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9 Water on Tap: Design Considerations for Outlet Controls Introduction Outlet works provide for the controlled release of water from reservoirs and may consist of channels, canals, or conduits. Outlet controls typically consist of gates or valves that control the rate of flow through the outlet works. The controls allow the owner/operator to regulate flow in order to accommodate downstream water needs, control reservoir pool levels or drawdown the reservoir at a controlled rate. Failure or improper operation of the outlet controls may induce failure of not only the outlet structure, but of the dam itself due to increased pressures through embankments or overtopping. Appropriate design and maintenance of outlet controls are important to ensure long-term operability and support dam safety. There are various types of outlet control structures and each serves a different purpose. Some systems are used almost continuously, with frequent adjustment to the outlet controls, while others are seldom used and serve only as backup to another primary outlet, such as an overflow spillway for emergency or lower-level releases. This article presents an overview of the commonly used systems, the key parameters to consider in the design and maintenance of the equipment, and the hydraulic regulation those systems typically provide. Since dam operations, obligations, and impacts are specific to individual circumstances, any decisions for outlet control design or rehabilitation should include appropriate engineering analysis. Previous Western Dam Engineering articles that complement this topic include: Outlet Works Air Vent Design; How Low Can You Go? The Needs and Considerations for Outlets; Letting It All Out: Hydraulic Design of Outlet Works; and Spillways on Small Dams. Structural and hydraulic analyses are an important component of outlet structure design but they are not addressed in this article. Design Considerations Whether a channel, canal, or conduit outlet is used, a variety of equipment and design characteristics can be considered. Among many factors that will impact the design, the most important are listed below:  Discharge capacity  Frequency of operation  Access for maintenance  Operational life  Rate of opening  Reliability  Flow control accuracy  Cost A best estimate of these parameters must be determined to select the most appropriate solution and design for the outlet. Power Supply for Control Systems Frequently used outlet systems usually rely on electric motors to operate the control system. Redundant power sources are often warranted for dam safety concerns. Redundant sources can be different independent power sources, generators, or manual operation. For smaller systems, or those with low frequency of operation, manual operation may be sufficient and can be accomplished with a wheel, lever, or hand pump connected to hydraulic cylinder. Control Operators Gates and valves can be operated by different mechanisms, depending on the physical configuration of the system. Historically, most gates were designed with an operator consisting of a hoist with chains, vertical screws or gear mechanisms such as worm gears or linear gears. Recent designs tend to favor wire rope hoists that offer a good combination of cost, longevity, and ease of maintenance. Photo 1: Wire Rope Hoisting Systems for Fixed Wheel Gates