Large irrigation projects have been constructed all over the world for the purpose of irrigating agricultural lands through gravity flow systems. These projects usually consist of one or more storage reservoirs, diversion structures, canals and related structures, outlets and finally drainage facilities.
The reservoirs constructed could also serve the purpose of flood control and hydroelectric power generation in which case they are referred to as multipurpose projects. The principles of the design of various structures are outlined in literature on hydraulic structures.
Structures in an Irrigation System:
The different structures in an irrigation system may be listed as follows:
1. Main Reservoir:
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(i) Masonry or concrete dams further classified as gravity dams or arch dams, or
(ii) Earth dams, or
(iii) Rockfill dams.
2. Diversion Head Works:
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(i) Weirs, and
(ii) Barrages.
3. Canals:
(i) Main canals,
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(ii) Secondary canals,
(iii) Tertiary canals or watercourses.
4. Canal Structures:
(i) Canal falls,
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(ii) Canal regulators,
(iii) Canal escapes, and
(iv) Metering flumes.
5. Cross-Drainage Works:
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(i) Aqueduct and syphon aqueducts,
(ii) Super passages,
(iii) Level crossings, and
(iv) Inlets and outlets.
6. Canal Outlets.
Main System:
The main reservoir stores the water collected from the catchment area and releases through the spillway or the power house. The structures which are constructed at the head of the canal, in order to divert the river water towards the canal are known as diversion head-works.
Weirs and barrages are used for this purpose. If a major part or the entire ponding water is achieved by a raised crest and smaller part by shutters or gates then the structure is referred as a weir and if a major part of the ponding is by gates the structure is referred to as a barrage also referred to sometimes as a river regulator.
The canal system consists of main canals, secondary canals and tertiary canals. The terms branch canals, distributaries, minors and watercourses are also used. The tertiary canals or watercourses deliver water to individual farmers. As the canals pass through varied topographical situations, different types of structures are needed in the canal system.
Whenever the natural ground slope is steeper than the designed bed slope of the channel, the difference is adjusted by constructing vertical “falls” or “drops” in the canal bed at suitable intervals. Regulators in the canal control the flow of water into the branch canals. A head regulator at the head of the off-taking channel controls the flow of water.
A cross regulator may be required in the main channel downstream of the off-taking channel in order to provide the necessary head of water to ensure flow into the off-taking channel. A canal escape is a side channel constructed to remove surplus water from an irrigation channel into a natural drain. The surplus water may result due to excess rainfall or closure of the canal outlets.
Cross drainage works are structures constructed at the crossing of a canal and a natural drain in order to dispose the drainage water without affecting the canal supplies. This is achieved in different ways depending on the topographical situation of the irrigation canal and the drainage channel. The irrigation canal can pass over the drainage channel (aqueduct or syphon aqueduct) or the drainage channel can pass over the irrigation channel (super passage).
Level crossing is a type of structural arrangement wherein the irrigation water and drainage water are allowed to mix. This is done when an irrigation canal and drainage channel (like a stream) approach each other practically at the same level.
Similarly, inlets and outlets are provided depending on the situation, to take in drainage water into the irrigation channel and letting it out when in surplus of the channel capacity. Details about the structures used in surface irrigation systems are given in Ankum (1991) and Garg (1987).
Canal Outlets:
A canal outlet is a structure built at the head of the tertiary unit to deliver water from the canal to the field watercourses. Water is taken from the field watercourses for irrigating individual fields. In many large irrigation systems, the system management is responsible for delivering water upto the canal outlet and it is usually the responsibility of the farmers beyond this point.
As water is to be delivered through a network of canals and related structures to these outlets, delivery of the right amount of water in right time is a difficult job. The outlets also have to function properly for delivering the designed amount of water.
There are several types of outlets used in different countries. Only a few of them are described here-
These outlets are referred to as modules in India and Pakistan and are classified as follows:
1. Non-modular modules – In these structures, the discharge depends on the difference of head between the distributary and the watercourse. The discharge of these outlets is variable.
2. Semi-modules or flexible modules – In these structures, the discharge depends only on the upstream water level and independent of the water level in the watercourse.
3. Rigid modules – These are structures in which the discharge is constant and fixed within limits, irrespective of the fluctuations of the water levels of either the distributary or of the water-course or both.
These could be in the form of a simple rectangular opening or a submerged pipe outlet. The submerged pipe outlets are simple and are extensively used in India. Sometimes, shutter is provided on the upstream side to control the flow. The pipe could be horizontal or inclined either upwards or downwards.
The discharge through the pipe can be calculated using the following equation –
The discharge can be more precisely calculated if the pipe is sloping upwards or downward. The discharge is influenced by HL and hence cannot be maintained constant under different operating conditions.
Semi-Modules or Flexible Outlet:
The pipe outlet can serve as a semi-module where it discharges freely into the atmosphere. The discharge can be calculated using Eq. 11.1, but HL will be the head upstream measured from the centre of the pipe to the supply level of the distributary. Free flowing conditions may be difficult to obtain as this will require a higher level of the distributary compared to the fields to be irrigated.
The open flume outlet is a weir type outlet with a contracted throat and an expanding flume on the downstream side. The constriction in the flume results in supercritical velocity in the throat and consequently a hydraulic jump in the expanding part. The formation of the hydraulic jump makes the discharge independent of the water level in the watercourse.
Adjustable Orifice Semi-Module (A.O.S.M.) consists of an orifice with a gradually expanding flume on the downstream side. The flow through the orifice is supercritical resulting in a hydraulic jump downstream. This type of outlet is also known as Crump’s adjustable proportionate module.
