How are Community Risks levels determined?

Learn about the methods used to calculate a community’s risk to flooding.

Flooding impacting the broader social, commercial, and community infrastructure surrounding your home can still have an impact on you even if your home does not flood. The First Street Foundation Flood Model is used to determine risk to infrastructure and roads from multiple types of flood events by taking into account flooding that may be caused by fluvial (river), pluvial (rain), storm surge, and tidal sources. The projected depth of flooding is used to determine which structures are at risk, the number of structures at risk, and the severity of operational risk within a community. 

Community Risk

The risk of flooding impacts more than an individual property. Community risk measures the impact flooding can have on day to day life within the surrounding community. Floods that impact infrastructure, cut off access to utilities, affect emergency services and transportation can impact business and the finances of homeowners. Community Risk shows how flooding can impact various community levels of your community, such as neighborhoods, zip codes, municipalities/cities, and counties. Because community risks can vary for different levels of geographies, Flood Factor allows you to compare risks between these levels. 

Communities available through community risk overview:

  • Neighborhoods - Encompasses macro neighborhoods, neighborhood, sub-neighborhoods and residential districts (e.g. subdivisions and apartment complexes) are not an official designation and vary in size depending on local usage
  • Zip codes are zip code tabulation  areas as provided by the U.S Census Bureau. 
  • Cities - Place (County Subdivision in New England) as provided by the US Census Bureau. Refer to a village, town, or city typically governed by a mayor and council.
  • Counties are territorial divisions of a state and are typically government units that sit below the state level. County or county-equivalents as provided by the US Census Bureau.

Flood Risk

The First Street Foundation Flood Model is used to determine the risk of flooding from rain, rivers, tides, and storm surge to infrastructure. Flood risk to each facility is determined by the likelihood of flooding and the annualized expectation of flooding that reaches the building footprint or parcel centroid.  The annualized expectation of flooding is the average total amount of flooding expected to reach a facility’s building footprint or parcel centroid over the course of year. Similarly, risk to roads is based on the depth and likelihood of flooding reaching the centerline of a road.

Flood risks consider the likelihood and annualized expected depth of flooding to each facility type and road 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 number of facilities and roads at risk of flooding increases, along with 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.

Operational risk 

Operational risk refers to the loss of operational functionality. In order to determine the point where infrastructure and roads can no longer operate, infrastructure type is broken down by functionality threshold. When a facility is flooded to the point where it can no longer function as intended, or a road is flooded to a depth that makes the road unusable and unsafe, it is considered to have operational risk. These thresholds vary depending on infrastructure type, as different types of infrastructure can withstand different depths of flooding while still maintaining some level of functionality. For this reason, functionality thresholds are used to determine the operational risk of infrastructure and roads caused by flooding. Operational thresholds used are as defined in the FEMA’s HAZUS methodology.

For example, hospitals have a functionality threshold of 42 inches, while police stations have a functional threshold of 12 inches. The operational threshold of hospitals is higher at 42 inches of flooding because hospitals are often designed to withstand some risk and provide emergency services, especially in the case of a national disaster. If both infrastructures are expected to flood 12 inches in the event of a 100-year flood, this depth of flooding would not exceed a hospital’s functionality threshold of 42 inches, but would exceed the 12 inch functionality threshold of a police station. This depth of flood could result in serious damage to a police station, causing it to no longer function as intended and likely close operation for a period of time. This is not to say a hospital would be undamaged - it may still suffer damage to its structure - but 12 inches of flooding would not cause a hospital to be disabled and operations could continue. 

Infrastructure Type

Operational Threshold (inches)

Category

residential parcels

0 in 

residential

airport

12 in

infrastructure

fire station

24 in

infrastructure

hospital

42 in

infrastructure

police station

12 in

infrastructure

port

12 in

infrastructure

power station

24 in

infrastructure

superfund / hazardous waste site

12 in

infrastructure

water outfalls

0 in 

infrastructure

wastewater treatment

0 in

infrastructure

government building

0 in

social

historic building

0 in

social

house of worship

0 in

social

museum

0 in

social

school

12 in

social

road centerlines

6 in 

road

commercial parcels

0 in

commercial

 

Calculation of Community Risk

To determine Community Risk to flooding, the number of facilities in each category with operational risk in 2021 and 2051 is separately calculated. This includes the number of airports, seaports, fire stations, police stations, hospitals, power stations, superfund sites, and wastewater treatment facilities with operational risk. Likewise, risk for all other categories is calculated based on operational flood thresholds and the depth of flooding for each facility.

Within a community, the percent of facilities with operational risk and the expected flood depth is calculated to determine categorical risk. The distribution of these expected depths within a community are considered at intervals of 5 from 0 to 100. For instance, the depth at which 5% of facilities within an area have operational risk less than or equal to this depth, the depth at which 10% of facilities within an area have operational risk less than or equal to this depth, and so on. The depth of flooding that threatens operational risk at each interval is added, resulting in a final numerical category score for a given community. 

Consider infrastructure risk for a neighborhood. If half of the facilities do not flood, the percentiles up to the 50th percentile will have a depth of 0. From there, the depth at which the next 5% of facilities (police and fire stations, airports, hospitals, power stations, etc) have operational risk is 0.5 ft, the depth at which the next 5% of facilities have operational risk is 0.7 feet, the depth at which the next 5% of facilities have operational risk is 1 foot, and so on. The depth at which a certain percent of facilities are flooded is  determined until a depth that floods 100% of facilities is reached. At each interval of 5 the depth of flooding expected is added, ranging from the least depth of flooding facilities experience (which is often 0) to the highest depth experienced of all facilities in an area. The sum of these depths, or the total depth, is used to determine that neighborhood’s infrastructure risk. 

Each category is given a ranking based on the percent of facilities or roads with operational risk at a given depth. Community risk rankings represent risk as Minimal, Minor, Moderate, Major, Severe and Extreme. Minimal risk is a case where no facilities within a category have flood risk. The total depth for each community level, neighborhood, zip code, city, and county; is then compared to the total depth for similar community levels within a state or across the country. County level risks are ranked based on how their total depth compares to counties across the county. While, neighborhood, zip code, city level risks are ranked based on how their total depth compares to communities within respective states.

The total depth for the neighborhood in the previous example would be compared to total depths for neighborhoods across their state. If this neighborhood’s total depth was greater than 90% of neighborhoods across the state, this means it would be among the neighborhoods most at risk in the state. 

The overall Community Risk encompasses the social risk, residential risk, commercial risk, risk to infrastructure and risk to roads for a given area. Once risk is determined for each of the 5 categories, overall risk for a community is calculated by averaging the risk for all 5 categories. Overall risk is also represented as Minimal, Minor, Moderate, Major, Severe and Extreme.

Changing Risk

As climate change alters the frequency and severity of extreme weather events, deteriorating infrastructure and the communities they serve can be especially vulnerable. Community Risk considers how flood risks may change over a 30-year period to account for the changing climate. For this reason, risk scores are reflective of both current and future risk of flooding.

Risk to each category -- social risk, residential risk, commercial risk, risk to infrastructure and risk to roads -- account for the effect climate change will have on a given community. Climate is highlighted by adjusting these risks into the future as “Increasing” or “Decreasing”. Risk increasing due to the climate may cause the number or percent of properties, facilities, and roads at risk of flooding to increase, increase the depth flooding, or a combination of both. 

 

Learn more

How is my Flood Factor calculated? 

What data sources are used to determine Community Risk ratings?

How to use the Community Risk map overview

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