Napatree Point – Shoreline Change

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), 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!

 

Installing a benchmark at Napatree, July 2013. Air temperature >90F!.  Did I mention I trust my colleagues? Also pictured, Josh Bartosiewicz (Eastern) and Jessica Cressman (Napatree Point Conservation Area, Watch Hill Conservancy & Watch Hill Fire District). Photo by Janice Sassi (Napatree Point Conservation Area, Watch Hill Conservancy & Watch Hill Fire District)

Installing a benchmark at Napatree July 2013. Air temperature >90F!. Also pictured, Josh Bartosiewicz (Eastern) Photo by Janice Sassi (Napatree Point Conservation Area, Watch Hill Conservancy & Watch Hill Fire District)

The final product; brass benchmark connected to 28 ft of 9/16″ stainless steel survey rod. In theory, these will survive a severe storm, allowing us to easily locate and reestablish transects.

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.

Last High Tide Swash (LHTS) (aka the wet-dry line) derived shoreline positions from 1939 (Boothroyd and Hehre, 2007), 2012 (Oakley et al 2013) and July, 2013 (collected using differential GPS)

 

Last High Tide Swash on the Little Narragansett Bay shoreline, Napatree Point: 18 December 2013

Beach profiles

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.

Eastern Connecticut State University student Josh Bartosiewicz collecting a RTK-GPS beach profile on 23 July 2013

Plotted profile of NAP-1, on the east end of the Napatree Barrier (see location map above). The portion of the profile in green represents a minor amount of berm deposition on both the Block Island Sound and Little Narragansett Bay shorelines.

Profile comparing the 2011 U.S.G.S. LiDAR data with RTK-GPS beach profile from 23 July 2013. Note significant erosion of the foredune (red), interpreted to have occurred during Hurricane Sandy.

 

While too early in the project to reach any significant conclusions, monitoring will continue quarterly throughout 2013 and 2014.

A summary of the project through the fall 2013 can be found in the 2013 State of Napatree report (SoN2013_Final)  compiled by Janice Sassi (Watch Hill Conservancy) and edited by Peter August, URI Environmental Data Center.

This project has been funded through August 2014 by the Watch Hill Conservancy and the Watch Hill Fire District

The Watch Hill Conservancy

 

The Napatree Headland, 16 Aug 2013

Napatree Lagoon, 16 Aug 2013

References

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.

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:

http://www1.easternct.edu/oakleyb/files/2014/10/NAP-1_July2013-Sept2014.pdf

http://www1.easternct.edu/oakleyb/files/2014/10/NAP-2_July2013-Sept2014.pdf

http://www1.easternct.edu/oakleyb/files/2014/10/NAP-3_July2013-September2014.pdf

http://www1.easternct.edu/oakleyb/files/2014/10/NAP-4_July2013-Sept2014.pdf

http://www1.easternct.edu/oakleyb/files/2014/10/NAP-5_July2013-Sept2014.pdf

 

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