INNOVATION January-February 2022

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A pipeline, elevated on a truss, escapes damage from a debris flow. P HoTo : B RIAN m ENoUNoS

Questions have arisen as to whether the summer 2021 wildfires and industrial- scale logging over recent decades may have exacerbated the flooding. Earth scientists will likely try to answer this question, but studies in burned forest lands elsewhere in western North America indicate that wildfires commonly produce a shallow hydrophobic layer just below the burned surface that greatly reduces infiltration of rainfall and runoff, and thus increases overland flow. Likewise, forestry cutblocks likely experience more rapid runoff than undisturbed forest because they do not have, at least for several decades, tree canopies that intercept rainfall, or undisturbed soils that facilitate infiltration. Some of the worst flooding in November 2021 happened in areas that were badly burned a few months earlier, but areas that had not been burned also experienced flooding. LANDSLIDES Many of the mountain valleys affected by the 2021 atmospheric river are surrounded by steep slopes, and some of these burned

One of the largest areas flooded was the Sumas Prairie area near Abbotsford. Sumas Prairie is a poorly drained surface that is only 1-12 metres above sea level. A large, shallow lake, termed Semá:th Lake (Sumas Lake), occupied much of this surface. In the 1920s, the lake was emptied to create farmland that, over the course of the next century, became an important area of poultry, livestock, and agricultural production. Numerous farms became established on this reclaimed land, and the communities of Vedder Crossing and Sardis grew along its former margin. Unfortunately, the diking system that protected Sumas Prairie from flooding failed during the November storm. Overflow from the stólaw7 (Nooksack) River watershed just south of the international boundary overwhelmed the Sumas River dikes and flooded Sumas Prairie, re- establishing Semá:th Lake (Sumas Lake), forcing the evacuation of rural properties and residents of Sumas, Washington, and blocking Highway 1 east of Abbotsford. This resulted in the death of numerous farm animals and disruption of local food chains.

Based on reconnaissance helicopter surveys along Highway 1 and Highway 5 north of Hope, along with other images published on social media, many slope failures initiated in gully systems that turned into debris flows. One debris flow on Highway 5 originated from road fill, and a debris flow on Highway 99

soil during intense rainfall, exacerbated by rapid snowmelt and antecedent moisture. All these elements were present in the November storm. Although debris flows originate in steep terrain, they become quickly channelized and overrun channels and impact bridge abutments or structures with high-impact forces.

and landslide damage. The Coquihalla Highway (Highway 5), Highway 99 between Pemberton and Lillooet, the Trans-Canada Highway (Highway 1), the Crowsnest Highway (Highway 3), and Highway 8 between Spences Bridge and Merritt were all damaged. The damage stemmed in part from washouts of roadbeds that occurred when culverts were unable to carry stream runoff or became blocked by gravel, boulders, and coarse woody debris. Waters ponded against the overwhelmed culverts and flowed across the roadbeds, easily cutting through them and the granular fill on which they were constructed. In some cases, bridges failed due to erosion around pilings. Damage to the road system was greatly exacerbated by the shifting of stream channels under the extreme flows they carried. Streams laterally eroded in some cases tens of metres against the fluvial terraces and adjacent valley walls. Sections of highways close to stream courses were totally removed due to this erosion.

during the previous summer. Steep, unstable slopes, in addition to high intensity rainfall, can trigger debris flows, one of British Columbia’s most dangerous types of landslides. Debris flows commonly initiate as small debris slides on gully head walls and sidewalls and are commonly triggered due to saturation of

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