In this article we will discuss about how to maintain canals.
Introduction to Maintenance of Canals:
The irrigation projects have to be continuously maintained, so that they may perform their functions efficiently. If they are not maintained properly, the canals of the systems may get silted up. Weed growth may also occur. Capacity of the canal is reduced both by silting and weed growth and such a canal will not draw its full discharge and as such will not be in position to feed all its outlets.
The canal may be scouring at some sections and if preventive measures are not taken in time, the canal may breach and cause lot of wastage of irrigation water. Hence canal system cannot work efficiently unless it is maintained properly.
Bed Bars:
Bed bars should be constructed at suitable intervals at the bed of the channels. The purpose of bed bars is to indicate the correct theoretical levels of the bed of the channel. They are constructed at an interval of say 200 m in case of small channels and 500 m in case of main and branch canals.
In case of small channels, the whole section of the channel is lined at intervals but in case of big canals, concrete blocks are used at mid-width of the canal bed. Bed bars indicate both silting as well as scouring of the canal. If channel section is scouring the bed bars will get exposed. The exposure of the bed bars indicates the extent of scouring. If the channel is silting the bed bars will get embedded in the silt. Height of silt above bed bars indicates the extent of silting.
When silt clearance is done, bed bars act as reference levels. The bed bar blocks should be sufficiently embedded in bed so that they are not unearthed by scouring. When complete profile is used as bed bar, the slope of the channel is also indicated by the slope of the profile. Bed bar profile should be properly anchored to the soil with the help of cut-offs.
Maintenance of Irrigation Channels:
For the efficient working of an irrigation system, all the canals and connected works should be properly maintained.
Following difficulties may hamper proper functioning of the irrigation canals:
1. Silting of Canals:
The capacity of the canal is reduced if its bed and banks are silted up. The best method of preventing the silting, is to adopt the measures that would not allow excess silt to enter into the canal. Silt excluders, silt ejectors, raised crest, King’s vanes etc. are the measures which can be adopted in this respect.
But it is seen by experience that in spite of all these measures, some silting definitely takes place. Silting may not be taking place when canal is run at full capacity but will silt when it is run at less than full capacity.
The following measures may be adopted to remove the deposited silt:
(i) By increasing the velocity of flow in the canal- For this either additional water will have to be admitted or canal escapes may be opened from time to time to develop increased velocity and scour out the deposited silt.
(ii) Loaded boats should be put in canal- It will decrease the area of cross- section of the canal and consequently velocity of flow will be increased and deposited silt scoured off.
(iii) Dredging- Dredger may be used to remove the deposited silt from the canal. Though it is a costly method but it can be employed where other methods fail to operate.
(iv) Silt is removed by manual labour- Silt clearance by manual labour is done during annual closer periods of the canal. If too much silting has taken place the canal may be closed for silt clearance.
(v) Grass which has grown into the canal at banks, should be cut from time to time.
(vi) The Bed and banks may be raked by iron bar rakers or thorny bushes. The raked silt is carried away by flowing water.
2. Breaches in the Canal:
The canals may breach in reaches of heavy filling. Breach in heavy filling reach is very serious, as breach develops into a very large opening at such places. At filling reaches, very large head of water is available and as soon as a certain breach occurs, it develops into a big gap, and lot of bank length is scoured due to water being passed through the breach with very large velocity.
Canal breaches may occur due to following reasons:
(i) Rats may dig through holes in the canal banks and cause canal breach.
(ii) The seepage line at the outer toe of the bank may get exposed. This exposure will cause escape of seepage flow ultimately leading to canal breach.
(iii) If canal bank is weak or is defectively constructed, the breach may take place.
(iv) Piping at D/S toe may cause settlement of canal bank and water may flow over the bank of the canal.
(v) Canal may be breached by deliberate cut. This mischief is generally done in small distributaries in which discharge is not much. Farmers whose lands are in the vicinity of the canal, are generally responsible for such deliberate cuts. This way they soak their lands and sow crops. This tendency of farmers is predominantly noticed when canals run very irregularly.
