Last month's disastrous flooding has a lot of people thinking about the flood risk in their own homes and neighborhoods. Even if they鈥檝e never experienced a flood in the past, could enough rain falling in the right (or wrong) place trigger one some day?
Floodplain maps are usually the best place to begin understanding that risk. Accessing them online is pretty straightforward. But different maps can provide different kinds of information, and Austin maps are undergoing some big changes.
What are these maps?
Floodplain maps, also simply called flood maps, show the percentage risk of a flood occurring in a given place in the course of a year.
Hydrologists and floodplain engineers arrive at these estimates by studying the topography and geology of the area, along with its built infrastructure, its weather history, its position in a watershed and its proximity to waterways.
The likelihood of flooding is often expressed on the maps in terms of 鈥測ears.鈥�
For example, an area in the 鈥�500-year鈥� floodplain has a 1 in 500, or about a 0.2% chance of flooding in any given year.
In a 100-year floodplain, there is a 1 in 100, or 1%, chance of a flood each year.
Any place with a 1% risk or greater is considered high risk. People who live in or own property in those areas should get flood insurance, though many do not.
Different maps for different things
The Federal Emergency Management Agency, or FEMA, runs the National Flood Insurance Program. It also maintains the national flood maps used to determine risk 鈥� and therefore insurance rates 鈥� for property owners.
To find out whether your home is located in a floodplain according to FEMA, you can visit . The shaded areas on the map designate places that are in flood-hazard areas, though not all are considered high risk.
You can find a key to what the different shaded colors mean (light blue signifies the 100-year floodplain, tan means 500-year) below the map or by clicking the menu in the upper right of the map.
Local governments also offer their own maps, which can be different from the federal maps. Cities have rules written into their development codes that may go beyond FEMA flood-protection standards. These rules dictate where buildings may not be allowed and how elevated a building in a floodplain might need to be. They also may require developers to ensure they won't increase flood risk to nearby properties.
Austin is one of those places with its own rules. It has a regulatory map that often shows a higher level of risk than the FEMA maps. You can find that map below or at . Just click 鈥淚 want to鈥� and enter an address to learn more.
Maps are changing
Flood maps are not static things. As cities grow, the climate changes and our understanding of risk evolves, so do the maps.
鈥淲e are doing studies on floodplains. We're paying for most of it on our own,鈥� Kevin Shunk, the city's floodplain administrator, said. 鈥淲e will use that in the city of Austin to regulate development.鈥�
Shunk said the city also provides the results of its floodplain studies to FEMA 鈥渢o create new floodplain maps that they will use for their insurance program.鈥�
In some cases, updated maps may show decreased risk after flood mitigation projects like tunnels or levies have been built. In other cases, new maps may show an increased risk due to development or a change in weather patterns.
Property owners can also provide FEMA with their own engineering studies to try to have their flood risk lowered, just as Camp Mystic, on the Guadalupe River, did in the years before last month's catastrophic flood.
At the local level, it鈥檚 also possible for developers to get permission 鈥� called a variance 鈥� to build in a floodplain. Sometimes that variance has to go to the City Council for approval, especially if the developer can't meet existing requirements for building in a flood plain.
In other cases, variances are simply granted by the Watershed Protection Department as long as the builders agree to do things to mitigate the risk to the property and those nearby.
Shunk said the city grants about six variances a year allowing people to build in a floodplain.
鈥淚f you see a house in a neighborhood that looks like it's on stilts, well, take a look around you," he said. "If you find a [water] channel, it's probably in the floodplain."
Hard rain gonna fall
Austin is currently in the middle of a major overhaul of its floodplain maps that has increased the number of buildings in high-risk areas from 7,000 to around 10,000.
This update started in 2018 when the National Oceanic and Atmospheric Administration added about a quarter century of new rainfall data to state weather records. That update showed big storms were more common than previously thought in Central Texas.
The new undertsanding of local rainfall greatly increased local flood risk, because it showed that severe rain events were more likely to happen more often.
Suddenly, most properties that had been in the 500-year floodplain were in the 100-year floodplain, according to city estimates.
Ever since the release of that rainfall data, Shunk and others at the Watershed Protection Department have been updating flood models for different parts of the city to get a more finely detailed understanding of how risks have changed.
After those studies are complete, Austin will submit its findings to FEMA for the agency to revise its maps as well.
鈥淲e're in the middle of that process right now,鈥� Shunk said. 鈥淚 would say that the first of those studies will be done mid-next year, 2026, and the remainding studies a year after that, mid-2027.鈥�
You can follow the department's progress at the .
Climate warms, risk grows
Despite the continuous updates, climate scientists say flood maps may still be underestimating risk because they rely largely on data from a time before greenhouse gas emissions began seriously warming the earth鈥檚 atmosphere.
The past is not the best predictor for future risk in a warming climate, because warmer air holds more water. That means the stronger climate change-charged rainstorms we are experiencing now will likely become stronger in the future, making flood risk greater, as well.
