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Converging stepped spillways: Simplified momentum analysis approach

Hunt, Sherry L., Temple, Darrel M., Abt, Steven R., Kadavy, Kem C., Hanson, Greg
Journal of Hydraulic Engineering 2012 v.138 no.9 pp. 796
concrete, dams (hydrology), engineers, models, momentum, prediction, topography, watersheds
Roller compacted concrete (RCC) stepped spillways are growing in popularity for providing overtopping protection for aging watershed dams with inadequate auxiliary spillway capacity and for the construction of new dams. Site conditions, such as limited right-of-way, topography, and geological formations, often dictate that these spillways converge. Convergence increases the flow depth near the training walls and alters the stilling basin design requirements as compared with traditional straight spillways. A simplified control volume momentum analysis is presented for predicting the minimum vertical training wall height necessary to prevent wall overtopping in converging stepped spillways. An expression is developed to predict vertical training wall height as a function of centerline flow depth, centerline velocity, chute slope, and convergence angle. A three-dimensional 3(H)∶1(V) sloping stepped spillway model with an ogee crest and convergence ranging from 0–70° was constructed to verify this relationship. The evaluation showed approximately 2% error for all combined convergences. Because the relationship is based on momentum principles, it provides design engineers with a method for determining minimum training wall height requirements under nonair entrained flow conditions for a range of design conditions (e.g., chute slopes, convergences, and step heights).