Impact of converging chute walls for Roller Compacted Concrete stepped spillways

Authors: Hunt, Sherry; Kadavy, Kem; ABT, STEVEN - COLORADO STATE UNIV; Temple, Darrel

Submitted to: Journal of Hydraulic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/9/2007
Publication Date: 8/1/2008
Citation: Hunt, S., Kadavy, K.C., Abt, S.R., Temple, D.M. 2008. Impact of converging chute walls for Roller Compacted Concrete stepped spillways. Journal of Hydraulic Engineering. 134(7):1000-1003.

Interpretive Summary: Roller compacted concrete (RCC) stepped spillways are growing in popularity as a method for providing overtopping protection for many aging watershed dams with inadequate spillway capacity. Land rights are often not obtainable for widening existing earthen spillways, and in other cases, landscape features and land use changes caused by urbanization limit the ability to modify the dimensions of the embankment or 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. Flow convergence was the primary point of interest during the tests. The results from the study showed that as the convergence angle increased the flow run-up near the wall increased. This flow run-up was significantly larger than the flow in the center of the chute. Based on this information, design guidelines were developed to determine the minimum wall height necessary to retain the maximum expected discharge. These guidelines do not account for the effects air in the water has on the flow. The wall height will be shorter if air is not present in the flow when compared to a wall where air is present in the flow. To determine whether air in the flow strongly influenced the depth of flow in the spillway, results from these tests were compared to work conducted by other researchers, and air was found to have little significance on the flow depth and therefore on the wall height needed to retain the flow for the given design parameters tested. This research is the first known attempt at developing generalized design criteria for vertical training walls for converging stepped spillway.

Technical Abstract: Roller compacted concrete (RCC) stepped spillways are growing in popularity as a method for providing overtopping protection for many aging watershed dams with inadequate spillway capacity. Land rights are often not obtainable for widening existing earthen spillways, and in other cases, topographic features and land use changes caused by urbanization limit the ability to modify the dimensions of the embankment or spillways. An advantage of stepped spillways is that they can be placed over the top of an existing embankment dam without increasing the height of the dam or without widening the existing auxiliary spillway. Furthermore, stepped spillways provide considerable energy dissipation in the chute, potentially reducing stilling basin size. 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 m**3/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 52 degrees were tested over a series of flow rates. As the convergence angle increased, the flow run-up near the wall increased. Additionally, the flow depth near the wall was considerably larger than the flow depth in the center of the spillway. This information gave an indication that the walls for a 52 degrees converging stepped spillway having a slope of 3(H):1(V) and a step height of 0.305 m (1ft) should be 3.0dc high. 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 study is the first known attempt at 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.