Sediment mobility in a gravel-bed river (Aragón Subordán River, Central Spanish Pyrenees) assessed using active RFID tags

Sediment transport is a key process in fluvial geomorphology, particularly in gravel-bed rivers, as it controls channel morphology and has important implications for river management and restoration. This process occurs in three main phases: entrainment, movement, and deposition, and is influenced by factors such as particle size, shape, density, angularity, imbrication, and the ratio between transported sediment size and bed material.  Although field data on sediment transport are essential, they are often difficult to obtain. To overcome this, a wide range of monitoring techniques has been developed, including direct samplers such as the Helley-Smith sampler and bedload traps, acoustic sensors such as geophones and hydrophones, and laboratory experiments that allow sediment dynamics to be studied under controlled conditions. In parallel, sediment transport models rely strongly on grain size distribution, either using the full distribution or representative metrics such as D50 or D84. Since the 1990s, sediment tracking has become increasingly important, with gravel tagging emerging as a widely used method for analysing sediment mobility and travel distances. Technological advances have significantly improved recovery rates, particularly through the use of electronic tags. Passive Integrated Transponder (PIT) tags are now commonly used due to their small size, low cost, long lifespan, and passive operation. Active tags, such as VHF and UHF, enable continuous tracking via fixed antennas but are larger and more expensive. In this work, we used active tags to estimate the volume of sediment mobilised by deploying RFID‑tagged gravels, thereby improving our understanding of sediment movement through the concept of virtual velocity. Tag seeding was carried out at four sites upstream of the first fixed antenna. Preliminary results show that gravels in the 45–181 mm size range (85% of all seeded material) have been mobilised over distances of approximately 700 m and detected by the antennas, whereas gravels in the coarsest size classes have not yet been recorded. In conclusion, higher‑magnitude events are required for the coarsest particle sizes to become mobile and be detected by the first antenna. Although several low‑magnitude events have occurred, very few of the mobilised gravels have been detected by the second antenna located downstream of the first, suggesting that they may have become buried or trapped in pools.

ACKNOWLEDGMENTS: This work is funded by the European Research Council (ERC) through the Horizon Europe 2021 Starting Grant program under REA grant agreement number 101039181 - SEDAHEAD.