6.3 Cross Drainage Works

A cross drainage work is a structure which is constructed at the crossing of the canal &natural drain (river, stream, nalla) so as to dispose of drainage water without interrupting continuous canal supplies. A CD Work I s a costly construction and must be avoided as far as possible. When canal is aligned along watershed CD Works are generally not required, till it reaches watershed.

Ways of disposing off drainage water.

By passing the canal over the drainage

An Aqueduct

· Siphon Aqueduct

By passing the canal below the drainage

· A super passage

· Canal syphon or syphon

By passing the canal through drainage

· A level crossing

· Inlets & Outlets

Drainage Aqueduct

Sketch

The canal is taken over the natural drain such that the drainage water runs below the canal either freely or under symphonic pressure.

When the HFL of the drainage is sufficiently below the bottom of the canal so that drainage water flows under gravity the structure is known Line Planning Crossing.

Aqueduct & Syphon Aqueduct

Canal water is taken across the drainage in a trough supported on piers. An inspection road is provided as shown. An aqueduct is just like like a bridge except that instead of carrying a road or railway it carries canal at its top. An aqueduct is provided when sufficient level difference is available between the canal & the natural drainage and CBL Is sufficiently higher than correct level.

Siphon Aqueduct

Sketch

If the HFL of the drain is higher than the canal bed and the water passes through the aqueduct barrels under syphonic action the action the structure is known as syphon aqueduct.

In this case drain bed is generally depressed & provided with pacca floor on u/s side drainage bed may be joined to floor by vertical drop. The Canal remains open to inspection throughout and damage caused by floods are rare.

Super Passage

Sketch

In these types drain is taken over the canal such that the canal water runs below the drain either freely or under syphonic pressure.

When FSL of canal is sufficiently below the bottom of the drain through, so that canal water flows freely under gravity, the structure is known as super passage.

It is reverse of aqueduct. Only inspection road cannot be provided along the canal & a separate bridge is required for roadway.

Siphon

If the FSL of the canal is sufficiently above the bed level of the drainage through so that canal flows under siphon action under the trough the structure is known as canal siphon or siphon

Sketch

Level Crossing

Sketch

In this type of CD Work canal water & drain water is allowed to mix with each other. Level crossing is generally provided large canal & huge drainage approach each other practically at same level

A regular is provided across drainage just on downstream side of crossing so as to control the discharge passing into drainage. A regular is also provided to control the discharge into the canal.

Inlets and Outlets

An inlet is a structure constructed in order to allow drainage water to entire canal to augment canal supplies. This is adopted when drainage discharge is small and drain crosses canal equal or slightly higher level.

But when drainage discharge is high and canal can not take entire drainage water, an outlet is constructed to escape out additional dischage at a suitable site, a little downstream along the canal. Escape discharge may not be equal to admitted discharge, also number of inlets and number of outlets may not be same.

Inlets and outlets are essentially open cuts in canal banks suitably protected by pitching.

This type of CD works are (allows mixing of two waters) interior to adueduct and superpassage but are cheaper.