1. Silting of canal
2. Branching of canal due to weak banks.
3. Weed growth
4. Over flow of canal banks Silt Removal When the silt is deposited on the bed and sides the capacity of the canal reduces. It is better to exclude silt by providing silt excluder and ejector as explained. Curved wing-cum silt vanes and silt tunnels-cum curved wings can be fitted near the head of an off-taking canal for exclusion of silt. However, none of these methods can put a complete check over the entry of silt in the channel and hence silt is deposited in the channel bed even after the best care.
The following measures are adopted to remove the silt;
a) flushing: Flushing of the canal with clear water will lift up deposited silt. Absolutely clear water should be used for flushing but if this is not available then the water which contains minimum quantity of silt should be used for flushing. Flushing should generally be over done to cause some scour. This will create room for further silting and will thus reduce the frequency of flushing.
b) Silt scouring fleet The method consists of having three lower barges connected to the upper barge by a cable operated by winch. The lower barges have movable shutters. The silt is kept agitated by maneuvering the barge up and down. The method was used in Punjab but was unsuccessful.
c) Bundle of thorny bushes tied together and pressed down hi’ weight of stones are pulled inside the channel by animals. They are quite useful in dislodging the tin muddy silt.
d) Iron rakes are also dragged in the channel to dislodge silt.
e) Reduction of area of flow. Loaded boats are put across the section to reduce the area of flow and increase the velocity of flow.
f) Stirring of silt by water jet. A pump fitted with a pipe and nozzle is placed on a barge. The high velocity jet is directed to the bed to stir the silt and prevent silting.
g) Dredging. A dredger is very rarely employed for removing silt from canal as it is very costly method
h) Excavation: The silt deposited in a channel is cleared off by manual labour. The method is quite costly as it requires recurring expenditure. This method is generally adopted for silt clearance in distributaries and minors.. The silt must he deposited clear off the channel so that it does not find its way back to the channel. Strengthening of Canal Banks: To prevent breaching of the canal bank, it should be strengthened Properly so valuable loss of irrigation and property is prevented due to breaching a canal section There are two methods of strengthen of a canal bank:
a) External silting system.
b) Internal silting system.
c) Formation of berm by internal silting.
d) Formation of back berm. (a) External silting system In this method subsidiary banks arc constructed which run parallel to the main banks. Cross bunds are constructed within the subsidiary bank and main bank at a distance of 150 to 1500 m to form compartment of silting or silting tanks. Water is allowed to get into the compartment from upstream side and is held there for some time before discharging it back to the canal from outlet end. When the cross bunds are separated at a distance of 150 to 300 m, the capacity of the bank is small and only a portion of full supply discharge is taken into the compartment. It is then known as into out system. But when the length of, the compartment is large say 1200 to 1500 m, full discharge of the canal can be taken inside the compartment and it is then known as long reach system. The method is practised only when no water is required for irrigation downstream of the reach. (b) internal silting system In this system the canal banks are set back away from their original positions. The section of the canal provided is large than required and, therefore, its velocity is low. The section, therefore, gets silted up very quickly. To induce silling and accelerate the process, low submersible spurs are constructed. The silted berm is shown. Hanging groyne or suspended groyne is also very useful for inducing silting.
There are a variety of weeds growing on canal bed, water surface and water marks. They tend to thrive better in a range of 20° to 300 C. Weed growth is not possible in channels having high velocity of flow but when the channel has velocity less than 0.6 m per second weed growth is generally possible. The deposition of silt has no direct effect on weed growth, yet profuse weed growth is known to take place where silt is deposited. Light has a considerable effect on weed growth. Weed growth is accelerated in presence of light.
The weed growth can be checked by passing higher velocity than regime velocity in the channel. This will keep the silt in suspension and will make water turbid. Thus the light rays are cut off and silt is not deposited on channel bed.
Yet another way of weed control is rush rotation. In the process of rush rotation, the channel is run with full supply discharge for some time and then it is left completely dry for some time. This helps in excluding more light when higher depth is flowing, thus reducing the weed growth. During closare weed is unable to resist scorching rays of sun. Long duration closure has killing effect on the weed growth.
Weed removal may done by plucking them by hand and burning them when canal CANAL BREACHES: Canal breach or the tearing of the canal bank takes place when canal is in filling. During a canal breach a big gap forms on the side and the large canal discharge rushes out doing unlimited damage. Reasons for canal breaching 1. Breaching due to defective design and construction of the bank. The canal bank is not strong enough to resist water pressure and a breach occurs.
2. Breaching due to exposure of saturation gradient. The soil particles on the outer slope are dislodged and flown away along with the seeping water. A cavity is thus formed in the bank which ultimately develops into a breach.
3. Breaching due to rush of water through rat holes or Stories.
4. Piping near downstream toe may ultimately cause subsidence of canal banks.
5. A willful cut in the canal bank either for unauthorized irrigation or for allowing the flood water to get into canal to save an area from submergence. CANAL BREACHES: A breach can be repaired by the procedure described below.
1. Immediately after a breach occurs, the canal discharge should be diverted to a nearby escape channel on the upstream side or the canal should be closed from the head to stop the outflow completely. Many a times it is not possible to close the canal from head as the irrigation of the area in between may suffer.
2. Provide steps in the sides of the gap so that the earth to be deposited for closing the gap forms a good bond with old surface
3. Derive two lines of ballapiles in the gap in the, inner side of the bank and fill the space in between by planks and bushes and with sand bages on top.
4. Step 3 should cut down the outflow considerably and velocities will reduce, If a second line of defence is needed, the same may be provided by- two rows of sand bags with earth in between behind the pile line.
5. Collect a sufficient amount of earth on both sides of the gap. The earth collected should either be from spoil bank nearby or from borrow pits. This earth material is then dumped up instantaneously from both sides of the gap to form a ring bund on outer side.
6. The opening in between the pile line and the gap is then filled with earth in layers, which should be properly compacted at optimum moisture content and the new bank is prepared.
7. The bank so formed is inspected carefully for few days to watch its proper functioning. Terms relating to canal section: 1. Berm. The narrow strip of land at the ground level between the inner toe of the bank and top edge of cutting is known as berm. The berms provide additional strength to banks and thus make it safe against breaches. A gap created in the bank of a canal is called a breach. The berms when fully formed have the following functions:
a) They provide a bigger water way.
b) They protect the bank from erosion because of wave action.
c) They reduce the absorption losses and prevent leaks.
d) They provide a scope for future widening of the canal.
e) They bring the saturation line within the embankment
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