EPTB Seine Grands Lacs

Paris, France

EPTB Seine Grands Lacs

Paris, France

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In the middle 1990's, the Seine Normandy's Water Board made an atlas of the biggest floods known, in order to define a hazard reference for main rivers of the Seine basin. This study was ordered to two major consultancies. The results confirmed that references are floods that happened at the 20th century, notably in January 1910, 1924 or 1955. Thousands of pages of official documents were written about 1910's flood, and the first films related to a catastrophic flood are due to this event. That flood still serves as a reference for the current policies (PPRI). Without minimizing this extraordinary flood, some other important elements appear from the analysis to the datas of official archives. Indeed, other events have equaled or exceed this reference, sometimes widely. Our research has unearthed some higher events on the Seine catchment, as March 1784, May 1836, or September 1866). If one of these floods occurred in late winter, we were mainly surprised by the season in which took place the two other events. Indeed, it is widely accepted by officials that most of the large floods of the Seine (since 1650) take place between December and February. Actually, the canopy and evaporation prevent generally the occurrence of major floods in the summer months. It appears that their locally impacts were far more important than 20th centuries flood. Their velocity of propagation sometimes surprised flood forecasting services, which has also forced them to revise their flood forecast formulas. Despite uncertainties about the real discharges of some of these flood (most of which still been calculated by the state utilities), they represented local references. The analysis of archives shows that memory of floods was transmitted continuously in the State utilities until the 70's. However, from that date, a trouble is maybe occurred; they rarely speak, and no longer use them in calculations of flood frequency. However, shouldn't we no better consider old events at the time of the "preliminary risk assessment" of the flood Directive and also the flood assessment of climate change on the Seine basin's hydrology? © Société Hydrotechnique de France, 2013.

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