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Title: RCC stepped spillways for Renwick Dam - a partnership in research and design

Author
item Hunt, Sherry
item REEP, DENNIS - USDA-NRCS
item Kadavy, Kem

Submitted to: The Journal of Dam Safety
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/15/2008
Publication Date: 7/1/2008
Citation: Hunt, S., Reep, D., Kadavy, K.C. 2008. RCC stepped spillways for renwick dam - a partnership in research and design. Dam Safety Journal. 6(2):32-40.

Interpretive Summary: The Small Watershed Program is a program within the United States Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS, formerly the Soil Conservation Service). Passage of legislation allowed the NRCS to provide technical and financial assistance for the construction of nearly 11,000 dams across the U.S. The construction peak for these structures occurred during the 1960s. Many dams provided flood protection for agricultural land, but today after years of urbanization and development, the hazard classification of many of these structures has changed. Many structures no longer have adequate spillway capacities to meet dam safety criteria in transferring flood waters safely downstream. Altering the dimensions of the existing spillway(s) to increase spillway capacity is often limited by urbanization, 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. This manuscript provides details related to a specific stepped spillway proposed for the rehabilitation design of Renwick Dam, a flood control structure originally constructed in the 1960s with the assistance of the NRCS. The proposed stepped spillway for Renwick Dam is a unique structure that lies on a rather flat (4H:1V) slope. Questions regarding the design of this spillway arose from the placement of the proposed stilling basin floor and the amount of tailwater necessary to have a properly functioning stilling basin. To assist the NRCS in the design of this spillway, a model was constructed and tested at the USDA-ARS Hydraulic Engineering Research Unit (HERU) in Stillwater, Oklahoma. This paper is intended to provide design engineers with information regarding the design of stepped spillways on 4(H):1(V) slopes. Specifically, this manuscript will discuss the original design of Renwick Dam, the reasons behind the rehabilitation and the selection of the proposed rehabilitation design, the modeling efforts of the USDA-ARS Hydraulic Engineering Research Unit, and the selection of the final design.

Technical Abstract: The Small Watershed Program administered through the United States Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS, 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 for these structures occurred during the 1960s with many originally designed to protect agricultural land. Sixty years of progression has led to changes in the hazard classification of many of these structures. Increased urbanization upstream and downstream of the structure and in the immediate vicinity of the embankment have caused many spillways to no longer meet hydrologic criteria. 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 urbanization, 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. Nearly 10% of NRCS structures are expected to use stepped spillways as a design solution for rehabilitation. Design guidelines for RCC stepped spillways are limited. The majority of the research to date is related to small-scale stepped spillways modeled with rather steep slopes (theta greater 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). Stepped spillways create significant energy dissipation, which leads to questions related to the design of the spillway training walls and the energy dissipating stilling basin. As a result, the NRCS requested a model study from the USDA-Agricultural Research Service (ARS) Hydraulic Engineering Research Unit in Stillwater, OK, on a relatively flat-sloped (4H:1V) stepped spillway. Specifically, this model study addresses the design of the spillway training walls as it relates to the flow bulking created by the air entrained flow and the design of the energy dissipating stilling basin. This manuscript provides details related to a specific stepped spillway proposed for the rehabilitation design of Renwick Dam. The proposed stepped spillway for Renwick Dam is a unique structure that lies on a rather flat (4H:1V) slope. Results from this model studied shows that the air entrained flow has little significance on the design of the spillway training walls. Additionally, the energy dissipating stilling basin and riprap for the protection of the structure and downstream channel dramatically differs between a 30 ft and 40 ft spillway drop. This article is intended to assist engineers in the design of similar structures.