USBR Type III and Type IV stilling basin and rock apron associated with stepped chutes

Authors: Hunt, Sherry; Kadavy, Kem

Submitted to: Applied Engineering in Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/27/2017
Publication Date: 4/15/2018
Citation: Hunt, S.L., Kadavy, K.C. 2018. USBR Type III and Type IV stilling basin and rock apron associated with stepped chutes. Applied Engineering in Agriculture. 34(2):389-394. https://doi.org/10.13031/aea.12638.
DOI: https://doi.org/10.13031/aea.12638

Interpretive Summary: Embankment dams across the U.S. are aging. In some cases, these aging dams may be at risk for overtopping. USDA-Agricultural Research Service (ARS) scientists are developing overtopping protection alternatives to address this overtopping potential. Stepped spillways applied to embankment dams are a common practice among the engineering community to provide overtopping protection. Stepped chutes like other structural chutes require outlet works to safely route flood waters to the downstream channel. Scientists with the U.S. Bureau of Reclamation (USBR) developed classical design criteria for outlet works associated with smooth chute spillways. Scientists at the USDA-ARS conducted research to verify whether the criteria developed by the USBR was applicable to stepped chute spillways. Research results indicated outlet works designed using USBR criteria performed well for stepped chutes.

Technical Abstract: Stilling basins are commonly used as energy dissipators for structural chutes. Classical research conducted by scientists of the U. S. Bureau of Reclamation (USBR) led to the development of design criteria for a variety of stilling basin configurations as the outlet works for smooth chutes, but little is known about the performance of these dissipators when paired with stepped chutes. Research at the USDA-ARS Hydraulic Engineering Research Unit (HERU) in Stillwater, OK was conducted on USBR Type III and Type IV stilling basins with rock aprons located downstream of a near prototype stepped chute. Data indicates Froude numbers based on the incoming clear water flow depth to the stilling basin range from 3.3 less than or equal to F less than or equal to 5.5. Visual observations noted during the tests indicate acceptable performance of the Type III and Type IV stilling basins when operated at recommended or greater tailwater settings as set forth by the USBR. Three methods, the Isbach, USBR, and USGS, were chosen for evaluating the rock stability of the rock aprons placed downstream of the end sill of the stilling basins used in association with stepped chutes. Data indicate each method is suitable for riprap sizing of the rock apron. The intent of this research is to provide design engineers with knowledge regarding the performance of USBR Type III and Type IV stilling basins with rock aprons designed in conjunction with stepped chutes.