Feds' maps unreliable in predicting flood risk. Disclosing a building's history might be better

The Metropolitan Area Planning Council is calling for the state to require flood history disclosure for potential homebuyers and renters, after the council's analysis of stormwater flooding in 2010 showed existing tools to predict flooding were unreliable.

The MAPC report, released Wednesday, showed greater Boston is susceptible to unpredictable stormwater flooding. The council examined Federal Emergency Management Agency (FEMA) and the Massachusetts Emergency Management Agency disaster claims records from a March 2010 rain events, which dropped 1.5 feet of water on eastern Massachusetts over 19 days.

The flooding caused by the 2010 storms was widespread and eventually cost $59 million in disaster assistance to Massachusetts residents. The average amount awarded per claim was $1,762, with a maximum award of $29,900. But, FEMA flood maps — the primary source of flood risk information for homebuyers — were poorly predictive of where the stormwater flooding was most likely to occur, the report says.

The large majority, 96%, of the claims from the storms came from areas outside of the FEMA Special Flood Hazard Areas.

"The vast majority of the 19,395 approved and unique disaster assistance and flood insurance claims we analyzed were outside the SFHA," said Rachel Bowers, regional planning data analyst at MAPC. "As a result, most residents were unaware of their risk and damages were much greater than they otherwise might have been."

The council says this data is especially relevant as the number of rainfall events is on the rise. The number of intense two-day storms has increased by 74% from 1901 to 2016, the report says, and the heaviest rain events of the year now drop 55% more precipitation than the rainiest days of the midcentury.

"Potential home and business owners look to FEMA flood maps as a primary source of flood risk information. Our analysis raises significant concerns about the region's current understanding of and ability to prepare for stormwater flooding of any magnitude," Bowers said.

Massachusetts is one of 15 states that has no flood disclosure requirements for potential homebuyers.

In addition to requiring flood history disclosure, MAPC also recommends that the state provide more funding for stormwater management to repair aging infrastructure and add new infrastructure. The council also says the Massachusetts Emergency Management Agency should apply for funds through the federal Storm Act, which provides funding for hazard mitigation revolving loan funds.