When using a surface float to measure stream velocity, the mean velocity of the stream is estimated as:
Choose the correct answer
Exactly equal to the surface float velocity
The surface float velocity multiplied by a reduction factor of 0.85-0.90 (because surface velocity > mean velocity due to the velocity profile)
Two times the surface float velocity
The float velocity / depth
Correct Answer
B. The surface float velocity multiplied by a reduction factor of 0.85-0.90 (because surface velocity > mean velocity due to the velocity profile)
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Surface float measures surface velocity. Mean velocity in a vertical = 0.85 to 0.90 times surface velocity (empirical). Correction factor depends on channel shape and flow conditions. For accurate work, use a double float (surface + sub-surface) or current meter. Float method is used when current meter gauging is impractical (very high or turbulent floods).
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