Sea Level Rise Effects on Roads & Marshes



The total area of Connecticut’s 21 largest coastal marshes is expected to increase through the end of the century. However, the projected increase is contingent upon marsh migration areas remaining undeveloped and subject to regular tidal water inundation.

It is expected that some potential migration areas will be unavailable to support new marsh due to development and additional restrictions to the flow of tidal water through practices such as the placement of fill, construction of seawalls and tide gate modifications.

Change in Total Area of Connecticut's 21 Largest Marshes*

(only marshes >33% probability of occurring)

*Results include only marsh with greater than 33% probability of occurring in future determined using SLAMM’s uncertainty module, as described in How SLAMM Works.


Although SLAMM predicts that the total area coastal marsh will increase, the model predicts that Connecticut’s future marshscape will be quite different. Modeling results indicate that Connecticut’s coastal marshes will change from high marsh to low marsh dominated systems.

CT Marsh Type 2010 (acres)
CT Marsh Type 2100 (acres)
   Other Marsh    High Marsh


Although all the implications of such change are not fully understood, predicted changes in marsh composition will alter some saltmarsh ecological services. For example, habitat for high marsh dependent species, such as the saltmarsh sparrow, is expected to decline.


As of 2010, SLAMM identified approximately 5.75 miles (not continuous) of Connecticut’s coastal roads that are regularly (at least every 90 days) inundated by tidal water from tidal (non-storm) flooding. SLAMM projects that at a rate of about 4 feet of sea level rise by 2100 (SLAMM’s medium-high SLR scenario described at Information), tidal road flooding will increase to approximately 43 miles by 2040 and 79 miles by 2055 (see the following table).  By 2055, 95 percent of tidal road flooding is expected to occur on municipally-controlled roads placing a significantly greater burden on municipal highway, public works and emergency service personnel responsible for managing road flooding. It’s important to recognize that SLAMM’s road flooding data does not identify road flooding depths or duration. Therefore, predicted future coastal road flooding cannot necessarily be equated with vehicle access/egress restrictions. Limited field verification of existing conditions (year 2010/2025) coastal flooding identified road floodwater depths ranging for less than an inch to depths that have resulted in temporary road closures by municipal departments of public works.

Tidal and Storm Road Flooding Frequencies

Tidal Flooding Frequency (days) and extent (miles) Total Tidal Flooding* Storm Flooding
Year 30 day 60 day 90 day 10 year 100 year
2010 N/A 2.50 3.24 5.75 139.84 218.93
2025 6.32 5.38 4.78 16.48 164.92 216.25
2040 24.32 9.73 8.41 42.46 180.33 214.92
2055 53.56 12.28 13.47 79.31 185.13 208.76
2085 173.35 16.33 13.91 203.59 175.83 193.93
2100 242.21 16.76 12.61 271.58 169.89 189.16

* SLAMM road flooding processing was unable to assess 30-day frequency road flooding in 2010 and is therefore reported as not available (N/A). It is estimated that less than 1 mile of 30-day frequency flooding occurred at this date. Total tidal road flooding differs slightly from the sum of road flooding frequency intervals due to rounding.


Flooding Frequencies Explained

The road flooding frequencies represent tidal flooding occurring at least every 30, 60 or 90 days as well as storm-influenced flooding with 10 year and 100 year frequency of recurrence intervals. Each flooding class should be interpreted as a histogram bin with the frequency of flooding being less than the upper limit of the preceding bin but more than (or equal to) the upper limit of the flooding bin.

  • 30 days - areas flooded at least once every 30 days
  • 60 days - areas flooded less frequently than once every 30 days but at least once every 60 days (e.g. areas flooded once every 40 days would belong to this flooding class
  • 90 days - areas flooded less frequently than once every 60 days but at least once every 90 days
  • 10 years - areas flooded less frequently than once every 90 days but at least once every 10 years. Therefore, areas that get flooded once every 2 years belong to this flooding class.
  • 100 years - areas flooded less frequently than once every 10 years but at least once every 100 years


A significant increase in regular tidal flooding of coastal area roads is expected to occur as early as 2025, with a dramatic increase after 2055 (graph, below).

Statewide Road Tidal Flooding Frequency Change
Floods at least once every:
   30 days
   60 days
   90 days