Hydraulic Modelling of Urban Flooding in Toronto: A 2D Approach to Evaluating Nature-based Solutions under Climate Change

Urban flooding poses growing threat to cities due to climate change, requiring effective and context-specific adaptation strategies. This thesis evaluates to what extent Nature-based Solutions can contribute to climate change adaptation for flood hazard in Toronto, Canada, using a two-dimensional hydrodynamic model of the Don River catchment developed in HEC-RAS 2D. The model simulates flooding under a historical baseline rain event and climate change scenarios for multiple Shared Socioeconomic Pathways and climate model percentiles. Nature-based Solutions were implemented through a Multi-Criteria Analysis and represented via changes in infiltration, Manning’s roughness, and terrain. The contribution of Nature-based Solutions to climate change adaptation is evaluated through changes in flood extent, depth, and hazard patterns. The results demonstrate that the 2D model provides an improved representation of flood extent and identifies high-hazard areas not captured by a 1D approach, although at the cost of increased computational demand and calibration constraints. Climate change simulations showed increases in flood depth and inundation extent, with flood behaviour strongly influenced by variability within the climate model ensemble such that differences between percentiles of a single pathway exceeded differences between pathways themselves. Implemented Nature-based Solutions reduce local flood depths and peak discharges, particularly near river channels and downstream reaches, but their effects remain spatially heterogeneous and limited in magnitude under extreme rainfall, especially in urban areas away from channels. The findings indicate that Nature-based Solutions can support urban flood adaptation as complementary measures within broader, integrated strategies, but cannot offset climate-driven increases in flood hazard on their own. Overall, the results underscore the need for ensemble-based planning, low-regret and adaptive management approaches, and critical, context-sensitive interpretations of the role of Nature-based Solutions in climate change adaptation.