Renwick Dam RCC stepped spillway research

Authors: Hunt, Sherry; Kadavy, Kem

Submitted to: State Dam Safety Officials Association Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 9/1/2007
Publication Date: 9/11/2007
Citation: Hunt, S., Kadavy, K.C. Renwick Dam RCC stepped spillway research. In: Dam Safety 2007. Proceedings of the Association of State Dam Safety Officials Annual Conference, September 9-13, 2007, Austin, Texas. 2007 CDROM.

Interpretive Summary: The Small Watershed Program administered through the USDA-Natural Resources Conservation Service (formerly the Soil Conservation Service) has provided technical and financial assistance for the construction of nearly 11,000 embankment dams across the U.S. The construction peak in the Small Watershed Program occurred in the 1960s with many of the dams originally designed to protect agricultural land; however, forty-seven years of progression has led to hazard classification changes for many dams. Consequently, many have inadequate spillway capacity. Altering the dimensions of the existing spillway(s) to increase spillway capacity is often limited by urbanization or unobtainable land rights. To provide adequate spillway capacity for these structures, many design engineers are choosing to place roller compacted concrete (RCC) stepped spillways over the existing embankment. Nearly 10% of NRCS structures are expected to use stepped spillways as a design solution for rehabilitation. Renwick Dam in North Dakota no longer meets state and federal dam safety guidelines due to a hazard classification change. At the request of the North Dakota NRCS, the USDA-ARS Hydraulic Engineering Research Unit constructed and tested a model section replicating the proposed stepped spillway for the rehabilitation of Renwick Dam. The goal of the study is to provide design recommendations for the stepped spillway chute and associated energy dissipating stilling basin. This research is expected to impact the development of design guidelines for stepped spillways and associated stilling basins planned on relatively flat slopes (Theta less than or equal to 22 degrees).

Technical Abstract: The Small Watershed Program administered through the USDA-Natural Resources Conservation Service (formerly the Soil Conservation Service) has provided technical and financial assistance for the construction of nearly 11,000 embankment dams across the U.S. The construction peak in the Small Watershed Program occurred in the 1960s with many of the dams originally designed to protect agricultural land; however, forty-seven years of progression has led to hazard classification changes for many dams. Consequently, many have inadequate spillway capacity. Altering the dimensions of the existing spillway(s) to increase spillway capacity is often limited by urbanization or unobtainable land rights. To provide adequate spillway capacity for these structures, many design engineers are choosing to place roller compacted concrete (RCC) stepped spillways over the existing embankment. Nearly 10% of NRCS structures are expected to use stepped spillways as a design solution for rehabilitation. Due to limited design guidance, a specific study utilizing a two-dimensional, 1:8 scale physical model was conducted to evaluate the hydraulic performance of a 4(H):1(V) chute slope, stepped spillway and the associated energy dissipating stilling basin. The model study came at the request of the North Dakota Natural Resources Conservation Service (NRCS) because of the limited design guidance available for flatter sloped stepped spillways (Theta less than or equal to 22 degrees). The steps from the spillway create tremendous energy dissipation resulting in flow bulking. As a result, higher training walls for stepped spillways as compared to smooth spillways are necessary to contain the design flow. According to literature, the increased energy dissipation in the stepped spillway chute allows for a reduction in the length of the energy dissipation stilling basin as compared to the stilling basin length required for a conventional smooth spillway chute. The riprap size expected to protect the downstream channel from scour has an approximate D50 of 14 cm (5.5 inches). The results of the study are presented in a format that will assist engineers in designing stepped spillways with similar design parameters.