Flood Gates for Drainage Outlets

New Zealand Journal of Agriculture, Volume 100, Issue 5, 16 May 1960, Page 473

Flood Gates for Drainage Outlets

By

D. F. SCOTT,

Machinery Instructor, Department of Agriculture, Auckland A FLOOD GATE is a simple flap valve so arranged that water in a drain can flow only in one direction. Though flood gates are mainly used with outlets affected by tide, they can be used for drains discharging into rivers where water levels may rise temporarily owing to flooding. 'T'HE flood gate prevents foreign water from backing up through a farm drainage system, but it has an obvious weakness in that while the gate is shut owing to high water level outside, drainage water will be trapped within the system. This may mean that internal drains may gradually fill up before the flood gate opens. The only way of overcoming this serious disadvantage is to use a pump in addition to the flood gate. At least the flood gate will prevent outside water from loading the internal drains, and this is most important if the outside water is salty. Types of Flood Gates Satisfactory flood gates may be made from a wide range of materials, concrete, cast iron, and timber all being quite suitable. Concrete gates, though they are reasonably resistant to rotting and corrosion, are very heavy, and this may account for a greater loss in head owing to the pressure of water required to open them. Cast-iron gates are very popular and the rate of corrosion, even in salt water, is not very serious. Where it is feared that corrosion of the flap and its seat may cause leakage these parts may be faced with bronze to give a corrosion-resistant seal. Timber gates are not very often used these days because, unless they are treated with preservative, they are not generally as resistant to corrosion and rotting as is concrete or cast iron. To assist gates to seat efficiently they are usually hung on special hinges which are free to move at both ends. Gates made of heavy material may be fitted with a counterbalance to reduce

the head loss due to the weight of the gate. Installation Most flood gate installations are associated with protective banking works, any weak or low place in which may give way and let water in in times of strain. Expensive flood gate installations can be completely wasted if there are any such weaknesses in the associated bank. Most flood gate installations contain a considerable weight of materials and require adequate foundations to prevent movement or subsidence. It is sometimes necessary to drive piles to support a flood gate installation, but flood gates of a size likely to be installed in farm drainage works do not generally require such precautions. A flood gate can be fitted to the end of a concrete or steel pipe, and as long as provision is made to prevent erosion by the water on the discharge side a fairly simply constructed outlet is quite satisfactory. If a gate is fitted to a culvert, it is important to provide some support along the length of pipes to prevent them moving out of

line. A good method is to bed the pipes on two lengths of railway line, hard wood, or preservatised timber. Commercially produced cast-iron or concrete flood gates may be either mounted on the end of a culvert pipe or fastened to a facing wall at the discharge end of the culvert. It is a good idea to allow plenty of pipe length to pass through the bank and to face the sides of the bank surrounding the pipe with concrete, stone, or timber to prevent subsidence and to prevent water leaking parallel with the pipe. Some gates are fitted so that they hang vertical when closed and others are hung on a slight angle. A reasonably well fitting gate should be perfectly satisfactory if hung vertical, but on the other hand some gates have been installed with batters of up to 1 in 14. The more batter on the gate the greater loss of head there will be owing to the pressure required to lift the gate as the water flows through it. Size of Gate Commercially made flood gates in both cast iron and concrete are available from 6 in. up to 4 ft in diameter. The size of gate will depend on the area of land in the catchment feeding the flood gate outlet and on whether the catchment is steep and hilly or flat and swampy. One authority suggests that for average New Zealand conditions a fair guide to flood gate size would be 1 sq. ft. of flood-gate area per 20 acres of hill country or per 30 acres of flat country. Operation and Maintenance Few flood gates provide a complete seal against leakage, but this is not really important, because their main

function is to prevent large quantities of outside water from entering an interior drainage system. Driftwood or other material lodging in a flood gate may prevent it from closing and have serious results, as a gate jammed 3 to 4 in. open will be completely ineffective. A screen or strainer should be fitted on the inside end of the pipe to the flood gate, as most blockages occur from material that is carried through the gate with the drainage flow. A flood gate fitted with a debris screen should require little attention apart from an occasional inspection and cleaning of the screen.

Diameter of Area of gate gate opening in. sq. ft. 6 0.2 12 0.8 18 1.7 24 3.1 30 4 Q 36 7.0 48 12.5

Table of Flood-gate Areas