Two-point method for mean stream velocity For a stream of depth h, the average mean velocity is often estimated as the average of current-meter readings taken at depths:

Introduction / Context:Accurate discharge measurement in open channels often uses vertical velocity profiles. The two-point method is a practical compromise between accuracy and effort.

Given Data / Assumptions:

• Relatively uniform vertical velocity profile (no extreme stratification).
• Current meter provides point velocities at specified depths along a vertical.
• Depth h at the vertical is known.

Concept / Approach:Empirical studies show that averaging velocities at 0.2 h and 0.8 h approximates the depth-averaged velocity well. When only one point is feasible, the 0.6 h method is common; with three points, 0.2 h, 0.6 h, and 0.8 h may be used with weights.

Step-by-Step Solution:

Verification / Alternative check:The one-point 0.6 h method yields v̄ ≈ v(0.6 h) and is slightly less accurate than the two-point average for many natural streams.

Why Other Options Are Wrong:

• 0.1/0.9 or 0.3/0.7 and 0.4/0.6 pairs are not the standard two-point recommendation and can bias v̄.
• Surface-only readings neglect lower-depth shear and are unreliable.

Common Pitfalls:Ignoring secondary currents near banks or measuring too close to bed/surface where meter errors increase.

Final Answer:0.2 h and 0.8 h