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Those in the real estate business in Texas—and Texas homeowners themselves—have probably heard of FEMA floodplains maps. But what exactly are floodplains, and why do they matter?

The implications of FEMA floodplains are as far-reaching as they are important. There are safety and financial considerations to take into account when buying and selling property located in or near floodplains. Those working in real estate in Texas must have a strong understanding of the implications of building, buying, and selling in those areas.

To that end, we’ve put together this guide to FEMA floodplains, including the answers to questions such as:

  • What are floodplains, and how often are floodplains maps updated?

  • What is the impact of global warming on models used to make FEMA floodplains maps?

  • Why does it matter if floodplains maps are incorrect or outdated?

  • How do we know that some current floodplains maps are incorrect?

  • What have Austin and Houston done about changing floodplains, and what’s the impact?


Floodplains refer to the areas close to bodies of water that have a chance of flooding due to rainfall. In the United States, FEMA (the Federal Emergency Management Agency) is responsible for mapping floodplains and the relative risk of flooding in different areas.

These maps are used to determine the price of flood insurance—or, in some instances, to make the purchase of flood insurance mandatory.

These flood maps are made in conjunction with state, local, and tribal officials using both current and historic flood-related data including, but not limited to:

  • Existing flood maps and floodplains

  • Land use

  • Infrastructure

  • Hydrology

Floodplains maps indicate both 100-year and 500-year floodplains, or floodways. These refer to the amounts of rainfall that occur, on average, within the given time period. 

Essentially, the 100-year floodplain indicates the amount of rainfall that has a 1% chance of happening in any given year; the 500-year floodplain indicates the amount of rainfall that has a 0.2% chance of occurring within one day in any given year.

By law, FEMA floodplains maps must be updated every five years. However, this isn’t always a reality. 

Recent reports show that FEMA maps are often based on out-of-date models. The National Oceanic and Atmospheric Administration (NOAA) released updated floodplains estimates in 2018 that replaced the existing Texas floodplains estimates formerly based on 40-to-50-year-old data.


Atlas 14, the NOAA analysis used to update Texas floodplains estimates, shows that rainfall will likely be significantly higher. In fact, some floodplains are expected to grow by 45% by the end of the century due to the rise in sea level caused by global warming.


FEMA floodplains maps are based on prior rainfall data, not projections of the future. Because rainfall levels are changing more rapidly due to global warming, it’s important to look at how rapidly these changes will come.

Atlas 14 recalculates what was previously a 500-year floodplain—about 13.5 inches—as being closer to a 100-year floodplain. It updates the predicted 25-year floodplain from 7.6 inches to 9 inches.

These increases of a few inches may not seem like much, but collectively mean quite a bit more water causing dangerous flooding.


Obviously, some existing floodplains are significantly different than what floodplains maps would have us believe. But why does it matter?

Incorrect floodplains maps have serious implications in several areas, including:

  • Risk assessment. The new higher estimates for potential rainfall indicate that people and property already located within floodplains are at a higher risk than was previously thought. Additionally, some land and people outside of pre-established floodplains are now in danger of flooding.

  • Insurance rates. Residents in floodplains with federally-backed mortgages are required to purchase flood insurance. These flood insurance rates will likely increase for everyone involved, as the chance of major flooding has increased. While changes based on Atlas 14 likely won’t become official via FEMA until at least 2023, those in the real estate arena can start planning now for the future financial impact.

  • Development and infrastructure. There are existing guidelines in place for development, residents, and activity within floodplains. As the floodplains themselves are redefined, the development and infrastructure within them will be too. This can occur because those in the area are preparing more or differently to combat the risk of flooding or because fewer people and businesses choose to move to or stay in that particular area.

Overall, there is a huge financial cost when floodplains maps are inaccurate. People and property are put at risk, insurance rates can skyrocket, and protective measures quickly become useless. 

Updating floodplains maps regularly, using accurate models and data, can save money, property, and lives.


“NOAA’s new rainfall frequency values for Texas will help state and local authorities better understand their flood risk and more accurately plan and design infrastructure to minimize the threat of flooding,” said Thomas Graziano, Ph.D., director, NOAA Office of Water Prediction upon the release of Atlas 14 in 2018.

But have authorities responded that way? How has the Austin government, specifically, responded to this new information?

The Austin City Council had made and approved several changes to their code by November of 2019 in an attempt to better prepare the city for future flooding. These changes included, among others:

  • New definitions of “100-year floodplain” and “25-year floodplain”

  • Revised methodology for determining 100-year floodplains based on Atlas 14

  • Revised methodology for determining 25-year floodplains based on Atlas 14

  • Replacement of an existing construction exception with a broader development exception for any residential dwelling to take the place of an existing, legally-built residential dwelling within the 100-year floodplain if it won’t increase the total number of structures on the property

  • Expanded exceptions for development within the 100-year floodplains of the Colorado River, Lady Bird Lake, Lake Austin, and Lake Travis

  • Permission for staff to allow the construction of parking areas in the 25-year and 100-year floodplains if they are part of property holding buildings that fall under existing exceptions

  • Clarification that the 500-year floodplain that is represented on the FEMA Flood Insurance Rate Map is based on the most recent effective date of these maps (January 6, 2016) or maps that will soon be updated for the South Brushy Creek watershed and Onion Creek watershed

Overall, the City of Austin has essentially written into their code that the existing FEMA 100-year floodplain should be treated and defined as a 25-year floodplain. This change calls for stricter measures on development in these areas in an attempt to protect people and property.

In a nutshell, this change means that areas within the newly-defined 25-year floodplains are prohibited for development.


It makes sense why those in the real estate arena should know the ins and outs of FEMA floodplains. After all, knowing when and where property can be developed is a big part of pointing clients towards the property that’s right for them.

However, as we’ve seen, FEMA floodplains maps aren’t always correct or current. 

Taking other resources, such as Atlas 14, into account can give your clients an edge up on financially risky decisions. Being familiar with the terminology can streamline advice and decisions, so you’re never left floundering.

The City of Austin has different floodplain-related regulations than many other areas of Texas. If you’re operating within Austin, it’s a good idea to read up and find other resources that can help you find the right property for your clients—resources like FuseGIS.


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