Techniques de l’eau et calcul des réseaux

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Techniques de l’eau et calcul des réseaux. partim2 Michel Verbanck mikeverb@ulb.ac.be 2012. Velocity determination: Float Method. Inexpensive and simple Measures surface velocity Mean velocity obtained using a correction factor

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### Techniques de l’eau et calcul des réseaux

partim2

Michel Verbanck

mikeverb@ulb.ac.be

2012

Velocity determination: Float Method

Inexpensive and simple

Measures surface velocity

Mean velocity obtained using a correction factor

Basic idea: measure the time that it takes an object to float a specified distance downstream

The electromagnetic sensors use the Faraday Principle to measure the water speed.

As a conductor (water) moves through an electromagnetic field (produced by the sensor), it generates a voltage that is measured by the sensor electrodes.

The Acoustic Doppler Current Profiler (ADCP) is a kind of sonar: it operates by transmitting acoustic pulses into the water column. These acoustic pulses propagate through the water column and are “reflected” by suspended particles which are carried by water currents. The reflected signals (echoes) are received by the ADCP. Echoes arriving later, from deeper in the water column, are assigned greater depths in the velocity profile. This allows the ADCP to form vertical profiles of current velocity.

Particles within the current flow moving towards the instrument exhibit higher frequencies while those moving away exhibit lower frequencies. This is the well-known Doppler shift, which enables precise measurement of current speed and direction, allowing to reconstitute the full velocity field.

(around 15 kEuros HTVA)

The radar flow-meter is also based on the principle of the frequency shift due to the Doppler effect. The radar sensor is installed pointing in a defined angle on the water surface. Via the sending antenna, a constant frequency of 24 GHz is emitted. This signal is partly reflected at the water surface (minimum 3 mm waves) and returns with a specific frequency shift to the receiving antenna. With support of spectral analysis, filters and statistical methods, the surface velocity is obtained. Significant progress has been reached recently in these data processing techniques, improving the applicability of this non-intrusive measurement method.

Monitoring of suspended sediment concentrationnovel densitometry method: SETUP SKETCH

pressure

sensor in air

ultrasound

probe

rw, Cv

L’

pressure sensor &

thermometer

L

rs

Monitoring of suspended sediment concentrationNovel densitometric method - principle

The suspended sediment concentration is extracted from the measured parameters using the formula

rSsuspended sediment density, typically 2650 kg/m³ in the case of very

fine quartz sands

L <<True>> water depth

g gravitational constant

In the illustrated configuration (rio Parapeti), the immersed pressure transducer is hanged in the internal part of a (perforated) protection tube.