Conserving Floodplains to Mitigate Future Flood Risk
Protecting undeveloped lands in areas at risk for flooding avoids needlessly placing more people and property in harm's way and can reduce future flood damages while also preserving habitat, improving water quality and providing a range of other valuable benefits.
This year was the wettest year on record in the conterminous United States. Flooding was widespread and destructive as rivers overtopped their banks and broke through levees. Intense rains inundated farms, cities, and towns, especially in the Midwest. Flooding is one of the most common, and the most costly, natural disasters. Flood insurance losses in the United States have averaged more than $3B annually since 2000 and total costs and impacts to lives and livelihoods is much greater. And population growth and climate change mean these impacts are likely to grow.
So, what can be done to reduce the damages and make people and property less vulnerable to recurring, costly flood events? In some areas, flood defenses need to be shored up and repairs to levees and other infrastructure and floodproofing of homes and other structures could reduce vulnerability. But in many places the simplest and most cost-effective way to reduce future flood damages is to simply not develop in floodplains where people and property would be put in harm’s way.
Drawing upon census projections and mapping from the Environmental Protection Agency and a hydrodynamic model for the conterminous United States, we identified 40,000 km2 of land in the “100-year” floodplain (areas with at least a 1% chance of flooding annually) projected to be developed by 2050 as populations grow and cities and towns expand. Builders understandably would like to build in relatively flat open areas near communities, and floodplains often seem like promising locations for development.
However, official flood maps are often incomplete or out of date, and local development decisions often give too little weight to future flood risk or ignore it altogether. What if new developments were directed away from these vulnerable lands to low-risk areas instead? To understand the potential for land protection to cost-effectively mitigate future flood damages, we compared the cost of buying and conserving undeveloped floodplains today relative to the benefits of avoiding flood damages to projected development in the future. We found that this strategy is cost-effective overall and is particularly sound in areas exposed to relatively frequent flooding (i.e. areas with a 5% chance or greater of flooding each year, the so-called 20-year floodplain). Even more striking is that for an area of floodplain across the country equivalent to twice the size of the state of Massachusetts, significant population growth in high-risk zones means investing $1 in protecting these floodplains today could save at least $5 in potential future flood damages. Retaining natural floodplains through zoning would be even more cost-effective and therefore would be warranted over an even larger area.
Not only would investing now to conserve undeveloped lands in floodplains likely save tens of billions of dollars in avoided flood damages but protecting these lands would also provide a host of additional benefits for habitat, wildlife, water quality, and recreation, further strengthening the economic rationale for floodplain conservation.
 U.S. Department of Homeland Security, Federal Emergency Management Agency. National Flood Insurance Program Losses Paid by Calendar Year https://www.fema.gov/loss-dollars-paid-calendar-year#
retrieved on November 25, 2019 and adjusted for inflation.
 O.E.J. Wing, P.D. Bates, A.M. Smith, C.C. Sampson, K.A. Johnson, J. Fargione, P. Morefield, Estimates of present and future flood risk in the conterminous United States, Env Res Letters 13, 034023 (2018). DOI: https://doi.org/10.1088/1748-9326/aaac65
Johnson, K. A., Wing, O.E.J., Bates, P.D., Fargione, J. Kroeger, T., Larson, W.D., Sampson, C.C., Smith, A. in press. A Benefit-Cost Analysis of Floodplain Land Acquisition for U.S. Flood Damage Reduction. Nat Sustain (2019) doi:10.1038/s41893-019-0437-5