Report Shows Greater Boston is Susceptible to Unpredictable Stormwater Flooding

May 1, 2023 – BOSTON – While images of waves crashing over seawalls and motorists braving flooded streets may attract more attention, stormwater flooding is devastating in its own right and often leaves unexpecting homeowners in a costly  predicament. Sometimes it takes a historic event – like the rainfall in March 2010 – to bring light to an issue that affects thousands of homeowners across the state.

As part of the Metropolitan Area Planning Council’s (MAPC) hazard mitigation planning efforts, the agency entered into an unprecedented data sharing agreement with the Federal Emergency Management Agency (FEMA) and the Massachusetts Emergency Management Agency to access disaster claims records from the March 2010 storm, which dropped 1.5 feet of rain in three historic storms over 19 days. These data allowed a first-of-its kind analysis of stormwater flooding in Eastern Massachusetts.

Stormwater flooding, sometimes referred to as inland or urban flooding, occurs when the volume of water on land exceeds the capacity of natural and built drainage systems. Unlike their coastal counterparts, inland property owners do not have access to a predictive flooding model, nor do the FEMA flood maps capture much of the risk of stormwater flooding.

The flooding caused by the March 2010 storms was widespread and distributed throughout the MAPC region. However, “Water, Water, Everywhere: The Increasing Threat of Stormwater Flooding in Greater Boston” shows that FEMA Flood Maps are poorly predictive of where stormwater flooding is most likely to occur. Ninety-six (96) percent of the disaster claims arose in areas outside of the FEMA Special Flood Hazard Areas (SFHAs), also known as the 1% chance flood zones.

“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 financial toll was significant. In total, residential properties in the MAPC region that filed disaster assistance claims experienced an estimated $34 million in damages initially identified as potentially eligible for reimbursement. The average amount awarded per claim was $1,762, with a maximum award of $29,900. Overall, more than $30 million was awarded to residential property owners in the MAPC region.

Of the flood claims granted, 86% were for flooding levels of less than a foot and 71% were for flood heights of less than six inches; this suggests that even moderate levels of flooding can cause significant, widespread damage to properties. In addition, the March 2010 storms may not be an anomaly. The number of intense two-day storms has increased by 74% from 1901 to 2016, and the heaviest rain events of the year now drop 55% more precipitation than the rainiest days of the midcentury. And with the increasing impacts from climate change, we can expect more intense storms in the future.

“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,” said Bowers.

“Water, Water, Everywhere” also looked at other flooding indicators related to the locations of flood claims beyond presence within the FEMA SFHAs, including proximity to water and wetlands, slopes, soils, and the year a home was built. While evidence was found that stormwater flooding is relatively more common in homes built during the mid-20th century and those near existing or filled wetlands, more research is needed to clarify these connections.

The report recommends several actionable measures for federal, state, and local levels:

• Enable more widespread access to flood claims data. Federal privacy requirements privilege the privacy rights of current property owners over municipalities' needs to identify and respond to flood risk and over the public's rights to be informed of risk.
• Require flood history disclosure. Massachusetts is one of only 15 states that has no disclosure requirements for potential home buyers. Renters also need to be aware of risks to their properties.
• Finance property retrofits and repairs. MEMA should apply to the federal Storm Act that provides funding for hazard mitigation revolving loan funds. The state and municipalities can also set up programs that provide financial and technical assistance to property owners at risk of flooding.
• Provide more funding for stormwater management. Repairs to aging infrastructure and additional green and gray infrastructure is required to meet the increasing flood risks we face. This is especially important in highly urban locations that are home to environmental justice populations.
• Continue to investigate the causes and impacts of stormwater flooding.

“We hope this report will help serve as a wake-up call about the widespread risks and disruption that stormwater flooding poses to our residents and business,” said Anne Herbst, one of the report’s primary authors. “As our research shows, there’s a huge economic cost that impacts people of all stripes across the region.”