A rising Coldwater River overtook parts of Merritt on Monday, with flood waters remaining on Nov. 16, 2021, as seen from aerial drone footage.

Image Credit: YOUTUBE/Greg InBC

Merritt resident Russ Wardell has been raising the alarm over Coldwater River dikes for years, but it started with his opposition to a residential development.

Wardell lives near the Coldwater River, and while his home was spared from extensive flooding damage, it was largely because another dike failed and the river forced a new channel over Pine Street.

When a developer proposed a new residential project to Merritt city council on 1330 Pine Street, Wardell simply believed it would be an "eyesore" from his house across the river, but his concerns changed after researching the history of the Coldwater dikes and the low-elevation proposed development.

As reported by the Merritt Herald, the proposal first came to city council in February 2018.

He started to speak with neighbours in the area and looked through archives at the local museum, where records showed Wardell that the dikes on the west banks between Main Street and the confluence were built haphazardly in the early-1970s.

"Some guy with a bulldozer showed up and started pushing the dike," Wardell said, adding that there was an ice jam that year which backed up the river.

On Nov. 15, 2021, the Coldwater River breached its dikes, flooding multiple homes, damaging critical infrastructure and forcing the evacuation of the entire city. The force of the river didn't just breach the banks and move up the floodplain, it also uprooted the Voght Street bridge and carved a new channel along Pine Street.

The more Wardell learned about the construction of the dike, the more he brought the issue to First Nations Bands, Merritt city council and other provincial regulators, hoping a government entity would address not only the dike but also a "choke point" where the river narrows at the north end of Pine Street.

"Every year we watch pieces of the dike fall away," Wardell said. "The reality is that we've always known this would happen. We knew this was going to happen because the dike was garbage to start with."

In a Fraser Basin Council report prepared for the City of Merritt in June 2021, engineers determined the existing Coldwater River dikes would likely be overtopped in the event of a 20-year flooding event.

Preliminary estimates suggest the Nov. 15 flood might have been a 1 in 1,000-year flooding event, according to City of Merritt corporate services director Greg Lowis.

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The Fraser Basin flood study, which was conducted by BGC Engineering Inc, recommended dike improvements in Merritt along both the Coldwater and Nicola rivers, but even the most pessimistic estimates of water levels was less than half what they got when the city flooded last month.

Dikes in B.C. are generally built to a 1-in-200-year flood event standard, for which the Coldwater River was estimated to have a peak discharge of 185 cubic metres per second.

Preliminary federal government data from Nov. 15 measured 405 cubic metres per second at the Merritt station, but when that much water flows through, those measurements are less reliable.

"The Coldwater River clearly exceeded the anticipated the 200-year impact," David Campbell, head of the B.C. River Forecast Centre, said.

That data comes from the federal hydrometrics, which he notes can take a couple years to be finalized and archived. Over the last few weeks, he's seen the river discharge figure go up for that date as the hydrometric data is updated.

Regardless of the current uncertainty around the actual measured amount of water that flowed through Merritt along the Coldwater River, it's clear that it was a flooding event previous engineers thought extremely unlikely.

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Campbell noted flooding events, like the 200-year return period, are based on the statistical likelihood of those flooding events occurring.

For example, a 200-year return period does not mean it will happen every 200 years. Instead, it means there is a one in 200 chance there will be that amount of water in any given year.

A major contributor to those figures is historic recorded data, and there is 58 years of data for the Coldwater River. The highest river discharge measured on record was in 1980 when there was 122 cubic metres per second flowing through the Coldwater River, which Lowis said is now a "footnote" compared to the November flood.

A changing climate makes predicting those flooding events even more difficult, Campbell said.

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"It's really based on that historic data... but that might not be the same world we live in now," Campbell said.

He added that statistical estimates for flood events can vary, so the 200-year flood event modelled in the Fraser Basin Council report may be different than models at the provincial government.

The report published in June was given to City staff in Merritt, according to Fraser Basin Council Thompson regional manager Mike Simpson.

It hadn't yet been formally presented to Merritt city council, but they were in the process of scheduling a time in the recent weeks before the Nov. 15 flood.

When asked whether the new river path down Pine Street can be rerouted back to its original channel, Lowis said the City will be working with other governments and utility suppliers to determine what the best course of action will be. He added there is a FortisBC pipeline through that street, which is why the river was temporarily rerouted during flood recovery.

Given the provincial dike standard is to prepare for a 200-year event, it may be unreasonable to expect that Merritt could prepare for a flood of the same magnitude to come again. However, the sheer volume of water that rushed through in November is likely to change exactly what a 200-year flooding event is for the Coldwater River.

Wardell's home was spared of extensive damage in the recent flooding, but he hopes the "choke point" between the Main Street bridge and the river confluence can be widened to mitigate future flows.

"Widening the river is definitely good for the fish and good for the people around it," he said. "Being proactive is always better than reactive."

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