Design flood assessment on the upper Rhine using historical data and accounting for discharge uncertainty

Abstract

Design floods on the upper Rhine River are a major issue for French-German agreements on flood embankments. As intense river training has modified the Rhine River morphology during the 19th century, with a significant rectification of the river course, some disagreement does exist on the interest and value of including information on historical floods. Some hydrological studies are using discharge series from the 20th century only, arguing that it is not possible to estimate flood discharge correctly during and before the river works of the 19th century. Other studies include a set of large historical floods during the 19th century. Unfortunately, design flood estimates with or without historical information are significantly. We used two long discharge series in Basel-Switzerland (1808-2018, with a set of historical floods since 1268) and Maxau-Germany (1815-2018. A preliminary analysis allowed crossing different archive sources, correcting data to account for the main river corrections (1714 and 1890), checking the homogeneity of data and assessing the uncertainty of the stage-discharge relationship. We present a comparative study of design floods with several distributions and several series lengths (a few decades to several centuries). A statistical Bayesian framework allows comparing design floods and the corresponding uncertainties.