Undersized culverts found to be a main source of Wood-Pawcatuck flooding

Richmond, R.I. — Nearly half of all the culverts in the watershed of the Wood and Pawcatuck rivers are too small to allow for adequate water flow during intense storms, contributing to the risk of flooding in the 317-square-mile area.

“We’re got a problem here,” said Erik Mas, vice president of the engineering firm Fuss & O’Neill, during a presentation Thursday to about 25 people at the H.L. Arnold Fire & Safety Complex. About 38 percent of the more than 400 culverts and bridges his firm inspected are undersized, and another 11 percent are expected to be undersized by the year 2070 as annual rainfall increases with climate change, he said.

The information about culverts and bridges was part of Mas’ presentation of findings of a study commissioned by the Wood-Pawcatuck Watershed Association of how to make the watershed less vulnerable to flooding. Funded by a $720,000 federal grant, the work began in 2015 and will result in creation of a plan for projects the 14 communities in the watershed can undertake to lessen damage from future floods like the one that occurred in 2010. Input from local officials and community residents at the meeting Thursday and another next week will be incorporated into Fuss & O’Neill’s final report, to be called the Wood-Pawcatuck Watershed Flood Resiliency Management Plan.

Denise Poyer, project coordinator for the watershed association, said a draft plan will be presented to the community in January, and a final plan issued in March. After that, the association will host training sessions for municipal officials about how to implement the recommendations of the plan and obtain funding for projects.

"It's not just about culverts and dams," she said, "but also about putting in green infrastructure projects." Creating swales, wetlands and gardens to store water would help minimize flooding and improve water quality. The plan will also make recommendations for changes in town regulations to discourage building in flood plains, she said. In addition, it will identify areas where stream channels could be restored with their natural meanders to slow floodwaters, and provide preliminary engineering plans for proposed projects. In all, about 30 proposed "green infrastructure" projects throughout the watershed are described in the preliminary plan. 

"All of these will be recommendations that the towns can do what they want with," Poyer said. "It gives them something to start with."

Mas said that during the field work portion of the study, Fuss & O’Neill staff inspected 420 culverts and bridges and 43 dams in person, and reviewed reports on another 27 dams they could not access. In addition to protecting buildings and people from future flood damage, the purpose of the work is also to protect and enhance the quality of the water and wildlife habitat in the watershed, Mas said. He said only 40 percent of the culverts and bridges on the 380 miles of rivers and streams in the watershed are large enough to allow for passage of fish and other aquatic organisms.

The watershed extends into 10 mostly rural towns in Rhode Island and four in Connecticut — Stonington, North Stonington, Voluntown and Sterling.

Mas emphasized that making the watershed better able to prevent, plan for and quickly recover from flooding is increasingly important as the intense storms become more common due to climate change. Climate and storm data from the past 60 years shows that the six-state New England region has experienced a 71 percent increase in these events, he said.

“New England is experiencing the greatest increase in intense storms,” he said.

Mas said the study will give each of the culverts, bridges and dams needing work in each of the towns a ranking of high, medium or low, so that the communities can prioritize projects. About 155 of the culverts and bridges that were assessed were designated "high priority" for upgrade or replacement, he said, based on hydraulic capacity, vulnerability, potential for causing damage to surrounding areas if flooding occurs, and ability for aquatic life to pass through. Another 12 of the dams assessed were rated "high priority" for removal or repair. 

Recommended actions in the final report may include replacing undersized structures with larger ones, removing or repairing dams or adding a fish ladder, buffering streams with natural vegetation, stabilizing stream banks and restoring wetlands.

The presentation will be given again from 10 a.m. to noon on Oct. 20 at the Westerly Library.

j.benson@theday.com

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