For over two decades, Barr has been providing engineering services at Xcel Energy’s Cedar Falls hydroelectric dam on the Red Cedar River. The mostly hollow Ambursen dam was constructed in 1910 and creates the 1,800-acre Tainter Lake located on the river. The dam, which is classified as high hazard, is about 60 feet tall, has an overflow spillway roughly 250 feet in length, and has a gated spillway with two small tainter gates.
Because the project was incapable of passing the current inflow design flood (IDF) prior to overtopping, Xcel hired Barr to conduct an initial study to compare options to safely pass the IDF. The option selected consisted of a new ogee spillway with six large tainter gates and a smaller crest gate to pass debris and lower return-period flows.
Barr worked with the St. Anthony Falls Laboratory at the University of Minnesota to construct a 1:36 scale physical model of the entire structure, including the proposed spillway replacement and stilling basin. The physical model confirmed the modified spillway’s ability to pass the IDF as well as the stilling basin’s function to contain the hydraulic jump for a range of flow conditions. It also helped to understand preferred gate operations as well as model water control during stages of the planned construction. Additionally, the physical model was used to test optimized stilling basin designs, which allowed the overall basin length and construction cost to be reduced.
Final design was completed in July 2020 and included the new spillway, stilling basin, and gates. Phased construction for the spillway modifications and stilling basin is planned for 2021 through 2023.