The rain started around 7 p.m. It came down harder and faster than most Leominster residents had ever seen. Soon, motorists started pulling over because they could not see the road. Then the flooding began. In downtown, a building crumbled. Businesses reported chest-deep waters. Along Lancaster Street, sinkholes opened up, swallowing cars, garages, and front yards. Elsewhere, roads and railway tracks were washed away. By the time the rain let up around midnight, eleven inches had inundated Leominster, leaving tens of millions of dollars in damages and compromised infrastructure, including the Colburn Pond Dam in Barrett Park on the western fringes of town.
One of 24 dams in Leominster, the Colburn Pond Dam is a 15-foot tall earthen structure built in the 1800s. It creates the nine-acre large Colburn Pond, which feeds into Fall Brook and flows past Lancaster Street. According to Leominster mayor Dean Mazzarella, the dam was slated to undergo repairs and upgrades in the near future, having received a $163,500 grant from the Massachusetts Executive Office of Energy and Environmental Affairs in 2021.
"This particular dam is one that we're actually about to replace," Mazzarella told CBS.
Mazzarella said that the city had not seen such widespread damage since a 1936 hurricane, estimating that the flooding caused between $25 million and $45 million in damage to city infrastructure.
Leominster's director of emergency management, Arthur Elbthal, told CBS that two dams out of 24 in the city, including Colburn Pond Dam, sustained damage but held. Now that the dams have been stabilized, the city will prioritize repairing major road damage and the railway, but full recovery will take some time.
“It was just Leominster that drew the short straw on this one.”
The rainfall was a "200-year event," Matthew Belk, a meteorologist with the National Weather Service (NWS) in Boston, told CBS, noting that a trained weather spotter near Leominster recorded 9.5 inches of rain.
Unpredictable and highly localized rainfall makes it impossible to predict when and where flooding events might occur. During the flooding event of Sep. 11, 2023 in Leominster, the weather station at Fitchberg Municipal Airport 2.5 miles away recorded only 5.1 inches of rainfall.
Rob Megnia, a meteorologist at the NWS Forecast Office in Norton, Mass., noted that the timing of the storm that inundated Leominster was unusual given the time of the year. However, thunderstorms are common to southern New England in the summer months when hot, humid air and weak high-altitude winds favor long-duration locally-intense storms.
"The storm could have easily set up over Boston or any other of the surrounding areas," Megnia said. "It was just Leominster that drew the short straw on this one."
Megnia was cautious to draw correlations between climate change and the increased risk of extreme rainfall given a lack of precedent and the difficulty in predicting summertime thunderstorms. But he conceded that warmer air holds more moisture which fuels the kind of storms that led to the flooding in Leominster.
Through the deluge, the Colburn Pond Dam held firm but could not contain the sheer amount of rain that fell that evening. Whether better maintenance could have prevented much of the damage along Lancaster Street would never be known. What is clear is that in this new age of extreme weather caused by climate change, aging dams like the Colburn Pond Dam, commonly found throughout the state, are no longer adequate to prevent disasters that were once considered improbable.