## 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.

U.S.B.R. DRUM GATES

## U.S.B.R. Drum gate

U.S.B.R. DRUM GATES This gate remains hinged at the top of the crest of the spillway. This gate when open i.e. when not in position remains swung into a ditch formed at the crest of the spillway. In open position drum of the gate itself forms the crest of the spillway while the framework supporting the skin plate remains in the ditch. See Fig. 14.22 dotted position. The gate remains hinged at point A. The gates is put in closed position with the help of power driven winches only. BEAR TRAP GATE This gate consists of two parts, one part slipping over the other for some length. It is mostly use for low Navigation dams.

Shaft spillway

## Shaft spillway

SHAFT SPILLWAY This spillway consists of a vertical shaft and a horizontal conduit. The top of the shaft is specially designe, through which water enters and then drops through a vertical shaft and finally gets carrie to the D/S side by a horizontal conduit. This spillway is also sometimes know drop inlet or morning glory spillway. The shaft spillway may be standard crest type or flat crest type. Both types are shown in Fig. 14.10. Small shaft spillway may be made from steel or concrete pipes, but large spillways are made from heavily reinforce pipes. Diversion tunnels use for diverting the river water during the construction of the dam may be plugg and then use as shaft spillway by attaching vertical shafts to the horizontal tunnels. Top flared inlet of the vertical shaft is know morning glory and is considered essential feature of the shaft spillway.

VERTICAL LIFT GATES

## Vertical lift gates

VERTICAL LIFT GATES This is the gate that is in most common use in all the irrigation structures. It consists of a Rectangular skin plate of mild steel. This plate remains supported on a framework of I-girders or channel sections. Grooves are constructed on the crest as well as along the inside of piers. The gates slide or move, vertically up end down in these grooves. Water pressure acting on the gate is transferred to the crest and piers. In order to facilitate lifting and lowering of the gates, rollers are fitted in the grooves. The gates are lifted with the help of winches. Counterweights are also used to ease the lifting of the gates. Counterweights remain suspended with one end of the ropes. The other end of the ropes being connected to the gates. If gates are very big in size, they may be constructed in more than one…

Rolling gate.

## Rolling gates

ROLLING GATE This grate is show in Fig. 14.24. It consists of a steel cylinder as large in diameter as the height of the opening and spanning between the piers. Each pier has an inclined rack. The gate is wound around the periphery of the cylinder. The gate is also attach to the incline rack with the help of toothed gears. ItGate can be roll while opening and unroll which closing the gate.

Trash rack

## Trash rack gate

TRASH RACK It is not a gate but a sort of permanent structure that is install at the mouth of the opening. Its main purpose is to prevent the entrance of debris into the openings. A framed trash rack structure of steel sections or of R.C.C. members is a construct and iron bars openings of the framework to prevent the entrance of debris. See Fig. 14.25. This structure is usually construct at the entrances of sluiceways or penstocks etc.

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…

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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