(vi) If canal happens to pass through or by the side of the village or town, the canal banks are generally weakened by continuous use of the canal by the people. The canal may breach if banks are not continuously looked after.
(vii) If flow in the canal is somehow obstructed, the water level in the U/S of the canal may over top the banks, and breach may occur.
(viii) Large deep erosion pits are developed at the canal banks during rains. If they are not immediately filled with soil, breach may occur.
Breaching of canals can be prevented by strengthening the canal banks.
Following are the methods of strengthening the banks of canals:
(a) External Silting:
In this case additional banks are constructed parallel to the main banks, by leaving some open space between them. The intervening space is divided into silting compartments by constructing cross bunds. The canal water is allowed to enter these compartments from U/S side, held there for some time and then discharged back to the canal.
The silt in the canal water settles in the compartment and in due course of time the compartments get silted up upto F.S.I. of the canal and thus a wide bank is developed which cannot breach on its own. Additional banks are constructed outside of the main banks.
(b) Internal Silting System:
In this case the original banks are constructed slightly away from their intended position. Thus section of the canal becomes larger than the designed section and consequently velocity of flow is reduced from the designed velocity. Because of reduction in velocity, the banks of the canal get silted up.
This process of silting continues till canal acquires the designed section. Thus by silting on the internal side of the banks, the banks become thick and thus strengthened. Sometimes, to accelerate the process of silting, low submerged spurs are constructed projecting towards the canal from banks.
(c) Internal Berms:
Normally canal sections are excavated with 1:1 side slope. In due course of time, sides assume 1 : 1 slope due to silting. Such additional thickness of the berm is known as internal silted berms. This process also strengthens the canal banks.
(d) Back Berms:
If saturation line or seepage line gets exposed on the outer slope of the bank, additional soil may be deposited in form of a counter berm. Counter berms are known as back berms.
3. Weed Growth in the Canals:
Aquatic weed growth hinders the flow in the canals. The result is that adequate supplies of irrigation water cannot be delivered to the consumers. The growth of weed is very rapid. The problem of weed growth is more accuse in south India. Weeds may be annual, biennial, and perennial.
Annual weeds live only for a year and then die automatically. Biennial weeds live for two years, but perennial weeds live for longer periods. Annual and Biennial weeds propagate by seeds alone and once the top is chopped, the roots have to power to regenerate. Perennial weeds are the most troublesome weeds. They can grow on canal bed, water surface and water marks.
The methods of weed control can be summarized as follows:
1. Run the canals with relatively higher velocities of flow. The velocity of flow should never be allowed to fall down, 0.6 m/sec.
2. The canal should be deep and run with full supply discharge. This will exclude most of the sun rays, and weed growth will be prevented.
3. Application of diesel oil, dilute solution of sodium chlorate, Ammate and Benochor also destroy the vegetation.
4. During closure periods, the canals should be dewatered and dried. The dewatering of the canal can be done by providing drainage outlets.
5. The weed destructive insects if introduced in the canals will kill certain varieties of weeds. Cochined inset is one such insect.
6. The weed growth is cleared by manual labour during closure periods of canals. If weed growth is very heavy, continuous mechanical methods may be adopted to clear it.
Closing the Canal Breach:
Canal breach can be closed in following steps:
1. If the size of the channel is small, the breach can be closed without closing the discharge in the channel. For small breaches, the discharge escaping from the breach is temporarily stopped by using wooden planks, and then breach is closed by taking soil either from outer face of the bank or from the top of the bank.
2. In case, size of the channel is moderate, the breach cannot be plugged without diverting the discharge.
3. As soon as information of the breach is received, the canal escape, U/S of the breach if any, is opened immediately. At the same time the discharge in the canal from head regulator is also reduced. Both these measures will cause considerable reduction in the discharge passing through the breach.
