A Watershed Moment: the November 15, 2021 flood in the Coldwater River
An atmospheric river (AR) brought two days of intense rainfall to southwestern British Columbia (BC) on November 14, 2021. This rainfall resulted in extreme streamflow the following day on November 15 and extensive flooding and river planform changes in watersheds across numerous rivers in the lower Fraser River watershed, including the Coldwater River at Merritt. Numerous infrastructures, notably roads and bridges, were destroyed or inoperable. This destruction led to a near complete isolation of the Lower Mainland from road and rail access.
ARs are long, conveyor belts of warm, moist air that typically result in intense rainfall during the late fall and early winter. AR-related floods are generally larger than non-AR-related floods in coastal watersheds in BC. During the November 14, 2021 AR, the streamflow generated by rainfall was augmented by melting snow, associated with a rapid rise in temperature.
Following the November 15, 2021 flood, an urgent need emerged to estimate the peak flow of the Coldwater River to inform long-term reconstruction and mitigation efforts. In support of ongoing programs and recovery from November 15, 2021 flood, BGC was retained by several interested parties to complete hydrotechnical hazard and risk assessments and flood hazard mapping in the Coldwater River and Nicola River watersheds.
We developed a flood frequency-magnitude relationship for the Coldwater River at Merritt by combining statistical models for AR-related and snowmelt-related peak flows. BGC’s current best estimate of the 200-year (0.5% Annual Exceedance Probability [AEP]) flood event is 445 m3/s (90% confidence interval 240 m3/s to 980 m3/s) calculated using peak flows recorded over the 1965 to 2021 period at the Coldwater River at Brookmere (08LG048) hydrometric station. To account for climate change, the peak flow distributions (AR-related and snowmelt-related) in the Coldwater River were scaled to account for the trends in rainfall-related (AR and non-AR) and snowmelt-related peak flows as projected by the Pacific Climate Impacts Consortium (PCIC). The climate-adjusted 200-year (0.5% AEP) flood event was estimated to be 730 m3/s (400 m3/s to 1600 m3/s for the 90% confidence interval) assuming a 75-year future time horizon from present. This estimate corresponds to a 64% increase compared to the stationary case (445 m3/s).
These findings show that climate change effects are profound and will influence the design of flood protection structures, flood construction levels (FCLs), and the design of infrastructure alongside or crossing watercourses.