Quick estimate of extreme floods water levels

Irstea, UR HHLY, 5, rue de la Doua BP 32138, 69616 Villeurbanne Cedex, France

^a Corresponding author: andre.paquier@irstea.fr

Abstract

In order to map extreme floods (return period as high as 500 or 1000 years) for which the discharges are uncertain, it appears cost effective to use a simplified method in order to obtain estimations of water levels. Generally, maps of the 100-year flood or an equivalent historical flood are available. The method presented here exploits the information of water levels contained in these maps. To ensure consistency with the map of the 100 year flood, the method calculates the additional water depths due to the difference in peak discharge between a 100 year flood and the studied extreme flood using a 1-D Manning equation. Two examples located in France permit to discuss the most suitable choice of the model parameters among which the more uncertain one is the slope. The error is as high as about 50% for the additional water depth but the error on the flooded area is about 10%. The method appears a relevant tool to define extreme flooded area if associated to the assessment of a corresponding extreme discharge.