For an expansive effort to remap floodplains across the United States, the Federal Emergency Management Agency and U.S. Army Corps of Engineers are drawing on a University of Wisconsin-Madison engineer’s flood modeling research as the backbone of the project.
Daniel Wright, a UW-Madison associate professor of civil and environmental engineering, developed the RainyDay software, which uses a method called stochastic storm transposition to calculate rain impacts on different areas. Storm transposition is, essentially, moving a storm from its original location to observe its effects in a different environment.
Daniel Wright
Stochastic storm transposition ramps that idea up by orders of magnitude, allowing researchers to identify hundreds of storms across decades and move them around more or less infinitely. “That lets us see possible outcomes that could happen for a particular river or location that we’re interested in,” Wright says. “We can look at things like 100-year storms, run those through flood simulation models, and get data that can be used to map out floodplains.”
FEMA is responsible for developing and maintaining maps of floodplains across the United States that are also the basis of the federally administered National Flood Insurance Program. The Army Corps of Engineers is using its hydrologic modeling software in partnership with FEMA, and drawing on Wright’s RainyDay methodology, to improve the nation’s floodplain maps.
The current maps, Wright says, employ hard boundaries based on outdated computer modeling that doesn’t properly account for the uncertainty that should be inherent in predicting floods. So for example, a resident could live just inside of a 100-year floodplain boundary for a flood that has a 1% chance of happening in any given year, and yet walk into or out of that boundary with a single step.
“In reality, the quality of the models we have right now is nowhere near the level that would allow you to set a boundary with that degree of certainty,” Wright says. “That’s very much an artificial line. So we should—and can, using more modern data and methods—quantify the amount of confidence we have in how these maps are drawn.”
FEMA is overhauling its mapping from the ground up and including simulations of what happens when rainfall hits the ground. That’s where experts like Wright, who studies extreme rainfall and hydrological infrastructure, come in. The hope is that, with a more holistic approach and modern data, FEMA can not only create better floodplain maps, but also more accurately reflect uncertainty in areas that are at risk of flooding.
Wright and his students now are updating RainyDay as FEMA broadens its use of the methodology beyond the original target environments. “When I started developing this during my PhD, the idea was that it would work for small watersheds, small river systems, and relatively flat terrain. FEMA needs to be able to apply it across the entire United States, including to mountainous areas and very large watersheds.”
One way Wright and his students are adapting their methodology is to incorporate snow melt in mountainous and northern watersheds where spring thaws can trigger flooding. They’re also trying to determine how to handle differing rainfall data quality, which can vary based on where and when it was collected—with older data typically being of lesser quality than more recent data.
Ultimately, the researchers’ task comes down to finding (and then pushing) the limits of stochastic storm transposition. Look at the Cascade Mountains on Google Maps, for example. The mountain range’s western side is verdant, while the east is visibly dry and brown. That’s a rain shadow—an area where less precipitation falls on the leeward side of a mountain range.
“You probably can’t move most storms from one side of the mountains to the other, because the mountains themselves change the meteorology,” Wright says. “So how do you do this in different places in a way that respects the real physics of how storms occur, but also in a way that you can repeat for 500 watersheds all around the country? That’s what we’re going to be working on.”
Wright is the Arno Lenz Memorial Associate Professor of Water Resources Engineering in the Department of Civil and Environmental Engineering.
FEMA is funding Wright and collaborator Antonia Sebastian, an assistant professor of earth, marine and environmental sciences at the University of North Carolina at Chapel Hill, $500,000 in 2024-25 to support the project. Given the substantial need and interest at FEMA and other government agencies, Wright anticipates additional funding opportunities and research support in the near future for continued efforts at modernizing federal approaches to flood risk management.