floodgate - Types of floodgates, Floodgate valves, The physics of flood gates

A moveable barrier which can be raised or closed to prevent flooding at times of bad weather or very high tides. Flap gates are metal barriers which are buried in special housings in the sea or river bed. When not needed they are full of water. With a warning of a tide exceeding 100 cm (40 in), compressed air is pumped into the flapgates. This empties them of water and they rotate on hinges so that they swing upwards to block the flow of tide. When the tide ebbs and the water is at a safe level, the flapgates fill with water again and sink down to their housings. There are many other different designs of floodgates, such as drum-shaped sector gates, bear-trap type gates, floating sail gates, segment gates below the water level, and inflatable cylindrical rubber gates. Other means of blocking water flow are simple floating caissons or air chambers.

(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. 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 of floodgates

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.

Hinged crest gates, are wall sections that rotate from vertical to horizontal, thereby varying the height of the dam. flap gate fish-belly flap gates Bascule® gates Pelican® gates

Radial gates are rotary gates consisting of cylindrical sections. Tainter gates are a vertical design that rotates up to allow water to pass underneath.

Drum gates are hollow gate sections that float on water.

Floodgate valves

Valves used in floodgate applications have a variety of design requirements and are usually located at the base of dams. Since water is very heavy, it exits the base of a dam with the enormous force of water pushing from above.

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. 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.

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:

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: ρ (rho) is the density of fresh water: 1000 kg/m3); 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.