Assessment of the joint impact of extreme rainfall and storm surge on the risk of flooding in a coastal area

In coastal areas, flood events can result from the interaction of several factors such as rainfall, river flow and the classical tidal asymmetry to mention but a few. Therefore, flood risk assessment in these areas involves not only the estimation of the extreme values of each variable, but also their probability of occurring simultaneously. This study investigates the combined effect and dependence between a "heavy" rainfall with a high tidal levels forcing on the occurrence and severity of floods in the urban neighborhood close to the estuary of the Bouregreg River (Morocco). The methodology used for this analysis is based on a bivariate copula model to evaluate the joint risk probability of flood events. The estimated joint probability is used to define the boundary conditions for a hydraulic model to quantify the water levels and the extent of floods caused by the combination of both extreme rainfall and storm surge. The considered variables reveal a not negligible correlation, and the copula approach seems to be suitable and enough flexible, for analyzing separately marginal distributions of the source variables and their structure of dependence. Results show that the joint probability of rainfall and tide both exceeding their critical thresholds remains low to moderate, and the biggest threat to this area might be caused by heavy rainfall. However, high tide adds an extra risk by reducing the capacity of the urban drainage in absorbing storm water; especially when rainfall intensity exceeds 100 years return period. Although rainfall and tide introduce a wide range of time scale meteorological forcing, this won't prevent storm surge and extreme rainfall events resulting from climate change, to take place in the future, on the same day leading to some of the most critical flooding scenarios in this area.