CANAL FALL
For main and branch canals which do not directly irrigate, the site is determined
on the basis of economy in earth works. All the excavated earth is utilized in
making up banks.
For the distributaries and minors, falls may be located d/s of the outlets as this
helps in increasing the command area, and in improving the efficiency of outlets.
Notch fall (trapezoidal fall):
This type of fall was started around the late nineteenth century and continued to be constructed due to
its property of being able to maintain a constant depth-discharge relationship, until simpler and
economical alternatives were designed.
In case of trapezoidal notch falls, a high crested wall is built across the channel and
trapezoidal notches are provided in that wall. Trapezoidal falls are very economical
and suitable for low discharges. Now a days this type of falls are using widely
because of their simplicity and popularity.
The trapezoidal fall was verysuccessful and was adopted in India for many years. It was also copied all
over the worldwhere it is still in use. There was one serious defect in these falls that they could not be
usedas regulators in addition.
Vertical fall/Sarada fall:
Simple vertical drop fall or sarda fall consists, single vertical drop which allows the
upstream water to fall with sudden impact on downstream. The downstream acts
like cushion for the upstream water and dissipate extra energy.
he Sarda type fall developed on the Sarda canal Project in UP and CDO type
falldeveloped in Punjab are some of the recent types of vertical drop falls
Fluming is not allowed in vertical fall. Two types of crest
Rectangular low discharge <14m3/s
Trapezoidal large discharge >14m3/s
Rapid Fall: (small discharge)
These were quite effective but, the cost being prohibitive was gradually phased out.
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.
Rapid fall consists a long sloping glacis. It is constructed if the available natural
ground surface is plane and long. For this, a bed of rubble masonry is provided and
it is finished with cement mortar of 1:3 ratio. To maintain the slope of bed curtain
walls are provided at both upstream and downstream. Rapid falls are high priced
conThe rapid fall is suitable when the slope of the natural ground surface is even and long.
It consists of a long sloping glacis with longitudinal slope which varies from 1 in 10 to 1 in
20.
Curtain walls are provided on the upstream and downstream side of the sloping
glacis.
The sloping bed is provided with rubble masonry.
The upstream and downstream side of the fall is also protected by rubble
masonry.
The masonry surface is finished with rich cement mortar (1: 3).
Stepped fall/cascade fall: (small discharge):
Stepped fall is the modification of rapid fall. It is suitable for the canal which has it
upstream at very high level as compared to downstream.
This fall is suitable in places where the sloping ground is very long and requires long glacis
to connect the higher bed level with lower bed level.
This fall is practically a modification of the rapid fall.
For flumed conditions vertical type is not suitable because effective
dissipation is difficult and harmful scouring is always expected. In
such cases the glacis fall either with straight sloping floor or with
baffle wall may prove useful.
Ogee fall:
There was considerable draw down effect on the u/s resulting is bed
erosion
Ogee curve is the combination of convex and concave curves. So, Ogee fall consists
of both convex and concave curves gradually. This gradual combination helps to
provide smooth transition of flow and also reduce the impact. If the canal natural
ground surface is suddenly changed to steeper slope, ogee fall is recommended for
that canal. Stone pitching is provided in the upstream and downstream of the fall.
The Ogee fall was first constructed by Sir Proby Cautley on the Ganga Canal. This
type of fall has gradual convex andconcave curves, with an aim to provide a smooth
transition andto reduce disturbance andimpact. This preserved theenergy (with out
dissipating it)
Straight Glacis Canal Falls (flumed or un flumed)
Energy dissipation is achieved by the formation of the hydraulic jump at the
toe of the glacis plus friction blocks. The height of blocks is 1/4 of total depth
of water on the d/s side.
This is the modern type of construction, in which a raised crest is
constructed across the canal and a gentle straight inclined surface is
provided from raised crest to the downstream. The water coming from
upstream crosses the raised crest and falls on inclined surface with sufficient
energy dissipation.
n Punjab which was called the CDO type fall,
Glasic type fall:
The efficiency of the hydraulic jump as a very potent means of destroying the
energy of canalfalls is used in glacis falls. The glacis type of fall utilizes the standing
wave phenomenon for dissipation of energy. Theglacis fall may be (i) straightglacis
type, or (ii) parabolicglacis type, commonlyknown 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.
The glacis type falls may be modified in the following ways: (a) Flumed or un-flumed, depending
upon the crest width being smaller or equal to the bed width of the canal (Figure17). (b) Meter
or non-meter fall depending upon whether the canal fall may be used to measure the discharge
as well
Montague Type Canal Falls
Montage fall is similar to straight glacis fall but in this case the glacis is not
straight. It is provided in parabolic shape to introduce the vertical
component of velocity which improves the energy dissipation to more
extent.
English or Baffle Canal Falls
In this case, straight glacis fall is extended as baffle platform with baffle
wall. This is suitable for any discharge. The baffle wall is constructed near
the toe of the straight glacis at required distance in designed height. The
main purpose of the baffle wall is to create hydraulic jump from straight
glacis to baffle platform.
