Experimental bed active-layer survey with active RFID scour chains: Example of two braided rivers (the Drac and the Vénéon) in the French Alps

An innovative scour chains device composed of active UHF RFID tags is proposed to survey the bed active layer. This device is tested on three cross-sections deployed along two large Alpine braided rivers: the Drac and the Vénéon. A specific field deployment procedure is developed, using a technique of drilling with a tube and a jack-hammer. After each flood, the device allows recording presence/absence of active tags inside a scour chain column. This provides the maximum scouring depth of the bed at the position of the column, and the topographic resurvey of the channel provides the net fill depth. Results show that bed active layer can reach up to 1.43 m during high flow conditions and remains <0.1 m during low and moderate flows. The main advantage of this technology against traditional scour chains is the rapidity and easiness with which the scouring depth is obtained after a flow event, notably under conditions of massive net deposition after the flow. This technology is therefore particularly suitable for large braided rivers, where traditional scour chains necessitate too much field efforts for their relocation, and can be rapidly lost under deep active layers.