CS15B - Concurrent Session 15B: Low-Level Outlet Retrofit at Lake Lure Dam

Abstract Description: Lake Lure Dam is a concrete, multiple-arch buttress dam owned and operated by the Town of Lake Lure, a small town in the mountains of western North Carolina. The dam, constructed in 1926, has performed well over its service life, but does not meet modern dam safety standards. In 2019, deficiencies in the dam were identified related to its hydraulic capacity, structural stability, and lack of a low-level outlet. To address immediate safety and operational concerns, the owner opted to install a low-level outlet in the existing dam while planning for a new dam that would address the other deficiencies.

A key project requirement was maintaining a full reservoir throughout construction. This requirement, along with a lack of subsurface information, limited construction access, and close proximity to a frequently operated gated spillway, influenced the design approach and required creative solutions. Additionally, the low-level outlet was designed such that portions of the system could be integrated into the replacement dam, planned for construction immediately downstream.

To create a low-level outlet, a 72-inch hole was cored into the base of one of the unreinforced concrete arches – a particularly vulnerable part of the dam. To mitigate risks, the lower portion of the arch was buttressed with shrinkage-compensating concrete anchored into bedrock to structurally deactivate the arches before the penetration was made. Prior to the arch penetration, divers installed a steel portal door hinge-mounted to a precast concrete base on the upstream side of the dam. The portal door was machined and included an O-ring to seal against approximately 80 feet of head pressure from the reservoir without using bolts and with no leakage. Once the pipes, valves, and other system components on the downstream side were installed, the portal door jack—mounted underwater on the upstream trash rack—was operated opening the portal door and activating the system. This project is considered a success in that the low-level outlet operated as designed, addressed a critical dam safety deficiency, and provided the owner with greater reservoir level control. This capability will support broader dam rehabilitation efforts and capital improvement projects upstream of the dam.

This case study offers valuable insights for engineers, contractors, and owners undertaking dam rehabilitation projects, especially those involving retrofitting a low-level outlet, buttress dams, underwater construction, or sealing applications where leakage would be problematic. It highlights creative design and construction solutions for modifying infrastructure to meet modern dam safety standards.