Need to Save Millions on your Spillway Rehabilitation? Consider Combining CFD Modeling and Rock Scour Analysis

Lake Roberts Dam, like so many dams around the country, had a spillway capacity that was significantly undersized to safely convey the inflow design flood (IDF). Several alternatives were previously developed to increase the spillway capacity; however, the costs of these alternatives exceeded the funds available for rehabilitating the dam. AECOM (formally URS) developed a new alternative for rehabilitating the spillway using a combination of computational fluid dynamics (CFD) and rock scour analyses. The new alternative was constructed at a savings of more than $5M, which was approximately half the cost of a previously developed alternative. This paper summarizes the alternatives which were evaluated, rock scour analyses and CFD modeling that were used during final design, and briefly discusses some of the lessons learned.
Lake Roberts Dam is located in southwestern New Mexico approximately 30 miles north of Silver City. The dam is a high hazard, zoned, earthfill structure. Prior to completion of modifications completed in 2014, the dam had a structural height of 56 feet and a hydraulic height of approximately 35.7 feet. Appurtenant structures included an outlet works and an emergency spillway. The capacity of the emergency spillway was approximately 16,700 cubic feet per second (cfs). The inflow design flood (IDF) was estimated to be approximately 63,500 cfs.
The new alternative selected for final design included a dam raise with two new emergency spillways; a concrete-lined spillway and an unlined spillway. The final design analyses included rock scour evaluation under various flood conditions to confirm rock characterizations used in the Erodibility Index Method. CFD modeling was performed to optimize the spillway approach conditions and stilling basin hydraulic performance, which reduced construction costs by more than ten times the cost of performing the CFD model.