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.

Time series of Nasa satellite images over the November 12 to 16, 2021 with Merritt, BC labels with the red pin. The Coldwater River watershed considered in this study is located upstream of Merritt.

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.

Click here to view a copy of the draft report for this work.

Melissa Hairabedian, M.Sc., P.Geo. (BC, ON)


Melissa is a senior hydrologist with expertise in hydrotechnical hazard identification, assessment, and management. Her interdisciplinary academic background and professional consulting experience reinforce her comprehensive set of technical skills including statistical hydrology, hydrological modelling, and climate change assessments. Melissa has experience in a wide range of climate and geographical contexts underpinning her practical professional judgement. 

Using lidar change detection to support the flooding recovery efforts in British Columbia

In November of 2021, southwestern British Columbia, Canada and northwestern Washington State were affected by a series of atmospheric rivers that caused widespread geohazards and destruction of critical infrastructure. This weather event resulted in massive precipitation leading to flooding, landslides, and debris slides that impacted many communities. Highways, pipelines, energy transmission lines, and railways all experienced damage and were inoperable – at the date of writing, some still are.

In the immediate aftermath of the event, BGC worked collaboratively with our clients to develop an understanding of the damage and chart a path to recovery. One of the many techniques we deployed was regional scale three-dimensional lidar change detection. Numerical processing can be used to quickly identify and visualize areas of topographic change where multiple lidar datasets are available for the same areas. In the case of the British Columbia atmospheric river events, BGC used airborne lidar scanning change detection to find and quantify the resulting geohazard activity, which took the form of landslides, flooding, bank erosion, and debris slides.

Working closely with our partners at McElhanney, we collected over 500 square kilometres of airborne lidar scanning data between Hope, BC and Merritt, BC. The post- atmospheric rivers data was compared to earlier datasets available from prior work for clients in the area. BGC was able to deliver digital change detection results within hours of receiving the lidar data from McElhanney using our patent processing method (patent has been allowed and is currently in the process of being granted). Results were immediately available to our clients and their partners through Cambio, our secure online platform, to support in recovery efforts. The resulting data was used to identify impacts on assets, prioritize field inspections, develop new designs, and understand future risks.

In the past year we have processed over 50,000 square kilometres of lidar change detection data, serving up tens of billions of lidar change detection points in Cambio to clients globally. We are proud that this work has been able to rapidly deliver high-value information used to save lives, reduce costs, and protect the environment.

Matthew Lato, PD.D., Eng., P.Eng. (AB, BC, ON, SK)

Innovation Lead

Matthew Lato is a Senior Engineer at BGC. His technical expertise is in the application of 3-dimensional remote sensing, specifically LiDAR and photogrammetry, for geotechnical mapping, change detection monitoring, and stability assessment and geohazard risk management. He is the lead author of the Site Investigation, Analysis, Monitoring and Treatment chapter of the Canadian Technical Guidelines and Best Practices related to Landslides, the recipient of the 2018 Canadian Geotechnical Colloquium Award, and an author or co-author of over 31 journal papers and 90 conference papers.

Landslide susceptibility map

Landslide Mapping for the Dominican Republic (NASA)

Landslide susceptibility map

Location: Dominican Republic
Partners: NASA’s DEVELOP Program and Landslide Team

NASA’s DEVELOP Program has the mission of integrating earth observations with society to foster future innovation and cultivate the processionals of tomorrow by addressing diverse environmental impacts today. As part of this program, NASA’s DEVELOP and Landslides Team worked with the Servicio Geológico Nacional and Oficina Nacional de Meteorología of the Dominican Republic to integrate various earth observation data sets and local data to map landslide susceptibility and exposure of infrastructure in parts of the northern region. BGC supported the NASA team with providing local knowledge on geology and landslide mapping to support calibration of these models.

Bangladesh refugee camps

Landslide Hazard Assessment for Rohingya Refugee Camps (UNCHR)

Bangladesh refugee camps

Location: Cox’s Bazar, Bangladesh
Partners: United Nations High Commissioner for Refugees (UNCHR)

The United Nations High Commissioner for Refugees (UNHCR) and the International Organization for Migration (IOM) administer the Rohingya refugee camps located south of Cox’s Bazar, Bangladesh. These camps are constructed in rugged hilly terrain where the steeper slopes are prone to landslides that pose a credible risk to life for camp refugees. At the request of UNHCR, BGC Squared carried out a desktop and field assessment of landslides in the camps to develop a preliminary understanding of the landslide hazards and provide recommendations for risk management. BGC identified that landslides in the camps were nearly all, at least to some degree, a result of human activity or human alterations of the landscape. Most notably, this included the removal of vegetation, slope regrading with construction of cut and fill slopes, and disruption of natural or man-made drainage patterns. BGC recommended that landslide hazard management focus on human factors, which are controllable to some extent, and that UNHCR adopt a landslide risk management approach.

Pedestrian Bridge Design for Mallkuchusi Community in Bolivia (NDSEED)

People crossing a bridge in Bolivia

Location: Mallkuchusi Community, Bolivia
Partners: Notre Dame Students Empowering through Engineering Development (NDSEED), Engineers in Action (EIA)

BGC Squared provided technical mentorship to Notre Dame Students Empowering through Engineering Development (NDSEED) and Engineers in Action (EIA) to design and build a pedestrian suspended bridge for the Mallkuchusi community in Bolivia.  BGC Squared supported in the design of bridge foundations/abutments in rock slopes through geotechnical mapping and evaluation of the rock walls to assess long-term stability of the piers and anchors.

Stream Restoration on Vancouver Island (CWFS)

Location: Vancouver Island, British Columbia

Partners: Central Westcoast Forestry Society (CWFS)

BGC is supporting the CWFS on two stream restoration projects at Ah’tap’apq Creek and Tranquil Creek on Vancouver Island, BC. Both creeks have been impacted by poor logging practices that have led to increased sediment load within the channel transport resulting in reduced habitat for spawning salmon. At Ah’tap’apq Creek, BGC is supporting CWFS in placement of in-stream wood structures and design of a sediment basin. At Ah’tap’apq Creek, the project is in partnership with the Hesquiaht Nation. At Tranquil Creek, BGC is supporting the CWFS in the design of in-stream and bank structures to stabilize the channel and reduce sediment mobility. At Tranquil Creek, the project is in partnership with the Tla-o-qui-aht Nation. These projects support the Surface Water Team’s river restoration goal.

At Tranquil Creek, the project is in partnership with the Tla-o-qui-aht Nation. These projects support the Surface Water Team’s river restoration goal.