Understanding Cross Drainage Works

Cross drainage works are structures built when a canal and a natural drain (like a stream, river, or nullah) cross each other. These structures are essential in irrigation systems to prevent the waters from mixing and to ensure the smooth flow of both the canal and the drain.

The type of cross drainage work chosen depends mainly on the relative levels of the canal bed, the canal water surface, the drain bed, and the drain water surface.

Types of Cross Drainage Works

Based on the relative levels, common types of cross drainage works include:

• Aqueduct: The canal passes over the drain. This is typically used when the canal bed level is much higher than the drain water level.
• Syphon Aqueduct: Similar to an aqueduct, but the drain runs below the canal under syphon action (under pressure). This is used when the drain water level is higher than the canal bed level, forcing the drain flow to dip below the canal.
• Super Passage: The drain passes over the canal. This is used when the drain bed level is much higher than the canal water level.
• Syphon: Similar to a super passage, but the canal runs below the drain under syphon action (under pressure). This is used when the canal water level is higher than the drain bed level, forcing the canal flow to dip below the drain.
• Level Crossing: Both the canal and the drain are practically at the same level. At the point of crossing, the waters of the canal and the drain are allowed to mix, and control structures are provided to regulate the flow into their respective channels.

Level Crossing for Canal and Stream at Same Level

The question describes a situation where a large canal and a stream approach each other practically at the same level. In this specific scenario, where the levels of the canal and the stream are nearly identical, allowing the waters to mix temporarily at the crossing point becomes a viable and often necessary solution.

A Level crossing structure is designed precisely for this condition. It consists of regulatory gates or control structures on both the canal and the stream channels near the crossing. When the stream is flowing, the canal gates might be closed to allow the stream water to pass, and vice versa. This requires careful management but is the most suitable type of cross drainage work when the levels are essentially the same, avoiding the need for expensive structures to carry one flow above or below the other.

Let's consider why the other options are less suitable:

• Aqueduct/Super Passage: These are used when there is a significant difference in levels, allowing one channel to pass clearly over the other without mixing. This is not the case when levels are "practically at the same level".
• Syphon Aqueduct/Syphon: These are used when the level difference is such that one channel must pass under the other under pressure (syphon action). Again, this implies a notable difference in levels, not the "practically at the same level" condition.

Therefore, for the condition where a large canal and a stream are practically at the same level, a Level crossing is the appropriate cross drainage work.