CROSS DRAINAGE WORKS
A cross drainage work is a structure which is constructed at the crossing of a canal and a natural drain, so as dispose of drainage water without interrupting the continuous canal supplies.
Type of Cross-drainage works The drainage water intercepting the canal can be disposed of in either of the following ways:
(1) By passing the canal over the drainage. this may be accomplished either through (i) an aqueduct,; or through a (ii) syphon- aqueduct.
(2) By passing the canal below the drainage. This may be accomplished either through (i) a super -passage; or through a (ii) canal syphon generally called a syphon.
(3) By passing the drain through the canal, so that the canal water and drainage water are allowed to intermingle with each other, This may be accomplished through (i) a level crossing; or through (ii) inlets and outlets.
All these different types of cross drainage works are described below in details.
1. SYPHON AQUEDUCT
When the HFL of the drain is sufficiently below the bottom of the canal, so that the drainage water flows freely under gravity, the structure is known as an aqueduct. However, if the HFL of the drain is higher than the canal bed and the water passes through the aqueduct barrels under syphonic action, the structure is known as Syphon Aqueduct .
2. SUPER PASSAGE
When the FSL of the canal is sufficiently below the bottom of the drain trough, so that the canal water flows freely under gravity, the structure is known as a super passage.
However, in this type of cross-drainage works, the inspection road cannot be provided along the canal and a separate bridge is required for the roadway. For affecting economy, the canal may be flumed, but the drainage trough is never flumed.
Level crossing:- In this type of cross -drainage work, the canal water and drain water are allowed to intermingle with each other, A level crossing is generally provided when a large canal and a huge drainage (such as a stream or a river) approach each other practically at the same level. A typical layout of a level crossing is shown figure .
The arrangement is practically the same as is provided on a canal headworks.
Inlets and outlets:- An inlet is a structure constructed in order to allow the drainage water to enter the canal and get mixed with the canal water and thus to help in augmenting canal supplies. may sometimes be constructed to escape out the additional discharge at a suitable site, a little downstream along the canal.
An inlet essentially consists of an open cut in a canal bank, suitably protected by pitching, to admit the upland drainage water into the canal.The aqueduct or super passage type of works are generally used when high flood drainage discharge is large and continues for a sufficient time. A level crossing is used when the high flood drainage discharge is large but short lived. Inlets and outlets are used when the high flood drainage discharge is small.