Probabilistic assessment and seasonal characterization of compound flood hazards in the Fraser River Estuary

Communities and critical infrastructure in the Fraser River Estuary of British Columbia are exposed to flood hazards from multiple drivers, including extreme river discharges and coastal water levels from high astronomical tides and non-tidal residuals (NTRs). The magnitude and dependence of these drivers vary seasonally, influencing the potential for compound interactions that amplify flood hazards. This study applied a statistical-hydrodynamic modelling framework to investigate a range of joint river-coastal extreme events and their contribution to compound flood hazards across three seasons: summer (S1), winter (S2), and transitional seasons (S3). Dependence analysis indicated that extreme river discharge and NTR events were largely independent within a given season. Univariate extreme value analysis and copula-based joint probability modelling were used to define twelve discharge-NTR event combinations across the three seasons, from which twelve synthetic event ensembles (115 simulations) were generated. S3 scenarios were less severe than S1 and S2 and excluded from simulation. Events were simulated with SFINCS, a reduced-physics hydrodynamic model, for the Fraser River from the Salish Sea to Hope. The model was calibrated and validated against observations from four flood events, including data from a 2025–2026 field campaign, yielding peak water-level RMSEs of 0.13–0.25 m. SFINCS was benchmarked against two full-physics models and achieved comparable performance with greater computational efficiency. Analysis of simulated water levels, inundation extents, flood transition zones and compound amplification revealed distinct seasonal patterns. In summer, extreme discharges dominate hazard in the upper estuary (above river km 25), compressing the transition zone from 40 km to 4 km at the mouth. In winter, extreme coastal water levels become the dominant driver in the lower estuary, and coastal influence may extend to Chilliwack as river flows recede. While individual drivers govern the most severe flooding throughout most of the domain, compound interactions can amplify water levels by up to 34 cm and, in some localized areas, compound events may produce higher water levels than the most extreme single-driver events, threatening communities across South Vancouver and Ladner. These results provide new insight into the spatial and seasonal variability of compound flood hazards in the Fraser River Estuary.