The Elm Lake Dam, constructed by the Works Progress Administration in 1937, is owned by the South Dakota Office of School and Public Lands. It serves as both a recreational lake and a supplemental water supply for the city of Aberdeen. Given its age and the risk to the community in the event of catastrophic failure, the dam was classified as high hazard.
The dam’s primary spillway is a 150-foot-wide reinforced-concrete chute, with a concrete-lined approach channel upstream. The structure had suffered extensive damage over the years, leaving sections of the chute broken or displaced and exposing the underlying subgrade. In addition, because there had been no stilling basin or other feature to dissipate the energy of high flows, a large void had developed near the chute’s bottom.
Following the flood of record in July 2020, the state of South Dakota decided to replace the spillway, but wanted to make the structure smaller without compromising its functionality. Barr was hired to conduct site investigations, assess alternatives, and identify options for replacing the spillway and upgrading the dam facility to meet modern safety standards.
During the site investigation phase, we estimated the probable maximum flood (PMF), performed dam-breach modeling, and evaluated a series of alternatives that paired embankment modifications with combinations of spillway sizes and types. The concept chosen for detailed design called for widening the primary spillway to 170 feet, adding a stilling basin at the bottom of the chute, raising the embankment crest by 4.5 feet, and adding a grade-control wall and riprap protection downstream.
Barr’s computational flow dynamics (CFD) experts used a program called FLOW-3D to develop a model that informed hydraulic design of the primary spillway. CFD simulations helped us predict flow patterns through the spillway’s curved approach channel and measure the impact of “superelevation”—the rise in water surface at the outermost point of the curve.
Those results showed that both the superelevation and its impact on conveyance were significantly less than standard empirical calculations had initially predicted. With that information on the hydraulics of the spillway chute, we were able to design a much shorter and shallower stilling basin than originally conceived—significantly reducing the construction cost of the primary spillway structure.