Canal Fall

Canal Regulation Works
170602
Module-IV
Part-II

Syllabus
• Canal Regulation Works:
• Canal Fall- Necessity and Location- Types of
Falls- Cross Regulator and Distributory Head
Regulator- Their Functions, Silt Control
Devices, Canal Escapes- Types of Escapes.

What is Canal Fall?
• Whenever the available natural ground slope
is steep than the designed bed slope of the
channel, the difference is adjusted by
constructing vertical ‘falls’ or ‘drops’ in the canal
bed at suitable intervals, as shown in figure
below. Such a drop in a natural canal bed will
not be stable and, therefore, in order to retain
this drop, a masonry structure is constructed.
Such a structure is called a Canal Fall or a
Canal drop.

Canal Fall
Irrigation canals are designed for a prescribed bed
slope so that velocity becomes non silting or non
scouring. But if the ground topography is such that in
order to maintain the canal designed slope, indefinite
filling from falling ground level is to be made. This
indefinite filling is avoided by constructing a hydraulic
structure in the place of sudden bed level. This
hydraulic structure is called canal fall or drop. Beyond
the canal fall, canal again maintains its designed slope.

Canal Fall
• Thus, a canal fall or drop is an irrigation structure
constructed across a canal to lower down its bed
level to maintain the designed slope when there is a
change of ground level to maintain the designed
slope when there is change of ground level. This
falling water at the fall has some surplus energy. The
fall is constructed in such a way that it can destroy
this surplus energy.

Necessity of Canal Falls
• When the slope of the ground suddenly changes
to steeper slope, the permissible bed slope can
not be maintained. It requires excessive
earthwork in filling to maintain the slope. In such
a case falls are provided to avoid excessive earth
work in filling

• When the slope of the ground is more or
less uniform and the slope is greater than
the permissible bed slope of canal.

• In cross-drainage works, when the difference
between bed level of canal and that of
drainage is small or when the F.S.L of the
canal is above the bed level of drainage
then the canal fall is necessary to carry the
canal water below the stream or drainage.

Types of Canal Fall
• Depending on the ground level conditions and shape
of the fall the various types of fall are:
Ogee Fall
• The ogee fall was constructed by Sir Proby Cautley on
the Ganga Canal. This type of fall has gradual convex
and concave surfaces i.e. in the ogee form. The gradual
convex and concave surface is provided with an aim
to provide smooth transition and to reduce
disturbance and impact. A hydraulic jump is formed
which dissipates a part of kinetic energy. Upstream
and downstream of the fall is provided by Stone
Pitching.

Types of Canal Fall
Stepped Fall
• It consists of a series of vertical drops in the form of
steps. This steps is suitable in places where sloping
ground is very long and require a long glacis to
connect the higher bed level u/s with lower bed level
d/s. it is practically a modification of rapid fall. The
sloping glacis is divided into a number drops to bring
down the canal bed step by step to protect the canal
bed and sides from damage by erosion. Brick walls are
provided at each drop. The bed of the canal within the
fall is protected by rubble masonry with surface
finishing by rich cement mortar.

Types of Canal Fall
Vertical Fall (Sarda Fall)
• In the simple type, canal u/s bed is on the level of
upstream curtain wall, canal u/s bed level is
below the crest of curtain wall. In both the cases,
a cistern is formed to act as water cushion. Floor
is made of concrete u/s and d/s side stone
pitching with cement grouting is provided. This
type of fall is used in Sarda Canal UP and
therefore, it is also called Sarda Fall.

Types of Canal Fall
Rapid Fall
• When the natural ground level is even and
rapid, this rapid fall is suitable. It consists of
long sloping glacis. Curtain walls are
provided on both u/s and d/s sides. Rubble
masonry with cement grouting is provided
from u/s curtain wall to d/s curtain wall.
Masonry surface is finished with a rich
cement mortar.

Types of Canal Fall
Straight Glacis Fall
• It consists of a straight glacis provided with a
crest wall. For dissipation of energy of flowing
water, a water cushion is provided. Curtain
walls are provided at toe and heel. Stone
pitching is required at upstream and
downstream of the fall.

Types of Canal Fall
Trapezoidal Notch Fall
• It was designed by Reid in 1894. In this type a
body or foundation wall across the channel
consisting of several trapezoidal notches between
side pier and intermediate pier is constructed.
The sill of the notches are kept at upstream bed
level of the canal. The body wall is made of
concrete. An impervious floor is provided to
resist the scouring effect of falling water.
Upstream and downstream side of the fall is
protected by stone pitching finished with cement
grouting

Types of Canal Fall
Well or Cylinder Notch Fall
• In this type, water of canal from higher level
is thrown in a well or a cylinder from where
it escapes from bottom. Energy is dissipated in
the well in turbulence. They are suitable for
low discharges and are economical also.

Types of Canal Fall
Montague Type Fall
• In the straight glacis type profile, energy
dissipation is not complete. Therefore,
montague developed this type of profile
where energy dissipation takes place. His
profile is parabolic and is given by the
following equation,

Types of Canal Fall
Inglis or Baffle Fall
• Here glacis is straight and sloping, but baffle
wall provided on the downstream floor
dissipate the energy. Main body of glacis is
made of concrete. Curtain walls both at toe
and heel are provided. Stone pitching are
essential both at u/s and d/s ends

Canal Escape
• It is a side channel constructed to remove
surplus water from an irrigation channel (main
canal, branch canal, or distributary etc.) into a
natural drain.
• The water in the irrigation channel may become
surplus due to -
• Mistake
• Difficulty in regulation at the head
• Excessive rainfall in the upper reaches
• Outlets being closed by cultivators as they find
the demand of water is over

Canal Escape
• It is the structure required to dispose of surplus or
excess water from canal from time to time. Thus, a
canal escape serves as safety valve for canal system. It
provides protection to the canal from possible damage
due to excess supply which may be due to mistake in
releasing water at head regulator or heavy rainfall
that makes sudden regular demand of water. The
excess supply makes the canal banks vulnerable to
failure due to overtopping or dangerous leaks.
Therefore, provision for disposing this surplus water
in form of canal escapes at suitable intervals along the
canal is essential. Moreover emptying canal for repair
and maintenance and removal of sediment deposited
in the canal can also be achieved with the help of
canal escapes.

