Le risque d'inondation et son évolution sur la rivière Châteauguay

Abstract

This article presents the first results of a three-year study that aimed at studying, understanding, and characterizing the evolution of flood risk in Quebec. In this study, flood risk is defined as the product of the return period of an event and the damages caused by this event. It is therefore important that both these components of the flood risk be assessed historically. The two components have been evaluated for a 32 km reach of the Châteauguay River located between the Canadian-American border and Ormstown, Quebec. A flood frequency analysis was undertaken on historical flow data for two gauging stations on the river and the data fitted with a log-Pearson type III distribution. The flood risk was then established using a three-step methodology. The first step was to establish flood levels over a range of discharges using a hydraulic model. Then the computed water levels were processed to define the flooded area and determine the property damage. The last step established the global flood risk, taking into account the complete flood distribution function. The results show that over the last 60 years, the global flood risk has increased for all of the study sites along the reach of interest. When the global flood risk is standardized based on population, the evolution of the risk differs greatly between study sites. For one site, the standardized global flood risk has increased by one order of magnitude over the period studied. The results also demonstrate that 75% of the global flood risk is due to floods having a return period of 4 years or less.Key words: flood, risk, damages, numerical modelling, flood forecasting.[Journal translation]