Using Census Data to Support Floodplain Management

August 28, 2020

Our team often meets with floodplain managers looking to leverage flood risk insight to conduct better outreach, plan dynamically for emergencies, and distribute resources more intelligently.  Drawing links between sociodemographics, economics, and flood exposure to develop a holistic understanding of residents living in floodplains is core to these goals. While effective floodplain management is dependent on these linkages, making these connections is not always easy. 

If you’re thinking about bringing additional data into your floodplain programs to inform your work, you might find it helpful to start with data from the U.S. Census Bureau. Census data is widely available, easily accessed online, and used by local governments throughout the country. Below we outline how you can mobilize these federal datasets to enrich your floodplain programs.

Integrating community census data analysis into your floodplain programs can help you...

  1. Make better decisions about where to invest funds. Natural disasters do not impact everyone equally. Incorporating census data into decision-making around flood risk can help you begin to address historic discrepancies in vulnerability and resource allocation. While taking into consideration socioeconomic variables in project funding decisions can be challenging, communities across the country are increasingly diversifying decision-making criteria.
  2. Plan better for emergencies. Tracking census data can greatly inform emergency preparedness. Integrating key metrics like population growth and age distribution into your emergency planning can enable you to respond more equitably and efficiently. Hampton Roads, VA uses census data to plan evacuation routes and update emergency operation plans.
  3. Tailor emergency and risk communication. Flood risk might not be the first thing on your residents’ minds each day. Understanding the makeup of your community can help you address this pressing issue with different segments of the population in ways that are more specifically relatable. As the FEMA Flood Risk Communication Toolkit points out - "it is tempting to think of your target audience as one group: the community’s residents. However, there are many subgroups with unique priorities and concerns. As you craft your social media messages, consider which of your audiences you are trying to reach and which social media platform, if any, is the best channel." You might concentrate mailings in areas where you know residents are less likely to use the internet to find flood risk information. Census data can also help you identify what languages to communicate important information in, and where. 
  4. Complete risk assessment and qualify for CRS credit. Socioeconomic flood risk analysis is a crucial part of conducting a risk assessment for your local flood mitigation plan. Again, from FEMA’s flood mitigation handbook - "The risk assessment can identify areas of greater population density, as well as populations that may have unique vulnerabilities or be less able to respond and recover during a disaster". Census data is cited by FEMA as a resource for conducting assessments. 

Accessing census data

You can access census information online using the Census Bureau’s Data Explorer. Through the explorer, data is searchable by topic (such as housing) or by geographic unit. It’s a large amount of information, so it can be difficult to know how to begin. The easiest first step would be to search for data for your specific region to see what’s available. Every census dataset has a geographic unit identifier (county, census tract, block etc.). To visualize the data in a mapping software such as QGIS or ArcMap, you can join the census data to one of these geographic boundary shapefiles based on the geographic unit identifier. Learn more about accessing the data and the different datasets available here.

As you’re diving into the data, you’ll need to hone in on which fields are relevant for your uses. This might be a tricky task, because there’s a lot of it! For floodplain management in particular, you’ll probably be interested in fields like population count, population growth, age distribution, median household income, income levels, race, and ethnicity. Digging deeper, you might find additional fields that are able to describe your community more precisely and provide greater insight. Some that might be of interest are: accessibility to public transit, unemployment rates, residential units occupied by an owner versus a renter, and education levels.

Integrating data into Floodplain Management Plans (FMPs)

Once you identify the data fields most relevant for your community, FMPs are a good place to start using census data in your work. To get you started, here are a few ideas for analysis and integration of census information in FMPs:

  • Add a detailed socioeconomic profile of your community. In this example from the City of Gresham, OR’s natural hazard mitigation plan, you’ll find a good example of a community profile that describes different census metrics including population growth, race, ethnicity, education level, income levels, employment rates, and transportation and commuting patterns. The FMP uses these characteristics to paint a more holistic picture of the city and answers questions like: Who lives in my community? How are residents employed? Who is at higher risk?
  • Display maps of population density and population growth. A population map can help you illustrate both existing community conditions and potential future growth. Combining population mapping with a flood map can help you visualize population density relative to flood risk. You can find an example of a population density map in this Louisville, KY natural hazards mitigation plan. 
  • Consider social vulnerability in your mitigation plan. Demographic information can be used to delve into your community’s capacity to respond to flooding or other natural hazards. Understanding vulnerability can, in turn, help inform how you address gaps in resource and capital distribution. Fairfield County, SC’s hazard mitigation plan incorporates a Social Vulnerability Index developed at the University of South Carolina. To learn more about social vulnerability, take a look at this resource.

Limitations

As you utilize census data to inform FMPs and your floodplain management practices, it’s important to keep in mind some limitations of federal demography. Data available through the American Community Survey (ACS) is based on samples and estimates. As such, it has the potential to misrepresent a community. Error values are available for most data points in the ACS. This can help you evaluate accuracy. In general, much of the data available through the Census Bureau is presented as an average or median value for a predefined area (such as a census tract or block). If there’s a large variance within that area, the data point might be misleading. This article describes how measuring values based on varying geographic boundaries might significantly affect the result data.

Lastly, not all population groups are equally represented in census data. Historically, communities of color and marginalized populations have been critically underrepresented in census-taking. Because census data is widely used for decision-making across government scales and agencies, undercounting over time has contributed to imbalances in funding, investment, and resource distribution throughout the country. 

This blog post was written by our amazing data analysis intern, Kama Lee-Tal! Have questions about integrating census data into your floodplain management work? Send us an email, we'd love to chat.

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