In B.C.'s forests, a debate over watershed science with lives and billions at stake

Ross Muirhead stood at the edge of a forestry cut block filled with stumps, rain pelting down as he watched water rushing over the barren ground.

The environmental advocate was storm watching during the atmospheric river disaster that swamped southwestern British Columbia in November 2021.

Muirhead says that without a healthy forest to help absorb the excess water, it was gushing toward a creek near the Sunshine Coast community of Halfmoon Bay.

"It was just complete surface run-off," he says.

Muirhead went to see what was happening near the outlet of the creek and found highway crews already working — water and debris had caused a "complete engineering failure" of a culvert and the road on top of it, he says.

It was one of at least six washouts along a 40-kilometre stretch between Halfmoon Bay and Gibsons, says Muirhead, who lives in neighbouring Roberts Creek. He's the founder of the group Elphinstone Logging Focus, named after the local mountain.

"These culverts are undersized for climate-change conditions, with atmospheric river events," he says. "All across the Sunshine Coast, the majority of them were designed and put in place in the 1950s, when the highway was engineered."

Now, Muirhead says he's worried about plans for additional logging on the slopes of Mount Elphinstone, about half an hour's drive north of his home.

Muirhead isn't alone. The harvesting plan has caught the attention of local officials, concerned about a situation that represents a case study on the impact of logging on forest hydrology and flooding — and how such risks are assessed in B.C.

Scientists say the stakes in getting it right are huge, with lives and billions of dollars in the balance during climate-related extremes and in a province where clear cutting has been a dominant practice for decades, affecting large swaths of the landscape.

The province's logging agency, B.C. Timber Sales, was set to decide by the end of this month whether the Mount Elphinstone harvesting rights will be put up for auction in April.

The agency reduced the potential area available for logging to a total of 13 hectares over two cut blocks following a watershed assessment last year, and says it has committed to harvesting only half of what the Mount Elphinstone area can "sustainably support."

A public information bulletin says the method would be "partial cutting," with 500 trees left standing in addition to substantial patches set aside for wildlife and reserves.

But Muirhead is still concerned about the effects of additional harvesting and the extension of logging roads on a landscape that he describes as "dying from a thousand cuts" sustained over more than a century of development.

Last month, the board of the Sunshine Coast Regional District wrote a letter to provincial officials asking for the cut blocks to be removed from the B.C. Timber Sales operating plan. It also requested further hydrological studies to evaluate the potential effects of logging on properties downstream.

The agenda for the board's meeting in March includes a reply from Pierre Aubin, a professional forester with B.C. Timber Sales on the Sunshine Coast. His letter says the agency is implementing all of the recommendations from the existing watershed assessment completed by the consulting company Polar Geoscience.

The study did reveal infrastructure that was "potentially undersized in light of climate change projections," including crossings on "urban" roads, Aubin says in the letter dated Feb. 22. 

The information has been shared with that ministry, he added.

Aging or undersized infrastructure is part of the problem, but Muirhead and others suspect another challenge may be looming over B.C.'s watersheds — scientific methods that underestimate the role of industrial logging in elevating flood risk.

Probabilistic vs. deterministic methods

A recent peer-reviewed study led by researchers at the University of British Columbia (UBC) says "deterministic methods," which are long-standing and widely used, result in projections that don't reflect the true risk of flooding after logging.

Deterministic modelling makes projections based on a set of factual inputs and is not designed to consider randomness or chance, the study says. 

It says this traditional method leads to results that are "diametrically opposite" to the alternative approach that the study's authors advocate.

Commonly used in other scientific disciplines, their preferred approach is known as "probabilistic" modelling, and the paper says it forecasts "larger effects" on flooding.

The study, published in the journal Science of the Total Environment, aims to guide the introduction of probability to forest hydrology in B.C., the authors say.

It's the latest volley in the debate about the use of probabilistic versus deterministic methodologies in forest hydrology that's been playing out for years.

But study co-author Younes Alila, a professor in the UBC forestry department, says the significance of the debate is more than just academic.

The scientific methods behind watershed assessments inform the design of dikes, bridges and highways, with safety and cost implications for aging infrastructure built in a way that doesn't reflect the risks, says Alila, who is also a professional engineer.

Alila says deterministic methods aren't designed to draw conclusions about the frequency of extremes and tend to underestimate the effects of industrial logging.

He has been calling for a shift away from deterministic methods in forest hydrology for two decades, saying they represent "the science of convenience."

"It's the only way they could justify the way they're logging in the form of clear cuts," he said.

By contrast, Alila says probabilistic modelling takes into account the random nature of the forces influencing flooding — a complex interplay he calls "the power of the forest" —  and produces projections about the likely severity and frequency.

On the Sunshine Coast, Muirhead's group hired Alila to review the assessment commissioned by B.C. Timber Sales to inform how much more harvesting should be allowed on Mount Elphinstone.

The logging agency has described the existing study by Polar Geoscience as "one of the most comprehensive produced for the industry." 

Polar Geoscience says the study was "conducted at a higher level of detail than is normally practised" in B.C. given the proximity to residential areas and says its approach aligns with guidelines released by the Association of BC Forest Professionals and Engineers & Geoscientists British Columbia in 2020.

It provided its recommendations "with the specific objective of minimizing risk" and incorporated "a degree of conservatism" beyond that of previous studies in the area, the company says in a response posted to a B.C. government web page.

However, Alila says the study took a deterministic approach that underestimates the risk posed by additional logging.

'Gaps' in science of forest hydrology

Worsening climate change is shining a spotlight on "gaps" in the science of forest hydrology, especially around extremes such as flooding, says Martin Carver, a registered professional engineer and geoscientist who previously worked as a hydrologist for the B.C. government. 

The past can no longer serve as a reliable estimate of the future, Carver says. 

Climate-related extremes are getting worse and they're happening more often, he says, raising the consequences of any limitations in the science used to predict and respond to the risks.

Alila and Carver say the B.C. government has no set prescriptions for how exactly watershed risk assessments should be conducted ahead of forest harvesting.

Instead, the province has been using a system known as professional reliance to manage forestry and other resources industries since the early 2000s. It's up to engineers, geoscientists and other professionals to decide how to assess risks.

Carver is one of the primary authors of the joint professional guidelines that Polar Geoscience says it adhered to with its Mount Elphinstone study.

He says the guidelines are not prescriptive when it comes to scientific methods.

"It's like a community of practice out there, and we're all meeting certain scientific expectations, and if you're going to depart from that, then explain yourself."

In its response last fall, Polar says Alila should approach the professional groups if he believes the guidelines require revision. It suggests that Alila offer "practical, defensible and cost-effective solutions to implementing the concepts he promotes."

Carver says he believes there's a growing recognition of the differences between deterministic and probabilistic approaches among professionals in B.C.

But he says the province is "lagging behind" when it comes to incorporating the science.

And as climate extremes become more frequent and severe, Carver says, assessing the risks incorrectly could lead to greater damages and losses of life.

"When our systems get larger, both in terms of the size of the watershed, or the events get larger … you cannot do that analysis deterministically anymore," he says.