By TIM FAULKNER/ecoRI News staff
PROVIDENCE — The historic floods of 2010 is a vivid memory for many Rhode Islanders. The natural disaster was unlike the hurricanes or other powerful weather events to which the region is accustomed. Instead, it was the culmination of 20 inches of rain in 38 days. The biggest and most destructive event was the 8.8-inch deluge between March 29 and April 1.
The Pawtuxet River watershed was one of the hardest hit in all of southern New England, with the river rising 11.8 feet above flood stage. Two shopping malls flooded, while roads, bridges and vehicles were submerged. Interstate 95 was closed for two days. The flooding washed out sewage treatment plants in Warwick and West Warwick. Across the state, hundreds of millions of gallons of untreated sewage and stormwater poured into Narragansett Bay.
The Pawtuxet River watershed has drawn considerable scrutiny since 2010. The river itself has two branches. One starts at the Scituate Reservoir. The other runs from the Flat River Reservoir in Coventry, R.I. to West Warwick. R.I. where both branches merge. The river flows another 12 miles as the border between Cranston and Warwick before emptying into Narragansett Bay.
The Scituate Reservoir, which provides drinking water for 60 percent of the state, has been a point of debate. According to the National Weather Service, the reservoir was filled before the rains began in March 2010. Much of the rain collected on its 5.3-square-mile surface flowed quickly to its collection basins and into the Pawtuxet River.
According to Dave Vallee, hydrologist-in-charge at the National Weather Service in Taunton, Mass., the reservoir was simply doing what it was built to do.
“It’s not a flood-control facility; it was designed as a water supply,” Vallee said.
As such, the reservoir, completed in 1926, was intended to reach capacity and increase water flow in the Pawtuxet River by June of each year, in time to meet irrigation needs. But, Vallee said, due to climate change the reservoir has been filling earlier in the year. Instead of June, it often reaches capacity by February.
“So you’ve got a lot more water there early in the season and that’s what exasperating flooding downstream,” he said.
The reservoir wasn't necessarily one of causes of the 2010 floods and in fact retained some of the rainwater. According to the University of Rhode Island, due to its large surface area, the reservoir likely reduced flooding by 40 percent.
Now the reservoir could be part of the solution. Preliminary research by URI and the Coastal Resources Management Council (CRMC) concluded that building a floodgate could decrease the volume and impact of future floods. The findings suggest that a new floodgate could add 4 feet of height to the reservoir. The additional volume could reduce major floods such as the one in 2010 by 80 percent, according to the research.
The findings suggest that a floodgate and new retention area, called a spillway, wouldn't compromise water needed downstream for sewage treatment plants and commercial and residential uses.
The research concluded that the gate “is not going to affect the drinking-water capacity of the reservoir," said Reza Hashemi, assistant professor with the Department of Ocean Engineering at URI’s Graduate School of Oceanography.
Hashemi spoke at a Nov. 9 presentation on the Pawtuxet River at the Department of Environmental Management (DEM). Hashemi concluded that the flood-control project is feasible but stressed that it requires more vetting.
Alisa Richardson, an engineer with the Water Resources Department at DEM and a member of the study’s stakeholder team, said the study still must examine dam safety, downstream drinking-water needs and funding source to pay for any project.
DEM director Janet Coit was enthusiastic about the proposal and suggested it could be one of the top resiliency projects for Gov. Gina Raimondo to add to her “call-to-action” list.
“When we are thinking about our resiliency plan, what are the four or five things we can do? Boy, this should be evaluated as one of those,” Coit said.
Hashemi noted that the dozen or so dams along the Pawtuxet River might also be contributing to the flooding. These decades-old relics of the Industrial Revolution also require safety inspections. Catastrophic failure of the dams, he said, could result in sending large quantities of contaminated soil lodged behind the dams flowing into Narragansett Bay.
Tree branches and other debris along the river is also a likely contributor to flooding, Hashemi said.
If the floodgate is built and other recommendations are followed, he said, “Then you can massively control the flooding. So that’s the message.”
By June 2017, the Pawtuxet River Watershed Modeling Study intends to complete a report on flooding and the Scituate Reservoir. It will analyze the effects of dams and dam removal. It aims to design an Internet-based modeling tool that uses stream gauges to predict floods along floodplains. The forecasting tool is expected to be a template for other river systems in Rhode Island and available on CRMC's Stormtools website.