Morphological changes and sedimentary processes induced by the December 2003 flood event at the present mouth of the Grand Rhone River (southern France)

Maillet, G.M., Vella, C., Berne, S., Friend, P.L., Amos, C.L., Fleury, T.J. and Normand, A. (2006) Morphological changes and sedimentary processes induced by the December 2003 flood event at the present mouth of the Grand Rhone River (southern France). Marine Geology, 234 (1-4), 159-177. (doi:10.1016/j.margeo.2006.09.025).

Abstract

The study, which is based on repetitive bathymetric surveys, assesses changes and effects of one of the most important floods recorded in the Rhône Delta area: bottom morphology and sediment distribution in the Rhône outlet after the December 2003 flood are discussed by comparison between Digital Terrain maps (DTMs) of November 2003 and January 2004.
The post-flood morphology shows that the whole of the system is active, mainly in the east. The channel of the Rhône has been hollowed out on the left bank by more than 5 m, the eastern coast of the mouth has retreated 400 m, the mouth-bar has prograded 200 m and the slope of the delta front has increased by 0.19°. The overlay of pre- and post-flood DTMs makes it possible to estimate the total volume deposited in the [0 to ? 20 m] zone as + 7.8 × 106 m3 (i.e. 0.88 m3 m? 2), which corresponds to 4 × 106 t of sediment. These values are compared with the average annual volume of 0.47 × 106 m3 yr? 1 of sediment deposited in this zone between 1995 and 2003, as well as the average sediment load estimated at the Arles station (50 km upstream) for this flood event (3.1 × 106 t to 5.3 × 106 t).
The acquisition of bathymetric measurements immediately before and after a major flood allows more than a simple morphological description; in addition, an analysis of the solid load transfer processes towards the prodelta is achievable. Three mechanisms are highlighted. Most of the material eroded in the channel and supplied to the delta front corresponds to fine sediment that drape homogeneously over the pre-flood morphology of the delta front. The transit of the coarsest sediment (primarily sand) is slowed down in the channel of the Rhône: this sediment builds up in relatively small areas, leading to the formation of gullies on the prodelta slope. The gullies have fixed positions and disappear gradually towards the west, following the progressive migration of the active band towards the east. These old features are not reactivated by floods occurring after their formation. During the flood, only a small volume of coarse sediment bypasses through the gullies, and the fine deposits are remobilised rapidly or compacted. Flood input concerns essentially the delta front, which traps 90% of the fluvial solid discharge between 0 and 20 m depth. The coarse prodelta supply is then due mainly to mass movements of unconsolidated material deposited beforehand on the top of the delta front. Consequently, the main nourishment of the prodelta does not occur directly during and/or immediately after a flood event, but is delayed by sediment being temporarily trapped on the delta front.

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