By Dan May
On Our Land
Dan May
Connecticut has more than 4,000 dams, with nearly all its rivers impounded to some level. They serve multiple purposes, from simple impoundments for aesthetic purposes or recreation to major structures for water supply, hydroelectric power and flood control.
Stevenson Dam in Monroe is the largest nearby dam. Route 34 crosses the Housatonic River on top of this large concrete dam, which impounds Lake Zoar, mitigates downstream flooding, and provides hydroelectric power. This is one of the stations used by the US Weather Service to issue river flood warnings for our region, and the dam’s spillway on the north bank dramatically discharges when the Housatonic is at flood stage.
Dams that impound water for water supply and/or hydropower are usually kept nearly full and thus have limited flood storage capacity in storms. Flood control dams, by contrast, are built to host reservoirs that are empty most of the time and fill up only during major flood events. One that is important for this region is the Thomaston Dam across the Naugatuck River about 30 miles north of Orange.
Thomaston Dam was constructed after Hurricane Diane in 1955 drenched northern Connecticut and southwestern Massachusetts, with some locales receiving nearly 20 inches of rain in a 24-hour period. That hurricane caused major shoreline damage in Milford (near what became Silver Sands State Park), but downstream flooding devastated communities all along the Naugatuck to Long Island Sound. This was the most damaging flood in Connecticut history; over 75 people died across the state. Thomaston Dam is a massive earthen dam designed to retain floodwaters for future comparable events.
Closer to home, the Wepawaug River is dammed at more than 10 locations from its headwaters in Woodbridge to its outfall into Long Island Sound in Milford. Two of its larger dams form reservoirs that are noticeable when driving through Orange – Lake Wepawaug west of Grassy Hill Road and the Wepawaug Reservoir north of Route 34. Part of Prudden Lane between Grassy Hill and Derby-Milford roads is an earthen causeway that retains the water for Lake Wepawaug. The spillway for this lake is barely noticeable when driving across the bridge on Prudden Lane – except in flood events that overtop the road.
The concrete spillway for the Wepawaug Reservoir is visible on the north side of Route 34 and was recently rebuilt. That construction project highlights one of the challenges for all dams: they are engineered structures that have to be maintained and ultimately torn down and/or rebuilt. For municipal or state-operated dams this ends up as a tax burden. But many small dams are privately owned and maintained, and private dams incur both liability and maintenance costs to owners.
One set of private dams are those maintained by the Regional Water Authority for drinking water supply. Closest to us are their dams and reservoirs on the West River in Bethany and Woodbridge, which provide water to RWA’s Woodbridge drinking water plant and then customers in nearby towns.
A good place to get an up-close look at a dam is the one that impounds Lake Bethany. This structure has been modified several times since it was first constructed in the 1890s. It has earthen, masonry and concrete components. Hatfield Hill Road crosses in front of the dam on top of an earthen substructure, with a masonry superstructure towering above, placing the reservoir well overhead. In a cold winter, many icicles emerge on the dam’s face from weep holes that are spaced through the masonry. All dams leak, and those leaks are one of the factors that age a dam. In major floods, aged dams can catastrophically collapse.
One of the oldest dams in the region is the RWA dam that impounds the Mill River to create Lake Whitney in Hamden. The spillway on the east bank of the dam is an attractive feature at the Eli Whitney Museum. The dam was built by Eli Whitney’s son in 1862 and is nearing the end of its safe operating life. Dam failure here could potentially devastate the neighborhoods of East Rock and Fair Haven in New Haven.
RWA is working on plans to rebuild the dam, as Lake Whitney is an important supply for drinking water. But the project has engineering and sociopolitical challenges. With regard to engineering, a temporary cofferdam must be constructed upstream so the reservoir can be drained and the dam rebuilt. A drained reservoir will generate nuisance odors and possible environmental hazards from sediment that has built up in the reservoir over 160 years. The existing dam is of historical significance as well.
Meeting the needs of all stakeholders is a challenge for this project, and ultimately for thousands of aging dams in the state. Balancing flood control with water supply needs over dam lifetimes that will encounter changing climatic conditions adds more uncertainty both to engineering design and the community discussions about this essential infrastructure.
Dan May is a geologist and professor of environmental science at the University of New Haven. He can be contacted at dmay@newhaven.edu.