Burn, D. H., & Whitfield, P. H. (2026). Shifting streamflow regimes and unusual flood events have an impact on flood frequency analysis for cold regions watersheds. In ICFM 10: Adapting to Global Change—Innovative Approaches to Flood Management and Resilience (pp. 39–46). Western University

Climate change is affecting flood events in complicated ways. In cold regions, the frequency of
different flood drivers has shifted causing important changes in flood distributions that lead to challenges
for flood frequency analysis (FFA). An assumption in FFA is that the flood series consists of independent
events that are identically distributed. This assumption is unlikely to hold if there are i): changes
occurring in the magnitude of flood events; ii) a mixture of flood generating processes; or iii) changes
with time in the mixture of flood processes. A particular concern for FFA is when events that are large in
magnitude occur outside the most common streamflow peak season. Such events can be characterized as
Rogue events that are distinct from the commonly observed flood generating process in a watershed.
Significant changes in flood type fraction were found such that nival events decreased in frequency
while mixed and pluvial events increased. These changes indicate a shift from nival events towards more
pluvial dominated systems in other seasons. Flood frequency analysis using a combined distribution
approach with the three flood types resulted in larger magnitude design flow estimates in comparison with
the results from considering the data to be from a single population. The characteristics of Rogue events
were explored using single dimension detection for magnitude and timing outliers, and multi-dimensional
detection of magnitude/timing density outliers. The methods identify large events that are outliers in
several distinct ways and are therefore considered Rogue events. The prevalence of Rogue events is
explored by applying the methodology to more than 2100 hydrometric stations from Canada and the
United States. The spatial and temporal distributions of these events are compared and the implications of
the Rogue events for FFA are investigated.

https://icfm10.com/proceedings/papers/papers_numbered/ch1.1_5.pdf