Canal instrumentation: Bed bars provided across a canal section are primarily intended for what operational purpose?

Introduction / Context:
Operational observation features are installed on canals to monitor bed levels and silt movement. One such feature is the bed bar, a small permanent marker or bar across the bed.

Given Data / Assumptions:

• Unlined or lined canal with potential aggradation/degradation.
• Need for long-term monitoring of bed elevation.
• Bed bars act as reference points, not flow control structures.

Concept / Approach:
Bed bars serve as fixed datum lines or reference features to periodically check bed profile changes. They are not meant to function as measuring weirs or to appreciably alter hydraulic grade line.

Step-by-Step Solution:
Install low-profile bar across bed at specified stations.Use sounding/levelling to observe bed level relative to bar elevations.Infer silting or scouring trends to guide maintenance and desilting.

Verification / Alternative check:
Instrumentation manuals treat bed bars as observation aids. Discharge measurement uses calibrated structures (weirs/flumes), not bed bars.

Why Other Options Are Wrong:

• Measure discharge: Requires hydrometric structures; bed bars are unsuitable.
• Raise supply level: Needs control structures (regulators/weirs).
• Control silting by throttling: Bed bars are passive; they do not control flow.
• All the above: Incorrect because only observation is valid.

Common Pitfalls:
Assuming any cross-bed feature acts as a weir; overlooking that bed bars are primarily for monitoring.

Final Answer:
To watch and record the general behaviour (bed changes/silting) of the canal