In this article we will discuss about:- 1. Introduction to Inundation Canals 2. Characteristics of Inundation Canals 3. Choosing Off Taking Points 4. Design Consideration 5. Advantages 6. Disadvantages 7. Maintenance.
Contents:
- Introduction to Inundation Canals
- Characteristics of Inundation Canals
- Choosing Off Taking Points for Inundation Canals
- Design Consideration of Inundation Canals
- Advantages of Inundation Canals
- Disadvantages of Inundation Canals
- Maintenance of Inundation Canals
1. Introduction to Inundation Canals:
Inundation canals are made under the following circumstances:
1. If river water level during floods remains high for considerable length of time.
2. If sufficient flood water reaches the river in March or April, the flood water may be used to submerge the land, so that Kharif crop could be sown.
3. If flood water remains available say up to late September, the Kharif crops can be irrigated upto this time and, even, areas may be submerged to sow early Rabi crops in October.
4. Areas to be irrigated when in the vicinity of the river banks the inundation canals may serve the purpose of irrigation.
5. When soil has good stabilizing power so that bed and banks of the canal remain stable.
Inundation canals are more or less similar to the permanent canals. The major difference is that, in the case of permanent canals, permanent masonry or concrete works like weirs, barrage, head regulator, fall, cross regulator, are constructed to regulate the supplies, but in case of inundation canals all these works are not there.
In order to take water into the canal, river bank is cut. The rising flood water in the river enters the inundation canal through the cut, made in the bank. Sometimes in worst floods, the cut made in the river bank is eroded and develops into a large deep cut. This may cause flooding of the adjoining areas as capacity of the canal may not be that much to deal with the situation. To avoid such possibilities of flooding, a crude type of head regulator or weir may be made at the off take point.
2. Characteristics of Inundation Canals:
Inundation canals are mostly used in deltic and alluvial regions of the river, as river course here is generally at a higher level and course is maintained between dykes or embankments.
The section of the canal is not regular. The banks are not very strong and may breach if not properly looked after. These canals have longitudinal, slope varying from 25 cm to as much as 1 m per kilometre length. Depth of the canal may be 1.5 m to 3.5 m. Like regular canal system inundation canal system may also have main canal and then smaller canals.
The bed level of these canals should be kept at the most at the level of minimum water level in the river. Lower the bed level of the canal more will be the time for which water will remain available. Inundation canal may be provided with two or three heads. It is done that if one head is washed away water may be taken from the other head.
The head reach of the canal may extend to several kilometres. At the end of the head reach, a flood regulator is constructed. An escape should be provided near the regulator, so that if more water gets entered into the canal, it may be taken out and re-discharged into the river D/S. Lot of silt is deposited in the canals and as such silt clearance should be done after every flood.
3. Choosing Off Taking Points for Inundation Canals:
Following point should be considered while choosing the off-taking point for an inundation canal:
1. The width of the river should be normal.
2. The river should be straight and stabilized.
3. If off-take point is to be on curve it should be on the outer bank of the river.
4. Off-take point should be as near the area to be irrigated as possible.
5. If one off-take point has been rendered unservicable, there should be no difficulty in locating the second off-take point.
6. The fluctuation in the level of river water should be minimum.
7. If a river bye-pass is available, off-take point should be located on the bye- pass. This measure will cause less silting of the canal.
4. Design Consideration of Inundation Canals:
1. The head regulator should be installed a few kilometres D/S of the off-take point. This measure would eliminate the risk of head regulator being washed away.
2. As escape should be located D/S of the head regulator and escape channel should be joined to the river D/S. If more water enters the canal at off-take point it can be taken out through the escape.
3. In order to reduce the chances of silting of the canal, the flood regulator should be provided with vertical lift gates which could be lifted in stages.
4. The full supply level of the canal should be fixed at a level at which the river water is more or less steady for a period of 40 — 50 days during which the canal can be run full to irrigate the lands. This steady level of water in the river is called fair irrigation level.
5. The bed of the canal should be as low as possible. Low bed level of the canal can draw some river water even when the river is in lower stage.
6. The section of the canal should be kept liberal to carry the entire required discharge as quickly as possible within the limited time factor.
7. The longitudinal slope should be such that scouring velocities are not developed longitudinal. Slope of the canal may be 0.25 m per kilometre length. The slope of the canal depends upon the general slope of the area also.
8. The flood water is heavily charged with silt. So these canals are more likely to be silted. Silt clearance must be done after every season.
9. The canals may be aligned with a number of bends so as to trap the silt at intervals.
5. Advantages of Inundation Canals:
Following are the advantages of foundation canals:
1. It is cheap, as no head work and other works have to be constructed.
2. Water being rich in silt, has good manurial qualities.
3. The area is less liable to be water logged.
6. Disadvantages of Inundation Canals:
Disadvantages of foundation canals are as following:
1. As there is no head works structure the head of the canal is liable to be washed away during floods.
2. Duty of water is very low.
3. Since availability of water is dependent upon the floods, the scarcity of water is always felt. Irrigation water may be badly required and the flood water in the river may not be available.
4. Due to lack of assured supply, the farmers take little interest in their work.
5. Canals will have to be frequently cleared from silt.
6. The bigger size of the canal is generally adopted. This is done to get as much of flood water as possible within limited time.
7. The alignment of the canal is not very precise. It may be silting at some section, and scouring at other section.
7. Maintenance of Inundation Canals:
Inundation canals always suffer from the difficulty of silting, scouring and scarcity.
In order to maintain these canals in fine shape, following provisions should be necessarily made:
1. Besides the main off-take point, subsidiary off-take points should be constructed. In case, main off-take point is closed somehow, water could be admitted in the canal from subsidiary off-take points.
2. A feeder canal should be constructed linking several foundation canals, taking-off from the same river. This measure avoids the necessity of constructing several subsidiary heads. By this method good head of water can be availed for considerable length of time.
3. Provision of a pucca head regulator, attached with an escape, a few km D/S of the off-take point, also enables exercise good control on the inundation canal.