Outlet Works
37.5 Outlet Works
Components and Layout
The purpose of the outlet works is to regulate the operational outflows from the reservoir. The intake structure forms the entrance to the outlet works. It may also include trash racks, fish screens, and gates. The conduit entrance may be vertical, inclined or horizontal. The conduit may be free flowing or under pressure. For low dams the outlet may be a gated open channel. For higher earth dams the outlet may
be a cut-and-cover conduit or a tunnel through an abutment. For concrete dams the outlet is generally
a pipe embedded in the masonry or the outlet is formed through the spillway using a common stilling basin to dissipate the excess energy of both the spillway and outlet works outflows. Diversion tunnels used for the construction can in some cases be converted to outlet works. Examples of layout of outlet works may be found in U.S. Department of the Interior, Bureau of Reclamation (1987).
Hydraulics of Outlet Works
When the outlet is under pressure it performs as a system of pipes and fittings in series as shown in Fig. 37.20 . It typically includes trash racks, an inlet, conduits, expansions, contractions, bends, guard gates that are usually fully open or fully closed for the purpose of isolating a segment of the system and control valves for the regulation of the flow. The total head H T , the difference in elevation between the reservoir and the centerline of the outlet, is used in overcoming the losses and producing the velocity head at the exit When the outlet is under pressure it performs as a system of pipes and fittings in series as shown in Fig. 37.20 . It typically includes trash racks, an inlet, conduits, expansions, contractions, bends, guard gates that are usually fully open or fully closed for the purpose of isolating a segment of the system and control valves for the regulation of the flow. The total head H T , the difference in elevation between the reservoir and the centerline of the outlet, is used in overcoming the losses and producing the velocity head at the exit
where Âh i = the sum of the applicable losses due to trash racks, entrance, bends, friction, expansion, contraction, gate valves
h v = the exit velocity head These losses are expressed as h l = KV 2 /(2g), except for the contraction and expansion losses which are
expressed as h l = K(V 2 1 –V 2 2 )/(2g). Appropriate values of the coefficient K may be found in Table 29.9 , Chapter 29 on Fundamentals of Hydraulics as well as in U.S. Department of the Interior, Bureau of Reclamation (1987) or in handbooks (Brater et al., 1996). Additional information on design of trashracks may be found in ASCE (1993).
When the outlet functions as an open channel, the flows are usually controlled by head gates. As the channel can be nonprismatic, the flow profile is calculated by the procedure described in the section on standard step method for gradually varied flow in nonprismatic channels ( Chapter 30 on Open Channel Hydraulics). An example of design can be found in U.S. Department of the Interior, Bureau of Reclama- tion 1987. Guidelines for the design of high head gates that may be used in outlet works have been reviewed by Sagar (1995). The experiences of outlet works in the UK have been reviewed by Scott (2000).