HADLEY — Rebuilding the current dike and levee system that protects Hadley center and areas to the west from Connecticut River flooding, or installing a new 13-foot-high levee for more than half a mile along Bay Road, are options that could have $25 million to $65 million price tags, according to conceptual cost estimates from the town’s consultant.
The Select Board received a presentation on Sept. 20 from Rich Niles, a project manager with Woodard & Curran of Andover, showing that the greater the investment, the more structures, including homes, garages and businesses, would be protected from the river overflowing its banks, with concern that the river is likely to rise higher as the result of both climate change and continued development along the riverfront.
The current flood control system includes the 1.7 miles of earthen levee embankment along the Connecticut River north of West Street, running from the intersection of Rocky Hill Road and River Drive to the Honey Pot section of town, and an additional 1.5 miles of the Norwottuck Rail Trail that parallels Route 9.
An upgrade to the current levee system by making it 4 feet taller and wider might cost around $25 million. This would also involve removing and reconstructing the rail trail berm. The current level of protection would be maintained for about 57 structures, houses or garages.
“We know it’s not cheap to repair levees,” Niles said.
The report shows there are some deficiencies in the levee, with the berm too deep and too narrow, and under some conditions it could become unstable, though under most conditions it meets some of the design criteria. “And there are some seepage and stability issues,” Niles said.
Niles said the consultants have suggested all along that the town should maintain what it has, do regular inspections and vegetation and drainage system maintenance, fix areas where animals have burrowed, and make sure trees don’t take root.
“It’s a challenging embankment to maintain,” Niles said.
A new levee along Bay Road, though, might cost around $65 million. It would be about 13 feet high and run 3,000 feet. “That’s substantial,” Niles said.
But that investment would protect 218 or 219 structures, including commercial sites on Route 9, such as hotels and gas stations, as well as Hopkins Academy.
“We know there are way more properties, commercial and residential, that are not protected now,” Niles said. “A new levee could provide protection for a lot of properties.”
Which option to pursue may come down to a matter of what the community, and those who own the land, prefer.
The consultants note that the Federal Emergency Management Agency base flood flow that the current levee system is designed for is 180,000 cubic feet per second, but that dates to 1978. Only in 1936, the year of the Great Flood, and 1938, the year of the Hurricane of 1938, has the river exceeded that flow.
FEMA is doing a remapping project and updated hydrologic and hydraulic analysis showing that there is a 2- to 3-foot increase in base flood elevation, and that the base flood flow has gone up to 182,000 cubic feet per second. “What that does is that changes the limits of the flood plain, the elevation against the berm,” Niles said.
More importantly, with estimates of future 100-year floods and through climate modeling and a study by University of Massachusetts researchers and the Massachusetts Department of Transportation, a 15% increase in river flow is projected. That means the updated 182,000 cubic feet per second flow will rise to 209,300 cubic feet per second flow, and create a lot more vulnerabilities.
“It gives you some sense of the magnitude change,” Niles said.
Originally built in 1928, the current dike was damaged and extensively repaired following the Great Flood of 1936 and the Hurricane of 1938, and has been improved at times by the Army Corps of Engineers. In 2009, a crack was found in a section of the dike and, during repairs, a section collapsed and was rebuilt. That led to the complete evaluation that began in 2014 and the effort to be recertified by FEMA.
The consultant suggests that a grant from the Massachusetts Vulnerability Preparedness program may pay for some of the work.