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.

 

Free Board & Canal Banks

FREE BOARD The vertical distance between F.S.L. and top of the lowest bank of the channel is know free board. It is provided to prevent waves or fluctuations in water surface from overtopping the banks. Freeboard depends upon the canal size, wind action, soil characteristics and location. According to USER, free board may be worked out from following formula, under ordinary conditions. \[F= \sqrt{CD}\] where F = free board C = a constant whose value varies from 0.46 to 0.76 D = the depth of water in metres Lacey gave following formula for the free board \[F= 0.20+0.15Q^{\frac{1}{3}}\] According to CWPC the free board should be as follows: CANAL BANKS The purpose of banks is to prevent spread of water beyond the specified limit. The width of the banks should be enough so that a minimum cover of 0.5 in soil is available everywhere above the saturation line. The hydraulic…

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Side slope Canal

SIDE SLOPE The side slope of the canal depends upon the type of soil. The canals in alluvial soils are designed assuming a 1/2 :1 side slope, irrespective of the actual initial side slope. It is assumed that after the due course of a run, the canal section would ultimately acquire a 1/2: 1 slope. This happens because silt gets deposited on the berms. Had section been designed with 1/2: 1 slope initially, the section would be reduced in due course of time due to silting and the section remaining would be inadequate. As per the recommendation of the Central Water. Power Commission (C.W.P.C.) the side slopes for various soils should be given in Table 19.2. BERMS It is a narrow strip of land, left on either side of a channel at G.L., between upper edge of the cut and the inside toe of the bank. The width of the…

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Channel cross section
CHANNEL CROSS-SECTION

Channel cross section

CHANNEL CROSS-SECTION The channel sections for an irrigation canal may be of the following four types. 1. Canal in cutting. 2. Canal in filling. 3. Canal in heavy filling. 4. Canal partly in cutting and filling. 1. Canal in Cutting. This canal does not require any bank as F.S.L. lies below the G.L. If F.S.L. is just at the G.L., small banks may have to be provided. In this section F.S.L. of canal lies just at G.L. or slightly below it. See Fig. 19.2 (a).   2. Canal in Filling. In such a section the bed level of the canal lies at the G.L. Section whose bed level is slightly above the G.L. also comes under this category. See Fig. 19.3 (a). 3. Canal in Heavy Filling. In this section, the bed level of the canal lies substantially above the G.L. Such a section should as far as possible be…

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Bed width and depth relationship

BED WIDTH AND DEPTH RELATIONSHIP Kennedy’s theory does not give any importance to B/D ratio. The very large number of sections with varying B/D ratio and all satisfying the C.V.R. are possible by this theory. But all these sections cannot be equally satisfactory. This drawback of Kennedy’s theory was made good to some extent by Mr. Woods, who gave B/D ratio table for various discharges. In Uttar Pradesh (U.P.) bed width, and depth are related by following equation. (i) For discharge of the channel up to 15 cumec \[D= \frac{1}{2}\sqrt{B}\] (ii) For discharge of 15 cumecs and above depths for various discharges should be as follows.

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Design of the canal

DESIGN OF THE CANAL In the design of a canal one has to find out bed width (B) depth (D), longitudinal slope (S), and velocity of flow (V). If discharge (Q) and silt factor (f) are given, the method of finding. This aspect limits only finding the sectional dimensions of the canal. But how to compute the discharge ata particular reach is an important aspect of canal design. Discharge in the off-taking canal does not remain constant throughout the length. Outlets fixed on the canal at regular intervals draw discharge from the canal and supply it to the fields for irrigation. Evaporation and percolation losses also go on increasing with length of the canal. Hence because of discharge withdrawn by outlets, and also continuous evaporation and seepage losses. The remaining discharge in the canal goes on decreasing as canal flows towards the tail. As discharge at various points on the…

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Design of Canals

LONGITUDINAL SECTION OF A CANAL The points which should be consider in fixing alignment of the canal. Actually, the whole of the area where irrigation is propose, is surveyed, and contour plans prepared. The other features of the area are also marked over the plans. The contour plan on which other features of the area are also marked in know shajra sheet. The alignment of the main canal is fixed on the main ridge. The area proposed to be irrigate so that irrigation is possible on both sides of the canal. Branch and distributory channels are aligned along the main ridges of the area allotted for their command. In this way, the whole of the area to be irrigated is divide into several parts and each part is command by a branch, distributory or minor depending upon the extent of the area. The area under. The command of each distributory…

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Tractive force theory

TRACTIVE FORCE THEORY In the study of mechanics of sediment transport, the soil particles are always considered as incoherent. Most of the river beds are made up of sand and gravel and as such assumption in regard to soil being incoherent is correct. The basic mechanism that controls the sediment transport, is the drag force exerted by water in the direction of flow on the channel bed. This drag force is also known as tractive force or shear force. This force is nothing but a pull of water on the wetted area of the channel. Consider a channel of length L and cross-sectional area A. The volume of water in this length of channel would be A × L. If w is the unit weight of water, then weight of water stored in this length (L) of the channel will be wAL. Weight of water acting in vertical direction =…

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Practical aspects of bed formation

PRACTICAL ASPECTS OF BED FORMATION The bed of the channel in which water is flowing, may adopt various shapes depending upon the velocity of flow. At very low velocity of flow, bed of the channel does not move at all. When velocity is slowly increased a stage is reached when the sediment load is just at the point of motion. This stage is known as ‘‘threshold of motion”. If bed is made of fine sand, having particles of less than 2 mm diameter, saw tooth type ripples develop in the bed on slightly further increasing the velocity. This phenomenon can be easily seen in sand at beaches. At still larger velocity, dunes with ripples appear at the bed, which on the further increase on the velocity take the shape of rounded dunes. When velocity is still further increased, the dunes are eliminated and a flat surface becomes available. If velocity…

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Sediment Transport and Sediment load

SEDIMENT TRANSPORT A flowing water in a channel, always tries to scour its surface. Silt, gravel or even larger boulders are first detached from its bed or banks, and then swept D/S by moving water. The phenomenon of detaching and then sweeping The silt or gravel from bed or banks is know is sediment transport. The phenomenon is of great economic importance. Following are some of the important works where sediment transport has its effects. The design and execution of flood control schemes is chiefly governed by the peak flood level. Which in turn depends upon the scour and deposition of sediment. Silting of channels and reservoirs also depends upon the sediment transport. Sediment deposited in rivers and harbours requires costly dredging. SEDIMENT LOAD The quantity of solids (silt) entering the channel, is know sediment load. It is a single important factor which controls the shape and cross-section of the…

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Bandhara irrigation

BANDHARA IRRIGATION

  • It is a special type of irrigation practiced in some parts of Maharashtra.
  • It is essentially a minor irrigation scheme consisting of diversion weir walls across small streams.
  • Small canals taking-off from the U/S of the weirs and commanding small tracks of lands.
  • The bandhara irrigation scheme is very economical and by constructing a number of such structures in series across minor streams.
  • The irrigation facilities can be economically extended to large areas.
BANDHARA IRRIGATION
BANDHARA IRRIGATION

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