A 2016 risk-based comprehensive modeling effort conducted by Lloyds of London/Nature Conservancy evaluated the effects of marsh systems on upland damage during Superstorm Sandy. The report estimated that coastal areas with large marsh systems contributed to a 10% average reduction in property damage within the associated census tracts, with damage reduction benefits in certain areas reaching as high as 29%.
Superstorm Sandy’s storm surge rose through the Back Bay and into the mouth of Mill River, flooding over 2,500 acres and 4,000 parcels in the LWTB project area. Nearly 3,300 parcels (80% of the total parcels) were residential properties. Based on research in the marsh areas of the project area, there has been significant loss of salt marsh in the Back Bay that hindered the marsh’s ability to attenuate wave action. Human-related impacts, such as upland urbanization and increased boat use have resulted in marsh loss since 1966; although several natural factors can also be correlated with marsh loss within the project area. Environmental conditions such as wind fetch, coastal storm impacts, and tidal flows have all played a role in marsh loss.
Existing marshes in the project area are facing two significant problems that must be addressed if the marshes are to maintain their storm protection capabilities and current natural function:
- Chronic erosion losses at the marsh fringes due to waves and boat wakes.
- Degradation and loss of marsh areas due to the effects of sea level rise.
Reducing the erosion of the marsh fringes and increasing the long-term stability of multiple marsh environments, are key RBD LWTB principals. Restoring the marsh will provide additional wave attenuation, while at the same time resulting in the co-benefit of habitat restoration.
FIGURE 8: DIFFERENCE IN WAVE ATTENUATION WITH AND WITHOUT TIDAL MARSH
Rock sills are a common living shoreline technique for protection of fragile marsh edges. They dampen wave energy that would otherwise erode the unstable marsh fringe area. The sills can be built with an edge to allow the use of dredged material to fill the marsh areas to higher elevations. Planting a diversity of vegetation helps the newly filled areas transition into high marsh habitat; offering improved resilience to changing environmental conditions and future extreme storm events.
Floating marsh islands are another technique to protect eroding marsh edges. Floating islands are designed to mimic the natural floating marsh systems found in Louisiana and other coastal locations. Marsh plants begin to grow on mats of floating reeds to form a tightly bound mass of vegetation that is not rooted in the bottom of the water body. Artificial floating islands are constructed of durable, recycled plastics and are vegetated with native plant materials. The floating island modules are bound together and the system is anchored immediately offshore of the marsh edge. They dampen wave energy that would otherwise erode the unstable marsh fringe area. This allows the marsh system to maintain its present level of storm surge and wave attenuation.
Fifteen existing areas (with a total area of 26.9 acres) will be preserved by using rock sill with the floating marsh islands. The marshland in these areas will also be enhanced by raising many of the marshlands. Major feeder channels, open water and ponds will be avoided to reduce the impact area and to maintain hydrologic connection for marsh sustainability. There are some open water areas that may be filled with a preliminary estimate of the post-improvement marsh being approximately 70% tidal flats and 30% open water. At the preliminary conceptual design phase, there are seven areas proposed for the rock sill alternative, totaling 5,572 linear feet (LF) and a total of five areas are proposed for the floating marsh alternative, totaling 6,858 LF.
FIGURE 9: NORTHERN MARSH IMPROVEMENT AREAS
FIGURE 10: SOUTHERN MARSH IMPROVEMENT AREAS