EXPLAINER SERIES
Most Texans think flood risk stops at the floodplain line. It doesn’t. Up to 47% of flood damage happens outside mapped high-risk zones, and the maps themselves are based on decades-old data that miss key risk factors like drainage and storm surge. IDRT’s Buyers Aware tool is helping close that gap, one Texas county at a time.
Flooding is the most prevalent and costly natural hazard in the United States.1 Texans experience about $380 million in flood-related property damages each year, adding up to more than $1.9 billion from 2020 to 2025.2 Many people believe the National Flood Insurance Program covers most of this damage. But as many as 70% of flood-related losses occur in uninsured households, many of them outside officially designated floodplains.3
The “100-year floodplain” has been the primary standard for identifying flood risk in the United States since the 1960s. However, maps based on the 100-year floodplain often fall short as risk communication tools. They rely on outdated data, omit key risk factors, and are widely misunderstood by the public. Newer tools such as those developed by the Texas A&M Institute for a Disaster Resilient Texas help address this problem by providing more detailed estimates of flood risk for communities, including those officially designated floodplains.
Since the 1960’s, the National Flood Insurance Program (NFIP) has used floodplain maps to estimate flood risk and guide flood insurance policy in the United States. To assess the flood risk of a particular area, scientists analyze data from stream gages placed throughout river systems across the country. They also examine an area’s topography and soil conditions to estimate how much water could flow through the area, and at what depth, under different rainfall scenarios.4,5 They designate areas with a 1% chance of flooding in a given year as “high risk” on official floodplain maps, known as 100-year floodplains. These maps define the boundaries used to regulate development and determine flood insurance eligibility and requirements.7
Communities located within high-risk (“100 year”) areas must adopt basic floodplain regulations to become eligible for federally backed flood insurance. This eligibility is critical because most private insurers do not offer flood coverage in these areas, and properties in high-risk flood zones must carry flood insurance to qualify for federally backed mortgages.8
Floodplain boundaries are based on elevation, not distance from water. The 100-year floodplain (1% annual flood chance) sits within the larger 500-year floodplain (0.2% annual chance) — meaning risk doesn’t stop at the mapped edge.
Source / American Flood Coalition
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Source / Adobe Stock
Increasing evidence shows that the 100-year floodplain does not adequately communicate flood risk for three key reasons:
1. Common Misconception Lead to Risky Decisions
Floodplain maps present flood risk as a simple “inside or outside” boundary, even though a significant share of flood damage occurs beyond mapped floodplains. This framing reinforces false assumptions about where flood risk ends. Research shows that up to 47% of flood-related damages occur outside mapped floodplains.9 Additionally, because the term “100-year floodplain” is widely used, many people assume a flood will only occur once every 100 years within these areas. In reality, floodplains have a 1% chance of flooding each year, meaning multiple floods can occur within a 100-year period.
2. Outdated Maps Leave Communities Unprepared
Floodplain maps are updated infrequently and can take years to revise.10 As a result, many communities rely on outdated information that does not reflect current development patterns or changing flood conditions.
3. Current Methods Overlook Important Risk Factors
Floodplain maps rely heavily on river-based data and often exclude other important drivers of flooding, such as inadequate draining, increased development, and rapid changes in sea levels during storms (e.g., storm surge). As a result, communities may face flood risks that the 100-year floodplain does not capture.
Many scientists and community planners are developing new tools to communicate flood risk alongside the federal maps based on the 100-year floodplain. Buyers Aware is one such tool working to complete the picture by providing Texas communities a way to more accurately estimate their potential flood risk.*
Developed by the Texas A&M Institute for a Disaster Resilient Texas, Buyers Aware combines traditional measures, such as impervious surface coverage, with additional contextual indicators, including historic flood losses and insurance policy counts, to better reflect flood risk at the community level, including in areas outside historically designated high-risk floodplains. Given the limitations of the current system for managing flood risk, tools such as Buyers Aware highlight the need for more flexible and adaptive approaches that better reflect real-world flood impacts and support proactive risk management.
*As of this article, Buyers Aware is available for 41 counties in Texas.
1. FEMA: Flood Insurance and the NFIP
2. NOAA: Storm Events Database
3. Federal Reserve Bank: Flood Underinsurance
4. USGS: What is a Flood?
5. USGS: Interagency Advisory Committee on Water Data
6. Highfield 2013
7. Highfield 2013
8. Highfield 2013
9. Highfield 2013
10. FEMA: Notice to Congress: Monthly Update on Flood Mapping