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Coastal Erosion Assessment for Maine FIRMs and Map Modernization Plan
This study has confirmed that shoreline change has occurred along many of Maine's beaches at a rate that has made a number of Flood Insurance Rate Maps in need of revision. The rate of erosion along many beaches is on the order of 1 foot per year, so any FIRM that is more than a decade old has the potential to be in need of updating if there are beaches present. Some of the FIRMs in York, Cumberland, and Sagadahoc Counties are more than 10 years old. The oldest maps are in the City of Biddeford; they are now 20 years old. The shoreline of Biddeford also has very significant beach erosion along heavily developed dunes, so it stands out as the community most in need of map modernization.
Within any particular community only some FIRM panels have beaches and erosion is not constant on any given FIRM panel. In order to prioritize the need for updating communities, the "worst case" in shoreline change on each panel was used to calculate an amount of change since the shoreline was made. This report identified individual panels that are most in need of revision. As might be expected from a panel-by-panel analysis, the maps most in need of revisions are not all in the same municipality. Only some of the panels in a community need to be remapped due to changes caused by erosion or accretion along the coast.
Analysis of coastal sediment budgets and their influence on shoreline positions has demonstrated the large role of human activities in affecting the erosion rate on beaches. The anthropogenic influence on the coastal sand budget in the Wells Embayment in the last 40 years is equivalent to redistributing half of the sand volume of the frontal dune. In Saco Bay the rate of longshore drift has accelerated 300% since the Saco River jetties were constructed. The jetty influence on the Saco Bay sand budget has been to move over 4,000,000 cubic yards of sand within the bay in the last century. This report demonstrated the importance of understanding coastal sediment budgets in order to correctly interpret historical air photos on which most Erosion Hazard Areas are based.
The ability to soundly project future EHAs to inland areas with development can be accomplished through the use of several data sets. Notably, high-resolution orthophotographs are needed for a base to register historical air photographs in a geographic information system. Maine should receive such a product within the next year. The advent of airborne laser (LIDAR) to measure beach and dune topography has allowed three-dimensional analysis of beaches and dunes for EHA assessments and sand budgets. Maine has had only one such survey, but efforts are underway to provide a second data set from which comparison for shoreline change and dune elevations can be measured over a period of about 4 years.
The Maine Geological Survey has begun to examine a new method of estimating the erosion hazard in areas that have been "stabilized" by seawalls. The comparison of LIDAR beach profiles seaward of natural and engineered dunes appears to be useful in determining how much "out of equilibrium" the engineered shoreline has become over the last half century as the beach profile has lowered and the dune profile behind a seawall has remained static. This new approach has the potential to complement traditional methods of estimating EHAs and to identify the hazard posed if there were to be a catastrophic failure of seawalls during a 100-year storm event.
Last updated on February 8, 2006.
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