Gates/Bulkheads

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Learn more about gates from this On-Demand Webinar

Gates and bulkheads are movable water barriers for the control of water. Gates are typically permanent features and include a wide range of types and 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 if 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. At 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”.

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.

“Gated spillways include drop inlet, free overfall, ogee crest, side-channel and bathtub, tunnel inlet, various shaped weirs, and orifice. These control structures are associated with regulated releases and suited for service, auxiliary, and emergency spillways. The most frequently used gates include radial gates, drum gates, wheel-mounted gates, and crest gates. Considerations include a firm foundation (typically rock), high degree of reliability of gate operation, limiting debris blockage potential, and favorable economics. In addition, gated spillways provide increased control of releases for a given reservoir water surface, allowing increased discharge where reservoir storage is limited, and/or to reduce the amount of reservoir water surface rise during a flood event. These considerations would apply to large volume inflows, where there is a relatively small reservoir storage capacity. Examples of these gated spillways include:

• "Gated drop inlet spillway - This type of control structure is mostly associated with a service spillway with radial gates. An example of a gated drop inlet spillway is Reclamation’s Gibson Dam (concrete). Other examples include the service spillways with ring gates at Reclamation’s Hungry Horse and Owyhee Dams (both concrete).
• "Gated free overfall spillway - This type of control structure is mostly associated with a service spillway with roller gates. An example of a gated free overall spillway is Reclamation’s Parker Dam (concrete).
• "Gated ogee crest spillways - This type of control structure is associated with a service and auxiliary spillway with gates. Examples of gated ogee crest spillways are Reclamation’s Shasta Dam (concrete) with drum gates and auxiliary spillway with radial gates at Reclamation’s Stewart Mountain Dam (concrete). Other examples include service and emergency spillways with radial gates at Reclamation’s Folsom Dam (composite) and the service spillway with fixed-wheel gates at Reclamation’s Keswick Dam (composite).
• "Gated side-channel spillway - This type of control structure is mostly associated with a service spillway with gates. An example of a gated side-channel spillway is Reclamation’s Arrowrock Dam (concrete), which has drum gates.
• "Gated tunnel inlet spillway - This type of control structure is mostly associated with a service spillway with gates. An example of a gated tunnel inlet spillway is Reclamation’s Seminoe Dam (concrete) with fixed wheel gates. Other examples include the service spillways with radial gates at Reclamation’s Glen Canyon Dam and the service spillways with drum gates at Reclamation’s Hoover Dam.
• "Gated various shaped weir spillway - This type of control structure is mostly associated with a service spillway with gates. An example of a gated various shaped weir spillway is Reclamation’s Hyrum Dam (embankment) with radial gates.
• "Gated orifice spillway - With this type of control structure, flow is typically released from the spillway by one of two approaches:
  • "A free jet is released downstream of the gates and typically is stilled by a plunge pool.
  • "Flows will be released to a conveyance feature (such as a chute, conduit, or tunnel) and/or terminal structure.

Life Cycle of Gates/Bulkheads

1. Design and Construction of Gates/Bulkheads
2. O&M of Gates/Bulkheads

Best Practices Resources

Hydraulic Design of Reservoir Outlet Works (EM 1110-2-1602) (U.S. Army Corps of Engineers)

Design Standards No. 6: Hydraulic and Mechanical Equipment (Ch. 6: Bulkhead Gates and Stoplogs) (Bureau of Reclamation)

Design Standards No. 14: Appurtenant Structures for Dams (Ch. 4: General Outlet Works Design Considerations) (Bureau of Reclamation)

Trainings

On-Demand Webinar: Introduction to Spillway Gates

Citations:

Revision ID: 4834
Revision Date: 12/02/2022