERWASH AT BALLSTON BEACH, TRURO MA: SYSTEM EVOLUTION AND MANAGEMENT IMPLICATIONS

Ballston Beach is an open ocean beach located in Truro, Massachusetts, on outer Cape Cod. It is subjected to episodic overwash events during times of high storm surge. These events occur where an area of low elevation abuts the back side of the beach. The low-lying area behind the beach is the Pamet River Valley, named for the geologic feature that was formed by the recession of the Laurentide ice sheet during the late Wisconsinan. A dike was built in 1869 by the U.S. Army Corps of Engineers to control the influx of tidal waters, which in turn prevented the introduction of new sediment into the Upper Pamet River Valley. This blockage of the tidal channel prevents the elevation increase associated with the sediment deposition.

Topography of the washover fan and surrounding area is being studied in both a historical and contemporary timeline to document changes in extent and elevation. Data from topographic maps dating back to the late 1800s are compiled and analyzed to quantify the historical evolution of the area in general and the washover fan in particular. Current topographic data is being collected monthly from August 2017 through the winter 2017/2018 season. This is done using a Trimble R8, Real-Time Kinematic GPS (RTK-GPS) to collect three-dimensional positional data. Multiple transects running from the waterline to the farthest landward point of the washover fan are collected using the RTK-GPS system to track elevation change. Points along the perimeter of the washover fan are also collected to track the contemporary changes to the feature.

At the time of writing there has been one overwash event that has deposited sediment on the existing washover fan during the ongoing field campaign. The intention of this study is to document past overwash events in the depositional layers within the washover fan to create a local record of paleotempestological data. If possible, the historical data will be correlated to the stratigraphy to create and accurate timeline of overwash events. If a timeline of these events can be made from the stratigraphy, as well as a seasonal accretion, the collected data could prove useful information for the creation and validation of a predictive model of wave energy and erosion related to overwash events. This in turn could give insight into local coastal evolution, and lead to better management policies.