Structure identification: In a siphon aqueduct, which relation between canal and drainage is correct?

Introduction / Context:
Cross-drainage works carry a canal across a natural drain/river. The type depends on relative levels of canal bed/full supply and drainage high flood level (HFL).

Given Data / Assumptions:

• Siphon aqueduct vs aqueduct distinction relies on whether canal water runs under pressure (closed trough) when drainage HFL is high.
• Terminology: FSL = full supply level; HFL = high flood level.

Concept / Approach:
In a siphon aqueduct, the canal is carried over the drainage, but the drainage HFL lies above the canal bed. Hence the canal is confined in a trough and may flow under pressure (siphonic action) when flood rises, distinguishing it from a normal aqueduct where the canal remains free-flow.

Step-by-Step Solution:
Identify who passes over whom: canal over drainage.Check level relation: drainage HFL above canal bed → pressure flow in canal trough.Therefore option (c) correctly describes a siphon aqueduct.

Verification / Alternative check:
Standard classification charts list: Aqueduct (HFL below canal bed), Siphon Aqueduct (HFL above canal bed), Super passage (drainage over canal), etc.

Why Other Options Are Wrong:

• Options (a) and (b) describe drainage over canal (super passage), not siphon aqueduct.
• Option (d) is an aqueduct (not siphon) case.

Common Pitfalls:
Confusing super passage with siphon aqueduct; mixing up FSL, bed level, and HFL definitions.

Final Answer:
canal passes over drainage and the drainage HFL is above the bottom of the canal trough