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Outlet Works Terminology

Common Terms

Access Shaft. - Lined (protected or covered excavation) or unlined (unprotected excavation) shaft portion of an outlet works between the shaft house and the gate chamber. The access shaft provides vertical access to the gates.

Active Storage. - The reservoir capacity normally usable for storage and regulation of reservoir inflows to meet established reservoir operating requirements. It extends from the highest of either the top of exclusive flood control capacity, the top of joint use capacity, or the top of active conservation capacity, to the top of inactive capacity. It is also the total capacity less the sum of the inactive and dead capacities. The reservoir capacity that can be used for irrigation, power, municipal and industrial use, fish and wildlife, recreation, water quality, and other purposes.

Approach Channel. - The channel upstream from intake structure of an outlet works. The channel is generally unlined, excavated in rock or soil. When the channel is lined, the lining could include riprap, soil cement or other types of erosion protection.

Bulkhead. - A one-piece fabricated steel unit which is lowered into guides and seals against a frame to close a water passage in a dam, conduit, spillway, etc. An object used to isolate a portion of a waterway for examination, maintenance, or repair. A wall or partition erected to resist ground or water pressure.

Cofferdam. - A temporary impoundment structure enclosing all or part of the construction area so that construction can proceed in the dry. A diversion cofferdam diverts a river into a pipe, channel or tunnel. A temporary barrier, usually an earthen dike, constructed around a worksite in a reservoir or on a stream, so the worksite can be dewatered/unwatered.

Conduit. - A closed channel to convey water through, around, or under a dam and/or dike. These portions of an outlet works are between the intake structure and gate chamber and/or the control structure. Conduits are located beneath embankment dams and within concrete dams. Conduits materials have included unreinforced and reinforced concrete lined, concrete/steel lined, high density polyurethane (HDPE), and corrugated metal pipe (CMP). A pipe, box, or horseshoe structure, or natural channel that is constructed by means of "cut and cover". A conduit can convey water or house other conduits or pipes.

Control Structure. - Portion of an outlet works, located along the outlet works which houses the control (regulation) gates.

Dam. - An impoundment structure or barrier built across a watercourse to impound or divert water. A barrier that obstructs, directs, retards, or stores the flow of water. Usually built across a stream. A structure built to hold back a flow of water.

Dead Storage. - The reservoir capacity from which stored water cannot be evacuated by gravity (generally associated with the reservoir storage below the sill elevation of the outlet works) .

Dike. - A low embankment, usually constructed to close up low areas of the reservoir rim and thus limit the extent of the reservoir.

Diversion Inlet. - Conduit or tunnel upstream from the intake structure. Diversion inlet may be integral with the outlet works or be part of a separate conveyance structure that will only be used during construction.

Gate. - A device that controls the flow in a conduit, pipe, or tunnel without obstructing any portion of the waterway when in the fully open position. As distinguished from valves, gates are constructed so that there is no obstruction of flow when gate is fully opened..

Gate Chamber. - A chamber in which a guard gate in a pressurized outlet works or both the guard and regulating gates in a free-flow outlet works are located. A room from which a gate or valve can be operated, or sometimes in which the gate is located. Portion of an outlet works containing gates between upstream and downstream conduits and/or tunnels.

Guard Gate (emergency). The first gate in a series of flow controls, remaining open while downstream gates or valves are operating. A gate used in the closed position to permit servicing of the downstream regulating gate(s) or valve(s) or the downstream conduit.

Hydaulic Jump A hydraulic jump is a phenomenon in the science of hydraulics which is frequently observed in open channel flow such as rivers and spillways. When liquid at high velocity discharges into a zone of lower velocity, a rather abrupt rise occurs in the liquid surface.

Intake Structure. - Upstream opening that conveys water from the reservoir into the conduit, tunnel and/or pipe (i.e., entrance to the outlet works). This feature may include debris and/or fish barriers such as trashracks and/or fish screens.

Outlet or Exit Channel. - Channel downstream from a terminal structure that conveys releases back to the "natural" stream or river. Channel can be excavated in rock or soil, with or without riprap, soil cement or other types of erosion protection.

Pipe. - A circular conduit constructed of any one of a number of materials that conveys water by gravity or under pressure.

Portal. - An opening into a tunnel.

Regulating Gate. - A gate used to regulate the rate of flow through an outlet works or spillway. A gate or valve that operates under full pressure flow conditions to regulate the rate of discharge.

Shaft House. - House at the top of the access shaft to gate chamber, which secures access to the gates.

Sill. - The crest of an outlet works reservoir opening generally associated with the intake structure.

Stoplogs. - Large logs, planks, cut timbers, steel or concrete beams placed on top of each other with their ends held in guides between walls or piers to close an opening in a dam, conduit, spillway, etc., to the passage of water; the logs are usually handled one at a time. Used to provide a cheaper or more easily handled means of temporary closure than a bulkhead gate.

Terminal Structure. - Lined or unlined portion of an outlet works downstream from a conduit, tunnel, or control structure. The structure dissipates or stills releases (i.e., rapidly flowing water) to protect the riverbed from erosion.

Tunnel. - Portion of an outlet works between upstream and downstream portals, excluding the gate chamber. Tunnels are generally located in the dam and/or dike abutments, and maybe lined or unlined. An enclosed channel that is constructed by excavating through natural ground. A tunnel can convey water or house conduits or pipes.

Valve. - A device used to control the flow in a conduit, pipe, or tunnel that permanently obstructs a portion of the waterway. As distinguished from gates, valves are constructed so that the closing member remains in the water passageway for all operating positions.