Evolution du plancher alluvial du Petit Rhône à l'échelle pluriannuelle (delta du Rhône, France du Sud)/Evolution of the channel bottom along the Petit Rhône river on a pluriannual scale (Rhône delta, Southern France)

摘要

Abstract The diachronic analysis of eight longitudinal profiles of the Petit Rhône (western branch of the Rhône delta) between 1989 and 1995 allowed the quantification of the vertical variations of the channel bottom. The overall sedimentary balance over six years shows a deficit of 164,849 m3. The upstream section of the Petit Rhône (from the diffluence to Saint-Gilles lock) has a deficit of 41,728 m3/year. Dominant accumulation periods (1993-1994) are correlated with exceptional floods that input a large sediment load into the hydrosystem. Incision periods (1990-1993 and 1994-1995) are related to more complex processes because low energy floods can also be morphogenous. A readjustment of the longitudinal profile between two hydrologie events has been revealed. This compensation in sediment is related to a longitudinal transfer of the bottom load according to the process of erosion in steps. The period necessary before the upstream section of the Petit Rhône returns to its initial state (sedimentary deficit) is 29 months. The sediment balance is highly variable at the scale of the hydrosystem. The sections showing an increasing sediment input will be more affected by rising water levels during strong floods. Sections undergoing regular incision (diffluence...) show a lateral instability of the channel (undercutting of the banks). The sections in sedimentary equilibrium are dominated by phenonmena of incision and accumulation that are compensated on the scale of the length of observation. The vertical variations of the channel bottom are under the control of the chronology, the succession and the intensity of flood events.