In an aqueduct, natural drainage is kept at the following of a canal
below
Understanding Aqueduct Structures and Cross Drainage
When an irrigation canal needs to cross a natural drainage channel, such as a stream or river, engineers design special structures. These are known as cross drainage works. The type of structure built depends on the relative levels of the canal bed and the drainage bed, as well as the nature of flow in both. One common type of cross drainage work is an aqueduct.
What is an Aqueduct in Civil Engineering?
An aqueduct is a hydraulic structure that allows an irrigation canal to pass over a natural drainage channel. Think of it like a bridge for the canal water, allowing it to flow above the river or stream below. This design is used when the bed level of the canal is higher than the bed level of the natural drainage.
Position of Natural Drainage Relative to Canal in an Aqueduct
In an aqueduct, the defining characteristic is the relative positioning of the two channels. The canal flows in a trough built supported by piers, effectively elevating the canal water level. The natural drainage channel passes underneath this trough.
Therefore, in a standard aqueduct:
- The canal passes above the natural drainage.
- The natural drainage passes below the canal.
The natural drainage maintains its natural course and flow pattern, simply passing beneath the elevated canal structure. The canal water flows by gravity in the elevated trough.
Summary of Level Relationship in an Aqueduct
To reiterate the key relationship in an aqueduct structure:
| Channel Type | Relative Vertical Position |
|---|---|
| Canal Water Level | Higher (Above) |
| Natural Drainage Channel | Lower (Below) |
This arrangement ensures that the canal water can be transported across the drainage path without interruption, and the drainage flow is also unimpeded as it flows underneath the canal.
Revision Table: Aqueduct Drainage Position
| Structure Part | Position Relative to the Other Channel |
|---|---|
| Canal | Above the natural drainage |
| Natural Drainage | Below the canal |
Additional Information: Other Cross Drainage Works
Besides aqueducts, other structures are used for cross drainage depending on the site conditions:
- Superpassage: This is the reverse of an aqueduct. The natural drainage is carried over the canal. This happens when the drainage bed level is higher than the canal bed level.
- Siphon Aqueduct: Similar to an aqueduct (canal over drainage), but the drainage level is high enough that it runs under the canal under pressure, acting like a siphon. The drainage bed is depressed.
- Canal Syphon: The canal passes under the drainage, with the canal flowing under pressure (siphoning) below the drainage bed.
- Level Crossing: Both the canal and the drainage are at the same level and intersect. This is less common for major crossings due to potential issues with water mixing and flow control.
Understanding these different types of cross drainage works helps clarify why, in an aqueduct specifically, the natural drainage is positioned below the canal.
Important Questions from Cross Drainage Works
A cross drainage work in which a canal is provided over the natural drainage is called:
Which of the following is not a cross-drainage work?
The cross drainage structure in which the canal bed level is kept much higher than the high flood level (HFL) of a stream is called
When the drain is over the canal, and FSL of canal is below the bed level of drain the structure provided is known as
Select the correct option for the given statements.
Statement 1: When the crossing site is such that the canal FSL is well above the stream HFL, the choice between aqueduct and siphon aqueduct is made depending on the stream discharge.
Statement 2: For larger stream discharges (i.e. when the stream bed is much wider), an aqueduct is more suitable than a siphon aqueduct which requires lowering of the stream bed by a drop.
