Canal breaches are the gaps created in the canal banks due to breaking up of the banks. Various reasons which lead to a canal breach are: 1. Breach Due to Faulty Design and Construction of Banks 2. Breach Due to Leakage or Piping 3. Breach Due to Ignorance of Cultivators.
Reason # 1. Breach Due to Faulty Design and Construction of Banks:
If the designed banks are not sufficiently high then the water may overtop the banks and a breach may occur. If the banks are constructed with poor material, the bank may not hold water without being damaged.
Reason # 2. Breach Due to Leakage or Piping:
The canal banks many times start leaking, through the holes created by rats or insects. The holes are smaller in size and do not pose a serious problem if timely inspection and correction is done. The natural correction may be accomplished by providing a sand core in the bank which settles and fills the holes. Of course, it is true that, then the banks may settle but it does not cause a breach. This method effectively closes the rat holes.
Sometimes water level in the canal raises above the designed F.S.L. for example when excessive rainfall takes place or when canal is run with the discharge which is more than F.S.D. In such cases hydraulic gradient line also rises. Then if the cover provided is less the new hydraulic gradient line may pass out of the banks (Fig. 6.28).
Under such situations the soil particles are dislodged on the outer slopes of the banks. Progressively a cavity is formed in the canal banks. It weakens the banks. The banks may give way and breach is formed and water rushes out of the canal.
Reason # 3. Breach Due to Ignorance of Cultivators:
Sometimes, when the area is flooded due to excessive rainfall, the cultivators cut the canal bank with a view to providing outlet to the flood water. But it actually has adverse effect. The whole canal discharge enters the low lying land and creates a serious situation.
Sometimes cultivators give cuts in the banks for obtaining additional unlawful supply for irrigating their lands. These small cuts take a shape of big breaches in a very short time. It consequently damages the land and crops.
When the canal is smaller in size (say minor or small distributary) the breach may be closed by dumping huge quantity of earth. The earth is dumped instantaneously from left sides of the gap. The soil for closing the breach may be obtained either from the spoil banks or outer faces of the banks or berms or from borrow pits. It is essential to store huge quantity of soil on both sides of the gap before closing the breach.
The obvious reason is outflow from the canal breach is large and all the soil dumped for closing the breach may be washed off.
The procedure generally adopted is mentioned below:
The first step is to stop or to reduce the outflow from the canal. It is accomplished by opening some other connected channel upstream or by closing the gates of a head regulator.
Second step is to cut the sides of the gap in form of steps. It gives proper bond to newly deposited material with the old bank.
Third step is to drive double pile line in the opening of the breach. The space between the pile lines is filled with planks, bushes etc. It retards the flow of water. The filler material is secured by placing sand bags on the top. If the breach is very wide another line of defence is provided. It may be in the form of a wall constructed by stacking sand filled gunny bags. This second line of defence is provided just behind the pile line.
Thus when the outflow is reduced sufficient earth is deposited on both sides of the gap. It is then dumped instantaneously from both sides to form a ring bond on the outer side of the breach as shown in Fig. 6.29.
Then the opening is properly filled with suitable earth in layers. Each layer is given proper compaction and breach thoroughly repaired.