4.2.1. Approaching Flow Measurements

The main restrictions for typical flow measurements at large velocities are related to instrument intrusion in the flow. For supercritical flows, instruments can only be deployed from a fixed point (a bridge or cable way), and intrusive measurements generally become impossible above a flow speed of 5 m/s (for a current meter on a torpedo). Acoustic Doppler Current Profilers (ADCP), which are currently used in the field to measure flow fields, are limited to a relative velocity of a few meters per second between the apparatus and the river, making their use generally impossible for supercritical flows. Indeed, these instruments are highly sensitive to air entrainment around the apparatus. From our own experience, the result quality using ADCP starts to decrease above a flow speed of 3.5 m/s. It should also be noted that such high flows are often associated with high suspended load concentrations and woody debris [69], which make such a measurement very dangerous.

The non-intrusive counterpart instruments used to measure large flow velocities during flood events are radar velocimeters ([70]; Figure 7a) and video analysis systems, such as Large Scale Particle Image Velocimetry (LSPIV; [71]). The main limitation of these systems is that they can only provide a surface velocity, so complex 3D flows cannot be described. Moreover, radar velocimeters are not able to provide a flow direction. LSPIV has been applied successfully to measure the velocity field in rivers with high discharges [72,73]. However, a difficulty that appears close to bridge foundations in supercritical flows is that the water surface may become 3D due to standing waves or antidunes. Since surface velocity estimations are made on a plane, this type of surface could lead to significant errors. On the other hand, stereovision-based techniques could be of interest to describe the water surface of such rapid flows.

**Figure 7.** (**a**) Radar measurement from a bridge (Photo: P. Belleudy), and (**b**) a system for pier erosion monitoring used at NARLabs (National Applied Research Laboratories), Taiwan.
