Preferred alignment of irrigation canals — when laying a new canal to minimize cross-drainage works and command both sides, which line is generally followed?

Introduction / Context:
Canal alignment determines hydraulic efficiency, cost of cross-drainage works, and the ability to serve the command area. The classical principle in irrigation planning is to choose an alignment that maximizes gravity command while minimizing structures needed to cross natural drains.

Given Data / Assumptions:

• New gravity canal planned in undulating terrain with natural drainage lines.
• Goal: provide maximum command on both sides and reduce cross-drainage structures.
• Standard alternatives: ridge, contour, or valley alignment.

Concept / Approach:

A ridge-line (watershed) alignment keeps the canal near the drainage divide. From here, the canal can command land on both sides with minimal crossings of streams (since streams usually flow away from the ridge). This cuts costs and hydraulic complications. Contour alignments can be long and sinuous; valley alignments face frequent cross-drainage and flooding risks.

Step-by-Step Solution:

Verification / Alternative check (if short method exists):

Preliminary reconnaissance profiles show fewer crossings and shorter structures along ridges compared with valley routing.

Why Other Options Are Wrong:

Contour lines lead to meandering alignment and longer lengths; valley lines require many cross-drainage works and suffer flood interference; straight or random alignments ignore topography and hydrology.

Common Pitfalls (misconceptions, mistakes):

Confusing watershed alignment with contour alignment; overlooking future drainage development that favors ridge routing.

Final Answer:

ridge line (watershed alignment)