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Over half of the global population and the majority of the cities in coastal zones are at risk of coastal flooding. Changes in the occurrence of individual extremes and the interactions between hydrological and coastal variables can exacerbate flood risks. While extensive research has been conducted to understand and predict different types of flood hazards in isolation, spatial and temporal trends and variability of compound flooding, that is, flooding caused by multiple drivers, remain an open question. This study investigates the individual and joint temporal variations of multiple drivers that can cause compound flooding in Canada's coasts including total water level, storm surge, precipitation, and streamflow. Long‐term changes in the frequency and intensity of extremes are analyzed over the Atlantic, Pacific, and the Great Lakes regions. Univariate and multivariate trend tests including Mann Kendall, Covariance Inversion Test, Covariance Sum Test, and Covariance Eigenvalue Test are applied. In addition, a new multivariate index based on the contributing flood drivers transformed into a probability space is proposed, and its application to study compound flooding is investigated. Overall, results show increased risks of individual and compound flooding over the Atlantic coast and varying trends in the Pacific and Great Lakes regions. The multivariate trend indices show consistent results in most scenarios. The proposed index provides a simple and flexible measure to analyze the spatial and temporal variation of compound flooding risks at different thresholds. The results highlight the importance of considering nonstationary compound flood events to develop resilience strategies in coastal environments.

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Flooding, as the most common natural hazard in the world (Kundzewicz et al., 2014), has affected more than two billion people and caused approximately USD 656 billion of damage between 1998 and 2017 (AghaKouchak et al., 2020;Wallemacq, 2018). From 1980 to 2019, flood events have accounted for 41% of all the 17,300 weather-related events, 28% of 890,000 lives lost, 27% of USD 4,000 billion economic losses, and 10% of USD 1,300 billion insured losses worldwide (Golnaraghi et al., 2020). The frequency of flood events has been increasing from 1960 to 2013, globally (Tanoue et al., 2016). Similarly, the magnitude of flooding shows increases in some regions around the world (Do et al., 2020). Around 0.8-1.1 million people experience flooding and its devastating socioeconomic consequences each year (Muis et al., 2016), especially the coastal communities. The population of the low-lying coastal areas was approximately 625 million in 2000, which is anticipated to reach 949 million by the 2030s and 1.4 billion by 2060s (Neumann et al., 2015), indicating larger exposure to different Abstract Compound flooding, caused by the simultaneous or successive occurrence of two or more flood mechanisms, is mainly associated with extreme precipitation, river overflows, and storm tides across coastal areas. The interdependencies between these components can increase the risks of flood impacts, threatening coastal communities and infrastructure systems. This study quantifies the corresponding multivariate hazard over Canada's coastal areas by characterizing the dependencies between multiple drivers of flooding based on the C-vine copula statistical approach. The joint return periods of compound flooding considering the AND, OR, and Kendall scenarios are estimated and the corresponding failure probabilities are assessed. Further, the compound hazard ratio (CHR) index is applied to quantify possible under-or overestimations of the flood hazards when individual drivers are assessed independently. Analyses are performed at 41 locations across the Atlantic, Pacific, and the Great Lakes coasts, and the uncertainties are quantified based on the Bayes theorem.Results show that at approximately 50% of locations (mostly at the Great Lakes), the flood hazard associated with the AND scenario increases considerably when the dependencies are characterized compared to the (unrealistic) independence scenario, indicating the potential for compound flooding in these regions. Besides, at more than half of the studied locations, the CHR index exceeds one highlighting the interrelationships between drivers of flooding. The results of this study provide a deeper understanding of the flood mechanisms and their interdependencies across Canada's coasts, which support the development of resilient structures and improved coastal flood management. Plain Language SummaryApproximately half of the global population lives within 200 km of coastlines. The communities and infrastructure systems in the coastal environments are at risk of flooding caused by one or multipl...

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