Canal alignment choice across a drainage: three crossings C1, C2, C3 (with bed levels C1 > C2 > C3) If a proposed canal takes off from a river at point B and three alternative alignments of approximately equal length cross the same drainage at C1, C2, and C3 (where the drainage bed levels satisfy C1 > C2 > C3), which structure is most appropriate at the lowest crossing point C3?

Introduction / Context:
Selecting the correct cross-drainage work (CDW) at a canal–drainage crossing depends on the relative elevations of the canal bed level, canal full-supply level, and the drainage bed/high flood level. When multiple crossing sites exist, the structure should be chosen to be hydraulically sound and economical.

Given Data / Assumptions:

• Three potential crossing locations C1, C2, C3 of similar route length.
• Drainage bed levels: C1 > C2 > C3 (so C3 has the lowest drainage bed).
• Canal alignment and full-supply level are essentially the same for each alternative.

Concept / Approach:
At a site where the drainage bed is low relative to the canal bed, the canal can pass over the drainage on a raised waterway—this is an aqueduct. If the drainage high flood level would submerge the canal waterway, a syphon aqueduct is used. If the drainage must pass over the canal (drain bed much higher), a superpassage is adopted. A syphon (inverted syphon) carries canal water under an obstruction when canal must go below.

Step-by-Step Solution:
Identify that C3 has the lowest drainage bed among the options.Lower drainage bed relative to canal favors canal carried over drainage: aqueduct.Economy and head-loss considerations usually make an aqueduct preferable at such low-bed crossings.

Verification / Alternative check:
Check flood levels: if drainage HFL is still below the canal soffit, a simple aqueduct suffices; only if the canal waterway would be drowned should a syphon aqueduct be considered.

Why Other Options Are Wrong:
Syphon aqueduct: Needed when canal must pass under a high HFL; not the favored choice here.Superpassage: Used when drainage bed/HFL is above canal; opposite situation.Syphon (inverted syphon): For carrying canal below an obstacle; not indicated by the low drain bed.

Common Pitfalls:

• Ignoring HFL and deciding solely on bed levels.
• Confusing aqueduct with superpassage due to naming symmetry.

Final Answer:
An aqueduct