Floodgate

Adjustable gate used to control water flow

Floodgates, also called stop gates, are adjustable gates used to control water flow in flood barriers, reservoir, river, stream, or levee systems. They may be designed to set spillway crest heights in dams, to adjust flow rates in sluices and canals, or they may be designed to stop water flow entirely as part of a levee or storm surge system. Since most of these devices operate by controlling the water surface elevation being stored or routed, they are also known as crest gates. In the case of flood bypass systems, floodgates sometimes are also used to lower the water levels in a main river or canal channels by allowing more water to flow into a flood bypass or detention basin when the main river or canal is approaching a flood stage.

Types

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Valves

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[clarification needed]

Valves used in floodgate applications have a variety of design requirements and are usually located at the base of dams. Often, the most important requirement (besides regulating flow) is energy dissipation. Since water is very heavy, it exits the base of a dam with the enormous force of water pushing from above. Unless this energy is dissipated, the flow can erode nearby rock and soil and damage structures.

Other design requirements include taking into account pressure head operation, the flow rate, whether the valve operates above or below water, and the regulation of precision and cost.[citation needed]

• Fixed cone valves are designed to dissipate the energy from a water flow during reservoir discharge. They are a round pipe section with an adjustable sleeve gate and cone at the discharge end. Flow is varied by moving the sleeve away or towards its cone seat. The design allows high pressure water from the base of a dam to be released without causing erosion to the surrounding environment. Fixed cone valves are able to handle heads up to 300 m.
• Hollow jet valves are a type of needle valve used for floodgate discharge. A cone and seat are inside a pipe. Water flows through an annular gap between the pipe and cone when it is moved downstream, away from the seat. Ribs support the bulb assembly and supply air for water jet stabilization.
• Ring jet valves are similar to fixed cone valves, but have an integral collar that discharges water in a narrow stream. They are suitable for heads up to 50 m.
• Jet flow gate, similar to a gate valve but with a conical restriction prior to the gate leaf that focuses the water into a jet. They were developed in the 1940s by the United States Bureau of Reclamation to allow fine control of discharge flow without the cavitation seen in regular gate valves. Jet flow gates are able to handle heads up to 150 m.

Physics

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The force on a rectangular flood gate can be calculated by the following equation:

  F = p A {\displaystyle \ F=pA}

where:

F = force measured in newtons (N)

p = pressure

= ρ g h {\displaystyle =\rho gh\,}

pascal (Pa)

where:

• ρ is the density of fresh water (1000 kg/m3);
• g is the acceleration due to gravity on Earth (9.8 m/s2 );
• h is the height of the water column in metres.

A = area = rectangle: length × height measured in m2

where:

length = the horizontal length of a rectangular floodgate measured in metres

height = the height of a non-submerged flood gate from the bottom of the water column to the water surface measured in metres

If the rectangular flood gate is submerged below the surface the same equation can be used but only the height from the water surface to the middle of the gate must be used to calculate the force on the flood gate.

See also

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References

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Sources

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• DeltaWorks.Org – project in the Netherlands on floodgates

floodgate, gate for shutting out or releasing the flow of water over spillways, in connection with the operation of a dam. Important safety features of many types of dams, floodgates and spillways direct excess water away from the dam and its foundation to prevent erosion that could lead to catastrophic dam failure.

Several forms of gates have been developed. The simplest and oldest form is a vertical-lift gate that, sliding or rolling against guides, can be raised to allow water to flow underneath. Radial, or tainter, gates are similar in principle but are curved in vertical section to better resist water pressure. Tilting gates consist of flaps held by hinges along their lower edges that permit water to flow over the top when they are lowered. Vertical lift and radial gates are generally placed at the top of the spillway crest.

Drum gates rotate backward, lowering their tops and permitting a measured flow over the top of the gates. Drum gates can control the reservoir level upstream to precise levels automatically and without the assistance of mechanical power. One drum gate design consists of a shaped-steel caisson held in position by hinges mounted on the crest of the dam and supported in a flotation chamber constructed immediately downstream of the crest. Water pressure in the reservoir and buoyancy of the caisson in the flotation chamber hold the caisson in rotational equilibrium. Raising or lowering the water level in the flotation chamber causes the caisson to rotate in the same direction, thus reducing or increasing flow from the reservoir over the gate. This action can be linked to and operated automatically by a float control device in the reservoir. Drum gates present the fewest problems in the passage of ice or drift over the spillway.