Sea Level Rise Effects on Roads & Marshes

Information

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Sea Level Rise and Coastal Roads

Low elevation coastal area roads are those just a few feet above the elevation of normal high tides. They are already vulnerable to coastal flooding. SLAMM’s road flooding results indicate that in some areas the extent and frequency of coastal area road flooding will increase significantly at a sea level rise rate of about 16 inches by mid-century, slightly less than the 20 inch rise by mid-century recently adopted for planning purposes pursuant by the State of Connecticut pursuant to Public Act 18-8.

 

SLAMM Model and Roads

SLAMM identifies future SLR-influenced road flooding frequencies by individually assessing 5 meter road segments. This is advantageous because even small areas of road flooding can potentially limit access and egress to neighborhoods served only by a single road. It is important to note that SLAMM currently does not provide road flooding depths. In some cases, road segments projected to be ‘flooded’ will only be wet, but passable, and in other cases, road flooding depths could preclude safe vehicular access and egress. The report A Vulnerability-Assessment Tool that Will Quantify the Future Risk of Inundation for the Road Segments and Critical Infrastructure contains more detail on how SLAMM’s road flooding results can be used to assess future road flooding risks. Additional investigation is required before likely approximate road flooding depths can be determined.

 

In some areas, like the neighborhood shown below, extreme high tide flooding not influenced by storms will increasingly limit dry access to areas served by low elevation roads.

 

Existing Road Flooding

Road Flooding by 2055 with 20" SLR

Although only small sections of the road shown below already flood as often as every 30 to 60 days, by mid century, all of the roadway is expected to flood at least once every 30 days. However, because SLAMM does not currently identify road flooding depths, the degree to which SLR will increasingly limit safe access and egress to neighborhoods served by a single low elevation flood prone road requires additional investigation.


See Public Act No. 18-82, An Act Concerning Climate Change Planning and Resiliency.

 

Connecticut-Specific Road Flooding Methods

The Connecticut Department of Emergency Services and Public Protection (DESPP) statewide road network was the starting dataset for roads. Warren Pinnacle Consulting, Inc., ran SLAMM’s road flooding module using 5-meter road segments to assign the following flood values for each road segment at four future time steps:

  no flooding
floods at east every 30 days
floods between every 30 and 60 days
floods between every 60 and 90 days
floods between every 90 days and 10 years
flood between every 10 and 100 years

 

The model provides five possible SLR scenarios: high, high-medium, medium, low-medium and low. These SLR scenarios, shown in the following table, are relative a base sea level elevation in 2002:

Connecticut SLAMM Sea Level Rise Scenarios

Time Interval

Low Projection (feet)

Low-Medium Projection (feet)

Medium Projection (feet)

High-Medium Projection (feet)

High Projection (feet)

2002

0

0

0

0

0

2025

0.167

0.335

0.499

0.666

0.833

2055

0.666

0.915

1.332

1.749

2.50

2085

1.083

1.499

2.418

3.251

4.833

2100

1.250

1.834

3.00

4.167

6.25

Source: Final Report - Advancing Existing Assessment of Connecticut Marshes’ Response to SLR, by Warren Pinnacle Consulting (2016)

 

Because it best fits with State of Connecticut's recently adopted policy of planning for 20 inches of SLR by mid-century (see Public Act No. 18-82, An Act Concerning Climate Change Planning and Resiliency) the high-medium scenario was chosen for analysis and display in the viewer. The years included in the viewer are 2010, 2025, 2040, 2055 and 2085. To simplify the GIS layer primarily for drawing speed, road segments that are not expected to experience coasting flood in the future were removed from each year's dataset and neighboring like-coded segments were combined. For example, 4 linear segments, each with the same flooding value in the same year are be combined to be a single, 20 meter length of road. To provide context to the flooded road segments, an All Roads layer is included in the viewer. It is the same road layer mentioned at the start before it was segmented.

State Roads

The original DESPP roads contain a lot of useful attribution but they do not indicate which roads are state roads and which are local. The Connecticut Department of Transportation (DOT) provided state roads but they were not a perfect geographic match with the DESPP road data. For ONLY roads in the coastal area, UConn CLEAR did some GIS analysis to populate the road segments with a state road attribute. We are confident in the results, although this data has not been approved by any state agency and is not considered authoritative.

The State Roads layer in the viewer is the DESPP roads that were identified as state roads after the analysis described above.