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CANAL REGULATION STRUCTURES
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CANAL REGULATION STRUCTURES



GENERAL
Canal regulation structures are hydraulic structures which are constructed to regulate the
discharge, flow velocity, or supply level in an irrigation channel. These structures are necessary
for efficient working as well as for the safety of an irrigation channel. Canal regulation structures
can be classified as follows:
(i) Canal fall: The canal fall (or, simply, the ‘fall’ or ‘drop’) regulates the supply level in
a canal by negotiating the change in its bed elevation necessitated by the difference
in ground slope and canal slope.
(ii) Distributary head regulator: This controls the supply to an offtaking channel from
the parent channel.
(iii) Cross regulator: This structure controls the water level of a channel and the discharge
downstream of another hydraulic structure.
(iv) Canal escape: Canal escape disposes of extra supplies when the safety of a canal is
endangered due to heavy rains or closure of outlets by farmers.

10.2. CANAL FALL
A canal fall is a hydraulic structure constructed across a canal to lower its water level. This is
achieved by negotiating the change in bed elevation of the canal necessitated by the difference
in ground slope and canal slope. The necessity of a fall arises because the available ground
slope usually exceeds the designed bed slope of a canal. Thus, an irrigation channel which is in
cutting in its head reach soon meets a condition when it has to be entirely in filling. An irrigation
channel in embankment has the disadvantages of: (i) higher construction and maintenance
cost, (ii) higher seepage and percolation losses, (iii) adjacent area being flooded due to any
possible breach in the embankment, and (iv) difficulties in irrigation operations. Hence, an
irrigation channel should not be located on high embankments. Falls are, therefore, introduced
at appropriate places to lower the supply level of an irrigation channel. The canal water
immediately downstream of the fall structure possesses excessive kinetic energy which, if not
dissipated, may scour the bed and banks of the canal downstream of the fall. This would also
endanger the safety of the fall structure. Therefore, a canal fall is always provided with measures
to dissipate surplus energy which, obviously, is the consequence of constructing the fall.
The location of a fall is primarily influenced by the topography of the area and the
desirability of combining a fall with other masonry structures such as bridges, regulators, and
so on. In case of main canals, economy in the cost of excavation is to be considered. Besides, the
relative economy of providing a large number of smaller falls (achieving balanced earth work
and ease in construction) compared to that of a smaller number of larger falls (resulting in



CANAL REGULATION STRUCTURES 353
reduced construction cost and increased power production) is also worked out. In case of channels
which irrigate the command area directly, a fall should be provided before the bed of the
channel comes into filling. The full supply level of a channel can be kept below the ground level
for a distance of up to about 500 metres downstream of the fall as the command area in this
reach can be irrigated by the channels offtaking from upstream of the fall.

10.3. HISTORICAL DEVELOPMENT OF FALLS
There was no theory or established practice for the design and construction of falls in the
nineteenth century. Falls were usually avoided by providing sinuous curves in the canal
alignment. This alternative increased the length of the canal. Obviously, this approach was
uneconomical and resulted in an inefficient irrigation system.
The ogee fall (Fig. 10.1) was first constructed by Cautley on the Upper Ganga canal with
a view to providing a smooth transition between the upstream and the downstream bed levels
so that flow disturbances could be reduced as far as practicable. The smooth transition of the
ogee fall preserved the kinetic energy and also resulted in large drawdown which caused heavy
erosion of bed and banks on the downstream as well as upstream of the fall. Later, this type of
fall was converted into a vertical impact type so as to cause more energy dissipation downstream
of the fall.



Vertical-impact Cisterns
In these cisterns (Fig. 10.5) there is an impact of a stream of water falling freely. The path of
such a stream is, obviously, parabolic. This type of cistern is very efficient for the dissipation of
surplus energy when the drop is sufficient so that the falling stream becomes almost vertical.
The dimensions of the cistern should be such that it serves the purpose of stilling and combing
out the residual eddies and disturbances.


Horizontal-impact Cisterns
In this type of cistern (Fig. 10.6), water, after passing over the crest, flows on a glacis whose
reverse curve at the downstream end turns the inclined supercritical flow to horizontal
supercritical flow before it strikes the subcritical flow of the downstream channel resulting in
the formation of a hydraulic jump. However, the position of a hydraulic jump on a horizontal
floor is very sensitive to the variations in the depth or velocity of the downstream flow.


Inclined-impact Cisterns
For such cisterns, the glacis is carried straight down into the cistern and reliance is placed
upon the effectiveness of the jump forming on the glacis for dissipation of surplus energy.
However, the vertical component of the supercritical jet is not affected by the impact and,
hence, energy dissipation is inefficient


No-impact Cisterns
Hydraulic impact is possible only when a supercritical flow meets a subcritical flow. In low
falls and falls with large submergence, hydraulic impact may not be possible. Hence, other
means of energy dissipation are adopted. A properly designed baffle wall along with suitable
roughening devices are useful means of energy dissipation for such cases. The depth of noimpact
cisterns cannot be calculated from theoretical considerations.
Reply

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