Canal alignment along watersheds: Under which circumstances is a watershed (ridge) alignment abandoned when planning an irrigation canal?

Introduction / Context:

A watershed (ridge) alignment is generally preferred for main canals because it minimizes cross-drainage structures and seepage into adjacent commands. However, field constraints often force deviations from the ridge alignment to balance constructability, hydraulics, and social impacts.

Given Data / Assumptions:

• Candidate canal alignment follows the watershed where feasible.
• Practical constraints include terrain geometry, river offtake conditions, and habitation corridors.
• Objective is to ensure hydraulic efficiency and reduce costs/risks.

Concept / Approach:

Watershed alignment is abandoned if it introduces excessive curvature (sharp loops), conflicts with mandatory offtake geometry from the source river, or intersects dense settlement corridors. Each of these can increase length, create unsafe embankments, or raise resettlement/land acquisition burdens.

Step-by-Step Solution:

Verification / Alternative check:

Feasibility reports compare alternatives with counts of cross-drainage works, excavation/embankment quantities, land impacts, and hydraulic losses; often these justify abandoning strict ridge alignment.

Why Other Options Are Wrong:

• None of the above is incorrect because all listed conditions are valid reasons to shift alignment.

Common Pitfalls:

• Over-valuing ridge alignment despite prohibitive bends or offtake problems.
• Ignoring long-term operation/maintenance costs of overly sinuous alignments.

Final Answer:

All the above.