Central Alabama Water signals commitment to original Lake Purdy Dam stabilization plan

Central Alabama Water’s board of directors heard differing expert views on how best to stabilize Lake Purdy Dam, with Vice Chair Phillip Wiedmeyer indicating the utility plans to maintain its current design approach. Wiedmeyer said reconsidering an alternative would require canceling the existing rehabilitation contract, redesigning, and rebidding, which he estimated would add about two years to a four-year project. He cited a study that found the 115-year-old dam would not remain stable during an extreme flood, noting that failure could threaten lives and thousands of homes.

Three independent experts, Donald Bruce, David Campbell, and Eugene Bentley Sorrell, examined the dam on November 5 and met the following day with project engineers. During Monday’s meeting, Bruce and Campbell supported the current plan to add concrete buttresses to the downstream face, while Sorrell urged the board to consider an alternative strategy that would allow water to overflow the dam with overtopping protection. Board members said it was the first time they had heard that option. “How’re we going to solve this mess we’re in, whether or not we should go one way or another way, and how much money is it going to cost us?” board member Jarvis Patton Sr. asked.

All three experts agreed the project should include micropiles to support areas of weaker foundation and a grout curtain to fill voids in the underlying limestone and reduce seepage. They also concurred that the overall design meets high engineering standards and would enable the dam to withstand a probable maximum flood, a designation for the most severe combination of conditions considered reasonably possible for the area. Last month, the board learned that engineers had determined the foundation on one side of the dam could not support the added weight of the buttresses, prompting the proposal to add deep foundation supports using micropiles.

Sorrell said overtopping protection could be significantly less expensive and would require fewer micropiles. He described it as a popular, lower-cost option in some current practices, in contrast to fully containing a probable maximum flood. Because Lake Purdy Dam is not designed for overtopping, uncontrolled overflow could erode earthen features and undermine the foundation; overtopping protection systems are intended to prevent such erosion and structural stress.

Wiedmeyer said other design choices should have been raised in 2019, when Sorrell was chief engineer and the current plan was presented to and accepted by the previous board. He said switching now would be more costly and the downstream impacts of an overtopping design have not been characterized. “We have a contract for the complete rehabilitation of this dam,” Wiedmeyer said, adding that he favors continuing under the existing plan.

After an executive session closed to the public, the board voted unanimously to authorize engineering firms to proceed with designing and overseeing installation of the micropile deep foundation under the current buttress-based plan. Patton said information presented during the executive session gave him confidence to support that step.

The current design is part of an $88 million rehabilitation project that began late last year. The board, which first met in May after a state law dissolved the previous board, voted on Oct. 29 to hire Bruce, Campbell, and Sorrell to review the overall design and the micropile proposal. The new board briefly halted the project in July before deciding to complete phase one at a projected cost of $28 million and then decide whether to continue. On Monday, engineering firm Arcadis North America presented an updated estimate indicating phase one will cost $44 million; it is expected to be completed in April. Wiedmeyer said the board will need to decide soon whether to continue with the project under the current design. “At some point, the board has got to give a green light,” he said.

Lake Purdy Dam, located in north Shelby County, has a capacity of about 21.2 billion liters (5.6 billion gallons) and supplies drinking water to much of Central Alabama Water’s service area, including Birmingham, Homewood, Hoover, Mountain Brook, and Vestavia Hills.