Floodgate

From Wikipedia, the free encyclopedia

Tokyo floodgates to protect from typhoon surges.

(Floodgate is also the name of a Christian rock band.)

Floodgates are adjustable gates to control water flow in reservoir, 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.

1 Types of floodgates
2 Floodgate valves
3 The physics of flood gates
4 References
5 See also
6 External links

[edit] Types of floodgates

A sluice gate on the Harran canal.

A flood wall gate at Harlan, Kentucky.

Bulkhead gates are vertical walls with movable, or re-movable, sections. Movable sections can be lifted to allow water to pass underneath (as in a sluice gate). Designs with removable sections allow water to pass over top and historically used stacked timbers, which are known as stoplogs or flashboards, to set the dam's crest height. Some floodgates in large levee systems slide sideways to open for various traffic. Bulkhead gates can also be made of other materials and used as a single bulkhead unit. Miter gates are used in ship locks and usually close at an 18° angle to approximate an arch.

A hinged crest gate during installation.

Fish belly flap gates at the Scrivener Dam, Canberra.

Hinged crest gates, are wall sections that rotate from vertical to horizontal, thereby varying the height of the dam. They are generally controlled with hydraulic power.

Variations;

flap gate
fish-belly flap gates
Bascule® gates
Pelican® gates

Tainter gate diagram.

Tainter gates and spillway.

Radial gates are rotary gates consisting of cylindrical sections. They may rotate vertically or horizontally. Tainter gates are a vertical design that rotates up to allow water to pass underneath. Low friction trunnion bearings, along with a face shape that balances hydrostatic forces, allow this design close under its own weight as a safety feature.

Drum gates are controlled with valves.

Drum gates on a diversion dam.

Drum gates are hollow gate sections that float on water. They are pinned to rotate up or down. Water is allowed into or out of the flotation chamber to adjust the dam's crest height.

A roller gate on the Mississippi.

Clamshell floodgates

Roller gates are large cylinders that move in an angled slot. They are hoisted with a chain and have a cogged design that interfaces with their slot.
Clamshell gates have an external clamshell leaf design.

[edit] Floodgate valves

Discharge from a Howell-Bunger valve.

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 are: operating pressure head, flow rate, whether the valve operates above or below water, regulating precision and cost.

Fixed cone valves, also known as Howell-Bunger 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 1,000 feet.

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 175 feet.

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 1940's 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 500 feet.

[edit] The physics of flood gates

In order to do a simple calculation of the force on a rectangular flood gate one can use the following equation:

$\ F = pA$

where:

F = force measured in the SI units kg·m·s^–2 which is called the newton (N)

p = pressure $= \rho g h\,$ measured in N/m², which is called the pascal (Pa)

where:

ρ (rho) 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 meters.

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

where:

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

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

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.