Report Finds FEMA Underestimates Flood Risk

A report recently released by the University of Bristol, U.K., and the Nature Conservancy entitled, “Estimates of Present and Future Flood Risk in the Conterminous United States,” concludes that FEMA’s flood hazard maps account for only one-third of the total population that is exposed to serious flooding.

Whereas FEMA estimates that 13 million Americans are currently exposed to the devastation of a “100-year flood,” the report puts that number at 41 million. A 100-year flood describes an extreme flooding event that has a one-percent chance of occurring in any year; it is a common benchmark for flood risk management.

The simulations run for this study used large amounts of data from the U.S. Geological Survey National Elevation Dataset. According to Kris Johnson, a scientist and one of the report’s authors, this data is more accurate and comprehensive than what was available before. FEMA’s appraisal of flood risk, on the other hand, relies on time-consuming local assessments of various catch basins and floodplains.

The report finds that FEMA maps overlook risk across the U.S., but the newly identified exposure areas are concentrated along the Pacific coast, in urban centers around the Great Lakes, and across the inland West. The researchers also projected future changes in population and housing density using the Environmental Protection Agency’s Integrated Climate and Land Use Scenarios. They found that the proportion of Americans living in flood-prone areas will increase over time.

Today, 13.3 percent of the U.S. population is exposed to a 100-year flood, but that number may rise to 15.8 percent by 2050 and 16.8 percent by 2100, according to the report. In some regions, the projected increase is stark. South Dakota, Nebraska, and New Mexico are slated to see a five-fold increase in flood exposure by 2100. In California, Florida, and Texas, exposure is predicted to triple or quadruple, according to the study.