Shoreline Change of Suffolk County’s Barrier Islands; A Coastal Erosion Analysis using Georectification of Historical Aerial Images Tom Parisi, Advised by Dr. Jase Bernhardt Hofstra University 2021; Dept. Geology, Environment and Sustainability, Hempstead NY, 11549 Introduction
Discussion
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100 Meters
• How has the shoreline of Long Island’s South Shore Barrier Islands changed over the past 100 years? • Which areas and inlets have experienced the greatest loss of land? • How can georeferencing be used to address coastal erosion?
Suffolk County Coastline, June 1938
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Research Questions
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Coastal communities worldwide are some of the most critically threatened regions by climate change. Global Sea Level Rise and subsequent increase in the erosion of sandy beaches is an increasingly pressing issue as the effects of Climate Change continue to take hold on our planet. In a recent report, the UN estimated that approximately 40% of the global population lives within 100 kilometers of the coast. These communities need targeted solutions in the short-term to reinforce their homes while the larger societal adjustments are made towards a more sustainable future. Like many climate solutions, the key is diversification of methodology—using various techniques to address regional issues that all work together on a larger scale. This project will focus on the ever-shifting barrier islands of Suffolk County, New York—highlighting their movement and suggesting new methods of addressing coastal erosion. Using historical aerial images from the 1938, we can arrange a mosaic that depicts the coastline of nearly 100 years prior. This can then be compared to modern-day satellite imagery and used to visualize the areas that have been most affected by coastal erosion and shoreline change.
GIS Analysis
Methodology
Georeferencing is a method of fitting raster datasets (i.e., hand-drawn maps, digital images, and aerial photographs) to a known spatial reference so that they can be used for spatial analysis. This process needs to occur for aerial photographs like the ones so that they are accurate for spatial analysis. The method of georeferencing that was used was georectification—matching well identifiable “control-points” on the aerial photographs to their respective locations on the present-day satellite imagery. These control points require careful consideration for aligning the pictures; they should be exact, unchanging points that are identifiable on both the photograph and the satellite imagery layers. Naturally, since these coastlines have changed significantly since 1938, this can cause uncertainty
When comparing the shorelines of June, 1938 to 2021 from Moriches Bay to Mauntauk Point, it is evident that many of the barrier islands have both accreted and eroded in different locations across the Island. Figure 1 shows the full extent of the picture set as a photomosaic of the 1938 Suffolk County Shoreline. The red outline represents the modern day shoreline underneath. Figures 2 through 7 show three examples of sediment change events: 2 & 3 show erosion thinning out the beach (West of Moriches Inlet), 4 & 5 show accretion widening the beach (Dune Beach) and 6 & 7 show the creation of a new inlet (Shinnecock Bay Inlet created a few months after these pictures were taken by a hurricane) connecting the Bay out to the Atlantic Ocean. Each of these examples are highlighted on Figure 1 with their corresponding border color square.
Conclusions Using georeferencing in this manner creates a unique view of shoreline change. In an area with coastal erosion as pronounced as Long Island, we can see the merits of this technique as a tool to visualize the effects. That being said, this is not a perfect method. Lack of control points in relatively low resolution images like these creates uncertainty in the precision of the spatial analysis. Barrel distortion is a concern with any georectification of images, especially those taken out of a plane nearly a century ago. An place that this might be useful in, however, is creating visualizations that help explain the issues associated with shoreline change to the communities it affects. Nearly 70% of the world’s sandy beaches will be affected by this phenomenon—it is crucial that the communities affected understand coastal erosion and how it pertains to their area.
References 1. Leatherman, S. P., Zhang, K., & Douglas, B. C. (2000). Sea level rise shown to drive coastal erosion. Eos, Transactions American Geophysical Union, 81(6), 55-57. Map data: ESRI World Imagery & Historical Aerial Photos from the Engineer Research & Development Center (https://rsm.usace.army.mil/shore/)