USBR Type III and Type IV Stilling Basins and Rock Aprons Associated with Stepped Chutes

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, Oklahoma, 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 <= F <= 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 Isbash, USBR, and USGS, were chosen for evaluating the rock stability of the rock aprons placed downstream of the endsill 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.