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15 Conduits or Tunnel Outlets Conduits and tunnels are used to convey spillway flows around or through a dam. These types of conveyance structures are typically used with a drop inlet spillway crest, although some overflow crests and side channel crests discharge into conduits. Conduits typically have a vertical or near vertical shaft section directly below the inlet and then a nearly horizontal section of conduit or tunnel through or around the dam to the outlet. Most conduits or tunnels are designed to flow partially full, although many drop inlets have a shorter section of the conduit that will remain pressurized at the control point. Proper aeration is critical for conduits to operate smoothly and avoid cavitation, burping, and surging. Conduit or tunnel linings are typically cast-in-place concrete. Some conduits consist of steel pipe that may have a cast-in-place concrete encasement to resist external pressure. Terminal Structures Terminal structures are required where energy from the spillway flow must be dissipated before being discharged to the downstream channel. In some cases, such as where the downstream channel is not erodible, a terminal structure may not be necessary. The more common types of terminal structures provided at small dams are described in the following subsections of this article. Hydraulic Jump Stilling Basins The most common type of stilling basin is the hydraulic jump stilling basin. A hydraulic jump stilling basin is used to dissipate kinetic energy by the formation of a hydraulic jump at the interface between a lined spillway and the downstream channel. The stilling basins are typically lined with concrete to avoid scour and erosion. Design of Small Dams (Reclamation 1987) documents typical jump style stilling basins for different ranges of Froude numbers (Fr). Froude numbers are related to the kinetic flow factor of the discharge entering the basin. Typical Reclamation stilling basins are summarized in Table 1, which also references Figures 14 through 17. It should be noted that stilling basins often have minimum tailwater requirements for correct operation. Table 1: Typical Reclamation Stilling Basins Basin Type Description Applicable Range of Froude Numbers Basin I (Figure 14) Hydraulic jumps on horizontal aprons 1.7 < Fr < 2.5 *Basin II (Figure 15) Stilling basins for high dams and earth dam spillways Fr > 4.5 **Basin III (Figure 16) Short stilling basins for small spillways Fr > 4.5 Basin IV (Figure 17) Transitional jump stilling basins 2.5 < Fr <4.5 * Not often applicable for small dams; ** Incoming velocities not to exceed 50-60 fps Figure 14: Type I Hydraulic Jump on Horizontal Apron Figure 15: Type II Stilling Basin Figure 16: Type III Stilling Basin Figure 17: Type IV Stilling Basin