Cross Drainage Works

CROSS DRAINAGE WORKS

It is a structure carrying the discharge of a

natural stream across a canal intercepting

the stream.

Classification based on

Relative levels

Discharges

Type I (C D works carrying canal over the

drainage)

Aqueduct

Syphon aqueduct

Type II (Drainage passes over the

irrigation canal)

Super passage

Syphon super passage

Type III (Drainage and canal intersect

each other at the same level)

Level crossing

Inlet and outlet

TYPE I

Canal is carried over the natural drain.

Opened to Inspection

Damage done by flood is rare.

The drain is very big comparison to the

section of the canal

AQUEDUCT

Irrigation canal is taken over the drainage Suitable when the bed level of canal is well

above the H.F.L of drainage Drainage water passes clearly below the

canal

SYPHON AQUEDUCT

In this canal is taken over drainage, but

drainage water not clearly passes below the

highest flood level.

It flows under syphonic action

Suitable when the bed level of the canal is

below the H F L

TYPE II

Drainage is carried over the canal

CD works are less liable to damage than the

earth work of the canal.

Disadvantages

Not open to inspection

If silt is deposited, it is difficult to clear it

out.

SUPER PASSAGE

FSL of canal is lower than the underside of the trough carrying drainage water.

Canal water runs under gravity

CANAL SYPHON

The drainage is taken over the canal but

water passes below the drainage under

siphonic action.

Suitable when bed level of the drainage is

below the FSL of the canal

TYPE III Level crossing

Bed level of canal and drainage are approximately same.

The quantity of water in canal and drainage is not that much different

In this water in canal and drainage is allowed to mix

With the help of regulator both in canal and stream water is disposed in reqd quantity.

Components Crest wall Canal regulator Stream regulator

DESIGN OF SYPHON AQUEDUCT

A. Hydraulic design Determination of max. flood discharge and

the HFL. Fixation of waterway of the drain Contraction of canal waterway Clearance and free board Head loss through syphon barrels Determination of uplift pressure on the roof

of trough Determination of uplift pressure on the floor Design of bank connections

B. Structural design Design of c/s of syphon barrrel and aqueduct

trough Design of piers and abutments Design of foundations

Determination of flood discharge

For small drains the high floood discharge

may be worked out from empirical

formulas.

Large drains- hydrograph analysis and

rational formula etc may be used.

Fixation of waterway of the drain

P = 4.75√Q

P- wetted perimeter of the river

Q- total discharge

Clearance and freeboard

Clearance /headway- vertical distance b/w

u/s HFL inclusive of afflux, of the drainage

and the bottom surface of trough.

Freeboard

Canals – FSL to top levels of banks

Drainage – HFL including afflux and top of the

embankment or guide banks

CONTRACTION OF CANAL WATERAY

Type I aqueduct – the canal cross section not

changed at crossing, so that the culvert

length is max. not economical on big drains

Type III- original width of canal is reduced.

Requires the provision of extra transition wings

for joining the flumed portion gradually to the

normal section.

Fluming is done in such a way velocity in the

trough is not more than 3 m/s.

The approach transition wings shd not steeper

than 300 ( corresponding to a splay of 2:1)

The departure transition wings shd not steeper

than 22.50 ( corresponding to a splay of 3:1).

Transition consist of curved and flared wings