4. When discharge passing through the breach has considerably reduced, the erosion of bank at breach would stop.
5. The loose and cracked soil of the breached bank is cut and solid bank is obtained at both the ends of the breach. Steps are cut in solid banks so that earth to be used for closing the gap develop good bond with old soil of the bank.
6. Cut the wooden ballies from nearby trees and drive them as piles on the inside of the canal bank. The piles are driven at 50 cm to 1 m interval, depending upon the size of the breach. The space between allies is filled with either tree bushes, planks or sand bags. The measure reduces the out flow through the breach considerably.
7. In the meantime, sufficient earth on both the sides of the breach is heaped. The earth may be taken from spoil banks, or borrow pits. Generally all the borrow pits are filled with water and bed of the canal heavily eroded and hence earth is not easily available. Earth is mostly taken from top of the banks and also from outer slope of the banks.
8. The closing of the gap should be started from both the sides by slipping earth from the heap in form of a ring bund. The last gap of about 1 m to 2 m width should be closed with a rush when enough earth has been collected on both the sides.
9. After having closed the gap by a ring bund which is on outer side of the bank, the open space between ring bund and pile line, is filled with earth in well compacted layers.
10. During filling, grass clods should not be allowed to be embedded in the earth otherwise water will again seep along clods and cause breach again.
11. When breach is closed to sufficient height, the water in the canal is again left. By the time water level reaches the F.S.L., the breach is sufficiently strengthened.
12. Even after strengthening the breach, a continuous watch is maintained for about 10 days so that water may not seep through any defect and cause breach again. During this period the gang keeping watch on the breach remains busy in further strengthening the breached section.
Maintenance of Service Roads:
The service road is an unmetalled road constructed at left bank of the canal. The surface of this road has to be continuously maintained as it is frequently used by irrigation department to carry out inspections. This road is maintained by gangs of labour employed to maintain the canal.
A particular length of the canal is entrusted to a gang of labour and all such gangs work under the guidance of a section officer. It is the duty of this gang, to maintain the canal and service road in proper order in the length allotted to it. Gang sprinkles water on the inspection road, fills pits developed by rains, removes grass, bushes and other unwanted vegetation from the road.
Duties of Maintenance Gang:
Certain length of canal is put in the charge of certain gang.
The duties of the gange, are the following:
(i) Gang daily takes a round of the length of canal in its charge and carries out all the functions that help in the efficient running of the canal.
(ii) Fills rain pits immediately after rains.
(iii) Strengthens the sections of the canal banks which has been rendered week due to certain reason.
(iv) Maintains service road in proper order by periodically leveling the road surface and sprinkling water.
(v) Removes unwanted vegetation.
(vi) Takes care that outlets are not tempered and siphons are not used by the farmers.
(vii) Arranges additional labour at the time of breaches.
(viii) Clears off an obstruction that might have developed in the fall or other regulation works, falling in its jurisdiction.
(ix) Daily report of the work is sent to the section officer incharge of the canal.
(x) Helps overseer in the leveling work, which is required for commanding the new areas or which may be required for fixing levels of water courses in villages.
Regulation of Canal System:
The main canal draws its full supply from the river. The main canal feeds the branches, distributaries and minors according to the demand of various off- taking channels and the total supplies available. The process of distribution of water into various sub-channels according to their demands and supplies available is termed as Regulation of canal system.
During keen demand, the main canal has to run with full supplies. If demand is less than the keen demand it may be run with part supplies. Main canal is generally not closed, as it commands a very large area and some demand, in different parts of the area, always remains.
When demand is limited to some particular distributary, it runs fully but others which do not require water are closed. The difficulty in distribution arises when demand is more and supply available is less than the demand. In such circumstances, either all the distributaries run with reduced discharge or distributaries run with full discharge, in rotation.
The later system is mostly adopted for regulation of supplies. In this method a group of channels run with full discharge at a time and remaining are kept closed. Then after some days the group of channels which was running is closed and closed group of channels run with full discharges. This process of regulation of supplies is known as rostering or rotation. Full control over regulation is exercised by the executive Engineer.