Development of a new morphometric to assess beach storm response and recovery

Wednesday, 17 December 2014
Owen Brenner and Cheryl J Hapke, U.S. Geological Survey, St. Petersburg, FL, United States
Various morphometrics are used to measure coastal change over a variety of time scales including shoreline, dune elevation and position, and beach profile volume. Each has limitations, many of which became apparent in the aftermath of Hurricane Sandy, including the juxtaposition of levelled dunes and a substantially prograded shoreline. In order to understand sustained beach behavior, including recovery after Hurricane Sandy, we develop a new morphometric – an upper beach change envelope (BCE) specific to Fire Island, NY. The upper beach better captures impacts from more frequent moderate storms during which there may be substantial beach change but less impact to the dune, and is less subject to the variable fluctuations nearer to the shoreline that only marginally influence future vulnerability and overall coastal resilience. The BCE can also be used to quantify the gradual recovery of the beach after storm events and is not reliant on the presence of a morphologic feature such as a dune, which may take many years to recover after a severe storm.
The BCE at Fire Island is based on a time series of historical response to storms. The BCE boundaries are elevation contours that capture the portion of the upper beach that experiences erosion during moderate nor’easter events but is above the influence of tides and lesser events.
In an application of the BCE concept, we use the BCE boundary elevations to quantify beach response from Hurricane Sandy and document the subsequent recovery, using a time series of post-Sandy elevation contours. The data include 10 profile sites from Fire Island that were surveyed multiple times from October 2012 to June 2014. Utilizing this time series we measure changes in the cross shore position of the BCE elevation boundaries. Initial assessments indicate the BCE successfully captures coastal response through time, including extensive change during Hurricane Sandy as well as subsequent seasonal changes. The recent data indicate there is a temporal trend towards widening of the BCE, due to sustained progradation of the lower boundary. This trend may represent a new “recovery state” of the beach which is wider than the pre-Sandy beach, providing an increase in the fetch that favors the aeolian processes of dune reformation.