Characterizing Compound Inland Flooding Mechanisms and Risks in North America Under Climate Change

Compound inland flooding (CIF) arises from the concurrent interaction of multiple hydrometeorological drivers. In this study, we characterize key CIF events across North America, including two preconditioned events, rain-on-snow (ROS) and saturation excess flooding (SEF) for historical baseline conditions and global warming levels of 1.5, 2, and 4 degrees C relative to the preindustrial level. Utilizing the high emission climate scenario (RCP8.5) from CanRCM4-LE with 50 members, the frequency and seasonality of compound events, along with the probability of these events leading to heavy runoff, and the relative role of external forcing and internal climate variability are assessed. We convert the identified hazards into risk levels by integrating them with exposure and vulnerability components. The results suggest that as global temperatures increase, the overall role of ROS events in causing significant runoff is projected to decrease compared to individual heavy rainfall. Concurrently, the impact of SEF occurrences is projected to become more pronounced. The signal-to-noise ratio highlights a high-confidence change signal for CIF events; however, uncertainty related to internal climate variability in future projections of joint probability with heavy runoff is more pronounced. These results underscore the need to consider compound mechanisms, dynamics, and risks associated with CIFs within systematic approaches to flood risk management.