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Current meter use by flow characteristics

Short answer

Measurement depth dictates method: one-point at 0.6d for shallow, two-point at 0.2 and 0.8d for moderate, three or five-point for deep. Unreliable in very low/high velocity, sediment, weeds; then use floats.

Describe about the use of current meter according to flow characteristics of channel.

A current meter (cup type Price meter, or propeller type) measures velocity from the rate of rotation of its rotor: v=aN+bv = aN + b. The way it is used depends on the depth and flow condition of the channel.

According to depth (points of velocity measurement in a vertical)

DepthMethodMean velocity
Very shallow (< 0.6 m)One point at 0.6 dvm=v0.6v_m = v_{0.6}
Moderate (0.6 to 3 m)Two points at 0.2 d and 0.8 dvm=(v0.2+v0.8)/2v_m = (v_{0.2}+v_{0.8})/2
Moderate to deepThree points at 0.2, 0.6, 0.8 dvm=(v0.2+2v0.6+v0.8)/4v_m = (v_{0.2}+2v_{0.6}+v_{0.8})/4
Deep and variableFive points (surface, 0.2, 0.6, 0.8 d, bed)vm=110(vs+3v0.2+3v0.6+2v0.8+vb)v_m = \frac{1}{10}(v_s + 3v_{0.2} + 3v_{0.6} + 2v_{0.8} + v_b)

According to flow characteristics

  • Shallow, small streams: the engineer wades with a rod-mounted meter (pigmy or Price AA) held at 0.6 d.
  • Wide and deep rivers: the meter is suspended from a bridge, cableway or boat with a weight (sounding weight) on a cable; the weight is chosen to keep the cable nearly vertical.
  • Very low velocity (< 0.15 m/s) or very high turbulent flood: a current meter is unreliable, so floats or other methods are used.
  • Sediment-laden or weedy flow: weeds and silt can foul the rotor, so it is cleaned and checked before each use.
  • Uniform flow: velocity is measured at 20 to 30 verticals spaced so that no strip carries more than 5 to 10 % of the discharge.
  • Time of observation: at least 40 to 60 s at each point, repeated when the velocity fluctuates.
  • The meter must be rated (calibrated) in a towing tank to give v=aN+bv = aN+b, and held pointing into the flow, away from the observer's body.

✓ Answer checked by Claude Sonnet 5.5

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