Understanding the Short and Long-term Shoreline Change of Napatree Barrier Using RTK-GPS Beach Profiles and Mapping of the Last High Tide Swash
The objective of this project, which began in July, 2013 is to begin to understand the geologic processes and will examine the short and long-term response of the shoreline to storms using beach profiles and the position of the last high tide swash (LHTS) (commonly referred to as the wet-dry line or wrack line). The lack of infrastructure and development on the barrier make Napatree an ideal location to examine shoreline change in the absence of the ‘line in the sand’ mentality inherent on more developed portions of the coastline. Storms (both hurricanes and extra-tropical cyclones), not sea level rise, are the driving force in shoreline change (Hayes and Boothroyd, 1987), particularly at the decadal scale (Morton, 2008). Napatree Barrier has migrated >60 m (200 feet) landward (towards Little Narragansett Bay) between 1939 and 2004 (Boothroyd. and Hehre, 2007; Boothroyd et al., 2016), via storm surge overwash and deposition of washover fans during hurricanes. This means the landform itself has migrated landward almost one barrier width since 1939!
Last High Tide Swash (LHTS)
The position of LHTS (also known as the wet – dry line) is being mapped quarterly using a Trimble 6000XH handheld differential Global Position System. By using LHTS as a proxy for shoreline position, it can be directly compared to shoreline positions digitized in historic vertical aerial photographs, as well as present and future orthophotographs. Additional shoreline positions will be collected quarterly, as well as before and after major storm events.
Beach profiles are being collected using RTK-GPS, which collects an X,Y, Z (northing, easting relative to Rhode island State Plane Feet, 1983 and elevation relative to NAVD88) for points along the profile (Figure 3). Elevation is collected relative to NAVD88, and is converted to mean lower low water (MLLW) using the V-Datum tool published by the National Oceanic and Atmospheric Administration. Profiles extend as close to MLLW as possible, and traverse the barrier from Little Narragansett Bay to Block Island Sound.
While too early in the project to reach any significant conclusions, monitoring will continue quarterly throughout 2013 and 2014.One of the very obvious trends is the steady increase in foredune elevation, which is what is expected during a period of fair-weather. The trend is very clear for profiles NAP1,2 and 5; less clear for 3 and 4, although why that is needs further investigation.
A summary of the project through the fall 2015 can be found in the 2013 State of Napatree report (SoN2015) compiled by Janice Sassi (Watch Hill Conservancy) and edited by Peter August, URI Environmental Data Center.
This project has been funded through August 2017 by the Watch Hill Conservancy and the Watch Hill Fire District
Boothroyd., J. C., and Hehre, R. E., 2007, Shoreline change Maps for the South Shore of Rhode Island: Rhode Island Geological Survey, scale 1:2,000.
Boothroyd, J.C., Hollis, R.J., Oakley, B.A., and Henderson, R.E., 2016, Shoreline Change from 1939-2014, Washington County, Rhode Island. 1:2,000 scale. Rhode Island Geological Survey. 45 maps
Oakley, B.A., Hehre, R.E., Rasmussen, S.A., and Boothroyd, J.C., 2013, Shoreline change Maps for the South Shore of Rhode Island 1939-2012: Rhode Island Geological Survey, scale 1:2,000 – In prep.
Plotted profiles can be downloaded here: