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Floodgates are adjustable gates used to control
water flow in 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
- 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
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- 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, although some are passive and are powered by the water
being impounded.
- Variations;
- flap gate
- fish-belly flap
gates
- Bascule gates
- Pelican gates
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Tainter gates and spillway
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- 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 to
close under its own weight as a safety feature.
Drum gates are controlled with valves.
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- 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.
- 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.
- Fusegates Fusegate System is an innovative
spillway control technology, which consists of free standing blocks
(the Fusegates) set side by side on a flattened spillway sill. The
Fusegate blocks act as a fixed weir most of the time and operate
independently without any remote control or energy source only in
case of excessive flood conditions. The System is developed and
patented by Hydroplus from Paris, France. It has been installed on
more than 50 dams around the world with sizes ranging from 1m to
more than 9m in height. Fusegate are typically used to increase the
storage capacity of existing dams or to maximize the discharge
potential of undersized spillways.
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 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.
- 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 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
In order to do a simple calculation of the force on a
rectangular flood gate one can use the following equation:
where:
- F = force measured in the SI units kg·m·s–2 which is called the newton (N)
- p = pressure
measured in N/m2, which is called the pascal (Pa)
-
- where:
- 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.
See also
References
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