Chapter Five-CANAL APPURTENANT STRUCTURES.pdf

ASTU/SoCEA/Water Resources Engineering Department
DESIGN OF CANAL
APPURTENANT STRUCTURES
CHAPTER FIVE
Compiled by: Tamirat D.

Canal Regulation Structures
▪ The structures constructed in order to control and regulate the discharge, full supply level
or depth, and velocity of flow in a channel are known as regulation works.
▪ Such structures are necessary for the efficient working and safety of the channel. Canal
regulation structures may be classified as:
1. Canal falls or drops
2. Canal regulators: Distributary
head regulator and Cross regulator
3. Canal escapes, and
4. Canal outlets

Canal Regulation Structures
i) Canal fall (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: controls the supply to an off-taking
channel from the parent channel.
iii) Cross regulator: controls the water level of a channel and the
discharge downstream of another hydraulic structure.
iv) Canal escape: disposes of extra supplies when the safety of a canal is
endangered due to heavy rains or closure of outlets by farmers
v) Canal outlet: provided at the junction of watercourses and the
distributary. It is built at the head of the watercourse to connect it
with a minor or a distributary channel.

➢Constructed across a canal to lower its water level whenever the available
natural ground slope is steeper than the designed bed slope of the channel.
➢This is achieved by negotiating the change in bed elevation of the canal
necessitated by the difference in ground slope and canal slope.
Necessity of Falls: Due to the natural ground condition, there is a condition
when the canal has to be constructed entirely in filling (embankment).
▪ But canal in filling has the following disadvantages
✓higher construction and maintenance cost,
✓higher seepage and percolation losses,
✓adjacent area being flooded due to any possible breach in the embankment,
and
✓difficulties in irrigation operations.
Canal Fall

➢A fall is a structure constructed across a channel to permit lowering down of
its water level and dissipate the surplus energy possessed by the falling water
which may otherwise scour the bed and banks of the channel.
➢When the natural slope of the ground over which channel is to be constructed
is greater than the designed bed slope of the channel, the difference in the
slopes is adjusted by providing vertical falls or drops in the bed of the channel
at suitable intervals.
Canal Fall

i. A fall may be provided at a location where the F.S.L. of the channel outstrips
the ground level but before the bed of the channel comes into filling;
ii. A fall should be so located that as far as possible there is no loss of command
area of the channel. In case of channels which irrigate the command area
directly, a fall should be provided before the bed of the channel comes into
filling.
iii. The location of the fall may be affected by the possibility of combining it with a
regulator or a bridge or some other structures. Such combination often results
in economy.
iv. A relative economy of providing large number of small falls Vs small number of
big falls should be worked out. The provision of small number of big falls
results in unbalanced earthwork, but there is always some saving in the cost
of the fall structure
Proper location of Canal Fall

➢In selecting a type of fall most suitable for a particular site, the main
consideration is the height of the drop and the discharge passing over the
fall or, in other words, the amount of energy to be dissipated downstream of
the fall. The type which dissipates this energy most satisfactorily is to be
preferred.
➢Where bed material is easily erodible, a baffle-type fall, though costly in
construction, may prove economical in the long run from the maintenance
point of view. But where the bed material is hard enough to withstand scour
action, the type of design sufficient to dissipate the surplus energy should be
adopted.
➢Drops that are usually provided with a low crest wall are subdivided into the
following types: (i) the vertical drop, (ii) the inclined drop, and (iii) the piped
drop.
Selection of Type of Fall

Types of canal fall
1. Ogee fall
➢This type of fall has gradual convex and concave curves, to provide a
smooth transition and to reduce disturbance and impact.
Defects
➢ Heavy draw-down on the u/s side, resulting in lower depths,
higher velocities & consequent bed erosion and affect the supply
in a distributary, situated u/s of the fall
➢ Due to smooth transition, kinetic energy of the flow was not all
dissipated, causing erosion of d/s bed and banks. It needs d/s
long protection

Types of canal fall
1. Ogee fall

Types of canal fall
2. Rapid fall
➢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

Types of canal fall
2. Rapid fall

Types of canal fall
3. Stepped fall
➢It consists of a series of vertical drops in the form of steps. These steps
are suitable in places where the sloping ground is very long and rehire a
long glacis to connect the higher bed level u/s with lower bed level d/s.
➢It is practically a modification of a rapid fall.
➢The sloping glacis is divided into a number of drops to bring down the
canal bed step by step to protect the canal bed and sides from damage by
erosion.

Types of canal fall
3. Stepped fall

Types of canal fall
4. Notch fall
➢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.

Types of canal fall
4. Notch fall

Types of canal fall
5. Vertical drop fall (Sarda fall)
➢It is a raised-crest fall with a vertical-impact cistern.
➢It consists of a crest wall, upstream and downstream wing walls, an
impervious floor, and a cistern, and downstream bank and bed protection
works
➢The crest length is normally kept equal to the bed width of the canal;
however, an increase in length by an amount equal to the flow depth takes
into account any future increase in discharge
➢For discharges of less than 14 m3/s, a rectangular crest with both vertical
faces is adopted
➢If the canal discharge exceeds 14 m3/s, a trapezoidal crest with sloping
downstream and upstream faces is selected.
➢The slopes of the downstream and upstream faces are 1 in 8 and 1 in 3,
respectively
➢The crest level must be so fixed that it does not create changes in upstream
water levels (backwater or drawdown effects)

Types of canal fall

Types of canal fall
➢The completed design consists of the design of the following component
parts:
1. Design of Crest
➢Two types of crest are used: Rectangular (<=14 cumecs) and trapezoidal (>14
cumecs)

Types of canal fall
▪U/S Cutoffs = (D1/3 + 0.6) m with a minimum of 0.8m
▪D/S Cutoffs = (D1/2 + 0.6) m with a minimum of 1m

Types of canal fall
Example

Cross Regulator and Distributary Head Regulator

Design of Cross Regulator and Distributary Head Regulator

Design of Cross Regulator and Distributary Head Regulator
Eqn above

Solution Design of cross regulator

Solution Design of cross regulator
On the next slide.

Solution Design of distributary head regulator

Solution Design of distributary head regulator
On the next slide

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