Community Flood Methodology

Use First Street to understand if flooding threatens access to utilities, emergency services, and transportation within your community. 

Even if your home is not at risk, flooding can still impact the broader social, commercial, and community infrastructure, and roads surrounding your home. As climate change alters the frequency and severity of extreme weather events, deteriorating infrastructure and the communities they serve will be especially vulnerable. The First Street Flood Model determines the depth of flood water likely to reach each facility in a community and checks if that depth is higher than the structure's operational threshold. When a facility is flooded to the point where it can no longer function as intended, it is considered to have operational risk. 

Community Risk

Floods that impact infrastructure cut off access to utilities, affect emergency services and transportation and impact the finances of businesses and homeowners. First Streets's community risk allows individuals to learn how flooding can impact various levels of your community. Because risks can vary for different levels of geographies, First Street™ allows you to compare risks between these levels. 

• Neighborhoods: Macro neighborhoods, sub-neighborhoods, residential districts, subdivisions, and apartment complexes with unofficial designations that vary in size depending on local usage. 
• Zipcodes: Tabulation areas as provided by the U.S Census Bureau. 
• Cities: A village, town, or city typically governed by a mayor and council.
• Counties: Territorial divisions of a state, typically government units that sit below the state level.

Flood Risk

The First Street 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 the year. Similarly, the 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 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. 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. In order to determine the point where infrastructure and roads can no longer operate, each infrastructure facility type is given a functionality threshold. 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 FEMA’s HAZUS methodology.

For example, hospitals have a functional 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. 

Community risk scores

To determine Community Risk to flooding, the number of facilities in each category with operational risk in 2021 and 2051 are 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, the 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 average expected flood depth are calculated for each category to determine categorical risk. All of the geographies are evaluated against each other based on their average expected flood depth and given a relative score from Minimal to Extreme.

Residential homes make up the majority of land use and provide a very important part of daily life. As such residential homes are weighted the highest. Other types of infrastructure, including non-residential properties, roads, and public and social infrastructure, have smaller weights.  

Once risk is determined for each of the 5 categories, the overall risk for a community is calculated by averaging the risk for all 5 categories. Community risk reflects the weighted percent of properties, facilities, or roads with operational risk at a given depth. However, flooding to police stations, hospitals, or schools, can disrupt daily life within a community. In communities with many different types of infrastructure, the weighted percent of non-residential properties, roads, and public and social infrastructure may add up to be bigger than the residential weight. Compared to a rural community, developed areas with a booming economy have more variety of infrastructure types. 

The overall Community Risk encompasses the social risk, residential risk, commercial risk, and risk to infrastructure for a given area. Minimal risk is a case where no facilities within a category have a flood risk. Weighting each category allows flood risk to be community-specific. 

Flood damage to public infrastructure increases a community's vulnerability

Public infrastructure includes utilities, emergency services, and transportation. Power stations, wastewater treatment, and hazardous waste sites are considered utilities. Emergency services are made up of fire stations, police stations, and hospitals. Transportation includes seaports and airports. As climate change alters the frequency and severity of extreme weather events, deteriorating infrastructure and the communities they serve will be especially vulnerable. 

Flooding can impact your social well being

Social infrastructure includes schools, historical buildings, government buildings, places of worship, and museums. Because flooding can impact the broader social and community buildings surrounding your home, social risk assesses the severity of flood risk facing social and cultural services. 

If commercial and residential properties flood, quality of life may be threatened

Flood damage to a commercial property can impact day-to-day life within a community and may impact the overall economic well-being of an area. Flood risk to commercial properties, directly and indirectly, threatens the quality of life, access to and the ability to work, and general safety within a community. Residential risk within a community may vary depending on each property’s elevation or distance to water sources, such as lakes, rivers, or oceans. Flood damage to residential properties can impact infrastructure, businesses, and roads, negatively affecting the finances of homeowners. 

Flooded roads can cut off access to utilities and emergency services.

In addition to damaging residential properties, floods that block roads can cut off access to utilities and emergency services, and affect transportation. Flood risk to roads 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.

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 risks of flooding.

Risk to each category - social risk, residential risk, commercial risk, risk to infrastructure, and risk to roads - accounts 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 of flooding, or a combination of both. 

Press Coverage

Financial Times: Flooding could leave billions of US municipal debt underwater

The New York Times: As Manchin Blocks Climate Plan, His State Can’t Hold Back Floods

Boston Globe: Flooding could shut down Logan airport, hospitals, and other critical infrastructure in Mass., and the risk is only growing, report says

Learn more

• How is my Flood Factor calculated? 
• What data sources are used to determine Community Risk of Flooding?