A Cross drainage works or structure is a structure that is constructed at the crossing of a canal and natural drain, so as to dispose of drainage water without interrupting the continuous canal supplies. Cross drainage structure may be of the following types.
Types of cross drainage structure
1) Irrigation canal passes over the drainage
- Siphon aquaduct
2) Drainage passes over the irrigation canal
- Super passage
- Siphon super passage (Canal siphon)
3) Drainage and canal intersection each other of the same level
- Level crossing
- Inlet and outlet
Selection of types of cross drainage works
However, in the actual field, ideal conditions may not be available and the choice would then depend upon the many factors, such as,
- Relative bed level
- Economical consideration
- Discharge of the drainage
- Construction problems
- Suitable canal alignment
The following considerations are more important for cross drainage works
- When the bed level of the canal is much above the H.F.L (High flow level) of the drainage an aqueduct is an obvious choice.
- When the bed level of the drainage is well above F.S.L of the canal, a super passage is provided.
- The necessary headway between the canal bed level and the drainage H.F.L can be increased by shifting the crossing to the downstream of drainage. If however, it is not possible to change canal alignment, a siphon aqueduct may be provided.
- When canal bed level is much lower but S.F.L of the canal is higher than bed level of drainage, canal siphon or siphon super-passage is preferred.
- When drainage and canal cross each other practically at the same level, a level crossing may be preferred. These types of work are avoided as far as possible.
- Suitable canal alignment
- Position of the water table and availability of dewatering equipment.
1) For passing the canal over the drainage
When the H.F.L. 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 this, canal water is carried across drainage in a trough supported on piers. This is the ridge carrying water. Provided when sufficient level difference is available between the canal and natural and canal bed is sufficiently higher than H.F.L.
b) Siphon aqueduct
In the case of the siphon Aqueduct, the H.F.L. (High Flow Level) of the drain is much higher above the canal bed, and water runs under siphonic action through the aqueduct barrel. The drain bed is generally depressed and provided with pucca 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 downstream 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 a flood is rare.
However during heavy floods, the foundations are susceptible to scour or the waterway of the drain may get choked due to debris, tress, etc.
2) For passing the canal below the drainage Super passage
a) Supper Passage
The hydraulic structure in which the drainage Is passing over the irrigation canal is known as super passage.
This structure is suitable when the bed level of drainage is above the flood surface level of the canal. The water of the canal passes clearly below the drainage.
b) Canal siphon
If two canals cross each other and one of the canals is siphoned under the other, then the hydraulic structure at the crossing is called “canal siphon”. In the case of the siphon, the F.S.L. of the canal is much above the bed level of the drainage trough, so that the canal runs under the siphonic action.
3) The For passing drain through the canal
a) Level crossing
The level crossing is an arrangement provided to regulate the flow of water through the drainage and the canal when they cross each other approximately at the same bed level.
The level crossing consists of the following components.
It is provided across the drainage just at the upstream side of the crossing point. The top level of the crest wall is kept at the full supply level of the canal.
It is provided across the drainage just at the downstream side of the crossing point. The regulator consists of adjustable shutters at different tiers.
It is provided across the canal just at the downstream side of the crossing point. This regulator also consists of adjustable shutters at different tiers.
b) Inlet and outlet
In the crossing of small drainage with a small channel, no hydraulic structure is constructed. Simple openings are provided for the flow of water in their respective directions. This arrangement is known as inlet and outlet.
In this system, an inlet is provided in the channel bank simply by open cut and the drainage water is allowed to join the channel.
At the points of inlet and outlet, the bed and banks of the drainage are protected by stone pitching.
Types of aqueducts and siphon-aqueducts
Depending upon the nature of the sides of the aqueduct or siphon aqueduct it hay be classified under three headings.
Side of the aqueduct in earthen banks with complete earthen slopes. The length of the culvert should be sufficient to accommodate both, water section of the canal, as well as earthen banks of the canal with aqueduct slope.
Sides of the aqueduct in earthen banks, with other slopes supported by a masonry wall. In this case, the canal continues in its earthen section over the drainage but the outer slopes of the canal banks are replaced by a retaining wall, reducing the length of a drainage culvert.
Sides of 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 in this section.
Suitability of Types of aqueducts and siphon-aqueducts
Type –I aqueduct or siphon will be economical only when the length of the aqueduct is small where the cost of bank connections would be large in comparison to the saving obtained from the reduction in the width of the aqueduct.
In Type-III the width of the aqueduct is minimum but the cost of bank connections is maximum. This type is, therefore suitable where the length of the aqueduct is very large and where the cost of bank connection would be small in comparison to the saving obtained from the reduction in the width of the aqueduct.
So, generally in very small drain type-I is most economical and in very large drain type-III is most economical.
I hope this article on “Cross Drainage Works | Syphon Aqueduct, Super passage, Canal Syphon” remains helpful for you.
Happy Learning – Civil Concept
Civil Engineer – Ranjeet Sahani