CANAL FALL/DROP STRUCTURES

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CANAL FALL/DROP STRUCTURES

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Necessity of Falls/Drops
A fall or drop is an irrigation structure constructed across a canal to lower down its water
level and destroy the surplus energy liberated from the falling water which may otherwise
scour the bed and banks of the canal.
We know that the canal requires a certain slope, depending upon the discharge, to
overcome the frictional losses. This slope may vary from 1 in 4000 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 an average value of 5 to 20 cm per
kilometre length (i.e., 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.

Location of Falls/Drops
The location of a fall is decided from the following considerations:
1. 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.

2. 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 filling.
After the drop, the F.S.L of the canal may be below the ground level for to
kilometre.

3. 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.

4. A relative economy of providing large number of small falls vis small number of 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.

Development of Falls/Drops
The ancient people always tried to avoid falls by aligning canals along zig-zag route in order
to increase the length of the canal and thus dissipate the excess energy head in friction. The
Eastern Yamuna Canal constructed by Mughal Emperors had no falls, and the canal, followed
a sinuous path. The falls were first constructed by the British in India in the nineteenth
century. The development of falls, since then, took place gradually. Among the earlier type of
falls are: Ogee falls, rapids and stepped falls. Later, notch falls, vertical falls and glacis type
falls were developed.

1. Ogee fall
The Ogee fall was first constructed by Sir Proby Cautley on the Ganga Canal. This type of
fall has gradual convex and
concave curves, with an aim to
provide a smooth transition and
to reduce disturbance and
impact. This preserved the
energy (with out dissipating it).
Due to this, the Ogee fall had
the following defects:
(i) There was considerable draw down effect on the u/s resulting is bed erosion.
(ii) Due to smooth transition, the kinetic energy was preserved till sufficient depth
was scoured out below the fall to ensure the formation of the hydraulic jump.

2. Rapid fall
Rapid falls were provided on Western Yamuna Canal and were designed by Lieut. R.F.
Croften. Such a fall
consists of a glacis
sloping at 1 vertical to 10
to 20 horizontal. The long
glacis assured the
formation of hydraulic
jump. The gentle slope
admitted timber traffic. Hence, the fall worked admirably. However, there was very high cost
of construction.

3. Stepped fall
Stepped fall was a next
development of the rapid
fall. One such type was
provided at the tail, of
main canal escape of
Sarda canal. The cost of this fall was also too high.

4. Notch fall
Soon after the development of stepped
fall, the efficiency of vertical impact
on the floor for energy dissipation
came to be recognized. The vertical
fall came in the field along with the
cistern. However, with greater
discharges, vertical fall gave trouble.
Hence, these were superseded for a
time by the notch fall. The trapezoidal
notch fall was first designed by Ried
in 1864.
The fall consists of one or more trapezoidal notches in a high crested wall. A flat
circular lip projects downstream of each notch to disperse water. The notches were designed
to maintain the normal water depth in the u/s channel at any two discharge values. The depth

discharge relation was thus maintained with close approximation. As the channel approached
the fall, there was neither drawdown nor heading up of water. The trapezoidal fall was very
successful and was adopted in India for many years. It was also copied all over the world
where it is still in use. There was one serious defect in these falls that they could not be used
as regulators in addition.

5. Vertical drop fall
In the vertical drop fall, the nappe impinges clear into the water cushion below. In the earlier
types of vertical falls, the dimensions of cistern were put in arbitrarily in light of experience
of the designers. Another
device in the form of grid was
usually used in the cistern
intercepting the dropping jet
of water. The grid consisted of
baulks of timber horizontal or
inclined and spaced some
centimeters apart. These were
later abandoned because the
timber grid got clogged and
rotted and had to be replaced
frequently.
The Sarda type fall developed on the Sarda canal Project in UP and CDO type fall
developed in Punjab are some of the recent types of vertical drop falls. In these falls, the high
velocity jet enters the deep pool of water in the cistern and the dissipation of energy is
affected by the turbulent diffusion.

6. Glacis type fall
The efficiency of the hydraulic jump as a very potent means of destroying the energy of canal
falls is used in glacis falls. The glacis type of fall utilizes the standing wave phenomenon for
dissipation of energy. The
glacis fall may be (i) straight
glacis type, or (ii) parabolic
glacis type, commonly
known as the Montague type.
The straight glacis fall may
be with baffle platform and
baffle wall. In such a case,
the formation of jump takes
place on the baffle platform. This type was first developed by Inglis and is called Inglis fall.

7. Miscellaneous Types
(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.