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.

 

Main canal head regulator
MAIN CANAL HEAD REGULATOR

Main canal head regulator

MAIN CANAL HEAD REGULATOR It is a masonry or concrete structure, constructe at the head of the canal taking off from the river. It is constructe U/S of the under sluices and located in one bank. Its alignment is kept at an angle varying from 90° to 120° with the axis of the weir. The head regulator consists of a number of spans separat by piers and each span is fitted with a steel gate which can be move up or down. In the grooves made in the piers, with the help of either manual labour or winches. In old regulators, the spans use to be quite small, but the modern trend is to use larger spans of 8 to 18 m. Following are the functions of a main canal head regulator. (i) To open or close the discharge in the canal and when require. (ii) To check the silt…

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Divide wall

DIVIDE WALL It is also called groyne, or groyne wall. It is an embankment constructed in the river, U/S of the weir. Its axis is kept at right angles to the axis of the weir. The embankment is protected from all the sides with the help of stone or concrete blocks. The divide wall separates weir from undersluices. It extends a little U/ S of canal regulator and on D/S end upto loose protection of the under sluices. It may be made of concrete or masonry, with top width of 1.5 m to 3 m. This wall should be designed for following conditions. (i) Silt pressure upto full tank level on the face opposite to the face lying towards the head regulator and minimum possible or no water on the face lying towards the head regulator. (ii) During high floods the water level behind the weir should be assumed about…

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Scouring or under sluices
Scouring or under sluices

Scouring or under sluices

SCOURING OR UNDER SLUICES These are the openings in the weir. They are forme by depressing the weir. They have gates fitt in them. The scouring sluices are locat on the side of the weir on which head regulation of the canal, taking off from here, is locat. If two canals are taking off on either side of the river two sets of under sluices should be locat on either side of the weir. Undersluices perform the following functions. A clear well-defined approach channel is preserve in the river just U/S of the canal head regulator. Silt entry into the off taking canals is controlled. When sluices remain close for a few days, silt gets accumulat in the get just U/S of the sluices. This can be easily scour to the D/S side by opening the under sluices from time to time. Low floods can be passed through them without…

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Method of independent variables
METHOD OF INDEPENDENT VARIABLES

Method of independent variables

METHOD OF INDEPENDENT VARIABLES Break up of a complex profile of a weir to simple three elementary. This break up shows the theoretical profiles. Actually, the usual weir section consists of a combination of all the three elementary profiles. In addition to this, the floor also has some thickness. Khosla solve the actual profile of the weir by an empirical method know the method of independent variables. According to this method, the actual complex profile is broken into a number of simple profiles know elementary profiles. Each elementary profile is then treat independently of the others. Each elementary profile is independently amenable to mathematical treatment. The pressures at key points are read from Khosla’s curves. The key points are the junction points of the floor and pile and bottom points of different piles. The pressures read from Khosla’s curves are true for individual elementary profiles. But when all the profiles…

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Weir
WEIRS

Weir

WEIRS Weir is a solid obstruction, put across the river to store water on its U/S. The store water is divert to the off-taking canal. Depending upon the criterion of design, the weirs may be gravity type or non-gravity type. The gravity weir is the weir in which uplift pressure below the weir due to seepage is fully resist by the self weight of the weir. In the case of non-gravity weirs the thickness of the floor is kept relatively small and uplift pressure is largely resist by the bending action of the reinforc concrete floor.   Depending upon the design features and available construction materials gravity weirs or simply, weirs can be further subdivide into the following three categories. Vertical drop weirs. Rockfill weirs. Concrete weirs with sloping glacis. 1. Vertical drop weirs. This weir consists of a vertical drop wall or crest wall, rectangular or nearly rectangular in shape.…

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Failure of weirs on permeable foundation
CAUSES OF FAILURE OF WEIRS ON PERMEABLE SOILS AND THEIR REMEDIES

Failure of weirs on permeable foundation

CAUSES OF FAILURE OF WEIRS ON PERMEABLE SOILS AND THEIR REMEDIES The weir failure may take place due to the following reasons 1. Piping or undermining. 2. By uplift pressure. 3. By suction due to standing wave. 4. By scour on the U/S and D/S of the weir. 1. Piping or undermining. When water seeping through the permeable foundation emerges out at the D/S end of the impervious floor of the weir, at hydraulic gradient or exit gradient greater than the critical value for the foundation soil, the soil starts boiling at the exit point. Boiling of the soil indicates lifting of the soil against gravity and it happens only when exist gradient of seeping water is greater than the safe limit for the foundation soil. The soil gets wash out with boiling water. With washing out of some soil from D/S side, the exit gradient increase and boiling of…

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Design of impervious floor

DESIGN OF IMPERVIOUS FLOOR OF THE WEIR FOR SUB-SURFACE FLOW Khanki weir failed in 1895. An expert’s committee under the Chairmanship of Lt. Col. Caliborn, then Principal of Thomson College, Roorkee was set up to investigate the causes of failure of this weir, which was founded on sandy soil. The committee confirmed Darcy’s law of seepage through sandy soils for low heads. Heads are generally low in the case of weirs. Later Mr. W.G. Bligh put forward his own theory know by his name “Bligh’s creep theory” for sub-surface flows. Later on Mr. Lane put forward is own theory in 1932 after having analyse a large number of dams. This theory is know ‘Lane’s weight creep theory’. A most elaborate scientific study base on facts was later carried out in 1936 by Pavlovsky and A.N. Khosla. This theory is know “Khosla theory of Mr. Khosla carrie out most of his…

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Bligh’s Creep Theory

BLIGH’S CREEP THEORY This theory is base on the assumption that seeping water through the soil below the weir Follows the path along with the contact of the base, with the underlying sub-soil. The length of the path of seeping water from the point of entry into the sub-soil from the U/S of the impervious. Apron to the point at the D/S end of the impervious apron is know creep length. Bligh also assumed that loss of head of the seeping water is proportional to the length of its travel irrespective of whether. The length of travel is in the horizontal or vertical direction. He also assumes that unless cut-off sheet piles extent to the impervious subsoil strata, no amount of sheet piling could stop the flow of percolating water. AB is the length of impervious apron l and H is the head of water-fill up to the top of…

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Khosla’s Theory
KHOSLA’S THEORY

Khosla’s Theory

KHOSLA’S THEORY Some siphons on the upper Chenab canal, which were design according to Bligh’s theory gave trouble. Khosla along with his associates was ask by the government to investigate the causes of trouble and to suggest remedial measures. Khosla and his associates insert some pipes on the D/S side of the weir through impervious aprons of some of the trouble giving works. The pipes insert to verify whether the pressures below the impervious apron were in accordance with Bligh’s theory or not. Based on his investigations A.N. Khosla drew the following conclusions: The outer faces of the end piles are much more effective than their inner faces and also more effective than the horizontal length of the impervious apron. Intermediate sheet piles, if equal to or smaller in length than the outer piles, are almost ineffective and do not provide any additional creep length. They may only cause a…

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Tainter gate or Radial gates
TAINTER GATES OR RADIAL GATES

Tainter gate or Radial gates

TAINTER GATES OR RADIAL GATES This gate is in the form of a sector of a circle. A curved plate is use to support water. The curved plate is given adequate support by a frame work of steel sections. The gate remains hinged at the piers, on both the ends. This gate can thus rotate about horizontal axis. Water pressure is ultimately transferr to the piers, through bearings fix in piers. The tales are generally very heavy and can be lifte or lowere with the help of power driven winches only.

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