Eagle Nest Dam: Hydraulic Model Study of Flood Overtopping

Eagle Nest Dam is a thin concrete arch dam near the town of Eagle Nest, New Mexico. The arch dam has a constant radius of 155-feet, crest length of approximately 400 feet, and a structural height of approximately 140 feet. The dam was completed in 1916, and used initially to impound water for irrigation and municipal purposes. Recently, the State of New Mexico purchased the dam for storage reliable storage on the Canadian River. Previous studies, by others, concluded that the inflow design flood (equal to the probable maximum flood [PMF]) will overtop the crest of the dam by approximately 13 feet, and the energy from the overtopping jet will probably result in significant rock scour downstream of the dam. Therefore, it was considered necessary to performed hydraulic model studies of the dam to determine the effects of overtopping on the existing foundation rock, and to develop alternatives that would mitigate the potential rock scour. This report discusses results from the hydraulic model studies. This paper will discuss the hydraulic model studies and present some of the results. The hydraulic model was built to a scale of 1:24 and was used to estimate the pressure coefficients on the foundtion rock due to overtopping with the existing (original) crest, and two modified crest shapes. The modified crests shapes had two primary purposes; first, breakup the overtopping jet to reduce the pressure on the foundation rock surface, which would reduce the potential for erosion, and second, to deflect the overtopping jet further downstream and away from the toe of the dam. The hydraulic model studies of the original (existing) crest configuration showed that the overtopping jet impacted very close to the toe of the dam. One note of particular interest was the observation of very high flow velocities along the toe between the jet and the downstream face of the dam. These velocities were more severe than initially estimated in the previous analyses. Therefore, the potential for erosion at the base of the dam near is probably greater than initially estimated. The modified crest designs included a Roberts crest and Falvey crest. Both crest designs resulted in more turbulence in the overtopping jet, and moved the impact area farther downstream. The resulting pressure coefficients on the foundation were less than original crest design. Additional modifications to the dam included blocking off flow through the existing spillway channel on the left abutment, building training walls on the right and left edge of the dam crest to deflect the flow away from the abutments, and developing a plunge pool downstream of the dam. The observations from these studies will also be discussed in the paper, as they redcuced the potential for erosion on the abutments. The problems associated with rock scour,particularly due to overtopping of existing dams, is a significant concern to all dam safety engineers and regulators. This paper provides interesting insite to a very complex problem. 14 pp., 15 figures, 9 references.