Canal over natural drainage: What is the correct name of the cross-drainage work when a canal is carried over a natural drain or river?

Introduction / Context:

Cross-drainage works (CDWs) allow a canal and a natural drainage to cross without interference. The relative elevations of the canal bed level (CBL), canal water level (CWL), and the high flood level (HFL) of the drainage decide the type of CDW used.

Given Data / Assumptions:

• Canal must pass over a natural drain.
• Canal prism stays above the drainage HFL.
• Objective is to maintain uninterrupted canal flow and pass drainage floods safely underneath.

Concept / Approach:

When the canal section is carried physically above the drainage, the structure is called an aqueduct. If the canal is above but has to be dropped below HFL through pressure flow, it becomes a syphon-aqueduct. Conversely, if the drainage is carried over the canal, it is a super-passage. An inverted syphon carries the canal below the drainage through a pressure barrel.

Step-by-Step Solution:

Verification / Alternative check:

Design charts list CDW type selection by comparing CBL/CWL with drainage bed level and HFL. Canal over drainage with freeboard clearance corresponds to an aqueduct.

Why Other Options Are Wrong:

• Syphon: Canal below drainage (pressure flow), not applicable here.
• Super-passage: Drainage over canal, opposite arrangement.
• Syphon-aqueduct: Used when canal must pass under HFL in pressure; not the case for open canal over drain.

Common Pitfalls:

• Confusing aqueduct with super-passage because both are “over” arrangements; the deciding factor is which flow is on top.

Final Answer:

Aqueduct