Mapping Dependence between Extreme Skew-Surge, Rainfall, and River-Flow

Flooding in coastal areas is a major global hazard, made worse during compound flood events, which occur when multiple flood-drivers, such as tide, sea surge, and fluvial and pluvial flooding, coincide. We use 12 sea-level, 2065 rainfall, and 81 river-flow records to assess the dependence of (1) extreme skew-surge and extreme rainfall (pluvial/surface runoff) and (2) extreme skew-surge and extreme river-flow (fluvial discharge) in New Zealand. We found that (1) skew-surge and rainfall and (2) skew-surge and river-flow are significantly, but not strongly, correlated in NZ. When spatially averaged to within 30 km of sea-level gauge location, the correlation was generally significant and positive, but weak with Kendall’s rank correlation coefficient τ< 0.3. We identify the weather types driving regional patterns of dependence. Trough weather types were the dominant driver of individual and coincident extreme events. Blocking weather types were associated with the highest extreme skew-surge and rainfall events along the northeast coast of the North Island and, consequently, were associated with a high proportion of coincident skew-surge/rainfall and skew-surge/river-flow events there. These findings have important implications for flood management, emergency response, and the insurance sector because impacts and losses may be correlated in space. Our findings add to a growing understanding of compound flooding worldwide for different geographical and meteorological settings. The positive dependence observed suggests that more attention to compound event probabilities is warranted when undertaking localized coastal-flood modelling.Scott A. Stephens, and Wenyan W