College of Agriculture and Natural Resources
Center for Land Use Education and Research
A CLEAR Mini Research Project

 

Contact the Geospatial Training Program for more information.

Researching Coastal Erosion at Griswold Point

 

Background

Griswold Point is a sand spit located in Old Lyme, Connecticut, just east of the mouth of the Connecticut River. Long Island Sound borders the spit to the south, and a shallow lagoon, marsh and estuary lie to the north. Over the winter of 1992-1993 a series of Nor'easters (extratropical storms) caused a breach in Griswold Point. Since this time the spit has rapidly eroded, leaving only an area approximately a third of the original length intact (though not unchanged). The rest of the spit exists as patches of shifting sand that span its original area and have accreted onto the marsh to the north and west.

The available historic record of Griswold Point indicates there has been general stability over its long term history, with periods of instability and change producing several different morphological configurations. The entire spit also has migrated tens of meters northward over its history, narrowing the marsh and lagoon space behind it. For tens of years prior to the breach, the spit experienced trends of lengthening and thinning. This may have made the spit more susceptible to breaching during the winter storms. Griswold Point has breached in the past with the last documented breach occurring in 1954. This breach, however, healed itself in only a short time. Though Griswold Point has always been a dynamic environment (as all beaches are) the amount and timescale of the present erosion is unprecedented.

This project aims

  • to document and evaluate the trends in the short-term and long-term erosional and depositional history of Griswold Point using both remote sensing and ground surveying techniques,

  • to develop a methodology to use these geospatial techniques in areas with similar conditions,

  • and to evaluate the effectiveness and accuracy of the study's methodology and use of technology.

1991 vertical aerial photo of Griswold Point
1995 vertical aerial photo of Griswold Point
2000 vertical aerial photo of Griswold Point

GIS and Remote Sensing

Orthorectified vertical aerial photographs can be used to obtain positional and temporal information about the size, shape and location of Griswold Point. Trends in the length, area and thickness of Griswold Point were obtained from a time series (1934-2000) of vertical aerial photographs. The outline of Griswold Point was digitized from each photo using ArcGIS and the outlines were overlaid to observe changes and trends. The length and area of the spit as well as other spatial information, such as the width of the spit or lagoon, can also be calculated in a GIS program. Using a reference photograph, and reference points within the photographs, estimates of error can be calculated.

The extension and morphological changes in Griswold Point for the years 1957, 1965, 1991 and 2000 using ArcMap software.

High resolution multi-spectral satellite images display information not found in black and white photography. The multi-spectral images contain blue (450-520 nm), green (520-600 nm), red (630-690 nm) and near-infrared (760-900 nm) bands. Different landscape features (vegetation, sand, water, etc.) reflect and absorb light at different portions of the spectrum. By analyzing the spectral data in multi-spectral images, landscape features can be differentiated and mapped. Two recent (July, 2003 and May, 2004) high-resolution multi-band images of Griswold Point are available for this study and additional images will be acquired during the summer and fall of 2004.


July 2003 Quickbird data displayed as a false-color infrared image.

Surveying

Positional data is acquired in the field with sub-centimeter accuracy surveying equipment. The surveying is accomplished by setting up a base station and a series of transects perpendicular to the beach. The surveying instrument is located at the base station and takes readings to a measured pole holding a prism. This pole is carried along each transect and stops at every change in slope or substrate to record a reading. The measurements are displayed as a series of X, Y and Z coordinates (a northing, easting and elevation coordinate) which can be plotted to make a topographic map. These data also can be graphed to show the profile of the beach and calculate the volume of sand. These data are being collected regularly to document the short-term changes at Griswold Point, including sites of erosion and deposition.


Surveying transects at Griswold Point

To convert the surveyed points to Connecticut State Plane Coordinates so they could be used in a GIS, a series of benchmarks (points with known coordinates and elevations) were installed near Griswold Point. These were established by Dr. Tom Meyer from the University of Connecticut using GPS (global positioning system) equipment capable of detemining a field position to within centimeters of its true position. The benchmark positions were established using a procedure where two GPS receivers occupy two different points to collect concurrent signals from orbiting satellites. These field positions were then adjusted using correction data collected at a nearby base station.


Dr. Tom Meyer, UConn Dept. of Natural Resources Management and Engineering, using a
high-precision GPS receiver to determine the coordinates for a project benchmark

This study was supported in part through a graduate student fellowship from the Connecticut Space Grant College Consortium.