How does flood risk to roads affect the community?

Understand how flooding on roads can affect your daily life.

In addition to damaging residential properties, flooding can impact the broader community. Floods that block roads can cut off access to utilities and emergency services, and affect transportation. As climate change alters the frequency and severity of extreme weather events, deteriorating infrastructure and the communities they serve will be especially vulnerable. 

How are operational thresholds used to determine flood risk to roads?  

Flood risk to roads are based on the likelihood of flooding, and the expected depth of flooding on a road. The First Street Foundation Flood Model is used to determine the risk of flooding from rain, rivers, tides, and storm surge to roads. This risk considers the amount of floodwater reaching the centerline of a road, the percent of the length of a road covered by water, and the depth of that water.

Operational thresholds refer to the point at which a piece of infrastructure would no longer be in operation. These thresholds are measured as the depth of flooding that renders a road unusable, making it unsafe for most passenger vehicles. Depending on the depth of flooding, roads may be able to maintain some level of functionality even when flooded. For this reason, operational thresholds are used to determine the operational risk to roads caused by flooding. The operational threshold for roads is 0.5 feet (6 inches), as passenger cars often have trouble passing through water at this depth. 

How is the risk of flooding to roads measured?

The annualized expected flood depth of each 100 meter road segment considers the projected depth of flooding a road may receive in any given year in the event of a 2-year, 5-year, 20-year, 100-year, 250-year, and 500-year flood. As climate change alters the frequency and severity of extreme weather events, the increasing number of roads at risk increases the vulnerability of a community. For this reason, the likelihood and depth of flooding expected to occur now and in 30 years from now is determined as a combination of all 6 flood events.

The expected depth of flooding to each road segment and the operational threshold of roads (0.5 feet) is used to determine if a road has operational risk from flooding in 2021 and 2051. 

For example, a neighborhood in Western New York may be projected to flood 7 inches in the event of a 100-year flood, while the depth of flooding caused by a 100-year flood event may only be 1.2 inches for a neighborhood in Arizona. Roads in both areas have an operational threshold of 6 inches. In Western New York 7 inches of flooding would exceed the 6 inch operational threshold of roads, however 1.2 inches of flooding in Arizona would not exceed the 6 inch operational threshold of roads in that neighborhood. This is not to say roads in Arizona would not be damaged; they may still suffer damage but 1.2 inches of flooding would not render roads in Arizona unusable.

Operational risk to roads in a geographic area is determined by adding the length of roads that are expected to be flooded at a depth that exceeds half the operational threshold, causing operational risk. Further, the severity of operational risk is considered, as greater depth of flooding will cause greater operational risk to roads.The length of roads with operational risk, and the depth of flooding, is compared to the total length of roads within a location to determine the overall risk to roads. These ratings are climate adjusted by representing the risk as Minimal, Minor, Moderate, Major, Severe and Extreme.


Learn more

What data sources are used to determine Community Risk ratings?

How are Community Risk levels determined? 

How to use the Community Risk map overview 

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