Impacts of Future Sea Level Rise on the Coastal Floodplain

Findings and Recommendations

Based on the results of this study, we are able to make the following findings:

With a 1-2 ft rise in sea level, high marsh, which currently dominates the study area, will be overtaken by a transgressing low marsh, steeper bank topography, and developed areas. The percentage of high marsh loss and low marsh gain is largest with a 1-ft rise in sea level.
In general, the Wells Beach portion of the study area appears to be able to sustain 1-2 ft of sea level rise without major flooding. Some minor flooding on HAT would be expected on the Webhannet River (marsh) side after 2-ft of sea level rise.
Drakes Island appears to be most susceptible to flooding after 1 or 2-ft rises in sea level due to its lower overall topography. Isolated flooding of private property can be expected during times of MHW and HAT after a 1 ft rise in sea level, especially along the low-lying areas adjacent to Drakes Island Road.
Public infrastructure (roads) on Drakes Island appears to be threatened by flooding after a 2-ft rise in sea level, mostly under HAT conditions.
The causeway leading to Drakes Island will be susceptible to flooding on +2 ft HAT conditions, and completely flooded under 3-ft sea level rise conditions.
Breach areas will likely become more susceptible to overwash and erosion as sea level rises. Key breach areas include: on Drakes Island, at the intersection of Drakes Island Road and Island Beach Road, and at the northeastern end of the seawalled properties along Island Beach Road; on Wells Beach, at the stretch of shoreline near the northern jetty that is unprotected by natural dune vegetation.

We also make the following recommendations:

Areas of potential flooding should be evaluated within emergency management plans in order to determine
1. designated evacuation areas;
2. breach points susceptible to flooding and overwash during storm events; and
3. specific water elevations that may trigger flooding of emergency transportation corridors (most notably Drakes Island Road).
We recommend identifying techniques that may help make these areas less susceptible to future flooding.
The existing bayside areas protected by seawalls should be evaluated in terms of their elevations in order to withstand future sea level rise, flooding, and storm events.
The Wells/Drakes Island jetties should be evaluated in order to determine appropriate future elevations of the jetties to withstand overtopping after sea level rises.
Communities should evaluate existing developed and open space areas in order to designate natural areas to allow for the natural transgression of marsh surfaces.
Upland areas identified as being vulnerable to marsh transgression could be targeted as prime areas for restoration, conservation and/or land-use planning.
Communities should evaluate the existing dune and beach areas in order to determine the best management techniques to help protect these natural resources (and habitat areas) from loss due to future sea level rise and the presence of sea walls.
Similar sea-level rise studies should be completed for other highly-developed and resource-valuable areas of the Maine coastline.
Future studies should be conducted to determine existing sedimentation rates of the marshes, and the rates that would be required to allow the marshes to keep up with sea level rise.
Future studies should be conducted to simulate non-static responses of the marshes and topography to sea level rise.
FEMA Flood Insurance Rate Maps may need to be updated in the near future as changes in sea level become more dramatic, causing the 100-year floodplain to migrate upwards and inland.

Contents Introduction Methodology Results Discussion Findings and Recommendations Conclusions References

Last updated on April 24, 2006