We have seen in chapter 14 that the canal requires a certain slope. Depending upon the discharge, to overcome the frictional losses. This slope may vary from 1 in 40000 for a discharge of about 1.5 cumecs to about 1 in 8000 for a discharge of 3000 cumecs. This slope is, therefore, quite flat in comparison to the available ground slope of in average value of 5 to 20 cm per kilometre length.
1. (in 200 to 1 in’50). Thus the ground slope in nature is always very much steeper than the design bed slope of irrigation canal, based on the silt theories. If an irrigation canal, taking off from its head, is in cutting, it will soon meet with condition when it will be entirely in embankment.
2. It has been stated earlier in chapter 15 that if the canal is in embankment, the cost of construction and maintenance is very high and at the same time the percolation and seepage losses are excessive Also, there is always a danger of the adjacent area being flooded if some cut or breach takes place in the canal banks. Hence, the canal should never be in high embankment. However, the divergence between the gentle bed slope of canal and the steep ground slope throws the canal in embankment after a certain distance though it started in culling at its head. To overcome this difficulty, falls are introduced at appropriate places, and the water surface of the canal is lowered. Arrangements are made to dissipate the excess energy liberated from the falling water ½ to ¼.
3. The Location of a fall is decided from the following considerations
4. For the canal which does not irrigate the area directly, the fall should be located from the considerations of economy in cost of excavation of the channel with regard to balancing depth and the cost of the falls itself.
5. For a canal irrigating the area directly a fall may be provided at a location where the F.S.L. outstrips the ground level, but before the bed of the canal comes into falling. After the drop, the F.S.L of the canal may be below the ground level for to- kilometer.
6. The location of the fall may also be decided from the consideration of the possibility of combining it with a regulator or a bridge or any other masonry works.
7. A relative economy of providing large number of small falls v/s small number big falls should be worked out. The provision of small number of big falls results in unbalanced earth-work, but there is always some saving in the cost of the fall structure. Classification of Falls: Falls may be divided into four principal classes with reference to the approach condition: Class I. Falls designed to maintain the depth discharge relation: In this class, the normal depth discharge curve of the channel is maintained, and there is neither draw down nor heading up of water as the channel approaches the fall. The fall under this category are: (1) trapezoidal notch falls, and (u) low-crested rectangular notch falls. In the former the depth discharge relationship is not so well maintained as the latter. The second type of fall is liable to cause silting if the channel is run at a lower level than designed, for long periods. However, if the periods of low water is small, the silt usually gets washed off.
Class II. Falls designed to maintain a fixed supply level in the channel above the work:
Such type of falls maintain a nearly fixed water surface level upstream of the work, and necessary under the following circumstances.
(i) When the hydraulic power station is combined with the fall.
(ii) When a subsidiary canal takes off some distance above the work.
The principal types of falls under this class are: (i) the Siphon falls (Siphon Spillways) and (ii) high crested weir falls. The fails under this category suffer from the defect of silting caused in the channel above the work. The crested falls are generally constructed without fluming, and hence the discharge intensity per unit length of the crest is low, making the energy dissipation simpler. It can also be used as a meter. Class III Falls designed to admit the variations of the surface level above the work at the will of the operator The falls under the category are designed to admit the variations of water level on the upstream of the work according to the requirements. They also serve as regulators. The regulation is done with the help of (i) sluice gates, (ii) horizontal stop logs, (iii) vertical needles. In practice, falls of this class are confined exclusively to rectangular notches as the trapezoidal notch cannot be used for this purpose. Class IV. Miscellaneous Types Falls under this class are designed from special considerations without any reference to the conditions of approach. The principal types under this class are as follows:
(i) Cylinder fall or Well fall
In this type of fall, water is thrown into a well over a crest from where it escapes near its bottom. The energy is dissipated in the well in turbulences. They are quite suitable and economical for low discharges and high drops, and are used at tail escapes of small channels.
(ii) Chute or rapids
They conduct the stream on open inclined troughs
(iii) Pipe falls
In this a pipe replaces chute.
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