Your Property Might Flood In A Storm

By: Alan Pollock

Topics: Erosion , Climate Change

A traditional FEMA flood map (left) and the FloodFactor map from First Street Foundation (right).

Even If FEMA Maps Say It Won’t

Time was, if you wanted to know which parts of town flooded during major hurricanes, you’d ask old timers. Then FEMA developed maps showing flood zones down to the property level. But both methods have a major shortcoming: they rely on past events to predict the future.

The problem is, the environment is changing quickly, and some of the baseline assumptions used in conventional flood maps are now up for debate. New maps published by the nonprofit First Street Foundation show that many properties not currently in official flood zones are actually susceptible, or will be in the decades ahead. That can have serious implications for individual homeowners, but also for coastal development on a macro scale.

FEMA flood maps, available at, are revised every few years as deemed necessary to reflect changes in environmental conditions, science or levels of development. The agency now classifies 8.7 million properties as having substantial risk, putting them within Special Flood Hazard Areas, but the First Street Foundation’s flood model puts about 70 percent more properties at the same level of risk.

“This means nearly six million households and property owners have underestimated or been unaware of their current risk” the Foundation’s news release reads. “This discrepancy exists because the Foundation uses current climate data, maps precipitation as a stand-alone risk, and includes areas that FEMA has not mapped.”

When adjusting for future environmental factors like sea level rise, warming air and ocean temperatures and new weather patterns, the Foundation models find that the number of properties at substantial risk of flooding reaches 16.2 million by the year 2050.

Available at, First Street Foundation’s models are based on peer-reviewed data, and were developed by more than 80 hydrologists, researchers and data scientists from Columbia, MIT, Boston University and a host of other non-government agencies, using data from NOAA, the USGS and other government entities.

“In environmental engineering, there is a concept called stationarity, which assumes that today is going to be like yesterday, and tomorrow is going to be like yesterday,” First Street Foundation’s Chief Data Officer Ed Kearns said. “This concept used to work, but with a changing environment it’s a poor assumption and no longer does. FEMA’s method assumes stationarity, First Street’s does not.”

Understanding a property’s flood risk is critical, since most homeowner insurance policies don’t cover flood damage. The First Street Foundation maps reflect risks both from storm surge, high tides and coastal storms, but also from freshwater flooding.

The Cape Cod Chronicle’s offices on Munson Meeting Way in Chatham have never experienced flooding, and are listed on FEMA Flood Insurance Rate Maps as being in a “Zone X,” or area of minimal flood hazard. The maps on predict that there is a 1 percent chance that the newspaper’s offices will flood with less than a foot of water this year, and show in detail which parts of the property are most susceptible to inundation. They can also be adjusted to predict the flood zones 15 and 30 years in the future.

That kind of detailed flood mapping has previously only been available to those who purchased flood risk information from private for-profit firms.

“First Street Foundation has not only taken this kind of data to the next level, using peer-reviewed science, but is correcting an asymmetry of information by providing free access to everyday Americans,” Foundation Executive Director Matthew Eby said.