Gates/Bulkheads
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Gates and bulkheads are movable water barriers for the control of water. Gates are typically permanent features and include a wide range of types of sizes. Bulkheads are typically used either for temporary closure of a channel or conduit before dewatering it for inspection or maintenance or for closure against flowing water when the head difference is small, e.g., for diversion tunnel closure.
Components of Gates/Bulkheads
While there are many different types and sizes of gates and bulkheads, they often share similar components. These can include gate slots, gate recesses, gate seats, passageway liners, and air vents. Additionally, it is often required to design and construct redundant systems with at least two gates to allow for partial operation of the system in an emergency should one of the gates become inoperable.
“Gate Passageway Requirements. Normally, when reservoir outlet flows require regulation, the following are provided:
- Two or more gate passages such that if one passage is inoperative, a reasonable flow regulation as pertains to project purposes is obtained.
- Emergency gate provision (tandem or transferable) for each service gate passage so that is a service gate is inoperative in any position, closure of the gate passage can be made with the emergency gate for any pool level.
- Bulkhead provisions for each gate passage for inspection and maintenance of the service and emergency gates. As a minimum, the bulkheads must be capable of being installed at the lowest pool elevation that has a reasonable frequency and length of occurrence sufficient for inspection and repair purposes. All judgement factors involved in the above should be fully discussed in the design memorandum presentation.
"Gate Slots. The guide slots of rectangular gates produce a discontinuity in sidewalls which may cause cavitation, unless specially designed. It has been common practice to use metal-liner plates downstream from the gate slots to protect the concrete from the erosive action of cavitation.
"Gate Recess. Hydraulically operated control gates recess into bonnets and cable-suspended gates into wet wells. The necessary dimensional clearances for gate operation are usually based on mechanical and structural requirements rather than hydraulic. The primary hydraulic consideration is the relative upstream and downstream clearance at the roof recess when the gate passage is operated at part gate opening. The upstream clearance at the roof should be appreciably larger than the downstream clearance to assure maintenance of a hydrostatic head in the well or bonnet for gate stability. If the downstream clearance exceeds the upstream clearance the gate well can be sucked dry and the gate may float or catapult or oscillate under certain operating conditions.
"Gate Seats. In general, the gate seat is flush with the floor of the gate passage.
"Steel Liners. Steel liners in concrete conduits have been used where experience indicates cavitation is likely to occur such as downstream from control gates and valves where a high-velocity jet occurs. For head above 150 feet, no liner should be required. If a liner is necessary, it should not terminate at a monolith joint or in a transition.
"Air Vents.
- Control valves and gates that are located a considerable distance upstream from the exit (i.e., do not discharge into the atmosphere) require air vents. An air vent is required for each service gate. Air vents are not required for emergency gates when those gates are located immediately upstream of air-vented service gates. Extreme caution must be observed if the emergency gate is used for regulation. Air demand will create very low pressures in the service gate recess. The attendant conditions must be carefully analyzed to prevent damage and/or danger to personnel.
- The size of air vents can be determined as per HDC 050-2n which assumes that the maximum air demand occurs at a gate opening of 80 percent fully open and the maximum air velocity in the vent does not exceed 150 fps. It is further suggested that air vents be designed so that the head loss through the vent not exceed 0.5 to 1.0 feet of water (i.e., air vent outlet pressure head of -0.5 to -1.0 feet of water). Although air vents are usually designed assuming incompressible flow, high-velocity local flow should be checked to determine if flow is incompressible.
- Air vent passages should use generous bend radii and gradual transitions to avoid losses and, particularly, excessive noise.
- Air vent intakes should be so located that they are inaccessible to the public and they should be protected by grills. The intake entrance average velocity should not exceed 30 fps.
- Interconnected air vents (one main vertical stem manifolded to vent more than one gate) should be avoided; but if they are necessary, the connections should be above the maximum possible elevation of the pressure grade line at the air vent exit opening to prevent crossflow of water.
- The air vent exit portal should be designed to assure spread of air across the full width of the conduit. The air vent should terminate into a plenum located in the conduit roof and immediately downstream of the gate. The plenum should extend across the full width of the conduit and should be vaned so that the air flow is evenly distributed”.[1]
Types of Gates/Bulkheads
As a result of varying purposes of dams, as well as the unique site conditions that accompany each particular dam, there are a wide variety of types of gates or bulkheads. Some of the most common types include vertical lift gates, roller gates, hinged or flap gates, or tainter or radial gates. In the design process of a gated spillway system, it is necessary to weigh the advantages and disadvantages of using each type of gates in the particular operating conditions expected at the dam.
Life Cycle of Gates/Bulkheads
Best Practices Resources
Hydraulic Design of Reservoir Outlet Works (EM 1110-2-1602) (U.S. Army Corps of Engineers)
Trainings
On-Demand Webinar: Introduction to Spillway Gates
Citations:
Revision ID: 4322
Revision Date: 11/18/2022