INNOVATION January-February 2022

F E A T U R E

T he year 2021 was not kind to western Canada. In the third week of November, British Columbians experienced one of the most damaging autumn storms on record. Preliminary estimates of insured and uninsured losses are estimated to be over $7.5 billion, and five people lost their lives due to a landslide that occurred during the event. The direct damage and secondary losses from disruptions in the transport of goods mean this disaster could be the costliest in Canadian history. The November storm followed the second- worst wildfire season that was kick-started by a record heat wave at the end of June 2021, referred to as the “heat dome,” which was linked to the deaths of over 526 British Columbians. Forest fires burned nearly 8,700 square kilometres of land and destroyed hundreds of homes, including the entire town of Lytton. The number of these disasters in such a short period of time was unusual, but not unexpected: the frequency and severity of these events are projected to increase in a warming climate. PHYSIOGRAPHY AND METEOROLOGICAL CONDITIONS OF THE NOVEMBER STORM British Columbia is a mountainous landscape with ranges that trend in a northwest-southeast direction. The

Precipitation map from reanalysis data (top); and moisture flux (bottom) for the November 2021 atmospheric river event.

government agencies were alerting the public about an impending “atmospheric river”: a phenomenon that delivers a narrow, long band of moist warm air, typically from lower latitudes. Previously, these phenomena were referred to by the media as “pineapple expresses,” so named because of their origin in the Pacific Ocean at about the latitude of the Hawaiian Islands. Atmospheric rivers are an important source of rainfall, supporting agriculture and other water needs for over 50 million people living in California, Oregon, Washington, and British Columbia, but they also can be damaging because of their long duration and the high intensity of rainfall. The rate of moist air delivery is referred to as “flux” ( kg m -1 s -1 ), and analysis of climate data reveals that the moisture flux delivered by the mid-November storm was high. Cumulative (two-day) rainfall records were broken at many meteorological stations beneath the atmospheric river as it moved eastward across southern BC. In addition, rainfall intensity ( m s -1 ) produced by the storm was high. Finally, given the advection of sensible heat sourced from low latitudes, the November storm generated runoff from the snowpack, and this additional source of water likely contributed to the severity of the event. Atmospheric rivers are common along the west coast of North America during autumn and winter, and an event similar to the November storm struck the BC south coast in October 2003. This storm caused a disastrous flood in Pemberton and destroyed highway and railroad bridges at Rutherford Creek, isolating the community from the rest of the country. The October 2003 event delivered 200-350 millimetres of rain in Pemberton and 325 millimetres in Squamish, larger amounts than the two communities received in the November 2021 rainstorm, but the northward path of the earlier atmospheric river spared areas to the south that received the full brunt of the 2021 storm. FLOODING AND ATTENDANT CHANNEL EROSION The heavy rains on November 13 and 14 resulted in unprecedented runoff from slopes that were saturated from well above average rainfall that fell in the 30 days preceding the storm. The runoff from the autumn storm rapidly entered streams and rivers that drain the southern Coast Mountains and interior highlands and raised their levels to flood stage. Witnesses described the rise in water levels as exceptional, akin to flash flooding. Small and mid-size rivers soon overtopped their banks and flooded many communities located in part on their floodplains. Merritt and Princeton were particularly hard hit. Nearly 7,000 people in Merritt were evacuated from their homes as the floodwaters inundated the town, incapacitating the water and sewage systems. The floodwaters had a devastating impact on highways and rail lines. All major highways that connect Metro Vancouver to the rest of the country became impassable to vehicles due to flood

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landscape is a product of the collision and amalgamation of fragments of Earth’s crust along the western margin of ancestral North America and their movement relative to one another along northwest- trending strike-slip faults, accompanied by uplift, intense metamorphism and brittle fracturing of the crust. Repeated glaciation and erosion by rivers and streams over the past 2.5 million years has sculpted this complex crustal block, producing deep valleys that have been carved into

glacially sculpted mountains and uplands, and the characteristic fiords and inlets along the BC coast. The high relief and steep slopes impose substantial challenges for the transport of goods, people, and electricity. Valleys serve as transportation corridors, but they are narrow and vulnerable to floods and landslides. Before November, snow began to accumulate at high elevations, as is commonly the case. At the end of the first week in November, meteorologists and

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