Fine Tuning a Piano Key Weir: The Story of the Lake Peachtree Spillway Replacement Project

Peachtree City, Georgia was chartered in 1959 and is one of the first planned communities in Georgia. Lake Peachtree is the center piece of the community, providing water supply and recreation for local residents. For many years, the dam was classified as a low hazard dam by Georgia’s Safe Dams Program (SDP). The SDP recently re-classified the dam to high hazard, but rescinded the re-classification after reviewing a detailed dam breach analysis that demonstrated the low hazard classification was justified. Despite the low hazard classification, the City proactively elected to upgrade the dam and spillway to meet criteria for a high hazard dam.
The proposed upgrades included increasing spillway capacity. Site conditions and economics created limitations on the spillway upgrade alternatives. The new spillway had to closely match the existing peak outflows for storm events up to and including the 100-year event. Designing a structure which would not increase flooding in the reservoir or the downstream floodplain, while still safely discharging the state required design storm was the primary objective of this project. Raising the dam significantly to provide additional storage of floodwater was not an option since the lake shoreline is well-developed and there were already flooding issues in the upstream reaches of the lake. Site restrictions limited widening the former spillway, and the existing spillway channel width was restricted by a downstream bridge. The City had concerns with installing a gated spillway due to maintenance, upkeep and the potential for unintended releases.
This paper explores the design and construction of the first multi-staged Piano Key Weir (PKW) weir put into service in the United States. These non-linear weirs provide significant hydraulic capacity under relatively low head, and the PKW’s unique geometry can provide advantages over more commonly accepted labyrinth weirs. This paper will be of interest to dam safety regulators, hydraulic designers, and other dam safety professionals, and will include a discussion related to the history of the PKW and the application of this unique structure for a small dam rehabilitation in the United States. The design, which included CFD modeling, structural analysis, and site layout will be presented, along with lessons learned from the design and construction.