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Title: Stepped chute training wall height requirements

item Hunt, Sherry
item Kadavy, Kem

Submitted to: Workshop Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 6/1/2016
Publication Date: 6/29/2016
Citation: Hunt, S., Kadavy, K.C. 2016. Stepped chute training wall height requirements. In: Crookston, B. and Tullis, B. (eds.) Hydraulic Structures and Water System Management, 6th International Symposium of Hydraulic Structures, June 27-29, 2016, Portland, OR, p. 260-267.

Interpretive Summary: The USDA-Natural Resources Conservation Service has provided technical and financial assistance for nearly 12,000 flood control dams. The age of a significant number of these dams has created a need for their rehabilitation. Over time many have experienced a change in demographics such that they no longer provide flood protection for agricultural land, but they provide flood control for communities and infrastructure that have developed in the vicinity of the dam. These embankments have developed deficiencies because more stringent dam safety regulations for their upgrade are required. To address these deficiencies, stepped chutes are often proposed to provide overtopping protection for the embankment and a means for safely conveying flood water downstream. Stepped chutes may initiate splash over the chute wall, and thus, the splash may result in erosion along the outside of the wall. Scientists, through their research at the USDA-Agricultural Research Service Hydraulic Engineering Research Unit, have found that the step height within the chute can influence the amount of the splash coming over the chute wall. The intent of this research is to develop conservative yet practical design guidance for stepped chutes.

Technical Abstract: Stepped chutes are commonly used for overtopping protection for embankment dams. Aerated flow is commonly associated with stepped chutes if the chute has sufficient length. The aeration and turbulence of the flow can create a significant amount of splash over the training wall if not appropriately accounted for in the design of the structure. According to the United States Department of Agriculture (USDA)-Natural Resources Conservation Service (NRCS) Technical Release 60 Earth Dams and Reservoirs, a structural spillway should be designed to pass the freeboard hydrograph such that it does not cause serious damage to the embankment or the structure itself. For stepped spillways, researchers have suggested the training wall height equal a design factor, ranging from 1.4 to 1.5, multiplied by the bulked flow depth, y90. Yet, literature suggests that the design factor is a recommendation with little to no research provided to actually quantify what the design factor should be. Researchers at the USDA-Agricultural Research Service (ARS) Hydraulic Engineering Research Unit (HERU) in Stillwater, OK conducted a series of tests in a near prototype scale stepped chute facility to determine the design factor for training walls necessary to minimize the erosive splash that could result from the aerated flow. Tests were conducted in a stepped chute with a slope of 18.4 degree for a wide range of skimming flow conditions. Data indicates for a step height, h, to critical flow depth, dc, ratio of less than 0.4, the design factor for the training wall is 1.4, thus the training wall height is equal to 1.4y90. For h/dc greater than or equal to 0.4, the design factor increases from 1.4 at h/dc = 0.4 up to 2.0 for h/dc = 1.0. These design factors are recommended for use on chute slopes ranging from 10 to 30 degrees. This research is intended to assist design engineers with appropriately designing stepped chute training walls to minimize the erosive splash that can occur as a result of aerated, turbulent flow.