Flood frequency analysis on the Ardèche river using French documentary sources from the last two centuries

Fitting statistical laws from a short time series does not give any guarantee of reliability on extreme flood estimation. Historical investigation through documentary sources can enlarge the record period. This paper presents a case study on the Ardèche river, based on collaboration between historians, archivists, hydraulic engineers and hydrologists for a better assessment of the flood risk. A list of historical flood levels from 1644 to the present has been drawn up and converted into discharge using hydraulic modelling. A sensitivity analysis provides error intervals on discharge estimates taking into account uncertainties on water level, roughness coefficient and channel geometry, and the impact of a nonpermanent discharge or the backward effect. A flood frequency analysis using systematic and nonsystematic data gives a practical example on how historical information can improve flood knowledge. It reduces the sampling uncertainty and shows also a good agreement with a hydrometeorological approach (Gradex model).