Velocity measurement: A current meter is used to measure the velocity of flow when held

Introduction / Context:
River and canal discharge measurements require point velocities. A current meter is a standard instrument for obtaining local velocity values in the wetted section.

Given Data / Assumptions:

• Proper calibration of the current meter is available.
• Instrument is submerged and aligned with flow.
• Depth and location can be selected as per measurement scheme.

Concept / Approach:
By placing the current meter (e.g., cup or propeller type) at specific points in the cross-section, the local velocity is measured. Multiple-point methods (0.2D and 0.8D; or 0.6D single-point) integrate these to obtain mean velocity.

Step-by-Step Solution:
Position the meter at a defined location (x, y) within the wetted cross-section.Record rotations to infer local velocity via calibration.Repeat at various points to compute the cross-sectional average if discharge is required.

Verification / Alternative check:
Hydrometry standards specify point, vertical, and area methods; all rely on point measurements taken anywhere within the flowing section.

Why Other Options Are Wrong:

• Surface or bottom only does not capture the velocity distribution.
• “Outside wetted area” is nonsensical for measuring flow.
• Only mid-depth on centreline is a special-case estimate, not a general rule.

Common Pitfalls:
Failing to align with flow; measuring too close to boundaries where instrument error increases.

Final Answer:
at any chosen point within the cross-section