Constant Head Orifice Outlet (CHO):
This type of outlet is used in Thailand and Malaysia as an outlet from canals to tertiary units. The submerged orifice principle is used in the design of this outlet. This structure has two sluice gates with a stilling basin in between the gates.
The upstream gate is set to give the required discharge with a standard differential head (usually 5 to 6 cm) across it. This head drop is obtained by adjusting the second gate. Depending on the size of the orifice and differential head, standard tables are prepared for estimating the discharge.
Romijn Weir:
The Romijn weir has been developed in Indonesia for use in relatively flat regions and for varied discharge supply to the tertiary units. The paddy crop is irrigated on a simidemand supply schedule.
The water demand varies through the irrigation scheme because of different growing stages. Moreover at same time, water shortages occur during a part of the day, while higher discharges may occur during part of the night.
The Romijn weir is a combination of a broadcrested weir and a gate. The broadcrested weir is mounted in a steel guide frame, and can be moved up and downwards to decrease or increase the discharge. Another (bottom) gate is often added in the steel guide frame for flushing sediment that has been deposited in front of the gate.
The standard widths of the Romijn weir may be selected at 0.50, 0.75, 1.00 and 1.25 m, for maximum discharges of 0.30, 0.45, 0.60 and 0.75 m3/s, respectively, at maximum heads of 0.30 m over the weir-table.
The Romijn weir has good sediment and floating debris capacity, is easy in operation and reading. It has a double function of both for regulation and for discharge measurement. However, the structure with gate is quite expensive and is not widely understood in many countries.
The hydraulic design equation of a Romijn weir equals the equation of the broadcrested weir with a rectangular control section –
The downstream water level may not rise too high to allow a modular flow and the non-submerged (modular) condition is satisfied for –
H (downstream) < 0.30 H (upstream).
Gauges:
The discharge measurement and the weir-setting is simple, although it needs three gauges viz.:
i. A water level gauge in the canal, the ‘counter-gauge’, which shows a reading in centimeters;
ii. Another fixed gauge on the frame, the ‘centimeter-gauge’, which is identical to the above counter-gauge;
iii. A gauge that moves with the weir, the ‘litre-gauge’, with a logarithmic scale of the “H” values according to the weir formula Q = 1.7 b H1.5.
For instance, a water level in a secondary canal is read at 63 cm at the counter-gauge and 124 1/s should be diverted into a tertiary canal. The movable weir is moved such that the ‘124 1/s’ reading at the litre-gauge corresponds with the ’63 cm’ reading at the centimeter-gauge.
Water Delivery Methods in Large Irrigation Systems:
In any irrigation system, water could be delivered to the fields owned by individual farmers using three principal methods viz.:
1. Demand,
2. Continuous flow, and
3. Rotational method.
1. Demand Method:
In the demand method water is delivered to individual farms or crops as per the requirement. This is possible in relatively smaller systems. In large systems because of the distances and the large number of farmers, it is practically impossible to implement this procedure.
2. Continuous Flow Method:
In the continuous flow system, water flows all the time in the mains, secondary and tertiary canals. Water can be taken by individual farmers at the time of need. This system is possible only when water is abundant and not limiting. By far in all large irrigation systems, rotational method is practiced where water is delivered to the farmers by a system of rotation predecided in the project.
3. Rotational Method:
Rotational system can be for main canals, laterals, tertiary units or within the tertiary units. Rotations within the tertiary units and among the tertiary units are the most commonly followed in the major irrigation systems in the Asian region. Different possible patterns of water deliveries to the tertiary units are shown in Fig. 11.6. An example of water allocation time within a tertiary unit.
Warabandi System of Water Allocation:
This method of water allocation has originated in the Indus system in India and Pakistan. “Warabandi” means a weekly rotation, each farmer getting water on a fixed day of the week. Each farmer obtains an equal share of available water volume per unit of area based on an allocated time to his field.
The time required for water to travel in field channels from one irrigator to the other and the seepage losses in the field channels are duly adjusted but the irrigation efficiencies in the farmers’ fields are not considered.
The warabandi is just an organised way of water distribution to a large number of cultivators in the irrigation system. The water deliveries may not be as per the crop water requirements.
There are several local variations of the warabandi system. The variations consist in calculating the time available for irrigation and allocation to farmers groups instead of a single farmer.
Time available in some areas is considered 24 hours a day meaning thereby night time irrigation has to be done, while in some areas 14 hours of irrigation during a day is considered. In some projects instead of calculating the time for each farmer, allocation time for a group of farmers is calculated and the group is allowed to adjust the time for each among themselves.
In spite of these variations, calculations are made broadly as indicated below. It is considered that certain time is required for filling the channel before the farmer can irrigate and some farmers could benefit as a result of the water in the irrigation channel draining into their fields. The time allotted to the ith field in hours (TTi) is given by-
Another approach is to estimate the daily water requirements and give an allowance for the distribution losses and calculate the time of irrigation for each block.
Example:
An area of 61 ha is to be irrigated from an outlet with a capacity of 60 1/s. It is divided into 5 rotational blocks, each having a particular value for distribution losses as given in the table. A 7-day rotation is to be followed.
The calculations are as follows:
If the water requirements of the individual units change, the time of allocation will change as illustrated in the following table:
By simple warabandi calculations, the time allocated to each block would be as follows:
In the example – the total time of irrigation is less than 168 hours in a week. If water is allowed to flow all the time and the warabandi timings are strictly followed, it will result in over irrigation.
The following points need to be noted:
1. For each tertiary unit, the cropping patterns, areas and water requirements need to be examined in relation to the water deliveries.
2. As far as possible, the crop water requirements and water deliveries need to be matched.
3. More areas can be brought under irrigation by selecting proper crop mixtures and reducing the conveyance losses.