INNOVATION January-February 2022

F E A T U R E

INTEGRITY OF DIKE SYSTEMS IS CRUCIAL

stationary system under one failure mechanism, extrapolation of historic frequencies may provide a rational approach. However, the current situation in BC calls for dynamic estimations of conditional return periods to support decisions on temporary protection measures and prioritization of investment. To make wise risk-based, economically optimal decisions on flood protection, information is key. Rare events have high information content. In these non-stationary conditions, it will become increasingly important to keep a close watch on the pulse of our water systems through the combination of a strong monitoring network and new advances in remote sensing, machine learning, large sample hydrology, and non-stationary flood frequency analysis.

data from Environment and Climate Change Canada, the flood peak is estimated to be 400 cubic meters per second. When converted to a water column over the catchment, the total volume in the flood peak over the base flow was about 43 millimeters. Due to the significant burnt area, an altered hydrological response, due to wildfire activity, may have played a contributing role here. A simple extreme value analysis on the historic record would determine a return period for this event of more than 2,000 years. Given all we know about the current state of the evolving hydrological system, the challenge is how much lower the best estimate of the return period might be. For design in a

The combination of the atmospheric river with the orographic effect of the mountains resulted both in precipitation of high intensity-duration and a release of latent heat, which caused potential for snowmelt. Because the precipitation in the preceding two months was already at the 98th percentile in many places, and high levels of antecedent soil moisture were combined with snowpack in some places, storage processes in the impacted catchments could provide little mitigation of the flood wave and thus exacerbated the flooding. Flood damage at Merritt impacted homes and business premises, roads, and the water treatment facility. Residents of Princeton were similarly impacted. In the case of the Coldwater River at Merritt, based on preliminary

British Columbia relies significantly on dikes for mitigation of floods. Studies commissioned by the Fraser Basin Council identify more than 1,000 kilometers of regulated dikes that are owned, operated, and maintained by about 100 diking authorities. An engineering report identified 35 of the nearly 200 dikes as “high consequence”, based on the impacts to people, buildings, critical infrastructure, and agriculture. The system includes the dikes intended to protect the Sumas Prairie landform that extends southwestwards from the Vedder Canal and Chilliwack River into Washington State. Another recent engineering report identified high flows in the Sumas River watershed,

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Catchment of Coldwater River at Merritt (grey), with burnt area (red). D ATA fRom moDIS/006/mCD64A1 DATA SET . m AP CoURTESy of g ooglE m APS

Sumas River Dike fail. The dike breached in the November 2021 storm, causing extensive inundation of farm buildings, damage to crops, and loss of livestock.

in combination with a Nooksack River overflow flood event in Washington State, as posing a risk of substantial flooding in the Old Sumas Lake Bottom should the

Aerial view showing the exposed Trans Mountain pipeline with construction underway as part of geotechnical/hydrotechnical protection works. P HoTo : T RANS m oUNTAIN

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