Impacts of Future Sea Level Rise on the Coastal Floodplain

Discussion

Implications for Marsh Habitat

Results of the sea level rise scenarios indicate that a 1-ft rise in sea level may have significant impacts on the existing marsh habitat by drastically increasing the area of low marsh (by over 100%), while substantially decreasing the area of high marsh by almost 50%. This is mainly due to pinching out of high marsh along steeper sloped upland areas. 2-ft of sea level rise would increase these percentages to 144% and -65%, respectively. This would dramatically change the overall dominant marsh type and habitat within the study area. Additionally, uplands within the study area (i.e., areas above HAT) would undergo 18-25% losses under 2-3-ft rise in sea level scenarios. It appears that the existing high marsh within the study area is near its maximum area, and cannot expand much more due to the steeper slopes that remain (hence the pinching out of high marsh).

The assumption that marshes would be able to keep up with sea level rise through increased sedimentation rates may not be correct. It is quite possible that increasing open water areas (which increase by 33% under a 2-ft rise in sea level scenario) would substantially alter channel morphology of the Webhannet and Little Rivers and their tributaries, which could subsequently change flood-and-ebb tidal current patterns. In turn, this could lead to increased erosion of marsh surfaces and possible removal of eroded material from the marsh system, resulting in increased open water, and a decrease in projected marsh growth. It is not within the scope of this project to simulate these occurrences, but it is important to note these limitations and possibilities.

Also, areas of marsh transgression are assumed to occur even where development (i.e., developed lots) may impede the natural transgression of marsh surfaces. This would play a large factor in the ability of the high marsh to successfully transgress onto higher topography; thus, it could be expected that the projected reduction of high marsh area may actually be lower than what would actually occur.

Flooding Implications

Results of the sea level rise scenarios raise some important issues regarding flooding along Wells Beach and Drakes Island.

The portion of Wells Beach within the study area appears, in general, to fare well under the different flooding scenarios of 1, 2, and 3-ft of sea level rise. The seawalls that line the Webhannet River side of Wells Beach appear to be of adequate elevation to prevent overtopping under 1 and 2-ft sea level rise scenarios. Minor inundation does occur in +2 ft HAT conditions, and is more widespread after 3 ft of sea level rise.

Even a 1-ft rise in sea level may have major implications regarding the future flooding of private property and public infrastructure, especially along the lower-lying Drakes Island Beach during times of MHW and HAT. With 2-ft of sea level rise, flooding becomes more pronounced and starts to inhibit emergency access to portions of the island. Should a coastal storm coincide with the HAT (or even MHW) and 1-2 ft of sea level rise, vehicular access to Drakes Island could be severely compromised, with possible flooding of the causeway completely cutting off Drakes Island from the mainland. Such information is vital to the preparation of future county and regional emergency management plans.

Although there does not appear to be any flooding impact on the oceanfront properties under the sea level rise scenarios simulated, none of the scenarios take into account superimposed storm water levels and waves on top of the water elevations simulated. One could expect 1-3 ft of additional water on top of these elevations during storm events. This could have a major impact on oceanfront lots along both Wells and Drakes Island, where overwash and minor flooding already are known to occur during storm events.

We have identified several potential breach locations (which already exist) which will become more susceptible to overwash and erosion as sea level rises. Most notably are those identified in Figure 13 and Figure 16 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, breach points are located at the stretch of shoreline near the northern jetty that is unprotected by natural dune vegetation, and just south of this location. Additionally, the inner-most portions of the jetties may be overtopped in scenarios of 2-ft sea level rise and HAT conditions (Figure 14 and Figure 17).

Contents Introduction Methodology Results Discussion Findings and Recommendations Conclusions References

Last updated on October 4, 2006