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Title: Stilling the waters: Stilling basin design for stepped chutes

item Hunt, Sherry
item Kadavy, Kem

Submitted to: State Dam Safety Officials Association Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 8/28/2015
Publication Date: 9/14/2015
Citation: Hunt, S., Kadavy, K.C. 2015. Stilling the waters: Stilling basin design for stepped chutes. In: Dam Safety 2015. Proceedings of the Association of State Dam Safety Officials Annual Conference, September 13-17, 2015, New Orleans, LA. 2 p. CDROM.

Interpretive Summary:

Technical Abstract: 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, the sequent flow depth, and tailwater. Research at the USDA-ARS Hydraulic Engineering Research Unit (HERU) in Stillwater, OK, has shown that Froude numbers for stilling basins designed for stepped chutes can range from 3.3 to 5.5. Based on current USBR stilling basin design criteria, Type III basins are recommended for Froude numbers greater than or equal to 4.5, and Type IV basins are recommended for less than or equal than 4.5. Current HERU 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, provided the tailwater is equal to or greater than the sequent flow depth. A Type IV basin shows acceptable performance for Froude numbers less than or equal to 4.5 with tailwater greater than or equal to 1.1 times the sequent flow depth. 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. Rock stability downstream of the stilling basin was also tested. Preliminary results indicate adequate rock size for stability and protection of the downstream channel for a Type III stilling basin using the Isbash, USBR, and USGS riprap sizing methods. The USGS rock stability method provides a more conservative rock size than the other two methods. For the limited tests conducted, the Isbash and USBR methods indicate suitable rock size for stability and downstream channel protection under normal design conditions. Observations indicate that undersizing rock could cause individual rocks to migrate back towards the stilling basin and over the endsill. This migration could cause significant damage to the stilling basin if rock were undersized. Ease of construction and feasibility will likely be key in stilling basin design selection for stepped chutes.