Community Risk FAQ

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Find answers to frequently asked questions about Community Risk.

What is Community Risk? 

In addition to damaging residential properties, flooding can impact day to day life within a community, cutting off access to utilities, emergency services, and transportation. The First Street Foundation Flood Model is used to determine the risk to infrastructure, properties, and roads from multiple types of flood events, including fluvial (river), pluvial (rain), storm surge, and tidal sources. The presence or lack of projected flooding, and the projected depth of that flooding, is used to determine community vulnerability to flood risk. The flood risk to each facility is based on the likelihood of flood waters  reaching the building footprint, or parcel centroid, of each facility, or  the center of a road -- and then combined with the expected depth of that flooding.

How are community areas defined? 

Community Risk denotes flood risk to the local services and infrastructure surrounding properties, and are grouped by neighborhood, zip, city or municipality, and county. It considers flood risk across 5 different dimensions: (1) Infrastructure (utilities, airports, ports, and emergency services); (2) Residential properties; (3) Commercial properties; (4) Roads; (5) Social services (churches, schools, museums).  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.

Is local adaptation taken into account?

The First Street Foundation Flood Model is used to determine the risk of flooding from the 4 major flood types of (tidal, pluvial, fluvial, and storm surge). Community Risk also takes into account the 23,000 adaptation features included in First Street’s Flood Model. Therefore, flood projections consider the risk-mitigating effects of levees, dams, open spaces, and other adaptation features included in the Flood Model. The model does not include personal adaptation such as sump pumps, or personal property drainage etc.

How can my home have minimal risk but my area has high flood risk? 

We understand it can be confusing if a property has a low Flood Factor yet Community Risk for your neighborhood is high. A property's Flood Factor and Community Risk scores are both  comprehensive risk assessments, which take into consideration the 30 year risk of flooding from high intensity rainfall, overflowing rivers and streams, high tides, and storm surge. 

The distinction is that Flood Factors reflect the likelihood of water reaching your individual home’s building footprint, while Community Risk determines the surrounding area’s vulnerability to flooding. Building footprints refer to the outline of a building structure on a property. Community Risk encompasses the social risk, residential risk, commercial risk, risk to infrastructure, and risk to roads of flooding within a geographic area.

Within a community, the likelihood of flooding, and the depth of flooding that causes a certain percent of facilities to have operational risk is used to determine categorical risk. Each category is given a ranking based on the percent of facilities or roads with operational risk at a given depth. 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. 

How are community risk scores determined?

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 average expected flood depth is 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.

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. 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, public and social infrastructure may add up to 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 flood risk. Weighting each category allows flood risk to be community-specific.

What is 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 the FEMA’s HAZUS methodology.

 

Learn more

Community methodology - Is your community at risk of flooding?

What data sources are used to determine Community Risk of Flooding?

Flood Model Overview 

 

 

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