Escapes are usually of the following
three types.
Surplus Escape
• It is also called regulator type. In this type sill
of the escape is kept at canal bed level and the
flow is controlled by a gate. This type of
escapes are preferred now-a-days as they
give better control and can be used for
employing the canal for maintenance.

three types.
Tail Escape
• A tail escape is provided at the tail end of the
canal and is useful in maintaining the
required FSL in the tail reaches of the canal
and hence, they are called tail escape.

three types.
Scouring Escape
• This escape is constructed for the purpose of scouring
of excess silt deposited in the head reaches from time
to time. Hence, it is called scouring escape. Here the
sill of the regulator is kept at about 0.3 m below the
canal bed level at escape site. When deposited silt to be
scoured, a higher discharge than the FSL is allowed to
enter the canal from the head works. The gate of the
escape is raised so as to produce scouring velocity
which remove the deposited silt. This type of Escape
has become obsolete as silt ejector provided in the
canal can produce better efficiency.

Head Regulator
• Regulators Constructed at the off taking point are called head
regulators. When it is constructed at the head of main canal it
is known as canal head regulator. And when it is constructed
at the head of distributary, it is called distributary head
regulator.
• Function:
• To control the entry of water either from the reservoir or
from the main canal.
• To control the entry of silt into off taking or main canal.
• To serve as a meter for measuring discharge of water.

Head Regulator
• Construction: The components of head regulator
depends upon the size of canal and location of head
regulator. It consists of one or more gated research
openings with barrels running through the bank.
For large canals head regulators are flumed to
facilitate the measurement of discharge.

Cross Regulator
• Cross Regulator
• A Regulator Constructed in the main canal or parent
canal downstream of an off take canal is called cross-
regulator.
• It is generally constructed at a distance of 9 to 12 km
along the main canal and 6 to 10 km along branch
canal.
• Functions:
• (i) To Control the flow of water in canal system
• (ii) To feed the off taking Canals
• (iii) To enable closing of the canal breaches on the d/s
• (iv) To provide roadway for vehicular traffic

Cross Regulator
Construction: For Cross Regulators abutments
with grooves and piers are constructed parallel
to the parent canal. The sill of regulation is kept
little higher than the u/s bed level of canal
across which it is constructed. Vertical lift gates
are fitted in the grooves. The gates can be
operate from the road.

Silt Control Devices
• Scouring Sluices or Under sluices, silt pocket and silt
excluders
• The above three components are employed for silt
control at the head work. Divide wall creates a silt
pocket. Silt excluder consists of a number under
tunnels resting on the floor pocket. Top floor of the
tunnel is at the level of sill of the head regulator.
• Various tunnels of different lengths are made. The
tunnel near the head regulator is of same length of
head regulator and successive tunnels towards the
divide wall are short. Velocity near the silt laden water
is disposed downstream through tunnels and under
sluices.

• Silt Excluder: The silt excluder is located on
the u/s of diversion weir and in front of the
head regulator. The object is to remove silt
that has entered in the stilling basin through
scouring sluices.
• Silt Ejector: Silt Ejector is located in the canal
take off from the diversion weir at 6 to 10 km
in the canal reach. It ejects the silt that has
entered in the canal

Silt Excluder

Silt Ejector

Canal Outlet/modules
• A canal outlet or a module is a small
structure built at the head of the water
course so as to connect it with a minor or a
distributary channel.
• It acts as a connecting link between the
system manager and the farmers.

Non-Modular Modules
• Non-modular modules are those through which the
discharge depends upon the head difference
between the distributary and the water course.
Common examples are:
(i) Open sluice
(ii) Drowned pipe outlet

Types of Outlet/modules
• Non-modular modules

Semi-Modules or Flexible modules
• Due to construction, a super-critical velocity is
ensured in the throat and thereby allowing the
formation of a jump in the expanding flume.
• The formation of hydraulic jump makes the outlet
discharge independent of the water level in water
course, thus making it a semi module. Semi-modules
or flexible modules are those through which the
discharge is independent of the water level of the
water course but depends only upon the water level of
the distributary so long as a minimum working head is
available.
• Examples are pipe outlet, open flume type etc.

Semi-Modules or Flexible modules

Rigid Modules or Modular Outlets
• Rigid modules or modular outlets are those through
which discharge is constant and fixed within limits,
irrespective of the fluctuations of the water levels of
either the distributary or of the water course or both.
• An example is Gibb’s module:

Exam Questions
Dec 2011, June 2012
• What do you understand by a fall in
canal? Why it is necessary?
• What are the functions of a canal head
regulator?
• Explain functions of cross regulator and
distributory head regulator.
• Write a S.N. on Types of Canal Falls

References
• Irrigation Engineering
– By Prof N N Basak
– Tata Mcgraw-Hill
• Irrigation Engineering & Hydraulic Structures
– By Prof. Santosh Kumar Garg
– Khanna Publishers
• Internet Websites
• http://www.uap-bd.edu/
• Lecture Notes By: Dr. M. R. Kabir
• Professor and Head, Department of Civil Engineering Department
• University of Asia Pacific (UAP), Dhaka

Canal Fall

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