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16 Roller Buckets and Flip Buckets Roller bucket and flip bucket terminal structures are not described herein, as they are typically provided with large dams/spillways and concrete dams. Impact Basins An impact style stilling basin does not depend on tailwater and can be used on either a conduit or open channel conveyance system. Energy dissipation is accomplished by discharging the high velocity jet into a vertically hanging baffle. Care must be taken not to submerge the outlet conduit with tailwater and inhibit downstream venting unless alternate means for venting the outlet conduit are provided. Typically, the area downstream of the baffle is protected with riprap. An impact basin is typically located so that the tailwater is approximately half way up the baffle. An example of an impact basin at the end of a conduit is shown as Figure 18. An impact basin can also be fitted to a rectangular chute. Figure 18: Impact Basin Plunge Pools A plunge pool is the terminal structure defined by the location where a free overflow spillway or a flip bucket discharges into the downstream channel. Plunge pools can consist of either naturally forming scour holes in the channel, or can be artificially created by construction of a downstream sill or excavation into the streambed. Plunge pools can either be lined or unlined, depending on streambed materials and erodibility of the material with respect to the energy that must be dissipated. Lining usually consists of either concrete or riprap. The volume and depth of the hole are related to the range of discharges, the height of the drop, and the depth of available tailwater. Downstream Channels The stability of the downstream channel must be understood to guard against headcutting or sidecutting at the toe of the spillway and the potential to threaten the integrity of the spillway, river channel, or the dam itself. The downstream channel can be described as natural or modified, and can be categorized as unlined rock or soil, armored, or vegetated or a combination of these. The downstream channel type, natural river geomorphology, and the behavior of the tailwater must be considered with regard to the tailwater requirements for the terminal structure. References [1] State of Colorado: Department of Natural Resources. (2007). Rules and Regulations for Dam Safety and Dam Construction. [2] United States Department of the Interior: US Bureau of Reclamation. (1987). Design of Small Dams. [3] United States Department of the Interior: Bureau of Reclamation. (1984). Engineering Monograph No. 25: Hydraulic Design of Stilling Basins and Energy Dissipators. [4] United States Department of the Interior: Bureau of Reclamation. (2011). Appurtenant Structures for Dams (Spillway and Outlet Works) Design Standards.