Submitted to: State Dam Safety Officials Association Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 8/15/2011
Publication Date: 9/27/2011
Citation: Hunt, S.L., Kadavy, K.C. 2011. One step at a time: Developing design criteria for stepped spillways. In: Dam Safety 2011. Proceedings of the Association of State Dam Safety Officials Annual Conference, September 25-29, 2011, Washington, DC. CDROM.
Interpretive Summary: U.S. Congress passed legislation creating the USDA Small Watershed Program. This program is one of the most successful conservation programs in U.S. history because it combines conservation practices and flood control dams on tributary streams as a way to control flooding on the American landscape. Originally designed to protect agricultural land, some of these dams today are located in residential communities. As a result, these dams come under stricter federal and state dam safety regulations. To address these regulations, dam rehabilitation is required. Stepped spillways are becoming a popular means for addressing the dam safety requirements for these dams located in residential areas. Stepped spillways are typically placed over the existing dam and used for overtopping protection and increased spillway capacity. The USDA-ARS Hydraulic Engineering Research Unit (HERU) in Stillwater, OK, is conducting research on stepped spillways in order to develop design guidance for engineers faced with designing these structures. The research program involves examining flow properties (i.e. velocity, flow depth, and air concentrations) within the spillway and the effect design elements (i.e. step height and slope) has on these properties. The goal of this research is to provide useful and practical design guidance to engineers faced with the design of these structures even if it means providing the guidance one step at a time.
Technical Abstract: The story of the USDA Small Watershed Program began seventy-five years ago when the U.S. was experiencing years of drought followed by years of flooding. The Flood Control Acts of 1936 and 1944 and the Watershed Protection and Flood Prevention Act of 1954 set the stage for one of the most successful conservation programs in U.S. history. The Program established the principle of combining conservation practices in a watershed with flood control dams on tributary streams. With the dams originally designed to protect agricultural land, we fast forward to today to find residential communities and other infrastructure in the immediate vicinity of the dams, resulting in hazard classifications changes for these dams. To meet the new hazard classification requirements and subsequent federal and state dam safety regulations, dam rehabilitation is required. The USDA-ARS Hydraulic Engineering Research Unit (HERU) in Stillwater, OK, is conducting generalized research on roller compacted concrete (RCC) stepped spillways to address rehabilitation needs. In rehabilitation, stepped spillways are typically placed as a retrofit over the existing embankment and used to increase spillway capacity. Research on stepped spillways has primarily been focused on gravity-style spillways with slopes steeper than 22 degrees, and insufficient to address the concerns related to stepped spillways applied to embankment dams where slopes are typically flatter than 22 degrees. The research program initiated by engineers at HERU involves the large-scale testing of a stepped spillway over a broad range of discharges and step heights. Inception point location, flow depth, velocity, air entrainment, and energy dissipation are important aspects for the design of these structures. The objective of this paper is to discuss the design parameters being developed in this research program, more specifically energy dissipation, with the plans to provide useful and practical design guidance to engineers faced with the design of these structures even if it means providing the guidance one step at a time.