What Is Cross Drainage Work?
Cross drainage works is a structure constructed when there is a crossing of canal and natural drain, to prevent the drain water from mixing into canal water. This type of structure is costlier one and needs to be avoided as much as possible.
It’s a structure, often referred to as a cross drainage structure, that carries the discharge of a natural stream across a canal intercepting the stream. This is essential in understanding the difference between aqueduct and syphon aqueduct.” When a canal is to be taken to the watershed, it crosses a number of natural streams in the distance between the reservoir to the watershed.
Once the canal is taken to the watershed, no cross-drainage works are normally necessary except when the canal leaves the watershed for some distance forming a loop. However, when the canal is aligned as a contour canal, a number of crass-drainage works are necessary. Cross-drainage work is generally costly and should be avoided as far as possible.
Cross drainage works can be avoided in two ways:
- By changing the alignment of canal water way
- By mixing two or three streams into one and only one cross drainage work to be constructed, making the structure economical.
Types of Cross Drainage Works.
Depending upon the relative levels and discharges, there are various types of aqueduct and cross-drainage works, which may be of the following types:
1. C.D. Works Carrying the Canal Over the Drainage
2. C.D. Works Carrying Drainage Over the Canal
3. C.D. Works Admitting the Drainage Water into the Canal
C.D. Works Carrying the Canal Over the Drainage
Plan of Crossing
In this type of C.D. work, often referred to as level crossing in canal, the canal is carried over the natural drain.
(See above Figure). The advantage of such an arrangement is that the canal, running perennially, is above the ground and is open to inspection.
Also, the damage done by floods is rare. However, sometimes during heavy floods, the foundation can be scoured, or the waterway of the drain may be chocked with trees, etc. This is the usual type of wort constructed when the drain is very big in comparison to the section of the canal.
Types of C.D. Works Carrying the Canal Over the Drainage
- Aqueduct.
- Syphon Aqueduct.
Aqueduct and Syphon Aqueduct.
In these works, the canal is taken over the natural drain, such that the drainage water runs below the canal (see above Fig.) either freely or under syphoning pressure.
Aqueduct
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 (See above Fig.)
Syphon Aqueduct
Syphon Aqueduct
However, if the HFL of the drain is higher than the canal bed and the water, often termed as drainage syphon, passes through the aqueduct barrels under syphonic action, the structure is known as siphon aqueduct or syphon aqueduct in irrigation. (See above Fig.). In this type of works, the canal water is taken across the drainage in a trough supported on piers.
An inspection road is generally provided along with the trough, as shown. An aqueduct is just like a bridge except that instead of carrying a road or a railway, it carries a canal on its top. An aqueduct is provided when a sufficient level difference is available between the canal and the natural drainage, and canal bed level is sufficiently higher than the torrent level.
In Sirsa, a city near Roper in Punjab, an excellent aqueduct having 20 spans of about 13 m each has been constructed to carry a canal having a bed width of 28 meters and discharge of about 360 cumecs, with a torrent discharge of about 4300 cumecs.
A difference of 3.3 meters was available between the bed level of the canal and that of a torrent in this case. In the case of a syphon aqueduct, the drain bed is generally depressed and provided with the pucca floor, as shown in the above fig.
On the upstream side, the drainage bed may be joined to the pucca floor either by a vertical drop (when a drop is of the order of 1 m) or by a glacis of 3: 1 (when a drop is more). The downstream rising slope should not be steeper than 5: 1.
In this type of cross-drainage works (i.e., when the canal is taken over the drainage), the canal remains open to inspection throughout, and the damage caused by floods is rare. However, during heavy floods, the foundations of the work may be susceptible to scour; or the waterway of the drain may get choked with debris, trees, etc.
C.D. Works Carrying Drainage Over the Canal
Crossing
In this type of C.D. work, drainage is carried over the canal (See above Figure). The advantage of this type is that the C.D. works themselves are less liable to damage than the earth-work of the canal. The major disadvantage of this work is that the perennial canal is not open to inspection. Also, if the silt is deposited in the barrels of the work, it is difficult to clear it out.
The structures that fall under this type are-
Types of C.D. Works Carrying Drainage Over the Canal
- Super Passage.
- Canal Syphon.
Super Passage and Canal Syphon
Drain took over the canal in a Superpassage or in a Syphon
In the case of works, the drain is taken over the canal such that the canal water runs below the drain (See above Figure) either freely or under syphoning pressure.
Super Passage
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 (See above Figure Super Passage).
Canal Syphon
Canal Syphon
However, if the FSL of the canal is sufficiently above the bed level of the drainage trough so that the canal flows under syphonic action under the trough, the structure is known as a canal syphon or a Syphon (See above Figure Canal Syphon). A super passage is thus the reverse of an aqueduct, and similarly, a syphon is a reverse of an aqueduct syphon.
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 road-way. For affecting the economy, the canal may be a flume, but the drainage trough is never a flume. In the case of a syphon, the canal bed is depressed, and a ramp is provided at the exit so that the trouble of silting is minimized.
