How is the new channel evolving flood after flood? How do sediments move during a flood? Over what distance? In what quantity?

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Geomorphological monitoring

 

A number of morphological changes expected in the new channel. They will depend essentially on the flows injected, themselves dependent on Rhine floods. These changes are important since they contribute to the diversification and improvement of the abiotic(1) characteristics of the alluvial environment.

Main evolutions expected in the main channel:

  • lateral evolution: modification of the shape of banks, erosion, deposits, etc.
  • longitudinal evolution: modification of the topography of the channel bed by adjusting the length of the profile, accentuation and multiplication of alternating thresholds - wet periods.


The transport of sediments is the main vector for these changes. Sediments are transported from supply zones (erosion of the concave bank, vertical incision) to accumulation zones (convex bank, lateral and median beds).

Measuring the intensity, rhythm and spatial distribution of morphological changes is the subject of close monitoring based on the latest techniques:

  • 3D imagery (in collaboration with INSA(2)), allows 3D models to be made to accurately measure sedimentary supply zones, deposit zones, volumes lost, even the particle size in a sector.
  • Passive transponders are radio-identified chips attached to stones of varying weights and sizes (photograph). They allow the distance travelled and trajectory of the particle to be determined following each flood.
  • Erosion chains are chains of a known length welded to a steel stake. They are fixed in the channel and the chain is laid along the bottom of the channel in the direction of the current. After each flood, erosion chains indicate the thickness of the deposit and/or erosion and also the thickness of the active layer (layer of sediments in movement during a flood).
  • A hydrophone is a microphone positioned in the water course. It records acoustic oscillations in the water and identifies the signal corresponding to solid particles (stones, gravel) impacting on each other and the bottom of the channel. Coupled with flow rates, it allows the exact flow rate to be determined from the start of the movement of coarse sediments.

The first visual observations have already allowed morphological changes to be observed (photographs).

(1) Abiotic: all the physical characteristics of an environment (non-living: water, air, soil).(2) INSA : Institut National des Sciences Appliquées - national institute for applied sciences.

Photo 1: Terrestrial laser scanner: this device allows 360° scanning. It generates clouds of dots, allowing 3D topographical models to be produced.

Photo 2: Several targets are fixed to trees close to study zones They allow two photogrammetry data acquisition campaigns to be spatially re-aligned.

Photo 3: Drone allowing aerial photographs to be taken.

Photographs 4 and 5: Lateral evolution at the first threshold level (artificial channel upstream), following a test injection at 35 cubic metres a second.

Photo credits: D. ESCHBACH, 2014