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<br /> <br /> <br /> <br />12 <br />Drop Inlet with Gated Low Level Intake on Upstream <br />End of Conduit (Continued) <br />Pros Cons <br />- Easy access to gate for <br />operation <br />- Relatively easy installation <br />- “Spilled” water is conserved (in <br />delivery system) <br />- Cost-effective <br />- Hydraulically efficient gate <br />position <br />- Stem is easily damaged by ice <br />- Need to drain reservoir to <br />work on gate in the dry <br /> <br />Drop Inlet with Inclined Low Level Slide Gate <br /> Pros Cons <br />- Hydraulically efficient gate <br />position <br />- “Spilled” water is conserved (in <br />delivery system) <br />- Gate stem must be buried to <br />protect from damage <br />- Easy to damage by mis- <br />operation – operator must be <br />careful not to bend stem <br />- Susceptible to clogging with <br />debris – trash rack important <br />Drop Inlet with Flashboards (No Gate) <br /> <br />Pros Cons <br />- “Spilled” water is conserved (in <br />delivery system) <br />- Good for remote locations <br />- Less expensive than systems <br />with gate & gate operators <br />- Difficult to mis-operate <br /> <br />- Limited control <br />- Making low level releases can <br />be difficult (must remove all <br />flashboards underflow) <br />- Only reasonable for small low <br />head dams <br /> <br />Hydraulic Analyses <br />Outlet Works Capacity – To identify the capacity of the <br />outlet works over the entire range of design reservoir <br />levels, it is necessary to analyze the hydraulics of each <br />condition individually. Generally, there are three <br />potential types of control within the system: <br />1) Weir inlet control <br />2) Orifice inlet control <br />3) Outlet control (full pipe flow within the <br />conduit) <br />Weir inlet control (drop inlet or conduit entrance) <br />typically occurs at low heads where free flow <br />conditions exist over the inlet crest. Weirs are very <br />efficient with capacity computed as: 𝑄 = 𝐶𝐶𝐻1.5 <br />where C is a weir discharge coefficient, L is the length <br />of the weir crest, and H is the head over the weir. At <br />some point as the reservoir level increases, conditions <br />transition from weir flow to submerged orifice flow. <br />Orifice flow is much less efficient with capacity <br />computed as: 𝑄 = 𝐶𝐶�2𝑔𝐻 <br />Where A is the area of the orifice entrance, 𝑔 is <br />gravitational acceleration, and H is the head above the <br />orifice. The coefficients associated with the weir and <br />orifice equations will vary depending on the shape of <br />the entrance and the control mechanism (slide gate, <br />valve, uncontrolled, etc.). See References [9], [10], and <br />[15] for more information. <br />A relationship between reservoir level (stage) and the <br />discharge volume can be developed for each flow <br />condition (weir and orifice) at each inlet based on the <br />above equations. The resulting stage-discharge <br />relationship for each inlet is based on the minimum <br />discharge of the weir and orifice curves. The combined <br />inlet stage-discharge relationship is the sum of the <br />individual inlet stage-discharges. <br />Full conduit control (outlet control) occurs when the <br />capacity of the conduit is exceeded by the combined <br />capacities of the inlets. This often results from <br />submergence at the downstream end due to tailwater. <br />Gate <br />Operator <br />Flashboard <br />Divider