C.D. Works Admitting the Drainage Water into the Canal
Level Crossing
In this type of work, often referred to as define cross drainage work, the canal water and the drainage water are permitted to intermingle with each other. The only advantage of this type of work is its low initial cost.
Such type of works have the following disadvantages :
Disadvantages of C.D. Works Admitting the Drainage Water into the Canal
- Regulation of such a work is difficult and requires additional staff.
- The canal has to be designed to carry the increased flood discharge of the drain.
- The faulty regulation of gates may damage the canal.
- There is an additional expenditure of silt clearance.
The following are the structures under this type of C.D. works :
Types of C.D. Works Admitting the Drainage Water into the Canal
- Level Crossing.
- Inlet and Outlets.
Level Crossing.
Typical Layout of a 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 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 above fig.
A regulator is provided across the torrent (drainage) just on the downstream side of the crossing so as to control the discharge passing into the torrent. At the outgoing canal, a regulator is also provided so as to control the discharge into the canal. A regulator at the end of the incoming canal is also sometimes required.
The arrangement is practically the same as is provided on a canal headworks. This arrangement is generally provided when a huge sized canal crosses, a large torrent carrying a very high but short-lived flood discharge. In this arrangement, the perennial drainage discharge is sometimes advantageously used, so as to augment the canal supplies.
During the dry season, when there are no floods, the torrent regulator is generally kept closed, and the outgoing canal regulator is kept fully open so that the canal flows without any interruption. During floods, however, the torrent regulator is opened so as to pass the flood discharge.
A beautiful level crossing has been provided in Eastern U.P. under Sarda Sahayak Pariyojna, where a canal carrying 370 cumecs crosses the Sarda river carrying a high flood discharge of the order of 10,000 cumecs.
Inlet and Outlets.
Inlet and Outlet (Plan)
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. Such a structure is generally adopted when the drainage discharge is small, and the drain crosses the canal with its bed level equal to or slightly higher than the canal F.S.L.
Moreover, for the canal to remain in the regime, the drain water must not admit a heavy load of silt into the canal. Thus, in an inlet, the drainage water is simply added to the canal.
But, when the drainage discharge is high or if the canal is small so that the canal section cannot take the entire drainage water, an outlet may sometimes be constructed to escape out the additional discharge at a suitable site, a little downstream along the canal.
It is not necessary that the escaped discharge should be equal to the admitted discharge. This type of cross-drainage work (i.e., those requiring intermingle of canal water with drainage water) is inferior to aqueduct or super passage type of works, but they are cheaper.
Hence, 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.
Frequently Asked Questions (FAQ) on Cross Drainage Works
What Are Cross Drainage Works?
Cross drainage works refer to structures built to carry the discharge of natural streams across canals. These structures ensure that the natural drain water does not mix with the canal water, maintaining the canal’s water quality and flow.
Why Are Cross Drainage Works Necessary?
Cross drainage works are essential to manage the interaction between natural streams and artificial canals, preventing the mixing of different water sources and ensuring the efficient flow of both the canal and the natural drain.
What Are the Main Types of Cross Drainage Works?
The main types of cross drainage works include:
- Works carrying the canal over the drainage
- Works carrying drainage over the canal
- Works admitting the drainage water into the canal
What Is an Aqueduct in Cross Drainage Works?
An aqueduct is a structure where the canal is carried over the natural drainage, allowing the drainage water to flow freely underneath by gravity. This type of structure is used when the High Flood Level (HFL) of the drain is below the canal bed level.
What Is a Syphon Aqueduct?
A syphon aqueduct is similar to an aqueduct, but the drainage water flows under the canal under syphonic pressure when the HFL of the drain is higher than the canal bed level. This structure ensures the canal water passes over the drainage without interruption.
How Do Super Passages and Canal Syphons Differ?
A super passage carries drainage water over the canal, allowing the canal water to flow freely beneath by gravity. In contrast, a canal syphon allows the canal water to flow under the drainage trough under syphonic action. These structures are essentially the reverse of aqueducts and syphon aqueducts.
What Is a Level Crossing in Cross Drainage Works?
A level crossing allows both canal water and drainage water to intermingle. This structure is used when a large canal and a significant drainage stream meet at approximately the same level. It is a cost-effective solution but requires careful regulation to manage water flow and prevent damage.
What Are Inlets and Outlets in Cross Drainage Works?
Inlets allow drainage water to enter and mix with the canal water, augmenting canal supplies, typically used when the drainage discharge is small. Outlets help to escape the excess water downstream when the drainage discharge is high, preventing overflow and maintaining canal integrity.
How Can Cross Drainage Works Be Avoided?
Cross drainage works can sometimes be avoided by:
- Changing the alignment of the canal to bypass natural drains
- Combining multiple smaller streams into a single larger stream, requiring only one cross drainage structure
What Are the Common Challenges with Cross Drainage Works?
Common challenges include:
- High construction and maintenance costs
- Potential for structural damage during heavy floods
- Difficulty in clearing silt and debris from the structures
- Complex regulation and management of water flow to prevent mixing and overflow
