1. What is a Cross Drainage work?
Explain the types of Cross
drainage work.
2. Explain the different types of aqueducts.
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WHAT IS CROSS DRAINAGE WORK
Irrigational Canals while carrying water have to cross few
natural drainage streams, rivers, etc..
A Cross Drainage Work (CDW) is a structure constructed
to cross the natural drainages like streams, rivers etc.,
safely by the canals.
At the meeting point of canals and drainages, bed levels
may not be same. Depending on their bed levels, different
structures are constructed and accordingly they are known
by different names.
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NECESSITY OF CDWs
The water-shed canals do not cross natural drainages. But in actual
orientation of the canal network, this ideal condition may not be
available and the obstacles like natural drainages may be present
across the canal. So, the cross drainage works must be provided for
running the irrigation system.
At the crossing point, the water of the canal and the drainage get
intermixed. So, for the smooth running of the canal with its design
discharge the cross drainage works are required.
The site condition of the crossing point may be such that without any
suitable structure, the water of the canal and drainage can not be
diverted to their natural directions. So, the cross drainage works must
be provided to maintain their natural direction of flow.
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TYEPS OF CDWs
Depending up on bed levels of drainage and canal,
CDWs are classified as:
(1) Type I (CDWs carrying canal over drainage)
(a) Aqueduct,
(b) Siphon aqueduct.
(2) Type II (CDWs carrying drainage over canal)
(a) Super passage,
(b) Siphon super passage.
(3) Type III (CDWs admitting the drainage water into the canal)
(a) Level Crossing,
(b) Inlet and outlet.
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(1) Type I (CDWs carrying canal over drainage)
A. Aqueduct
The hydraulic structure in which the irrigation canal is taken
over the drainage (such as river, stream etc..) is known as
aqueduct. This structure is suitable when bed level of canal is
above the highest flood level of drainage. In this case, the
drainage water passes clearly below the canal.
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B. Siphon Aqueduct
This structure is suitable when the bed level of canal is
below the highest flood level. In a hydraulic structure where
the canal is taken over the drainage, but the drainage water
cannot pass clearly below the canal, It flows under siphonic
action. So, it is known as siphon aqueduct.
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(2) Type II (CDWs carrying drainage over canal)
A. Super Passage
The hydraulic structure in which the drainage is taken over
the irrigation canal is known as super passage. The
structure is suitable when the bed level of drainage is
above the full supply level of the canal. The water of the
canal passes clearly below the drainage.
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B. Siphon Super Passage or Siphon
The hydraulic structure in which the drainage is taken over
the irrigation canal, but the canal water passes below the
drainage under siphonic action is known as siphon super
passage. This structure is suitable when the bed level of
drainage is below the full supply level of the canal.
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(3) Type III (CDWs admitting the drainage water into
the canal) When the bed level of canal and the
A. Level Crossings stream are approximately the same
and quality of water in canal and
stream is not much different, the
cross drainage work constructed is
called level crossing where water of
canal and stream is allowed to mix.
With the help of regulators both in
canal and stream, water is disposed
through canal and stream in required
quantity. Level crossing consists of
following components (i) crest wall
(ii) Stream regulator (iii) Canal
regulator.
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B. Inlet and Outlet
When irrigation canal meets a
small stream or drain at same
level, drain is allowed to enter
the canal as in inlet. At some
distance from this inlet point, a
part of water is allowed to drain
as outlet which eventually meets
the original stream. The bed and
banks between inlet and outlet
are also protected by stone
pitching. This type of CDW is
called Inlet and Outlet.
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Depending up on the arrangement of canal passing over the
Stream/drain, Aqueducts are classified as:
• Type-I: Sides of aqueduct in earthen banks, with complete earthen
slopes
• Type-II: Sides of the aqueduct in earthen banks, with other slopes
supported by masonry walls
• Type-III: Sides of the aqueduct made of concrete or masonry.
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SELECTION OF TYPES OF CDWs
Factors which affect the selection of suitable type of CDWs are:
Relative bed levels and water levels of canal and drainage
Size of the canal and drainage.
Suitable canal alignment (see slide below)
Suitable soil available for bank connections
Nature of available foundations
Permissible head loss in canal
Availability of funds
Note: Compared to an aqueduct, a super passage is inferior and should be
avoided whenever possible. Siphon aqueduct is preferred over siphon
unless large drop in drainage bed is required.
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Contd…
The following points should be remembered while fixing the alignment
of CDWs,
a) The crossing should be at right angle to each other,
b) Well defined cross-section of the river or drainage should be available.
c) At the crossing point the drainage should be straight for a considerable
length.
d) The width of the drainage should be narrow as far as possible.
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