Estimated splash and training wall height requirements for stepped chutes applied to embankment dams

Authors: Hunt, Sherry; Kadavy, Kem

Submitted to: American Society of Civil Engineers Journal of Hydraulic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/18/2017
Publication Date: 11/1/2017
Citation: Hunt, S.L., Kadavy, K.C. 2017. Estimated splash and training wall height requirements for stepped chutes applied to embankment dams. American Society of Civil Engineers Journal of Hydraulic Engineering. 143(11): 06017018. https://doi.org/10.1061/(ASCE)HY.1943-7900.0001373.
DOI: https://doi.org/10.1061/(ASCE)HY.1943-7900.0001373

Interpretive Summary: Age is taking a toll on embankment dams such that age can affect the functionality and potentially the safety of the dam. Stepped chutes are often constructed over the top of dams to protect them for many years to come. As the water flows down a stepped chute, the water may become erratic, and splash may be generated. To determine the appropriate stepped chute wall height requirements, research was conducted at the USDA-Agricultural Research Service in Stillwater, Oklahoma. Tests were conducted on a scale model of a stepped chute, and splash height was measured. While the splash can reach great heights, containing all of the splash within the stepped chute maybe unreasonable from a cost perspective especially if the splash is not viewed to cause a safety issue. Equations were developed to estimate the splash height and the stepped chute training wall height requirements. This research is expected to assist engineers design stepped chutes to address problems that may arise on aging dams and to design new stepped chutes for future dams.

Technical Abstract: Aging embankment dams are commonly plagued with insufficient spillway capacity. To provide increased spillway capacity, stepped chutes are frequently applied as an overtopping protection system for embankment dams. Stepped chutes with sufficient length develops aerated flow. The aeration and flow turbulence can create a significant amount of splash over the training wall if not appropriately accounted for in the design of the structure. 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 data 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 quantify the maximum splash height, ysp, expected from aerated flow and to determine the design factor for training walls necessary to minimize the erosive splash that could result from aerated flow. Tests were conducted in an 18.4 degree sloped stepped chute over a wide range of step height to critical flow depth ratios, h/dc, within the skimming flow regime. Data indicates the maximum ysp ranged between 1.1 and 3.7 times the critical flow depth and 2.3 and 5.5 times y90, but designing training walls to contain the maximum splash would be overly conservative and could impact the feasibility of the training walls. Based on engineering judgement, a reasonable amount of flow over the top of the training wall per unit length of wall, qsw, of 1.24 liter/(min•m) was arbitrarily selected. For this limit, data indicates for h/dc less than 0.4 the training wall design factor is 1.4. For h/dc greater than or equal to 0.4, the training wall design factor increases from 1.4 up to 2.0 at the lower limit of skimming flow. These design factors are recommended for use on stepped chute slopes ranging from 10 to 30 degrees.