Hazard creep for embankment dams can become problematic, limiting rehabilitation options. The most common deficiency for embankment dams due to hazard creep is inadequate spillway capacity. Roller compacted concrete (RCC) stepped spillways are a popular method to address this issue. Researchers at the USDA-ARS Hydraulic Engineering Research Unit (HERU) in Stillwater, OK have made great strides in developing generalized design guidelines related to surface inception point (L_i), flow depth (y), clear-water flow depth (y_(cw)), average air concentration (C_(avg)), and energy dissipation for stepped spillways applied to embankment dams (4(H):1(V) to 2(H):1(V)). Two large scale models with slopes of 4(H):1(V) and 2(H):1(V) and one near prototype scale model with a slope of 3(H):1(V) adequate to minimize the occurrence of scale effects were tested over a range of unit discharges (q) and step heights (h). Discharge was monitored throughout testing using a manually operated point gauge coupled with a gauge well as well as a string potentiometer coupled with a computer interfaced data acquisition system. Measurements of velocities, air concentrations, and flow depth were taken throughout testing. Digital photography and videography were utilized to assist with determination of the surface inception point (L_i). The average air concentration (C_(avg)) and flow depth are necessary parameters for determining training wall height; C_(avg) is key when aerated flow develops in the spillway chute. Air concentration (C) profiles were collected using a fiber optic probe. This study primarily focused on C downstream of the surface L_i where entrained air is the dominating contributor to the air measured in the flow. The C profiles were used to calculate C_(avg). The surface L_i is an important aspect in training wall design as well and was determined visually when aerated flow developed across the full width of the flume at the water surface. Upstream of the surface L_i, the flow appears smooth and glassy before developing a minor undulating flow pattern near L = 0.6 to 0.7 L_i. This undulating flow pattern is attributed to turbulence observed at the water surface as well as entrapped air in the flow near the surface Lj. At the surface Lj, the majority of the air observed in the flow is due to surface fluctuations due to turbulence and some entrapped air near the surface. Very little entrained air in the flow profile was observed at the surface L_i with C_(avg) ranging from 0.1 to 0.2 depending on the θ and h. Between 1.0 ≤ L/L_i ≤ 2.0, the flow behavior is more erratic, and entrained air develops in the flow profile resulting in a rapid increase in C_(avg). When L/L_i > 2.0, the flow becomes fully developed air entrained flow, and C_(avg) trends to a constant value for a given θ and h. The air observed in the flow downstream of the surface L_i is attributed to entrained air in the flow profile, entrapped air in the upper flow profile, and surface fluctuations due to turbulence. The value of C_(avg) for L/L_i ＞ 2.0 ranged from 0.2 to 0.45 for the θ and h tested. Data indicates that C_(avg) is a function of h/d_c, θ, and/or L/L_i. Data indicates that chute slope (θ), normalized step height (h/d_c), and the normalized length from the crest (L/L_i) are key parameters for determining flow depth. The flow depth decreases rapidly from the crest section to the surface L_i. Downstream of the surface L_i, the clear-water flow depth becomes relatively constant for a given θ and h. A relationship for the normalized clear-water flow depth (y_(cw)/d_c) downstream of L_i when L/L_i ＞ 1.0 as a function of chute slope (θ), and the ratio of step height to critical depth (h/d_c) was developed. Upstream of L_i from 0.1 ＜ L/L_i ≤ 1.0, the normalized flow depth (y/d_c) is a function of θ, h/d_c, and the normalized length from the crest (L/L_i). L/L_i is the ratio of length from the downstream edge of the broad-crested weir to the point of interest (L) to the length from the downstream edge of the broad-crested weir to the surface inception point (L_i). The objectives of this paper are to introduce 1) flow depth relationships capable of predicting the flow depth at any location along the chute and 2) generalized air concentration relationships for stepped spillways downstream of the surface L_i.
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