Quantifying overwash flux in barrier systems : an example from Marthas Vineyard, Massachusetts, USA

Abstract

Coastal barriers are particularly susceptible to the predicted effects of accelerated of sea-level rise and the potential for increased impacts of intense storms. Over centennial scales, barriers are maintained via overtopping during storms, causing deposition of washover fans on their landward sides. This study examines three washover fans on the south shore of Martha's Vineyard using a suite of data including vibracores, ground penetrating radar, high resolution dGPS, and LiDAR data. From these data, the volumes of the deposits were determined and range from 2.1-2.4 x 10 4 m 3 . Two overwashes occurred during Hurricane Bob in 1991. The water levels produced by this storm have a return interval of ~28 years, resulting in an onshore sediment flux of 2.4-3.4 m 3 /m/yr. The third washover was deposited by a nor'easter in January 1997, which has a water level return interval of ~6 years, resulting in a flux of 8.5 m 3 /m/yr. These fluxes are smaller than the flux of sediment needed to maintain a geometrically stable barrier estimated from shoreline retreat rates, suggesting that the barrier is not in long-term equilibrium, a result supported by the thinning of the barrier over this time interval.

Reply to the Discussion by Dillenburg et al. on “Evidence for a transgressive barrier within a regressive strandplain system: implications for complex response to environmental change” by Hein et al. (2013), Sedimentology 60, 469–502

“…Both features are readily apparent in GPR records as well as sediment cores/trenches through known washover deposits (e.g. Moller & Anthony, 2003;Wang & Horwitz, 2007;Carruthers et al, 2013). Such coupled geophysical and sedimentological records closely match the relative positions, radar morphologies and sedimentary characteristics of Unit IIIa-2 and the upper sections of Unit IIIa-3 of Hein et al (2013).…”