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<br /> <br /> <br /> <br />11 <br />stilling basins, plunge pools, hydraulic jumps, and cone <br />valves. <br />Stilling wells are concrete structures located at the end <br />of the outlet conduit. The outlet conduit typically <br />discharges horizontally at the bottom of the well and <br />energy is dissipated through turbulence and diffusion <br />within the well. The flow then rises upward and <br />discharges to the downstream channel through the top <br />of the well. <br />Stilling basins can be designed in several ways but <br />generally dissipate energy through a hydraulic jump, <br />which is the natural transition from supercritical flow <br />to subcritical flow. There are several variations of <br />stilling basins including impact, hollow-jet, and baffled. <br />Plunge pools are deep areas of water into which the <br />outlet conduit discharges. Turbulence within the pool <br />dissipates energy before the flow is released into the <br />downstream channel. The size and depth of the plunge <br />pool is determined by the velocity and trajectory of the <br />outlet conduit jet. <br />Cone valves release flow downstream in a highly <br />dispersed jet, dissipating energy in the process. Design <br />criteria for cone valve energy dissipaters vary by <br />manufacturer. Cone valves are generally not suitable <br />in very cold climates as the spray that is generated is <br />highly susceptible to freezing. <br />Flow exiting the outlet conduit is typically supercritical, <br />characterized by shallow flow depths and high <br />velocities. The high energy at the outlet must be <br />dissipated through properly designed energy <br />dissipators. Hydraulic design of stilling basins and <br />similar structures induces a hydraulic jump within the <br />structure to dissipate energy. This significantly reduces <br />the flow velocity exiting the structure. The structure <br />dimensions are related to the tailwater depth and the <br />Froude number of the flow at the exit of the outlet <br />conduit. Hydraulic jumps can dissipate 50 to 70 <br />percent of the outflow energy. The outflow conditions <br />should be evaluated carefully to ensure adequate flow <br />conditions in the downstream channel to prevent <br />erosion. See References [8] and [10] for more <br />information regarding the design of energy dissipation <br />structures. <br /> <br />Photo 5: A cone valve dissipates energy downstream of an outlet <br />works <br />Potential Configurations <br />Some potential outlet works configurations for small <br />dams are presented below. <br />Drop Inlet with Gate Near Dam Centerline <br /> <br />Pros Cons <br />- Easy access to gate for <br />operation <br />- “Spilled” water is conserved (in <br />delivery system) <br />- Gate is protected from ice <br />damage <br />- Easy to operate <br />- Can be hydraulically inefficient <br />if tailwater is present (high <br />losses) <br />- Expensive at larger diameter <br />installations <br />- Complicated hydraulics <br />- Only feasible for low head <br />dams <br />Drop Inlet with Gated Low Level Intake on Upstream <br />End of Conduit <br /> <br />Entrance Outlet <br />Conduit