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Streamflow Measurement and Calculation

Streamflow Measurement and Calculation. Professor Ke-Sheng Cheng Dept. of Bioenvironmental Systems Engineering National Taiwan University. Measuring the flow velocity. Current meter – propeller meter, price current meter. Digital flow meter – Marsh McBirney Model

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Streamflow Measurement and Calculation

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  1. Streamflow Measurement and Calculation Professor Ke-Sheng Cheng Dept. of Bioenvironmental Systems Engineering National Taiwan University

  2. Measuring the flow velocity • Current meter – propeller meter, price current meter. • Digital flow meter – Marsh McBirney Model • It measures flow using the Faraday law of electromagnetic induction. This law states that as a conductor moves through a magnetic field, a voltage is produced. The magnitude of this voltage is directly proportional to the velocity at which the conductor moves through the magnetic field. • When the flow approaches the sensor from directly in front, then the direction of the flow, the magnetic field, and the sensed voltage are mutually perpendicular to each other. Hence, the voltage output will represent the velocity of the flow at the electrodes.

  3. The electrodes on all sensors must be kept free from nonconductive coatings such oil and grease. Electrodes

  4. Discharge calculation

  5. Indirect method of discharge calculation

  6. Slope-area method

  7. Assume the following information is known: • U/S and D/S flow depth, y1 and y2; • Change of water surface elevation through the reach, F; • Length of the reach, L; • Average roughness n for the reach.

  8. Procedures of the slope-area method

  9. The stage-discharge relationship – rating curve

  10. Rating curve adjustment to account for backwater effect • For a given main-stage reading, the discharge under variable backwater condition is a function of the fall F, i.e., Q = f(h, F).

  11. Observe stage h1 and fall F1, • Read Qo* from the constant fall rating curve, • Read (Q1/Qo*) from the adjustment curve using F1/Fo • Q1 = (Q1/Qo*)Qo*

  12. Established using observed data

  13. Unsteady-Flow Effect • When a flood wave passes a gauging station the approach velocities are larger than in the steady flow at corresponding stage. Thus for the same stage, more discharge than in a steady uniform flow occurs. • In the retreating phase of the flood wave the converse situation occurs with reduced approach velocities giving lower discharges than in a equivalent steady flow case. • Thus the stage-discharge relationship for an unsteady flow will not be a single-valued relationship as in steady flow.

  14. Falling stage Rising stage

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