## Irrigation engineering

Irrigation engineering : in agriculture, artificial watering of the land. Although used chiefly in regions with annual rainfall of less than 20 in. (51 cm), it is also used in wetter areas to grow certain crops, e.g., rice. Estimates of total irrigated land in the world range from 543 to 618 million acres (220 to 250 million hectares), almost half of them in India, Pakistan, and China. The United States had almost 60 million acres (23.8 million hectares) of irrigated farmland in 1991.

In Irrigation engineering Methods of applying water include free-flooding of entire areas from canals and ditches; check-flooding, in which water flows over strips or checks of land between levees, or ridges; the furrow method, in which water runs between crop or tree rows, penetrating laterally to the roots; the surface-pipe method, in which water flows in movable slip-joint pipes; sprinklers, including large-scale center-pivot and other self-propelled systems; and a variety of water-conserving drip and trickle systems. In many cases irrigation is correlated with drainage

to avoid soil salinity, leaching, and waterlogging. Irrigation may also involve preliminary clearing, smoothing, and grading of land. Especially in areas of high evaporation rates, intensive irrigation can result in excessive quantities of salts accumulating in the upper layers of the soil as water evaporates from the surface, rendering the soil unfit for crop production.

Since prehistoric times water has been diverted from waterways to fields by ditching. Early improvements for raising water included counterbalanced poles with attached water vessels, and adaptations of the wheel and of a pump called the Archimedes’ screw. The use of canals, dams, weirs, and reservoirs for the distribution, control, and storage of water was probably initiated in ancient Egypt. A system of gently sloping underground tunnels (qanats) to deliver water from a subterranean source to distant areas where it is accessed through shafts was developed in ancient Persia and has been widely used elsewhere. In modern times pumps have facilitated the use of underground as well as surface water, but overuse of water in aquifers can exhaust their usable water. Large-scale 20th-century irrigation projects commonly also include water supply, hydroelectric power, and flood control.

FISH BELLY FLAP GATE

## Fish belly flap gate

FISH BELLY FLAP GATE This gate is show in Fig. 14.26. This gate is also know as the Bascule type of gate. This is use at the top of the weir crest to store extra stormwater. The gate is fitt on the crest with the help of a hinged joint. It is operated with the help of a lever rod. Example.1 The Head of water over the crest of the ogee spillway is 3 m and the coefficient of discharge 2.5. Weir is 100 m long and the height of the crest above the base of the approach channel is 10 m. The width of the approach channel is equal to the length of the weir. Find out the discharge passing over the spillway. Solution. $Q= CLH^{3/2}$   $Q= 2.5\times 100\times 3^{3/2}= 1300 cumec.$ Velocity of approach $V_{a}= \frac{Q}{Head\times width.of.channel}$ $= \frac{1300}{(10+3)100}= 1m$ $H_{a}= \frac{V_{a}^{2}}{2g}= \frac{(1)^{2}}{2\times 9.81}= 0.05m$ \[H= h+H_{a}=…

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Slipways Outlet works

## Slipways Outlet works

OUTLET WORKS The impounded water in the reservoir has to be regulat through outlet works to use it usefully. Outlet works are required for following purposes (i) Feeding water for power generation. (ii) Discharging heavily laden silty water D/S. (iii) Supplying water to irrigation channels. (iv) Supplying water for water supply purpose. (v) To evacuate the water from the reservoir in anticipation of flood inflow. Outlet works consist of three component parts. 1. Water way. 2. Gates to control the flow of water. 3. Intake structures. A pipe or tunnel passing though the dam is the water way. It is sometimes also know sluice way. In concrete gravity dams the sluice ways may be made in the body of the dam whereas in case of earth dam they are set up outside the limits of the embankment. If sluice way has to be provided in the body of the earth…

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BEST QUESTIONS SPILLWAYS

## Questions in this chapter

What do you understand by the term spillway? How the spillways are broadly classifie? On what factors the discharging capacity of the spillway is fix. What are the functions of a spillway? Prepare a list of main spillways. Make a neat sketch of an Ogee type spillway and describe its working in brief. Why emergency spillway is installed? Is it made of earth or cement concrete? Explain the formation of a hydraulic jump. Derive the expression for height of hydraulic jump on D/S side. What are various methods of energy dissipation? Compare jump height curve with tail water curve and suggest the protective works which would be most effective for dissipation of excess energy of flowing water. Discribe various types of siphon spillways with the help of neat sketches. 10. Draw the sketches of (i) Radial gates. (ii) Vertical lift gates. (iii) Needle gates.

