Column: Flood report highlights troubled waters ahead

Dikes could be vulnerable not only from lack of height but also erosion, a vulnerable foundation, or instability from earthquakes.

Given climate change, one thing is clear. Major floods in the Fraser Valley and Lower Mainland are expected to increase in magnitude and frequency because of sea level rise and larger peak flows.

That fact hasn’t gone unnoticed by federal, provincial and municipal governments as well as public and private organizations that all came together in a 43-member partnership to assess hazards, levels of risk, and the current state of protection  of communities along the lower Fraser River from Hope to Richmond and from Squamish to White Rock.

They launched the Lower Mainland Flood Management Strategy project. The Fraser Basin Council, which manages the project, retained Northwest Hydraulic Consultants assisted by several subcontractors to do the assessment.

On May 30, the Council released Phase 1 of the study. Clearly we’re underprepared and under protected with a lot of work ahead of us to shore up mitigation plans.

“The BC Lower Mainland is vulnerable to major, catastrophic floods from the Fraser River freshet (spring) and from coastal flooding (winter),” the authors wrote in the Phase 1 Summary Report.

Four major flood scenarios were assessed for comparative purposes, two on the Fraser River and two coastal, now and in 2100. For communities in the Fraser Valley, spring freshets pose the greatest risk of a flood and, should dikes be breached, flooding could inundate 99,300 hectares by present day measures, 110,300 hectares by 2100. A coastal flood would be expected to cover 54,700 hectares today and 61,100 hectares by 2100. Communities at greatest risk are those adjacent to the Fraser River from Chilliwack to Richmond.

The report said that current design standards of dike infrastructure are still based on water levels of the 1894 Fraser River flood of record that had a peak flow of 17,000 cubic metres/second at Hope. At the height of the 1948 flood, 50,000 acres were under water. Dikes broke at Agassiz, Chilliwack, Nicomen Island, Glen Valley and Matsqui. By the time the flood waters receded, 16,000 people had been evacuated and damages totalled $20 million.

Seventy-four dikes divided into 118 segments stretching 500 kilometres and managed by 35 diking authorities were assessed. They represented 50 per cent of all dikes in the province. But, despite upgrades, the current report found that 71 per cent of them are vulnerable to failure from overtopping. Only 4 per cent of the assessed dike segments meet current provincial standards for crest height including 0.6 metres of freeboard above the water’s elevated surface.

Dikes could be vulnerable not only from lack of height but also erosion, a vulnerable foundation, or instability from earthquakes. In addition, the past decade has seen more accurate design flood standards, more stringent geotechnical design and the fact that an increased number of buildings, roads, and buried utilities can interfere with maintenance. Then there’s the issue of dredging (or not) the Fraser River to remove excess sediment brought down with flows from the Interior. As well, many municipalities face similar challenges of funding for repairs, upgrades, and the costs of technical analysis.

Flooding means lives lost, lives threatened, and people injured. Flooding means widespread evacuation of residents to safety, land taken out of food production, loss or damage to buildings (homes, businesses, schools, hospitals, farms), roads, railways,  ports, utilities above and below ground, sewage plants, fresh water supply, and transportation access for essential goods such as food, equipment and repair materials.

The report estimated that the cost of a present day Fraser River flood scenario could be as much as $22.9 billion, almost five times the cost of the devastating Alberta flood in Calgary and High River in 2013.

The FBC report is a sobering look at a real future risk.

Chilliwack Progress

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