US towns limit new buildings in floodplains more than you think - and can do better

U.S. towns are building new housing and infrastructure in their floodplains – but not as much as expected, and the tools to stop it may be more routine than we imagined.

Those are the key takeaways from two new papers: one in Oxford Open Climate Change OUP Academic (Siders et al .) today and one in Earth’s Future last week (Agopian et al ). The findings could change how advocates, floodplain managers, and a new administration, think about policy reforms for flood risk management.

Not building in flood-prone areas is one of the most effective ways to reduce flood disasters. People have known this for decades (at least), but in practice, limiting new development in flood-prone areas has always been seen as a challenge. So: Which US towns are rising to this challenge? What can we learn from them?

The two papers out this week are the latest from a collaborative project by University of Delaware , University of Miami Rosenstiel School of Marine, Atmospheric and Earth Science , 美国北卡罗来纳大学教堂山分校 with co-authors from Georgetown University Law Center , 美国麻省理工学院 , The Citadel , and Embry-Riddle Aeronautical University ; and with funding from National Science Foundation (NSF) Humans, Disasters, and Built Environment program (#HDBE). Read more on the project website .

Floodplain Development is Less than Expected

We developed two new ways to measure floodplain development in towns across the United States, looking at housing and impervious surface changes 2001-2019. ?Contrary to previous studies and our expectations, we find that it is common practice for US towns to limit new development in floodplains. Using a relative measure – taking development rates and floodplain sizes into account – allows us to compare towns with big and small floodplains, high development pressure and slow growth. Armen Agopian and Miyuki Hino led the analysis .

• Over 2 million acres of floodplain were developed across the United States 2001-2019, including over 800,000 new residences.
• However, those numbers are much smaller than we would expect given the size of floodplains and rate of new development across US towns. ?This is surprisingly good news, and it complicates the common story we hear that building along flood-prone rivers and coasts is unchecked.
• 87% of US towns have limited new housing in floodplains, and 74% have limited new impervious surfaces (reflecting not just housing but infrastructure of all types).
• That said, a few towns are building five or six times more in their floodplains than would be expected; these exceptions are so severe that they can mask other success stories when they are averaged.
• Explore the data in Design Safe here . ?We are developing interactive maps to help view the data; we’ll update when those are available. ?
• A related paper on housing in floodplains, led by Chris Samoray earlier this year, showed that new housing is more likely to be built in floodplains in the most wealthy and least wealthy areas – a dichotomy that calls for targeted policy solutions.

Towns are Limiting Floodplain Development with Routine Tools & Practices

The second paper, led by A.R. Siders and Katharine Mach , focused on how local governments managed floodplain development. The analysis focused on New Jersey – a state that previous reports said was building rampantly in the floodplain. ?If we could find towns that were limiting or avoiding floodplain development in New Jersey, we could learn from what they had done: how they had resisted the pressure.? We found that keys to avoiding new buildings in floodplains are the unglamorous but consistent enforcement of common laws and codes. ?Commitment, not innovation, reduces flood-prone development in most cases. ??

• 85% of the 496 New Jersey towns we studied developed less than half as much new housing and infrastructure in their floodplains (2001-2019) as might be expected based on the relative size of the floodplains and rate of development.?

• 25% (126 towns) put none of their new housing in the floodplain 2001-2019.
• “They’re not doing anything special”: this quote from one of our interviewees nicely summarized our finding that most towns are able to limit floodplain development with a few commonplace local laws and codes and with typical levels of government capacity.

Results suggest that broad policy reforms or program innovations may be less important for limiting floodplain development than targeted improvements to existing rules. The minority of towns that are concentrating new buildings in the floodplains may require more specific supports or increased incentives.

If most towns are able to limit floodplain development—even with moderate capacity and in challenging contexts, using standard municipal ordinances, plans, and programs—then we should wonder not what has gone right in towns that limit development but what has gone wrong in towns that develop their floodplains.

Overall, it’s a surprisingly good news story: floodplain development is happening, but it’s less than we might expect and it’s possible.

Thank you to all our co-authors on these papers: Jennifer Niemann , Elizabeth Shields, MPP , Lidia Cano P. , Tess Doeffinger, PhD, EIT, LEED AP , Logan Gerber-Chavez, PhD , Ju-Ching Huang , Lexi Lafferty , Salvesila Tamima , Caroline Williams, PhD .

Disaster Research Center , University of Miami