Ecocity Snapshots

Expecting the Unexpected: Another Cautionary Example

A new model finds that almost twice as many US properties are at risk of flooding than the number estimated by the Federal Emergency Management Agency.
Written by Rick Pruetz

by Rick Pruetz, Vice President, Ecocity Builders

Official government maps identify roughly half of the properties actually at risk of flooding in the US. Unlike the approach used by FEMA, the US Federal Emergency Management Agency, a new methodology called Flood Factor incorporates the combined effects of rainfall (pluvial), riverine (fluvial), and coastal flooding due to storm surge, high tides and sea level rise. This model also incorporates grey and green infrastructure to estimate whether these adaptations are capable of managing flood water or simply pushing it somewhere else. As a result, Flood Factor finds that 14.6 million U.S. properties are at risk from a 100-year flood, meaning a flood that has a one percent chance of occurring in any given year. That is almost twice the 8.7 million at-risk properties identified in maps produced by FEMA. Furthermore, Flood Factor projects that the number of at-risk properties will grow to 16.2 million by 2050 (First Street Foundation, 2020; Flavelle et al. 2020).

FEMA flood maps use historic events to predict current risk. However, budget constraints have prevented FEMA from updating these maps and 75 percent are now more than five years old. Furthermore, FEMA’s reliance on the past fails to account for changing environmental conditions while Flood Factor uses global climate model projections to forecast changing flood risks over the next 30 years (Eby and Ensor 2019; First Street Foundation 2020).

Most of the concerns raised so far by the Flood Factor data relate to the millions of property owners who are uninsured/underinsured for flood damage and whether the federal government is prepared to fund relief efforts for a vastly increased scale of disasters. However, FEMA maps also typically inform the floodplain regulations necessary for communities to qualify their properties for obtaining insurance under the National Flood Insurance Program (NFIP). Consequently, FEMA’s floodplain designations form the foundation for the land use plans, zoning ordinances and building codes of many if not most of the 22,000 U.S. jurisdictions approved for the NFIP. For this reason, reliance on FEMA maps hamstrings public officials in their efforts to build better communities, as illustrated by considering the impact within ten categories of the Ecocity Standards.

Most obviously, developing in flood zones upends Ecocity Standards goals for providing safe housing, minimizing water pollution, and fostering ecological integrity by reducing the water-borne contaminants that are rapidly destroying aquatic environments. Perhaps less obvious, the continuity of floodplains allow for wildlife migration, making them ideal places to protect and hopefully restore biodiversity, as promoted by the Ecocity Standards. Floodplains are also preferred locations for multi-use trails, infrastructure that promotes Ecocity Standards goals for planet-friendly transportation, clean air, energy conservation, climate action and well-being from the health and recreational benefits of walking and bicycling. In addition, accurate flood data would help promote more compact communities as recommended in the urban design pillar of the Ecocity Standards, an outcome with the potential to single-handedly cut urban resource consumption in half (IRP 2018).

Many planners are aware of the shortcomings of FEMA flood maps. It remains to be seen whether the political will exists to build better plans and codes using more accurate modeling like the Flood Factor methodology developed by First Street Foundation. Failure to use the best available information squanders an opportunity to build better communities and exposes more people to preventable risks.


Eby, Matthew and Collen Ensor. 2019. Understanding FEMA Flood Maps and Limitations. Accessed 7-6-20 at

First Street Foundation. 2020. Flood Factor. Accessed 7-6-20 at

Flavelle, Christopher et al., 2020. New Data Reveals Hidden Flood Risk Across America. New York Times. Accessed 7-6-20 at

IRP (International Resource Panel). 2018. The Weight of Cities: Resource Requirements of Future Cities. Paris: United Nations Environment Programme.

About the author

Rick Pruetz

Rick Pruetz, FAICP, is Vice President of the Ecocity Builders Board and an urban planner who writes about sustainability, most recently Ecocity Snapshots: Learning from Europe’s Greenest Places and Smart Climate Action through Transfer of Development Rights.