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Developed by: Various Companies

Date:

Type: Device

Suitable for the following [[::Category:Measures|measures]]:

Introduction

The first devices to measure flow velocities appeared during the 1780s with the first mechanical current meter which counts the rotations of a propeller and links it to the flow velocity (by proportionality principle).

Today there are 3 different techniques to assess the flow velocity: mechanically, electromagnetically, and acoustically (Figure 73). The acoustic current meters using the Doppler Effect are not presented here, as these tools (ADCP, ADV, etc.) are presented separately.

Application

For a mechanical flow meter, the rotation of the propeller produces an electrical impulse detected and recorded by a counter connected to the current meter. The electromagnetic flow meter uses the Faraday law. The sensor creates a magnetic field between two electrodes located at the end of the probe. The flow velocity is deduced from the measurement of the electromagnetic force generated by the passage of water through the magnetic field.

The measurement conditions of discharge through flow velocity fields are regulated by the European standard NF EN ISO 748. The velocities are measured along a cross section at verticals. The number of verticals (n) depends on the width of the channel:

Width of channel < 0,5 m, n = 5 or 6
Width of channel > 0,5 m and < 1 m, n = 6 or 7
Width of channel > 1 m and < 3 m, n = 7 or 12
Width of channel > 3 m and < 5 m, n = 13 to 16
Width of channel > 5 m, n ≥ 22

One method among others to measure the mean velocity on each vertical is to measure the velocity in one or several measurement points at different water levels on the vertical. The number of measurements points for each vertical depends on the water level. For example, only one point is needed for water level <20 cm at 60% of the water level. If there are only 2 measurement points, they should be at 20 and 80% of the water level, and the mean velocity corresponds to the average of the two measured velocities. In the following example, 6 measurements were selected (Figure 74). The example is linked to the velocity profile inside a downstream migration channel, which is not logarithmic.

To do the measurements, the operator needs to be equipped with waders and to be in the stream (Figure 75).

By the method of mean section, the mean velocity between the banks and the closer verticals can be calculated. The mean velocity on each vertical is calculated that way, in the case of 6 measurement points:

V_(mean section)=0.1(V_s+2V_0.8+2V_0.6+2V_0.4+2V_0.2+V_pf)

Then a mean velocity is calculated for each flow section between the verticals and the closer bank; by making an average for the sections without including the banks, and by applying the following formula:

V_(mean section)=m/(m+1)*V_(mean closer vertical)

where m is a parameter depicting the characteristics of the river bed or of the wall. m is generally comprised between 5 and 7 but can be equal to 10 if the walls are really smooth. If m=4 the river bed and the walls are rough.

The discharge per section can be calculated according to standard EN ISO 748:2007:

Q_section=V_(mean section)*H_(water mean)*L_flow

Where Hwater mean is the mean water level and Lflow is the width of the channel.

In order to have the total discharge, all discharges per section have to be added.

The most recent devices have the possibility to directly treat the data and give the discharge on the cross section.

Other information

The total costs for the geophone and accelerometer sensors amount to approx. 885-1'330 €. The costs for the field computer, the analog-digital-converter, and the 3G modem are approx. 5'300-6'200 €. The total costs for the Teledyne RiverPro 1200 kHz, Teledyne Q-boat and DGPS from Hemisphere Atlas link amount to approx. 22’000 €, 21’200 € and 3’340 € respectively. The costs of shipping, VAT, some mounting apparatus and long-range radio modem are excluded. For current costs of the equipment, we recommend to ask the corresponding supplier. Note that Q-boat can also house Sontek RiverSurveyor M9. Furthermore, a rugged laptop for field use is recommended.

Relevant literature

Mueller, D.S., Wagner, C.R., Rehmel, M.S., Oberg, K.A., Rainville, F. (2013). Measuring discharge with acoustic Doppler current profilers from a moving boat (ver. 2.0, December 2013), U.S. Geological Survey Techniques and Methods, book 3, chap. http://dx.doi.org/10.3133/tm3A22.

Simpson, M.R. (2002). Discharge measurements using a broadband acoustic Doppler current profiler. Open-file Report 2001-1, https://doi.org/10.3133/ofr011.

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