The impact of two large floods (1993-1994) on sediment deposition in the Rhône delta: Implications for sustainable management.

In October 1993 and January 1994, two large floods with peak discharge of 9800 and 10,980m3 /s and total suspended solid transport of 10.7×106 and 9.7×106 tons, respectively, occurred on the Rhône River. Both floods led to multiple levee breeches in the Northern part of the delta resulting in the introduction of 131×106 and 54.9×106 m3 of river water, respectively. In both cases, the flood water drained to the southern lagoons and was partly pumped directly back to the Rhône or to the sea. Most of the 390,000 tons of sediment introduced remained in the Northern inundated area with accretion ranging from 70mm near the breaches to 4mm 6-8km away. This last value is close to the mean accretion value (3.7mm) inferred from the water budget and the estimation of the total quantity of sediment introduced in the flooded area. In a small area near the mouth of the Rhône river still receiving natural overflow from the river, total deposition during both floods was as high as 10cm. The Rhône delta is facing an uncertain future with projected sea-level rise. The results of this study show that large introductions of river water can help sustain the delta in the face of climate change. Controlled introductions of river water using riverside closable structures, as in being done in other deltas, could be done in a way that delivers water and sediments to the places where it is needed most and at the same time protect important infrastructure.