Compound flooding analysis over the Canadian coastal regions

The communities settling in the Canadian coastal regions are threatened by multiple flood-generating mechanisms including riverine, pluvial, and sea level forces. Reliable design flood estimation and risk assessment in these regions demand characterization of the interrelationships between different drivers as well as the corresponding compounding effects. In this study, as our first step, we assess the compound flood risks across Canada’s coasts considering eight bivariate flooding scenarios acquired from four flooding drivers including total water level, streamflow, precipitation and the skew surge at 41 sites located at three main regions of the Pacific, the Great Lakes (GL) and the Atlantic. For each scenario, an initial dependence test based on Kendall’s Tau is conducted. Their joint probability is constructed using copulas. Further, compound flood risks and the failure probabilities are analyzed considering the OR, AND, Kendall, and conditional hazard scenarios. Results suggest that most locations can be affected by compound flooding associated with at least two types of bivariate events.\nIn the second step, we characterize the dependence structure between the three drivers of total water level, streamflow and precipitation based on the C-vine copula statistical approach and create their multivariate joint distribution for different locations. This is followed by calculating the OR, AND, and Kendall compound flooding joint return periods (JRPs) and their corresponding failure probabilities (FPs) and comparing them with the univariate and independent JRP values. Further, the CHR index is applied to quantify possible under- or overestimations of the flooding risks when individual drivers are assessed, independently. The results show that multivariate JRPs are less than those of univariate and independent multivariate hazard estimates.\nIn our third objective, we try to explore the univariate and multivariate trends of four flooding drivers at all sites. The univariate Mann Kendall trend test and its extension to the multivariate case namely the Covariance Inversion Test, Covariance Sum Test, and Covariance Eigenvalue Tests are applied to see the univariate (change in the intensity and frequency) and joint nonstationary behavior of the three drivers, respectively. The results show increased risks of individual and compound flooding over the Atlantic coast, and various trends in the Pacific and the GL regions.\nFinally, we assess the compound flooding hazard under a nonstationary framework for all the locations. To this end, the time-varying behavior of the three drivers of step 2 and also the interdependencies between them are captured using linear and polynomial models. This process leads to producing a time-dependent joint occurrence/probability of the drivers. Then, the temporal variations of the compound flood hazard are assessed concerning the OR, and AND hazard scenarios and the CHR index. The results highlight the decline and increase in the AND JRPs and CHR values over time at 23 locations, especially in the Atlantic region.

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