Photo courtesy of Central Alabama Water.
Work is underway for a $28 million rehabilitation project at Lake Purdy Dam. The 115-year-old dam is undergoing structural reinforcement at the reservoir, a primary drinking water source for Central Alabama.
Construction continues on a $28 million rehabilitation project at Lake Purdy Dam, where Central Alabama Water officials have added new safety tests and increased liability insurance for the 115-year-old structure.
Lake Purdy Dam is undergoing structural reinforcement in Phase 1 of the long-term project, which includes installing roller compacted concrete to strengthen the existing structure. The reservoir serves as a primary drinking water source for communities across the Birmingham metro area, including Hoover, Vestavia Hills and surrounding areas.
“Lake Purdy Dam is a historic structure, and this project presents a generational opportunity to ensure it serves our customers for decades to come,” said Phillip Wiedmeyer, vice chairman of the Central Alabama Water board and a retired engineer.
In addition to the main structural work, the CAW board has authorized three new initiatives designed to enhance safety and gather more data on the dam’s integrity: a reservoir dye test, exploration drilling and the installation of a deep foundation system.
The dye test will involve releasing two biodegradable, NSF-approved dye colors upstream to check for seepage pathways. Photos will be taken downstream at regular intervals during the test, which is expected to cost $10,000. Some dye may briefly appear in the Little Cahaba River and possibly the Cahaba River, but officials said there is no risk to water quality or public health.
Schnabel Engineering, the firm overseeing the project, will also supervise exploratory drilling on the dam’s downstream left side to confirm geological conditions. The exploration is expected to take one to two days and cost around $25,000, covered under the firm’s existing contract.
“While we’re at this point in the project, it makes good business sense to conduct additional exploration before the area is covered with concrete,” Wiedmeyer said. “We can do this at minimal expense and gather data to help inform future decisions.”
On the right side of the dam, engineers will install a deep foundation using micro piles — steel pipes filled with rebar and concrete — topped with a concrete slab. The system is designed to strengthen the foundation ahead of further roller compacted concrete work. Design is expected to take two weeks, with construction finishing by the end of December. Costs for the deep foundation work will be covered by contingency funds already budgeted into the project.
CAW has stated that Lake Purdy Dam is safe, with no imminent threat to the public. The reservoir was nine feet below its full capacity as of mid-October due to typical seasonal dryness. According to CAW, it would take more than seven inches of rain in 24 hours — a 25-year storm — for water to reach the top of the dam.
The Federal Emergency Management Agency classifies the dam as a “high hazard” structure. CAW has emphasized that this designation is based on the dam’s location near residential areas, not on its structural condition.
In response to public concern over insurance coverage, CAW recently increased liability coverage for the dam from $1 million to $10 million. The new policy is set to go into effect Nov. 1.
The board also discussed third-party inspections of the dam, but that item was tabled to allow additional time to evaluate the credentials of proposed inspectors. The board is expected to revisit that decision in two weeks.
Lake Purdy Dam was completed in 1910 and raised to its current height in 1929. It stands 73 feet tall and spans 445 feet. The reservoir holds up to 5.6 billion gallons and plays a key role in supplying water to Jefferson, Shelby, St. Clair, Blount and Walker counties.
More information is available at www.bwwb.org/lakepurdy.
