Cross Drainage Work Types Explained

Cross-drainage works are structures built to manage the intersection of a canal (for irrigation or navigation) and a natural drainage channel (like a river or stream). The goal is to ensure that the drainage water does not interfere with the canal water, and vice versa, by either passing one over the other or under the other.

Key Cross Drainage Structures:

• Aqueduct: Used when the canal passes over the drainage channel. This is suitable when the canal bed level is significantly higher than the drainage channel's High Flood Level (HFL).
• Syphon: Used when the canal passes under the drainage channel. This is chosen when the canal's Full Supply Level (FSL) or its bed level is lower than the drainage channel's HFL. The water is conveyed through tunnels or pipes beneath the drainage.
• Super Passage: Used when the drainage channel passes over the canal. This is essentially the reverse of an aqueduct, suitable when the drainage channel's HFL is much higher than the canal's bed level.
• Siphon Aqueduct: A hybrid structure where the canal is taken under the drainage, but the canal's structure might be elevated, and the canal's FSL is higher than the drainage's normal water level, though still below the drainage's HFL. It typically involves tunnels for the canal passing under the drainage.

Analyzing Canal and Drainage Levels

Let's examine the data provided for the project:

We need to compare the relative levels of the canal and the drainage channel.

• Canal Full Supply Level ($FSL_{canal}$): 105
• Canal Bed Level ($BL_{canal}$): 100
• Drainage High Flood Level ($HFL_{drainage}$): 106
• Drainage Bed Level ($BL_{drainage}$): 102

Selecting Syphon Based on Levels

The primary factor in deciding the type of cross-drainage work is the relative vertical position of the canal and the drainage channel.

Comparing the Canal FSL to the Drainage HFL:

• $FSL_{canal}$ = 105
• $HFL_{drainage}$ = 106

Since the Canal FSL (105) is lower than the Drainage HFL (106), the canal water must pass under the drainage channel.

Specifically, when the canal passes under the drainage and its FSL is below the drainage HFL, the structure is classified as a syphon.

The canal bed level (100) is also lower than the drainage HFL (106), reinforcing that the canal structure must be built below the drainage's highest possible water level.

Therefore, based on the provided levels where the canal's water surface is below the drainage's highest flood level, a syphon is the most appropriate cross-drainage work.