Lake Peachtree Spillway Replacement

Designing a structure to occupy the same footprint as the existing spillway, maintaining a low hazard classification, and not increase flooding upstream or downstream of the lake.

Peachtree City, Georgia, chartered in 1959, is one of the first planned communities in Georgia. Lake Peachtree, the centrepiece of the community, provides water supply and recreation for the residents. In response to a pending reclassification from low hazard to high hazard, Schnabel performed detailed analyses to justify supporting a low hazard classification.

Despite remaining a low hazard structure, the City proactively elected to upgrade the dam and spillway to meet criteria for a high hazard structure. The proposed upgrades included increasing spillway capacity.

Site conditions and economic considerations placed limitations on acceptable alternatives. The upgraded spillway had to simulate the existing outflows for storm events up to and including the 100-year event. Designing a structure to occupy the same footprint as the existing spillway, maintaining a low hazard classification, and not increase flooding upstream or downstream of the lake while safely discharging the required design storm were the objectives of this project.

Raising the dam to provide additional storage of floodwater was not an option due to surrounding development and frequent upstream flooding of upstream properties. Site restrictions limited widening the former spillway, and the existing spillway channel width was restricted by a downstream bridge.

A gated spillway was not considered viable due to concerns related to maintenance and the potential for unintended releases.

The selected alternative was a multi-staged Piano Key Weir (PKW) – the first in service in the United States and the first known multi-staged PKW in the world. These non-linear weirs provide significant hydraulic capacity under relatively low head, and the unique geometry provides advantages over more commonly accepted labyrinth weirs within limited footprints.

The PKW structure was designed by Schnabel Engineering to simulate the existing storm discharges, occupy the footprint of the existing spillway to minimize environmental impacts, and discharge the State required design storm (one half of the Probable Maximum Precipitation).