Understanding Cross Drainage Structures in Canals

Cross drainage structures are essential components of irrigation systems where a canal needs to cross a natural stream, river, or drain. These structures allow the canal water to be carried across the path of the natural drainage, or vice versa, without mixing the waters. Different types of structures are used depending on the relative bed levels and discharges of the canal and the stream.

Types of Cross Drainage Structures

Let's examine the common types of cross drainage structures:

• Aqueduct: In an aqueduct, the canal is carried over the stream. This means the bed level of the canal is higher than the high flood level (HFL) of the stream. The stream flows below the canal structure.
• Super Passage: This is the inverse of an aqueduct. The stream is carried over the canal. Here, the bed level of the stream is higher than the full supply level (FSL) of the canal. The canal flows below the stream structure.
• Canal Syphon: Similar to an aqueduct in that the canal is above the stream, but the stream bed level is higher than the canal bed level. The stream water flows under the canal under pressure (syphonic action).
• Syphon Aqueduct: A type of aqueduct where the stream flows below the canal, but the stream waterway is depressed below its natural bed level, causing the stream to flow under pressure.
• Level Crossing: In a level crossing, the canal and the stream intersect at approximately the same level. A structure with regulators is needed to control the flow and prevent mixing of waters.

Analyzing the Question's Condition

The question describes a cross drainage structure where "the canal bed level is kept much higher than the high flood level (HFL) of a stream".

Let's match this condition with the definitions we discussed:

• If the canal bed level is much higher than the stream's HFL, it means the canal is flowing well above the maximum water level the stream can reach during floods.
• This configuration directly matches the definition of an aqueduct, where the canal is carried over the stream, and specifically, the canal bed is positioned above the stream's high flood level to ensure clear passage for the stream even during floods.

Let's consider why the other options do not fit:

• A canal syphon involves the stream flowing under pressure below the canal, and typically the stream bed is higher than the canal bed at the crossing. This contradicts the condition given.
• A level crossing involves the canal and stream intersecting at roughly the same level, which is also contrary to the condition where the canal bed is "much higher" than the stream's HFL.
• A super passage involves the stream flowing over the canal, meaning the stream bed level is higher than the canal's full supply level, the opposite of the condition described.

Conclusion: Identifying the Cross Drainage Structure

Based on the analysis, the structure where the canal bed level is significantly higher than the high flood level of the stream is known as an aqueduct. This allows the canal to pass safely over the stream, ensuring gravitational flow in the canal and unobstructed flow in the stream below.

Revision Table: Cross Drainage Structures

Additional Information on Cross Drainage Structures

The selection of a specific type of cross drainage structure depends on various factors including:

• Relative bed levels and water levels of the canal and stream.
• Discharge rates of both the canal and the stream.
• Topography of the site.
• Soil conditions.
• Economic considerations.
• Head losses and flow conditions required.

Detailed hydrological and structural analysis is performed to choose the most suitable and economical cross drainage structure for a particular site.