Cross Drainage Works
A cross drainage work is a structure carrying the discharge from a natural stream across a canal intercepting the stream.
Canal comes across obstructions like rivers, natural drains and other canals.
The various types of structures that are built to carry the canal water across the above mentioned obstructions or vice versa are called cross drainage works. There are many different factors involved in selection of a specific type of Cross drainage works and in selection of a suitable site for cross drainage works.
It is generally a very costly item and should be avoided by
Types of cross drainage works
Depending upon levels and discharge, it may be of the following types:
When the HFL of the drain is sufficiently below the bottom of the canal such that the drainage water flows freely under gravity, the structure is known as Aqueduct.
In case of the siphon Aqueduct, the HFL of the drain is much higher above the canal bed, and water runs under siphonic action through the Aqueduct barrels.
The drain bed is generally depressed and provided with pucci floors, on the upstream side, the drainage bed may be joined to the pucca floor either by a vertical drop or by glacis of 3:1. The downstrean rising slope should not be steeper than 5:1. When the canal is passed over the drain, the canal remains open for inspection throughout and the damage caused by flood is rare. However during heavy floods, the foundations are succeptible to scour or the waterway of drain may get choked due to debris, tress etc.
Cross drainage works carrying drainage over canal.
The structures that fall under this type are:
Selection of suitable site for cross drainage works
Factors which influence the choice / Selection of Cross Drainage Works
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.
Classification of aqueduct and siphon aqueduct
Depending upon the nature of the sides of the aqueduct or siphon aqueduct it may be classified under three headings:
Sides of the aqueduct in earthen banks with complete earthen slopes. The length of culvert should be sufficient to accomodate both, water section of canal, as well as earthen banks of canal with aqueduct slope.
Sides of the aqueduct in earthen banks, with other slopes supported by masonry wall. In this case, canal continues in its earthen section over the drainage but the outer slopes of the canal banks are replaced by retaining wall, reducing the length of drainage culvert.
Sides of the aqueduct made of concrete or masonry. Its earthen section of the canal is discontinued and canal water is carried in masonry or concrete trough, canal is generally flumed in this section.