The Hydraulic Design Toolbox: Theory and Modeling for the Lake Townsend Spillway Replacement Project
Association of State Dam Safety Officials (ASDSO)
Lake Townsend is the primary water supply for the City of Greensboro, North Carolina. The dam’s concrete gated spillway is suffering from severe deterioration due to alkali silica reactivity and has inadequate capacity to meet spillway design flood (SDF) requirements. To address these dam safety issues, the existing spillway will be replaced with a labyrinth weir located just downstream of the existing structure.
A combination of theory, computational fluid dynamics (CFD), and physical modeling were applied to the design of the replacement spillway for Lake Townsend Dam. In addition, updates related to ongoing research related to hydraulic performance of labyrinth weirs are presented.
Labyrinth discharge can be estimated using reliable empirical methods and through physical or numerical modeling. The hydraulic design for this spillway included several challenges, including weir submergence at high flows and providing energy dissipation over a wide range of flows and tailwater conditions.
Spillway hydraulic modeling included 2-D and 3-D CFD and a physical model study of a 1.5 cycle labyrinth weir with several stepped chute configurations. These models were used to:
• estimate discharge capacity and compare with empirical methods,
• develop a shortlist of stepped chute and stilling basin geometries (using 2-D CFD),
• evaluate four selected configurations (step heights and geometries) using the physical model,
• assess the hydraulic effects of the existing partially demolished spillway on weir performance,
• evaluate splash and wave run up to size training walls, and
• estimate pressures on the downstream face of weir for use in structural design,
The two stage, seven cycle labyrinth spillway is designed to pass 82,000 cfs without embankment overtopping. This flow (about 60 percent of the SDF) is roughly equal to the theoretical capacity of the existing gated spillway. To safely pass the SDF (143,000 cfs), the embankment will be armored with articulating concrete blocks (ACB). The ACB design also presented challenges due to the high tailwater and occurrence of a hydraulic jump on the downstream slope of the embankment.
Paxson, G., B. Crookston, B. Savage, B. Tullis, F. Lux (2008). “The Hydraulic Design Toolbox: Theory and Modeling for the Lake Townsend Spillway Replacement Project.” Assoc. of State Dam Safety Officials (ASDSO), Indian Wells, CA, Sept.