Why Falls are Provided
- When natural slope of the country is steeper than the designed longitudinal slope of the irrigation canal.
- The falls have to be provided.
- If falls are not provided, the canals run so much in filling.
- That it will be almost impossible to even construct the canals.
- If canal is constructed very much in filling.
- It will be a constant danger to the adjoining people, and also it will be impossible to mantain it.
- For example, let slope of a particular canal be say 1 in 5000 and general slope of the ground say 1 in 2500.
- If canal bed at the start i.e. zero distance, is say 1 m in cutting.
- The bed of canal will just coincide with ground level at a distance of 5000 m and thereafter.
- The bed of the canal will come in filling.
- The fill of the canal will go on increasing at the rate of 1 m per 5000 m.
- If total length of the canal from head regulator to tail is say 30 km.
- The canal bed which was 1 m below the G.L. at head, will come 5 m in filling at the end of the canal.
- The construction and maintenance of a canal in this much filling will be very costly.
- Besides this percolation and seepage losses will also be excessive.
- To overcome the difficulty of heavy embankments and also to reduce.
- The cost of construction and maintenance, falls are provided.
- The falls lower down the bed level of the canal.
- Since bed is lowered, the water surface, at fall is also equally lowered.
- Due to fall in the water surface excessive energy is developed on the D/S side.
- Which is dissipated by adopting suitable measures discussed in this chapter later.
Location of Falls
- The location of the fall on a channel depends upon the general slope of the country through.
- Which channel passes. Following points should however by considered while deciding the location of the fall.
- In the case of main canals which do not do irrigation directly.
- The position of the fall should be located by considering the economy in the cost of excavation of the canal.
- With regard to balancing depth and the cost of the fall itself.
- Main canal should not be allowed to run into much filling.
- The canal being very large, any breach in it may cause lot of damage to life and property.
- In case of branches and distributaries, which do direct irrigation.
- The falls are located considering command area.
- The F.S.L. of the canals should remain above the G.L. for most of the length.
- However the F.S.L. of channel D/S of fall may remain below G.L. for some length.
- The area D/S of the fall for which F.S.L.
- Of channel remain below G.L., is commanded by taking outlets from U/S of the fall.
- Location of the fall may be slightly varied so that its construction may be combined with a regulator or bridge.
- The falls may be provided large in number but of smaller fall or they may be smaller in number but of larger fall. Both the alternatives should be weighed and adopted.
History of Falls
- In old days, modern scientifically designed falls were not in existence.
- The excess fall of the general country was used to be adjusted or dispensed with by increasing.
- The length of the canal by giving circuitous alignment to the canals.
- Eastern Yamuna canal in India, constructed during Mughal period.
- Does not have any fall and canal alignment is very much circuitous.
- Development of falls started late in the 19th century.
- When large irrigation projects like Ganga, Cauvery, Eastern and Western Yamuna canals were constructed.
- A brief description of each fall which came into being one after the other is being given here.
1. Ogee Type Fall.
- This fall was first constructed on Ganga canal.
- The crest of the fall was kept at level with the U/S bed of the canal.
- A smooth concave surface was provided on the D/S side, to provide smooth transition and to reduce impact and disturbance.
- The energy of water passing over the fall, remained preserved and thus deep scourings for very long lengths on D/S side were used to develop.
- Crest of the fall being at the U/S bed level, there used to be considerable draw down effect on the U/S side.
- This fall consists of a sloping glacis.
- The slope is kept from 1 in 10 to 1 in 20.
- Because of long sloping glacis, the formation of hydraulic jump is assured.
- It is hydraulic jump formed on D/S side, which carries out the dissipation of excess energy.
- This fall worked very well but the major drawback of this fall was its high cost (Fig. 21.3).
3. Stepped Fall.
- As its name indicates, it consists of a number of steps.
- The total fall is broken into a number of steps of smaller falls.
- This fall also has the disadvantage of being very costly.
- As far as its performance is concerned it is very satisfactory.
4. Notch Type Fall.
- This fall consists of notches situated in a high crested wall, built across the channel.
- The fall may be having only one notch or a number
- of notches depending upon the discharge to be handled.
- All the notches have smooth entrance and flat circular lips projecting D/S side to disperse water.
- This fall maintains the depth discharge relationship and does not cause any draw down on the U/S side.
- This fall continued to be in use for quite a long time.
- One of the major defect of these falls was that they cannot be used as regulators.
5. Vertical Drop Falls.
- In such falls the nappe of falling water impinges vertically into the water cushion developed on the D/S side.
- Nappe does not remain in contact of the crest wall on D/S side.
- In such falls, excess energy of falling water is dissipated by turbulent diffusion in the pool of water.
- Sarda type fall and C.D.O. type fall, are the usual examples of vertical drop falls.
- The design of this fall has been given in detail later on in this chapter.
6. Glacis Type Fall.
- In such falls a sloping ramp is provided D/S of the crest.
- Because of a slope, formation of hydraulic jump is assured.
- The energy dissipation in such falls is carried out with the help of hydraulic jump.
- The glacis type falls may be divided into two categories:
(i) Straight glacis type, and
(ii) Parabolic glacis type.
(i) Straight glacis type.
- This fall consists of a straight sloping ramp at D/S.
- If a baffle platform and baffle wall are provided on the D/S side, it is know Inglis fall.
- In such falls the formation of jump is a must on the baffle platform.
(ii) Parabolic glacis type.
- The fall having parabolic D/S glacis is known as Montague type fall.