DROPS IN CANAL

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A drop has been defined to be a work designed to secure lowering of water surface in a channel and safe destruction of surplus energy so liberated. The velocity in channel is a function of slope and there is a limiting velocity which can be allowed in a channel depending upon the nature of soil through which it passes without causing it to scour its bed and sides. It follows therefore that there is a limiting slope which cannot be exceeded without causing any harm to the channel. The slope of the country which a channel has to irrigate is usually steeper than the slope required in the channel. To absorb their differential head, the introduction of drop at intervals becomes inevitable. The introduction of drop is required to protect from the ill effects such as erosion on the upstream, side and bed due to increased velocity because of the drawdown effect, to protect below the drop from impact force of falling water at the toe of the drop and to prevent the sides from erosion and bed from scouring due to the turbulent flow.

Structures – Inlet, Outlet and Bridge

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What is Inlet?
If a drain is crossing a canal alignment with its bed level just above the FSL of canal and the MFD of the drain is less than or equal to 10 percent of canal discharge, then an Inlet may be proposed by letting the discharge of drain into canal by accommodating that discharge in free board.
What is outlet?
If an inlet is proposed on canal and the discharge allowed from all previous inlets including this inlet exceeds 20 percent of canal discharge, then an outlet shall be proposed to let that additional discharge of canal into the natural drain.
What is Bridge?
When a canal alignment crosses a road alignment a bridge will be proposed to continue the road which will be interrupted due to excavation of canal. The road level may be modified suitably to accommodate free discharge of canal without any obstruction. The carriageway width of bridge varies depending on the traffic of road.

Economics of Detour Canal Alignments

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In major irrigation projects, the main canals are generally aligned as contour canals to give the benefits to the maximum extent possible. These contour canals have to necessary to cross number of ridges and valleys depending upon magnitude of the project. In general, it is noticed that canal alignments are finalized following the shorter routes available to minimize the ultimate length of canal. This would naturally result in the crossing of the canal over major ridges by means of deep cut flumes or tunnels and the negotiation of major valleys by means of level crossings, very high aqueducts or pressure aqueducts. Generally the authorities are hesitant to adopt detour alignments.

Types of Canals

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The canal system consists of the following
Main Canal
Main Canal takes off directly from the upstream side of weir head works or dam. Usually no direct cultivation is proposed. Most of the main canals are aligned as contour canals to derive benefit.
Branch Canal
All offtakes from main canal with head discharge of 14-15 cumecs and above are termed as branch canals.
Major Distributary
All offtakes from main canal or branch canal with head discharge from 0.028 to 15 cumecs are termed as major distributaries.
Minor Distributary
All offtakes taking off from a major distributary serving more than 40.47 hectares are termed as minor distributaries. They are named after a prominent place near about their tail ends.
Field Channel
All pipe offtakes serving less than 40.47 hectares of ayacut are calledfield channels and are denoted by numbering as left or right side pipes.

Super Passage, Under Tunnel, Canal Syphon, Aqueduct – STRUCTURES

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What is Super Passage?
When the canal is in full cutting and there is sufficient head way for the canal water a Super Passage may be proposed. That is if the Full supply level (F.S.L) of canal is below a depth of free board required for canal from bed level of drain crossing, at this point a super passage may be constructed to let the discharge of drain without causing any damage to the canal.
What is Aqueduct?
Aqueduct is a structure that may be proposed on canal that is running in full embankment and there is sufficient way to pass the discharge of drain. That is if the Maximum Flood Level (M.F.L) of drain is below a depth of free board required in drain from Bed Level Canal, at this point an aqueduct may be proposed to let the discharge of drain below canal without causing any damage to the canal.
What is Under Tunnel (Type-2 Aqueduct)?
When the canal is running in partial or full embankment and there is no sufficient head way to pass the discharge of drain below the canal then an Under Tunnel may be proposed. That is if the (MFL +Freeboard) of drain is above the bed level of canal, then a barrel will be provided below the canal bed level by depressing the bed level of the drain.
What is Canal Syphon?
When the canal runs partially in cutting, the canal syphon may be adopted. A barrel will be provided below the bed level of drain to pass the discharge of canal by depressing canal Full supply level (F.S.L) before the drain crossing and again lift it back after crossing the drain. Sufficient head may be provided to force the water from up stream of canal to down stream of canal. This is often provided for drain crossing that have huge discharge and an Under Tunnel may not be economic.

What is Irrigation?

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Irrigation is defined as the artificial application of water to arid land for growing crops. It is a profession as well as a science. A crop requires certain amount of water at certain fixed intervals throughout its period of growth. Irrigation is not required if this requirement is met with from sufficient rainfall as in England. In tropical countries like India the first two or three essential requirements of plant growth, that is, heat and light are available in abundance, but the third, that is, moisture needs to be supplemented frequently by artificial application of water. Thus, irrigation is supplementary to rainfall when it is either deficient or comes irregularly or at unreasonable times.