WMO / OMM Autorite du Bassin du Niger Niger-HYCOS and Volta-HYCOS Projects Training programme on flow measurements 2 nd part : Flow measurement techniques
Flow measurements and calculations Definitions Volumetric method Weirs Chemical dilution Electromagnetic method Ultrasonic or acoustic method Flow measurements with current meter Flow measurements with floats uncertainty, errors
Definitions global methods : Q is measured directly (volumetric method, weirs, chemical dilution, electromagnetisme); incomplete methods : velocity is partialy explored (ultrasonic, mobile boat, floats); complete methods : the 3 components Hm, L, U are measured separately (current meter, ADCP).
Global methods Volumetric measurements First principle : direct flow determination from the necessary time to fill a container of a given capacity
Global methods Thin plate weirs Objective : the flows passes through a hydraulic structure with known geometric characteristics Application of hydraulic principles Determination of the rating curve Principle : to obtain a critical regime Unique relation between flow Q and stage h (stage above control) : Q = K C H n With : K: flow coefficient linked to H; C and n : parameters depending on the characteristics of the weir
Global methods Thin plate weirs
Global methods Broad crested weirs
Global methods Weirs : Parshall flume
Global methods Weirs : Parshall flume Effect of a contraction section
Global methods Chemical dilution Cross section 1 Injection of the solution with constant flow q abstraction Cross section 2 Q : river flow q : rate of flow of the injected tracer solution C1 : concentration of the tracer solution injected into the stream C2 : concentration of the tracer at the abstraction section
Global methods Chemical dilution
Incomplete methods Ultrasonic method
Incomplete methods flow measurements with floats : principles Measurement of surface velocities in the upper surface (about first 20 centimeters) Floats can be artificial (plastic bottles) or natural (trees, big branches, etc.). The horizontal deplacement of a surface float during time t enables the determination of the surface velocity of flow. Several measures of float velocity must be carried out. The mean of these measurements is then multiplied by the appropriate coefficient to get the average velocity of the section
Incomplete methods flow measurements with floats : examples of floats All natural or artificial objects carried by water, partially or totaly immersed, whose vertical movements indicate the variations of water level and whose horizontal movements indicate the velocity of flow in the surface or at different depths.
Incomplete methods flow measurements with floats : measurement method Float Measurement of velocity
Incomplete methods flow measurements with floats : calculation Surface velocity Horizontal distances
Incomplete methods flow measurements with floats : calculation of floats velocity Float trajectory
Complete methods flow measurements with current meter
Complete methods flow measurements with current meter : flow measurement by exploration of velocities Determination of river flow by measuring, in a cross section, the current velocity on a number of points. For each point the depth is also measured. By summing the products of the mean velocity by the applicable elemental areas
Complete methods flow measurements with current meter : the supports of current meter Wading rod Reading of depths Current meter Current meter on wading rod
Complete methods flow measurements with current meter : Deplacements of the support of the current meter
Complete methods flow measurements with current meter : Hydrological sextant
Complete methods Hydrological sextant
Complete methods Hydrological sextant
Complete methods Deplacement between verticals
Complete methods flow measurements with ADCP
Complete methods flow measurements with current meter : Flow measurement sheet River Station flow measurement n Water level surface Date Propellor Comments Time D.. H. F.. H. Position Depth N T m.s -1 Comments Bottom = 3.25
Complete methods flow measurements with current meter : Flow calculation
Complete methods flow measurements with current meter : Flow calculation Great velocities Turbulent flow Low velocities Smooth bottom Rough and rocky bottom Very rough bottom Aquatic vegetation is important Velocity curve above a big rock Velocity curve above a hole downstream of a rock Low surface velocity (immersed branches )
Complete methods flow measurements with current meter : Flow calculation
Complete methods flow measurements with ADCP measurement based on the propagation and the reflection of acoustic waves by suspended particles in water. measure of current velocity using the Doppler effect. Suspended particles are used as reflectors. Does not work in very clear water (mountain lake, spring, )
Complete methods flow measurements with ADCP : the Doppler effect When the particles move away from the source, they receive the sound with a lower frequency. If the particles get closer to the source, they receive the sound with a higher frequency. The particles reflect the sound wave and behave like emitters. The Doppler effect is applied 2 times. The Doppler lag Fd is given by the following equation : Fd = 2 Fs(V/C) where : Fs is the frequency of emission, C : celerity of the sound V : particle velocity perpendicular to the face of the transducer.
Complete methods flow measurements with ADCP : the signal 2 pulses are separated by a time T apart They travel along the beam away from the ADCP 2 1 T apart 2 1 2 1 The suspended particles move towards the ADCP When it meets the first pulse, an echo is sent towards the ADCP 2 1 1
Complete methods flow measurements with ADCP : the signal Particles continue to move along the beam towards the ADCP. When they meet the 2 nd pulse, a 2 nd echo is created. 1 2 2 1 The 2 nd echo is closer to the ADCP than the first one. 1 2 1 2 The echos are detected by the ADCP. The difference between their time of arrival is lower than the difference between the 2 pulses emitted T apart -t 1 2
Complete methods flow measurements with ADCP : measurement of the current
Complete methods flow measurements with ADCP : deplacement
Flow measurement with ADCP on a catamaran RiverCat features include: Specially designed waterproof electronics housing. Available with rugged aluminum hulls or low-profile fiberglass pontoons. The Mini-ADP is available in 3.0-MHz (0.3 6 m), 1.5-MHz (0.5 25 m), or 1.0-MHz (1 40 m) operating frequencies (ranges). Completely integrated ADP processor, RS-232 radio-modems, DGPS, and 24-volt power supply. Powerful windows-based data-acquisition/post-processing software features discharge/flow measurements, and automatic ADP and DGPS setup wizard. RiverSurveyor software is compatible with Windows XP, 2000, NT, ME, 98, and 95. Comes powered and ready to go -- No wiring, No soldering, No radio-programming!
Flow measurement with micro current meter
Flow measurement with a truck
Sounding weight
80 kg sounding weight with cable and support
Flow measurement with cable way
Gauging truck (France, 1996)
Winch and computer for visualising velocity profile (France, 1999)
Cable car station (Chili, 1992)
uncertainty and errors uncertainty due to the control section uncertainty due to the equipment uncertainty due to calculation Conclusion