Selecting a site for canal headworks: Among the listed river stages, which location is generally most suitable for constructing canal headworks to ensure stability and a dependable intake?

Introduction / Context:
Headworks divert water from a river into a canal system. The site selection must consider river stability, foundation conditions, sediment entry, and hydraulic control to ensure a reliable and safe intake structure.

Given Data / Assumptions:

• Objective: minimize foundation problems, sediment ingress, and training works.
• Preference for stable bed and banks with manageable hydraulics.
• Avoid rapidly aggrading/degrading or highly mobile reaches.

Concept / Approach:

Rocky reaches (rock stage) are typically narrow, stable, and less prone to lateral migration and bed aggradation. They provide strong foundations and better control for weirs/barrages and intakes, reducing training works and maintenance.

Step-by-Step Solution:

Verification / Alternative check:

Historical headworks on major irrigation projects preferentially select rocky or constricted stable reaches to minimize works and ensure long-term functionality.

Why Other Options Are Wrong:

Boulder or trough stages in alluvium are mobile; delta stages are tidally influenced and unstable for irrigation intakes; shifting sand bars cause sedimentation and intake choking.

Common Pitfalls:

Ignoring seasonal regime changes; underestimating sediment management needs at alluvial sites.

Final Answer:

Rock stage (stable rocky reach)