Research provides design options for stepped RCC spillway

Authors: Hunt, Sherry; Kadavy, Kem

Submitted to: Popular Publication
Publication Type: Popular Publication
Publication Acceptance Date: 3/1/2008
Publication Date: 4/1/2008
Citation: Hunt, S., Kadavy, K.C. 2008. Research provides design options for stepped RCC spillway. Hydro Review. p. 81.

Interpretive Summary:

Technical Abstract: Increased urbanization upstream and downstream and in the immediate vicinity of small flood control dams have caused many structures to experience inadequate spillway capacities. To meet dam safety standards, some of these structures are in need of immediate attention. Altering the dimensions of the existing spillway(s) to increase spillway capacity is often limited by infrastructure, landscape, or unobtainable land rights. These constraints have led design engineers to select roller compacted concrete (RCC) stepped spillways as a design alternative to increase spillway capacity. To date, most research on stepped spillways has been related to those constructed with rather steep slopes (theta less than 25 degrees). Questions still linger about the design of RCC stepped spillways, especially those constructed on relatively flat slopes (theta less than 25 degrees). Additionally stepped spillways create significant energy dissipation, which leads to questions related to the design of the stepped spillway training walls, the energy dissipating stilling basin, and the downstream channel protection. The USDA-Natural Resources Conservation Service (NRCS) has proposed a stepped spillway for the rehabilitation of an existing flood control dam named Renwick Dam. The proposed stepped spillway is a unique structure that lies on a rather flat (4H:1V) slope. Questions related to the design of the spillway training walls and the elevation of the proposed stilling basin were brought forth by NRCS design engineers. A 1:8 scale model was constructed and tested at the USDA-Agricultural Research Service (ARS) Hydraulic Engineering Research Unit (HERU) in Stillwater, Oklahoma, at the request of the NRCS. The results from this study showed that the energy dissipation and air entrainment had little significance on the design of the spillway training walls under design flow conditions. Additionally, a 3.0 m (10 ft) difference in spillway drop (40 ft to 30 ft) and subsequently a 3.0 m (10 ft) difference in tailwater on the structure resulted in 42% difference in the length of the energy dissipation stilling basin (30 ft to 46 ft) and a 46% difference in the D50 of the riprap (10 inch to 16 inch) for the downstream channel. This research provides multiple design solutions for the NRCS for the design of this specific RCC stepped spillway.