CONVERGING RCC STEPPED SPILLWAYS

Authors: Hunt, Sherry; Kadavy, Kem; Temple, Darrel

Submitted to: American Society of Civil Engineers
Publication Type: Proceedings
Publication Acceptance Date: 3/15/2006
Publication Date: 5/21/2006
Citation: Hunt, S.L., Kadavy, K.C., Temple, D.M. 2006. Converging RCC stepped spillways. In: Proceedings of the American Society of Civil Engineers, May 21-25, 2006, Omaha, Nebraska. 2006 CDROM.

Interpretive Summary: To meet current dam safety requirements, roller compacted concrete (RCC) stepped spillways has become a popular choice is dam rehabilitation. In many cases, urbanization has changed the hazard classification of these aging watershed structures, and land rights are often not obtainable for widening existing earthen spillways. The research discussed in this document provides a summary of a small-scale converging stepped spillway tested at the USDA-ARS Hydraulic Engineering Research Unit. Researchers hope to gain a better understanding of how the convergence of flow affects some of the design components of the spillway. This research has the potential to provide design aids to field engineers faced with designing these structures. Flow convergence was the primary point of interest during the tests. Observations and results from this study showed that as the convergence angle increased the flow run-up near the wall increased. Because of the convergence, the flow near the wall is significantly larger than the flow in the center of the chute. Preliminary theoretical design guidelines are provided within this document to determine the minimum wall height necessary to retain the maximum expected discharge. Currently, these guidelines do not account for the effects air in the water has on the flow. When the results of these tests were compared to the work conducted by other researchers, air was shown to have little significance on the flow depth. Therefore, it was assumed that the wall height necessary to retain the flow for the given design parameters would not need adjustment to account for air. This research provides a more theoretical approach to developing generalized design criteria for vertical training walls for converging stepped spillways.

Technical Abstract: To meet current dam safety requirements, roller compacted concrete (RCC) stepped spillways has become a popular choice is dam rehabilitation. In many cases, urbanization has changed the hazard classification of these aging watershed structures, and land rights are often not obtainable for widening existing earthen spillways. RCC stepped spillways are viewed as a feasible alternative for dam rehabilitation because of the advantages that 1) they can be placed over the top of the dam, 2) they can permit a shorter construction schedule compared to other designs, and 3) they can provide substantial energy dissipation on the stepped chute, potentially reducing the size of the energy dissipating stilling basin. The U. S. Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS) has proposed to design a converging RCC stepped spillway for Big Haynes Creek watershed dam site 3 in Georgia. The design calls for a 100 m (330 ft) ogee crested weir section, with a 3(H):1(V) spillway chute converging 52º to the stilling basin located at the toe of the structure. The peak runoff from a probable maximum precipitation (PMP) event is expected to generate a spillway discharge of 763 m3/s (26900 cfs). To assist with the design of this spillway and with future designs based on similar design parameters (i.e. chute slope, step height, etc.), a study utilizing a three-dimensional, 1:22 scale, physical model was conducted to evaluate the flow characteristics in the spillway. Convergence angles ranging from 0' to 70' were tested over a series of flow rates. As the convergence angle increased, the flow run-up near the wall increased. Because of the flow convergence, the flow depth near the wall was considerably larger than the flow depth in the center of the spillway. To aid in the design of these structures, researchers provided a theoretical simplified, control volume approach to momentum analysis to determine the minimal training wall height required to retain the full probable maximum flood event. Air entrainment was not shown to significantly influence the depth of flow at the highest tested flow rate based on the calculated and observed locations of the air entrainment inception point. However, complete interpretation and/or extrapolation of the data for the prediction of prototype behavior for similarly designed structures may require the addition of an appropriate scaling factor to account for air entrainment depending on the location of where air entrainment starts. This research provides a more theoretical approach to developing generalized design criteria for vertical training walls for converging stepped spillways. The potential for these results to aid in the development of design guidelines and to reduce construction costs for converging stepped spillways is promising.