Estimated Splash and Training Wall Height Requirements for Stepped Chutes Applied to Embankment Dams

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 develop aerated flow. The aeration and flow turbulence can create a significant amount of splash over the stepped chute training wall if not appropriately accounted for in the design. Recommendations for stepped chute training wall height requirements are available in the literature with little to no supporting data available. Researchers at the USDA-Agricultural Research Service (ARS) Hydraulic Engineering Research Unit (HERU) in Stillwater, Oklahoma, conducted a series of tests in a near prototype scale stepped chute facility to quantify the maximum splash height, $y_{sp}$, expected from aerated flow and to provide design recommendations for training wall height necessary to minimize erosive splash over the wall. Data indicate the maximum $y_{sp}$ ranged between 1.1 and 3.7 times the critical flow depth ($d_c$) and between 2.3 and 5.5 times the bulked flow depth ($y_{90}$). For $0.035\le h/d_c<0.4$, the minimum training wall height required above the step edge downstream of the free-surface inception point, $y_{sw}$, is $1.4y_{90}$. For $0.4\le h/d_c\le 1.1$, $y_{sw}$ normalized by $y_{90}$ follows a simple power function of $h/d_c$. According to the data, an increase of $y_{sw}$ to approximately $2.0y_{90}$ may be needed. These minimum training wall height requirements are recommended for use in skimming flow conditions (i.e., $0.035\le h/d_c\le 1.1$) for nonconverging stepped chutes with slopes ranging 10–30°.