Site selection
The selection of a suitable measurement site for the RQ-30 is crucial for the reliability and accuracy of the measurement results. Several aspects related to the hydraulic situation and the mounting of the sensor have to be considered.
Hydraulic requirements
Cross-Sectional velocity distribution
In general, the cross-Sectional velocity distribution at the measurement site must not be changed. For example, fluctuating inflows and regulated weirs can influence the measurements. Therefore a mini-mum distance to such influences of 5…10x the channel width up- and downstream of the measurement site is recommended. The following drawings illustrate some examples:
Figure 8 Positioning of RQ-30 sensor under different flow conditions
Stationary waves
There must be no stationary waves present in the field of view of the RQ-30 as they can negatively influence the measurement accuracy. Stationary waves may be caused by big stones and other obstacles; their impact depending on water level. Stationary waves cause errors in angle as the radar impulse is partly reflected from them and not the plane water surface. Also, they may overestimate the water level.
Range with unchanging cross Section
The cross-Section of the channel/river in the vicinity of the flow measurement needs to be stable. Pillars of bridges and bends or corners in the flow channel represent changes of the cross-Section. The minimum distance with a constant cross-Section upstream and downstream of the sensor should be 5…10x the channel width.
Stable cross-Section
The calculation of the discharge requires the cross-Sectional area of the channel/river (see Determination of water discharge). There-fore, the cross Section of the river must not change as this causes the need of a new site calibration. Examples of a changing cross-Section are erosion of the channel bed, deposition of debris or the relocation of sediments. Changes of the cross-Section may be identified by changes in the W-v relation.
Adequate wave movements
Waves or ripples with a height of at least 2 mm have to be present at the water surface over the full expected water level range. Especially very slow flowing rivers often do not meet this requirement (see also Flow velocity).
Influence of wind
For slow moving, deep rivers the flow velocity measurement may be interfered by waves which are caused by wind. If such an influence is observed, the measurement site should be shielded from wind by proper means or an alternative site should be considered.
Installation requirements
Viewing direction
It is recommended to install the RQ-30 with its tip pointing upstream. This has some essential advantages: For installations on bridges the influence of pillars on the water flow conditions are avoided. Addition-ally, the influence of precipitation is eliminated by a direction separation obtained from the velocity spectrum (see Flow velocity).
Free field of view
The RQ-30 sensor interprets all movements in its field of view. Therefore, no moving objects shall be present in the field of view of the RQ-30. Figure 9 shows the size of the measurement spot and its distance from the RQ-30 sensor for different installation heights. Consider these dimensions when installing the sensor.
Figure 9 Measurement spot size for different mounting heights (dimensions in m)
Installation underneath bridges
When the RQ-30 is installed underneath a bridge it has to be assured that no rain or melt water is dripping through the field of view of the velocity radar. The occurrence of such events may influence the measurement considerably.
Installation on extension arms
The RQ-30 sensor can be mounted on an extension arm protruding from a river bank or channel wall. Sommer Messtechnik suggests installing a rotatable extension arm to facilitate maintenance.
Installation above open channels or rivers
The RQ-30 can be mounted in a range between 0.5 to 15 m above the water surface. With the extended measurement range the RQ-30 can be installed at a height up to 35 m above the water surface.
The sensor has to be mounted on a rigid structure that does not move, e.g. support beams or hand-rails of a bridge. An exception is the mounting on cable ways, which requires determination of the sensor inclination before each measurement to account for swinging (see parameter definition Inclination measurement).
Documentation
It is recommended to document the measurement site with pictures for future analysis. These may include:
- Measurement site with the installed sensor
- River or channel facing upstream and downstream
- Flow conditions at the measurement site
- Roughness of the river bed or channel walls