The D.C. region’s roads and other transportation infrastructure is more at risk from future flooding than previously thought, a new analysis found.
The National Capital Region Transportation Planning Board (TPB) outlined its findings to leaders of member jurisdictions, including Hunter Mill District Supervisor Walter Alcorn and Braddock District Supervisor James Walkinshaw representing Fairfax County, at a meeting last Wednesday (June 18).
Past efforts using Federal Emergency Management Agency (FEMA) mapping to determine flooding threats to transportation weren’t “totally sufficient,” TPB planner Katherine Rainone said. To augment FEMA’s available resources, TPB licensed data from flood maps issued by Fathom, a UK-based analytics firm.
When combining the Fathom and FEMA data, the TPB found that roads, bus stops, transit stops and Metrorail lines all face higher flooding risks than previously had been reported:
- A 25 percentage-point increase in the total roads and highway lane miles that are seen as at risk
- A 7 percentage-point increase in the number of bus stops impacted
- An increase of 13 percentage points in the number of rail stops imperiled
- An increase of 25 percentage points in the total number of rail-miles that could be endangered
The new analysis is “far from perfect,” Rainone acknowledged, but she said it marks an improvement in what has been reported in the past.
“We really wanted to get some better information … to broaden our understanding of what transportation assets are at risk,” Rainone said.
The new analysis drew praise from TPB board members.
“This is really groundbreaking,” said David Snyder, a member of the Falls Church City Council who represents that city on the body.
He said the additional data will be valuable, as leaders at the local and regional levels continue to confront the impacts of flooding and general stormwater management.
“We simply have to address this issue,” Snyder said.
Neil Harris, a representative from Gaithersburg who serves as the TPB’s vice chair, said the new report was one facet of stepping up on flooding.
“We’ve been very active and busy working on resiliency of the region’s infrastructure,” he said.
There are some limitations in the use of the new data, owing to its proprietary nature. The raw data will not be available to the public or even local leaders on the TPB, whose contract with Fathom is set to expire at the end of the year.
For now, the information will be useful to “choose which projects to prioritize” at a regional level, Rainone said.
The FEMA data previously used exclusively for the analysis found that, when counting both coastal/river flooding and urban flooding, about 14% of roads, 4% of bus stops, 7% of rail stops and 39% of rail line-miles were at risk.
But FEMA’s data only focused on areas within its designated floodplains. A map shown by TPB staff, comparing FEMA-recorded flooding events with those reported by local governments and other sources between 2008 and 2024, revealed that about 85% of the 56,000 incidents had not been incorporated into FEMA’s maps.
Though a federally designated, independent body, TPB is housed at and staffed by the Metropolitan Washington Council of Governments. Its coverage area includes D.C., plus nine Virginia and 12 Maryland localities.
