Sea Level Rise Effects on Roads & Marshes


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Sea Level Rise and Coastal Marshes


What are Coastal Marshes

Coastal marshes are the waving green fields of grasses along Long Island Sound’s shoreline and its saltwater tidal rivers, mostly within low-energy environments such as embayments. Here, protected from waves and strong currents, sediment accumulates providing conditions for salt marsh plants to grow. Located at the margin between land and sea, largely between the elevation of the average height of all tides and the highest tides of the year (see figure below), coastal marshes are among Connecticut's most susceptible ecosystems to the effects of SLR.

Connecticut's Coastal Marshes

Adapted from Warren Pinnacle Consulting


Coastal Marshes and Tides

Connecticut’s coastal marshes depend on a delicate balance between regular salt water inundation (high tide) and drying periods when tidal waters recede from the marsh surface (low tide). The heights reached by rising and falling tides and the marsh surface topography control the frequency and duration of saltwater inundation that define the marsh’s distinctive plant communities.

For centuries, Connecticut’s marshes have adapted to increases in SLR in two ways. The first is by increasing the elevation of the marsh surface. This happens through the accumulation mineral and organic material deposited on the marsh by tidal and riverine flooding as well as complex subsurface marsh expansion processes. The second is by migrating landward or upland onto undeveloped land not previously inundated by regular saltwater flooding. 


As SLR Out-paces Marsh Adaption

In some areas, as the rate of SLR out-paces a marsh’s ability to increase its surface elevation, the lowest part of the marsh flooded daily (called ‘low marsh’), becomes too wet for marsh plants to survive resulting in their conversion to tidal mud flats or open water. Similarly, as the less frequently flooded parts of the marsh (called 'high marsh') is flooded more frequently, this part of the marsh converts to low marsh which supports a distinctly different plant community. In some cases, the highest elevation areas of marsh, called transitional marsh or marsh border, transitions to high marsh and the transitional marsh migrates to previously dry upland.

However, where barriers to marsh migration such as steep slopes or developed land prevent migration, the total area of marsh may decline or change from a high marsh to a low marsh dominated plant community.

Although SLAMM assumes that existing undeveloped upland areas regularly inundated by saltwater will support new marsh, the model recognizes that developed upland areas regularly flooded in the future are not suitable to accommodate marsh migration. Therefore the model does not project new marsh within newly flooded development landscapes.. One of SLAMM’s limitations is that it does not recognize that some of the undeveloped lands that will become regularly flooded that it predicts will become new marsh will be developed, filled or otherwise fortified over time prevent the right soil conditions or regular tidal water inundation needed to sustain marsh migration . Given these limitations, SLAMM likely overestimates the total amount of future new marsh.

Barriers to Marsh Migration


Scientists are just beginning to understand all of the implications of these kind of changes to the ecosystem services coastal marshes provide, such as nutrient sequestration, shoreline erosion control. and breeding habitat for marsh-dependent wildlife species, such as the Saltmarsh sparrow that breeds only in the high marsh.