Suspended-load dynamics during floods in the river Saône, France
Résumé
With a 30,060 km2 catchment area and a mean annual discharge of 442 m3/s near the confluence in Lyon, the River Saône is the main tributary of the upper French Rhône. The 165 km-long lower reach shows very low gradients (about 0.06 m/km) and wide floodplains. As for most French streams, the River Saône suspended sediment concentration (SSC) is not continuously monitored. In order to get a first insight of the SSC behaviour during the 2006 spring floods, quasi daily measurements by surface water sampling and filtration were performed at the CNR St-Georges gauging station in Lyon. Water level, velocity and discharge time series were continuously recorded by a side-looking acoustic Doppler profiling system (H-aDcp). Besides, for contrasted discharge and SSC values, 8 water sampling surveys at different points of the cross-section showed that the SSC is quite homogeneous in the section. The SSC time series suggests that concentration peaks in Lyon stem from the resuspension of fine material stored in upstream reaches: pronounced SSC peaks were observed during each rising limb and the maximum levels of successive SSC peaks decreased with time order. Actually SSC peaks occur during phases of bottom shear stress increase, as expected for resuspension. An examination of upstream catchment hydrometric data helps understand the formation and propagation of the successive flood waves. However SSC and water velocity measurements at upstream gauging stations are lacking in order to assess precisely the mechanisms of sediment routing in the river Saône.
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