Stilling Basin Design for Stepped Chutes: More than one type to consider

Abstract Only - Stilling basin design criteria was developed by scientists with the Bureau of Reclamation for smooth chute spillways (Bradley and Peterka 1957). Advances in concrete technology has allowed for a more economic design of chute spillways through use of roller compacted concrete, a concrete with the similar compressive strength as reinforced concrete without the reinforcing steel rebar. In addition, stepped chutes provide advantages of providing increased energy dissipation as compared to traditional smooth chute spillways; and thus allows for increased spillway capacity for chutes of similar size. With increased energy dissipation, the stilling basin footprint may be reduced.
A stepped chute spillway requires design engineers to consider the hydraulic performance of stilling basin when highly aerated flow enters the basin. Traditional stilling basin design criteria is based on the following design parameters: Froude number, clear water flow depth (i.e. flow depth upstream of the jump) and sequent flow depth (i.e. flow depth downstream of the jump). Physical model studies of the stilling basin associated with a stepped spillway will ensure the basins operate as anticipated. USBR Type I, Type II, Type III and Type IV stilling basins were evaluated at the USDA-ARS Hydraulic Engineering Research Unit in Stillwater, OK in association with a state of the art near prototype 3(H):1(V) stepped chute facility. The stepped chute has step heights of 152 mm (6 inch). Incoming unit discharges of 0.23 m2/s, 0.46 m2/s, and 0.93 m2/s were tested. Tailwater ranged from 1.0d2 ≤ TW ≤ 1.4d2. Data collection included pressures on the basin floor, water surface elevations, and visual observations from still photographs and videography.
Based on the clear water flow depth entering (flow depth upstream of the jump, d1) the stilling basin, the Froude numbers ranged from 3.3 ≤ F ≤ 5.5. From visual observations, the hydraulic jumps within the stilling basin oscillated for F ≤ 4.5. Energy dissipation blocks or a dentated sill appeared to dampen the oscillations and the resulting waves propagating downstream. Increasing the tailwater above the USBR recommended value for each type of basin did not significantly improve the behavior of the oscillations and propagating waves. Pressure profiles indicate positive pressures relative to the stilling basin floor elevation and pressures peaks near the energy dissipation blocks and the end sills. For Froude numbers as low as 3.3, a Type III stilling basin was effective at dissipating energy and reducing wave action at the exit of stepped chutes provided the TW ≥ d2. Data and visual observations indicate that the Type II and Type III stilling basins effectively reduce the oscillations and the wave action for F< 4.5. For the Type I stilling basin, the ratio of d2 (i.e. TW) to d1 representing data from this study plotted with respect to the Froude number was in agreement with the published data and a relationship for the ratio of d2 to d1 presented by Bradley and Peterka (1957). Likewise, the data representing the length of jump to d1 ratio and the length of jump to d2 ratio for this study plotted with respect to the Froude number for the Type I stilling basin was in agreement with the findings presented by Bradley and Peterka. Based on these findings, the stilling basin design criteria developed by Bradley and Peterka (1957) appear to be a reasonable design when paired with stepped chutes. For the Type III stilling basin, the design criteria for stilling basin may be extended to Froude numbers as small as 3.3. This research is intended to provide practicing engineers guidance on designing stilling basins for stepped chutes.
References Bradley, J. N. and A. J. Peterka. (1957). “The hydraulic design of stilling basins: hydraulic jumps on a horizontal apron (basin I).” Journal of the Hydraulics Division, 83(5): 1401-1401-24.