Exploring the mechanics of water flow and bedload sediment transport and their interplay with river topography using seismic observations acquired along the 'Les Bossons' river (France)
Abstract
Seismic observations near rivers may be used to infer bedload sediment flux rates, a quantity that is particularly challenging to measure continuously through other means. However, there remain difficulties in applying the seismic technique, including understanding the correct mechanical interpretation for the various observations. This work aims at improving estimates of sediment bedload fluxes using seismic noise by developing appropriate mechanical descriptions of river mechanics such as bedload transport and water flow, and their connections with river morphology such as slope, roughness and degree of channelization. We conducted a field experiment at the Les Bossons river, a small mountain stream fed by a large Alpine glacier in the Mont-Blanc area (France). This river exhibits a wide range of bed roughness and average slopes, which vary from 1 to about 30 degrees. These features make the Les Bossons river an ideal place to test theoretical assumptions on seismic noise induced by both water flow and bedload transport. For water flow, we propose a mechanical description of noise induced by water flowing down large riverbed steps that decrease in size in the downstream direction. Our analysis is supported by in-situ measurements of riverbed roughness, topography, water discharge and water flow velocities, that were measured over scales of several tens of meters and also locally in plunge pools. To constrain the granular bed mechanics, we investigate the motion of individual sediment particles by isolating individual impact events from the seismic signal. We investigate the grain impacts and their mechanical properties such as grain size, impact velocity and impact directionality, which can be obtained by evaluating the ratio of Rayleigh versus Love waves. This information provides useful constraints that allow us to improve upon the mechanical model recently proposed by Tsai et al. (2012). Finally, we apply the theoretical concepts described above to analyze a flood event that induced a complete re-channelization of the river. Sediment transport and grain displacements were estimated by measuring river profiles before and after the event, and by analyzing PIT-tag grain trajectories. These estimates are compared with the sediment flux rates obtained from the seismic noise records. REFERENCES V.C. Tsai, B. Minchew, M. P. Lamb and J-P. Ampuero (2012), A physical model for seismic noise generation from sediment transport in rivers, Geophys. Res. Lett., 39, L02404, doi:10.1029/2011GL050255.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMEP44B..04G
- Keywords:
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- 5419 PLANETARY SCIENCES: SOLID SURFACE PLANETS Hydrology and fluvial processes;
- 7200 SEISMOLOGY