River bedload tracking using RFID technology : Detection range, tracer particle resistance, and tracer mobility results from in situ and ex situ experiments

International audienceLow-frequency Passive Integrated Transponders (LF-PIT tags) have been increasingly used to track bedload transport in gravel-bed rivers. Prior studies have reported high recovery rates in small streams. Recovery rates remained much lower in large systems, in large parts because of the limited reading distance of the tags (< 1 m), the problem of signal interference due to tags in close proximity (“clustering”), and potentially the particle abrasion during transport and subsequent PIT tag destruction. Experiments have been performed to assess controlling factors for detection ranges, develop new tracking techniques to go beyond the constraint of tracer clustering using High-Frequency Passive Integrated Transponders (HF-PIT tags) inserted in synthetic particles, and evaluate the resistance of natural and synthetic tracer particles. We tested different equipment (two types of PIT tags, three types of antennas) within different environments. We worked for the first time with real fluvial constraints, i.e., the gravel deposits and the aquatic channel. The three-dimensional detection envelopes were quantified as well as the effect of practices (inter-operator bias, battery power) on the detection. Best reading distances were measured along the (x, y0, z0) axis (antenna plane) with tag orientation a = 90°, reaching respectively 64, 91, and 127 cm with the small, medium, and large antenna. The influence of burial in dry and water-saturated sediment, and the influence of water immersion were shown to be minor: whatever statistically differences were detected between environments, the size of these differences was only between -4.0 and + 3.5 cm regarding median values of reading distances. We also tested the resistance of tracers during transport in an annular flume. We designed synthetic particles using corundum (density ~ 2.2) equipped with both passive (LF) and active (HF) tags. Transport distances of synthetic particles were found to be close to natural limestone particles used as a reference. Particle densities and forms slightly influenced tracer mobility. We showed that synthetic pebbles were more resistant to fragmentation than limestone, but less resistant to abrasion. LF-equipped limestone in the b-axis was also more sensitive to fragmentation than unequipped limestone. 10% of HF devices were damaged during the experiment whereas only 2% of LF ones were. Finally we summarized practical implications for RFID bedload tracking through in situ and ex situ experiments