Stilling the Waters: Stilling Basin Design for Stepped Chutes

Energy dissipation is a desired feature of stepped chute design because it may lead to a shorter length of stilling basin than that of a traditional smooth chute design. Design parameters for stilling basins include Froude number, clear water flow depth, and the secant flow depth. Research at the USDA-ARS Hydraulic Engineering Research Unit in Stillwater, OK has shown that Froude numbers for stilling basins designed for stepped chutes can range from 3.3 ≤ F ≤ 5.5. Based on current USBR stilling basin design criteria, Type III basins are recommended for F ≥ 4.5 and Type IV basins are recommended for F ≤ 4.5, these Froude numbers indicate that a Type III or Type IV stilling basin may be adequate to dissipate the remaining energy exiting a stepped chute. Current research indicates that a Type III stilling basin is effective at dissipating energy and reducing the wave action at the exit of stepped chutes for Froude numbers as low as 3.3. A Type IV stilling basin can be significantly longer than that of a Type III stilling basin, but a Type III stilling basin requires blocks and/or and end sill. Ease of construction and feasibility will likely be key in stilling basin design selection for stepped chutes.