Models and Observations of Overwash Processes

Overwash is one of the few mechanisms that transports sand across barrier islands. Because it occurs primarily during storms, the process is difficult to observe and measure, and quantitative estimates of resulting erosion and deposition rely on before/after surveys that may, or may not, be timed to closely encompass the event. Recent advances in unmanned aerial systems and structure-from-motion algorithms now allow accurate, precise, and relatively inexpensive barrier-scale elevation maps to be made in a few hours with little advance preparation. New technology has brought low-cost pressure sensors and current meters that can be rapidly deployed. Pilot experiments using these methods were conducted on a sandy barrier spit (Sandwich Town Neck Beach, MA, USA) after winter storm Juno (26-27 January, 2015), when offshore waves with significant heights of 8 m and peak periods of 14 s were recorded at National Data Center Buoy 44013 located about 75 km north in Massachusetts Bay. The barrier spit was mapped with an unmanned aerial system two days after the storm. Comparison with lidar maps made in 2014 indicated that the dune bluffs had retreated more than 8 m, a 150-m wide breach had opened in an older overwash channel, and approximately 10,000 m³ of sand was eroded from the bluffs and dunes and deposited behind the barrier and on the downdrift end of the spit. A pair of tilt current meters with co-located pressure sensors was deployed on the seaward and landward ends of the newly formed overwash channel. The measurements from these instruments indicated net landward tidal transport over several tidal cycles and revealed that incident wave energy had been dissipated or converted to lower-frequency (infragravity) fluctuations that modulated open-channel flow across the barrier. These observations help constrain geomorphic models of overwash contributions to barrier evolution.