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## Diversion Head Works

INTRODUCTION A structure whose main purpose is to supply water to the off taking canal is know as head works. When the off taking canal is to supply water for irrigation purpose, its headworks is call irrigation head works. Irrigation headworks may be divid into two categories. 1. Diversion headworks or simply head works. 2. Storage headwork. 1. Diversion head works. The purpose of this head work is to divert river water into the off taking canal. It is located across the river near the point of the take-off of the canal. A diversion headwork performs the following functions: It regulates the flow in the off-taking canal. Silt entrance into the canal is controlled. It raises the level of water in the river. This causes diversion of the river water into the off taking canal, under gravity flow. By raising the level of water in the river with the help…

types of diversion head work

## Types of diversion head works

TYPES OF DIVERSION HEAD WORKS Diversion works can be classified under the following two heads: 1. Temporary bunds or spurs. 2. Permanent weirs and barrages. Temporary bunds or spurs are temporary works, which have to be constructed every year after floods. These bunds cannot sustain the assault of floods and get washed away. Immediately after floods, they are again constructed. Temporary bunds can be constructed on small streams only. Such works are generally carried out by the nearby villagers jointly, so as to make arrangements for irrigation for their fields for lean months of flow. Such temporary spurs or bunds can be economically constructed in boulder reaches of the river. However, for important diversion works permanent weirs and barrages have to design and construct. Weir. Weir is a solid obstruction, construct across the river. It is used to raise the water level in the river and then divert it into…

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LOCATION OF DIVERSION HEAD WORKS

## Location of diversion head works

LOCATION OF DIVERSION HEAD WORKS The total length of the river may be divide into the following four stages. 1. Mountainous stage. The first length of the river originating from hills is know the mountainous stage of the river. The longitudinal slope in this stage is very steep and the velocity of the flow of water, very fast. The width of the river in this region is generally very small. This region is found most suitable for the construction of storage works like high dams. Large storages are possible very deep valleys are easily available here. Moreover, the length of the dam is also relatively small. Materials of construction are locally available. This stage is however not suitable for diversion works. 2. Sub-mountainous stage. The bed and banks of the river in this stage are made of gravel. The velocity of flow is considerable though smaller than first stage. The…

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LAYOUT OF A DIVERSION HEAD WORKS

## Layout of a diversion head work

LAYOUT OF A DIVERSION HEAD WORKS A typical layout of a diversion head work is shown in Fig. 15.1. Following is the list of component parts of a diversion headworks. The design and description details of all these components given one by one in subsequent articles. Weir or barrage. Scouring sluices or under sluices. Divide wall. Fish ladder. Log chutes. Canal head regulator. River training works. Silt control devices   Out of all these component parts river training works include lot of other elements also and hence have been discussed in a separate chapter later on. Before we take up description and design of all these component parts of a diversion headworks, let us first of all discuss location of head works.

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Flash board and stop log or needle gate

## Flash board & stop log or needle gate

FLASH BOARD It is a temporary arrangement which is adopted to store extra water during low flows in the river. At large floods they are either removed or allowed to drop on D/S side if hinged at the crest. Flash Boards are made by joining wooden planks which are fixed against the pins. The pins are installed over the crest at suitable intervals. Flash Boards when in position may be vertical or inclined. See Fig. 14.19. STOP LOG OR NEEDLE GATES In this case, piers are located at suitable intervals on the crest and stop logs Which are nothing but timber planks fixed horizontally in the grooves of the piers, one above the other. While opening, the stop logs are removed one by one. See Fig. 14.20 (b). In the case of needle gates, wooden planks are not used horizontally but vertically. A groove is developed at the crest of…

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SPILLWAY GATES

## Spillway gates

SPILLWAY GATES The spillways may be uncontrolled or controlled types. Uncontrolled spillways do not require any gate. Controlled spillways are provided with gates. Controlled spillways are considered superior to uncontrolled ones, as better control on the reservoir level can be exercised with gates. During low flows,   additional storage can be done in the reservoir with the help of gates. However, during high floods gates are fully lifted or opened so that the full discharging capacity of the spillway may be used to dispose of flood water. In case of earth dams, spillways are not provided with gates. Slakness on the part of lifting the gates, may cause over-topping of the earth dam and may lead to its failure. Following are the various types of gates, which are commonly used: 1. Flash Boards 2. Stop log or needle gates 3. Tainter gates or radial gates 4. U.S.B.R. Drum gates 5.…

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ENERGY DISSIPATION BELOW SPILLWAYS

## Energy dissipation below spillways

ENERGY DISSIPATION BELOW SPILLWAYS When stream of water moving with hyper-critical velocity, meets an other stream, moving with sub-critical velocity there is abrupt rise in the water level of the stream. This abrupt rise of water is know hydraulic jump. Hydraulic jump is a very useful measure of dissipating the energy of flowing water. Flood water at the level of the crest of the spillway has potential energy proportionate to the rise of the crest above the D/S floor of the spillway. When flood water passing over the crest of spillway reaches the bottom of spillway on D/S side, the potential energy of water is convert into kinetic energy, as velocity is greatly increase by the time water reaches from crest to D/S floor of the spillway. This high kinetic energy may cause very deep erosions on D/S if measures to disscipate it are not taken. There are several methods…

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