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10 Most valves are provided with an actuator that can be either rotary or linear, depending on the type of valve. There will often be a back-up manual operation with a wheel or lever. Small valves or valves with low frequency of operation may only have a manual operation system. Hydraulic cylinders do not require large space and can be installed on top of the gates. They can also easily be submerged and connected to an accessible location such as the side of the dam. Rising concern about oil leakage is driving the use of biodegradable oil or oil free electric linear actuators for submerged applications. Screening Screening at the inlet is used to prevent debris from clogging, impinging, or otherwise blocking the conduit or preventing the control from closing. A trashrack of appropriate size should be designed, that can be permanently fixed or removable for cleaning. A good rule of thumb for trashrack bar spacing is half the diameter of the pipe so small debris washes through and larger debris is arrested on the rack. Motorized trashrack cleaners are typically used in areas exposed to heavy organic loading and frequent debris build up that require regular cleaning. ‘Head Killer’ versus Head Loss Reduction An outlet system can serve different purposes. In the case of the outlet discharging to a channel or river, high velocity flow is usually undesirable due to its potential for channel erosion. Energy dissipation structures, such as impact basins, plunge pools, or baffles may be used to reduce the erosion potential of the discharging flows. Riprap or other armoring can also be used to protect channels from erosion. If the flow outlet is a pressurized pipe, a cone valve or a pressure reduction valve may be used. Pressure reduction valves keep turbulence inside the piping and release flow at the desired pressure and velocity. However, when the pressure head needs to be conserved for downstream use, minimizing friction losses is desirable and can be achieved by:  Smooth pipe entrance and smooth angles  Low friction factor associated with gates or valves along the circuit  Large diameter pipe to reduce velocity  Low friction pipe lining. Valve or Gate? There are several considerations that affect the choice of using a gate versus a valve. Valves may be preferable for the following reasons:  Valves tend to be chosen for smaller flows and pipes.  Valves are easy to operate and require relatively small space for installation and operation.  Valves can be installed anywhere along the piping system where access is easiest for operation and maintenance.  A variety of standard valves are readily available that can be chosen to match the conditions required for flow control, pressure, and reliability. Gates may be preferable over valves for the following reasons:  Valves are not feasible for open channel conditions  Gates are less expensive for control of large flows  Access constraints may make valve operation and maintenance difficult, (e.g., on an inclined upstream face with limited access) Location of the outlet control can be on the upstream face, downstream face or somewhere within the dam. It can also be located downstream of the dam if the water is conveyed through a pipe, in a vault, or a valve chamber. Embankment and earth filled dams tend to favor upstream control locations to avoid constant pressure conduits through the dam with the risk of leakage damaging the dam itself. Often times an upstream guard gate is accompanied by an in line control valve for redundancy and improved operation capabilities. Types of Control: Gates Tainter Gate: Also commonly called a segment or radial gate, these gates offer suitable control of large spillways due to their efficient structural designs that are generally less massive than other large gates. They are better suited for surface water control, although they can be used for pressurized conduits. They