The development of a newer, higher-resolution maps of the continental United States has a revealed a problem, according to researchers. At least 43 million people in the US may be affected by a 100-year flood, three times more than federal estimates currently predict, says a study release this week.

[Read the full study here.]

Current flood maps drawn up by the Federal Emergency Management Agency (FEMA) dramatically underestimate the number of Americans who could be at risk of a 100-year flood.

The Small Streams Matter

Last February, the University of Bristol’s School of Geographic Sciences revealed that its mapping model that uses the latest high-resolution maps of the US can produce much finer-grained flood-risk maps than FEMA can. These new flood maps can pinpoint risk even on the smaller streams ignored on the FEMA maps. These smaller streams should not be ignored because some of them run through heavily-populated areas. Among these new risk areas pin-pointed by the study are towns around the Great Lakes area and along the Pacific Coast.

The study which also uses maps supplied by the Environmental Protection Agency (EPA), can locate population centers much more precisely. Combined with population growth projections released by other sources, the study reveals that the number of US citizens at risk could exceed 60 million by 2025.

This total only counts those at risk from freshwater flooding. Communities at risk from sea level rise are not included in the study.

Because of the incomplete risk hazard maps, many of the people who reside in these flood-prone areas are not aware of the danger they face.

Average flood losses in the US have now risen to over $10 billion a year. This number is expected to rise even more steeply in the coming decades as climate change could intensify the risk of storms and make them even more severe.

By making accurate flood-risk maps and using them to guide development, local governments can minimize the loss of life and damage to property.

From CityLab:

Avoiding new construction in these areas will be key to minimizing future flood damage, Johnson said. It could also have another advantage: Instead of being clogged with concrete, these natural drainage areas could do what they’re meant to, and absorb a river’s excess water.

“We’ll get some additional benefit, because those flood waters will be able to spill out on the flood plain instead of being shunted downstream at higher velocity,” Johnson said. “In this country, we overlook and undervalue the roll of intact natural features in helping manage risk.”

For localities to make granular decisions, like how high to elevate buildings or where exactly to avoid new construction, planners will still have to conduct more specific local models. But the study points to the regions of the country that need to be more critical about their risk assessment. With more data, Wing said, this methodology will be able to better inform localities on flood risk: “With this new modeling, at this stage, we’re only scratching the surface.”