Investigation of pluvial flash flood loads on overpasses for the city of Montreal.

Abstract

Warmer temperatures in a future climate can lead to more frequent high intensity short-duration rainfall events, which can lead to frequent and severe flash floods. Such events pose significant threats to urban infrastructure, including urban overpasses, which have not been adequately explored. This study combines high-resolution (4 km) regional climate simulations from the Global Environmental Multiscale (GEM) model with two-dimensional hydrodynamic modeling, based on HEC-RAS, at 25 m spatial resolution to assess vulnerability of overpasses in Montreal, the second-largest city in Canada, under flood-induced hydrostatic, drag, and debris impact loads for different greenhouse gas emission scenarios. HEC-RAS simulations for design storms, developed following the Huff curve and Chicago methods, corresponding to 100-year return levels of 15-min, 1-h, and 6-h rainfall events for current and future climates obtained from GEM, suggest future increases in inundated areas by 13 to 31%, with higher changes being associated with shorter-duration events. Moreover, classification of overpasses into various risk categories (i.e., low, medium, and high) based on flood loads indicates potential increases in the number of overpasses in both high- and medium-risk categories in future climate. Risk categorization shows that 6-h duration events in current climate have the highest number of overpasses (30) in the high-risk category, with far future projections indicating increases of 17-200% in the number of high-risk overpasses across all storm durations. This foundational work will form the basis for detailed investigations focused on individual overpasses and infrastructure design considerations that account for the intensification of flash flood loads under future climate conditions to ensure climate resiliency.<h4>Supplementary information</h4>The online version contains supplementary material available at 10.1007/s44327-025-00172-1.