Applied Hydrology RSLAB-NTU Lab for Remote Sensing Hydrology and Spatial Modeling 1 Streamflow Measurement and Calculation Professor Ke-Sheng Cheng Dept. of Bioenvironmental Systems Engineering National Taiwan University

Lab for Remote Sensing Hydrology and Spatial Modeling RSLAB-NTU 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.

Lab for Remote Sensing Hydrology and Spatial Modeling RSLAB-NTU 3 The electrodes on all sensors must be kept free from nonconductive coatings such oil and grease. Electrodes

Lab for Remote Sensing Hydrology and Spatial Modeling RSLAB-NTU 4 Discharge calculation

Lab for Remote Sensing Hydrology and Spatial Modeling RSLAB-NTU 5 Indirect method of discharge calculation

Lab for Remote Sensing Hydrology and Spatial Modeling RSLAB-NTU 6 Slope-area method

Lab for Remote Sensing Hydrology and Spatial Modeling RSLAB-NTU 7 Assume the following information is known:  U/S and D/S flow depth, y 1 and y 2 ;  Change of water surface elevation through the reach, F;  Length of the reach, L;  Average roughness n for the reach.

Lab for Remote Sensing Hydrology and Spatial Modeling RSLAB-NTU 8 Procedures of the slope-area method

Lab for Remote Sensing Hydrology and Spatial Modeling RSLAB-NTU 9 The stage-discharge relationship – rating curve

Lab for Remote Sensing Hydrology and Spatial Modeling RSLAB-NTU 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).

Lab for Remote Sensing Hydrology and Spatial Modeling RSLAB-NTU Observe stage h1 and fall F1, 2. Read Qo* from the constant fall rating curve, 3. Read (Q1/Qo*) from the adjustment curve using F1/Fo 4. Q1 = (Q1/Qo*)Qo*

Lab for Remote Sensing Hydrology and Spatial Modeling RSLAB-NTU 12 Established using observed data

Lab for Remote Sensing Hydrology and Spatial Modeling RSLAB-NTU 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.

Lab for Remote Sensing Hydrology and Spatial Modeling RSLAB-NTU 14 Rising stage Falling stage