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The unpredictable intense rainfalls that are part of climate change mean people who design tailings facilities must adjust their design plans to ensure that those facilities can handle the increased water.
“Considering weather and climate is not new,” said Charles Dumaresq, vice-president of science and environmental management at the Mining Association of Canada (MAC). “It has been part of best practices around engineering, planning and design for tailings facilities for a long time. The change is that you can no longer rely on the past fifty years of climate to tell you what the next fifty years are going to be.”
The intensity duration frequency (IDF) curves, which are derived from past rainfall events and used as a source of data for designing tailings facilities, are becoming out of date because of climate change and no longer a reliable source of data.
“Unfortunately, the effects of climate change (means) storms are becoming more intense, more frequently,” said Ken Bocking, a geotechnical engineer with Golder Associates that has 32 years of experience in designing tailings facilities. “So clearly the design criteria we use for the spillway needs to be upgraded and changed.”
Spillways are designed to prevent the overtopping of a tailings facility, but not necessarily to handle a thousand-year return storm.
“Now hydrologists must look at more recent rainfall events and that would tend to shift the IDF curves,” he said. “The climate scientists have to give us guidance on that.”
While climate scientists can predict temperature rises for different scenarios, he added: “It’s not so clear in terms of rainfall.”
Luciano Piciacchia, director earth and infrastructure at BBA Consultants, has developed his own formula for calculating the capacity of new facilities.
“With new facilities, what we’ve taken to doing is we’ve increased the amount of water above and beyond the design criteria,” he said. “Typically, it’s around a twenty per cent increase that gets added to it.”
For example, the capacity for 2 million cubic metres of water is now increased to 2.5 million.
“But there’s not a scientific basis behind that,” Piciacchia said. “It’s a bit of guess work. There are no exact criteria (outlined) by regulations when we are looking at including climate change.”
Piciacchia believes that getting more accurate weather data from closer to the site is the key to designing new tailings facilities. Currently, meteorological stations could be as far away as 100 km from the site and not be accurate for the proposed mine site.
“There are ways of transposing that data, but every time you do that, it dilutes the accuracy of what you are doing,” Piciacchia said.
Meteorological stations at the site would provide more accurate data for the planning of tailings facilities and reduce the risk of building a facility that can’t handle extreme rainfall.
Piciacchia suggests pointing out at the prefeasibility level that the nearest meteorological station is too far away to get accurate data for designing the best facility. The may result in a new station being erected so that more recent and more accurate data can be collected.
“You can use the most recent data that you have,” Piciacchia said. “Then you come up with a model that says this is what we are going to design to.”
Snowfall and thaws
Extreme rainfall is not the only challenge for tailings facilities designers. Increasing rainfall, snowfall and thaws during the winter are also a problem.
“That typically happens in the spring when you have a big snowpack and then it rains on top of it and melts it,” Bocking said. “So, design must consider snow as well.”
Thaws as early as February are now possible.
For existing tailings facilities, upgrades could be completed to accommodate extreme rainfall.
“You can take an existing tailings basin and even one that’s closed, and you can look at its stowing capacity,” Bocking said. “You could upgrade the stow and make it wider, for example.”
A lack of rainfall can also be problematic, because some tailings facilities require rainfall to keep water on top of the tailings to prevent acid generation.
“A drying effect would be negative in that situation,” Bocking said.
With warming temperatures, mines in northern Canada, which is warming at about twice rate of the planet, have other concerns.
“There are a few dams, typically way up north, that have frozen cores,” Bocking said. “They rely on frozen temperatures to keep the dams impervious. As temperature rises, at some point they will thaw and become pervious.”
According to Bocking, only a few of these exist and they tend to be in the high Arctic, where the mean annual temperature is -12° Celcius.
“The temperatures would have to rise to about -4° Celcius globally before we have a lot of thawing,” Bocking said. “Going forward, I think people will stop designing frozen core dams.”
One solution to the management of tailings would be to significantly reduce tailings. But both Bocking and Piciacchia agree they can’t be completely eliminated.
“We can reduce tailings, but we can’t get rid of them,” Piciacchia said. “We always look at the opportunities, but the opportunities are going to be few and far between.”
Currently, there’s some emphasis on dry-stacking tailings, in which tailings are dewatered and placed in a tailings management facility.
“It makes for a small footprint facility, which is a good thing,” Piciacchia said. “But it’s not necessarily the best technology for acid-generating tailings.”
Another solution would be to backfill the tailings.
“When you take ore out of the ground and grind it up, you are increasing its volume, so you cannot possibly get all the tailings back underground,” Bocking said.
Bocking and Piciacchia both said the best they’d seen done was about 50% of the tailings backfilled.
“To be fair, people don’t do backfilling to reduce the volume on surface, they do it for other economic reasons,” Bocking said. “It’s very expensive, so they do it to increase ore recovery.”
With funding through Natural Resources Canada, MAC has been working on a guidance document, Guide to Assessing and Incorporating Climate Change into Decision Making for the Mining Sector.
“This document was developed to address a need for more detailed guidance in this area,” Dumaresq said. “Regulators and others are increasingly asking questions about climate change adaptation. Companies recognize the importance of it, but there has been a lack of guidance at this level of detail to help inform site-specific decisions.”
He hopes the document will focus the discussion more constructively around, “How do we do this?”
The goal is to have users understand they can use multiple tools, because each one has its own inherent strengths and weaknesses. Then users can choose the best data set to make informed decisions with climate change in mind.
“Collectively, they are going to give you a picture and an idea,” Dumaresq said. “If you use multiples, that essentially becomes your crystal ball.”
Working through the suggestions and best practices outlined in the document will help people conduct a risk assessment for each mining site, including many vulnerabilities, as well as climate change. More data collection will help people understand a particular vulnerability.
“We don’t want to put the document out on our website and forget about it,” Dumaresq said. “We need to think about it within our own membership and potentially within the industry more broadly, how do we increase awareness of the document? How do we actually get people using it?”
As people use the information in the guidance document, Dumaresq foresees the potential for updates down the road as more experience in implementing it is gained and as the broader science continues to evolve.
The guidance document will be available for free download from the MAC website in mid-summer.
The International Council of Mining and Metals (ICMM) also released its own document: Adapting to a Changing Climate – Building Resilience in the Mining and Metals Industry, in November 2019. MAC closely aligned its document with this one. Both address climate change as it relates to tailings facilities, as well as other issues around climate change and its effects on mining operations.
“Nothing out there covers best practices like our document,” Dumaresq said. “It’s more Canadian–specific, but applicable anywhere.”
To prevent overtopping of tailings facilities, the people designing tailings facilities must take climate change into account.
“As tailings engineers, we have to pay attention to climate change,” said Bocking. “It’s real and it has important effects on the rainfall events in particular, and we have to adjust our design parameters to deal with it.”