Irrigation Engineering

Unlined Canals

mpchekuri — Wed, 27 Jan 2010 17:31:49 +0000

Irrigation is an ancient art. Irrigation has been practised from time immemorial and so has been the construction of irrigation canals. There are constructions dating back to many centuries in India, Egypt and other countries. In earlier periods irrigation was restricted to comparatively smaller areas close to the rivers by means of inundation canals. Later with the taking up of large irrigation schemes, perennial irrigation came into being and with that the subject of design of irrigation canals, economical in first cost and maintenance with minimum liability to silting and scouring engaged the attention of engineers. However, formulation of laws governing the flow of water and sediment transport has been done in recent times.
It is only in the nineteenth century that canals have been designed on a more or less scientific basis. The formula of Chezy with coefficients derived from Kutter’s and Bazin’s formulae is currently in use in India and many other counthes. Manning’s formula is equally widely used. Kennedy’s formulae and the rational empirical Lacey’s regime theory, although originated in India, are used in many countries too. The tractive force theory, though not developed for general and practical adoption on a wider scale, is currently mainly used as a check over the parameters calculated by one or other empirical formula. Thus both empirical and analytical concepts are increasingly being used.

Drip irrigation

mpchekuri — Wed, 27 Jan 2010 17:28:17 +0000

Drip irrigation is adaptable to (i) all vegetables and a vast assortment of crops, except paddy which requires standing water, (ii) all climatic variations, and (iii) highly permeable soils, like sandy soils, sandy loam, gravelly soils and less permeable soils like clay, clay loam, silty clay or shallow soils underlain with impermeable strata. However, it is most suited to coarse sandy formation. Under the adverse condition of high salinity of both soil and water, high temperatures and low relative humidity, crops of record yields and superb quality are obtained.
A unique feature of drip irrigation is its excellent adaptability to saline water. Since the frequency of irrigation is quite high, the plant base always remains wet which keeps the salt concentration in the plant root zone below the critical.

Head Works

mpchekuri — Wed, 27 Jan 2010 17:06:25 +0000

Canals draw supplies from rivers. Rivers do not have constant flow; the maximum being during monsoon months and minimum in winter months but the canals require continuous and dependable supplies for irrigating the command areas. For this purpose, perennial canals are essentially provided with permanent head known as headworks. Headworks comprise a weir or a barrage on the river and head regulators on the offtaking canals, the former for local pondage and raisitig of water level for diversion of supplies into the offtaking canals as well as passing regulated supplies in the river downstream while the latter is meant to draw the water supply into the canals. A weir is defined as a barrier constructed across a river to raise the water level in it course, with or without low shutters.
The functions of a barrage or a weir are (i) Divert water from river into canal, (ii) Effect pondage and raise water level in the river in the order to feed the offtaking channel during low supplies, (iii) Effect control over fluctuations in water level of the river, i.e., impart permanency to the arrangement unaffected, within working limits, by vagaries of the river, (iv) Command the irrigable area by gravity flow, (v) Regulate the supplies into the offtaking canal and the river downstream, (vi) Pass extra supply into the canal when required and prevent flooding the canal during high floods in the river, and (vii) Control silt entry into the offtaking canal.

Types of Canals

mpchekuri — Sat, 06 Jun 2009 10:48:09 +0000

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.

Irrigation Engineering Basics

mpchekuri — Sat, 06 Jun 2009 06:49:36 +0000

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.

DROPS ON CANAL

mpchekuri — Fri, 05 Jun 2009 06:53:43 +0000

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.

Economics of Detour Canal Alignments

mpchekuri — Thu, 04 Jun 2009 07:43:45 +0000

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.

Structures – Inlet, Outlet and Bridge

mpchekuri — Thu, 04 Jun 2009 07:38:32 +0000

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.

Structures – Super Passage, Under Tunnel, Canal Syphon, Aqueduct.

mpchekuri — Thu, 04 Jun 2009 07:34:36 +0000

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.