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Mount Polley Dam Breach: discussing Tailings Dam Failure Frequency and Portfolio Risk

Mount Polley Dam Breach: discussing Tailings Dam Failure Frequency and Portfolio Risk

Feb 4th, 2015

Mount Polley Dam Breach: discussing Tailings Dam Failure Frequency and Portfolio Risk

We have read the recent Report on Mount Polley Tailings Storage Facility Breach by the Independent Expert Engineering Investigation and Review Panel, and we are discussing below the Appendix I: B.C. Tailings Dam Failure Frequency and Portfolio Risk.

Defining what constitutes a failure

The Appendix looks at dam failures where “failure is defined as breach of the dam resulting in release of tailings and/or water . Incidents meeting this definition were considered failures regardless of volume released, run-out distance, or whether released materials were contained on the mine site.” Over the 46 years the panel analysis looked at, a total of seven failures (ranging from 5,000m3 to millions m3) were recorded in B.C.

Selecting what constitutes a dam

As the panel had access to Ministry of Energy and Mines (MEM) records, they opted for a very careful selection of what constitutes a dam in their analysis. We will note that many recent accidents outside of B.C. have occurred on structures that were disregarded by the panel.

Evaluating failure frequency (and failure rates)

The panel were then able to derive the historic failure frequency expressed as failures per dam per year. The panel rightly explains that failure frequency is distinct from failure rate as the former accounts for the lifetime of dams in the population, and the latter does not.

In a paper we wrote in 2013 we looked at world-wide failure rate (per decades) as we had no access to the lifetime data of world-wide tailings dams. Going back to the panel’s work, we note that the lifetime of dams in the population is anyways estimated (beginning and end are both estimated, as explained in the Appendix). Just to get a feel for the gain in precision one may get let’s consider the following: the panel reports that the number of active tailings dams has fluctuated within a relatively narrow range between 110 and 120 for about the last 15 years in B.C. “Over the period 1969-2015 (46years), there were a total of 4,095 dam-years of active operation for the complete inventory”. If we were to assume an average of 100 dams over the 46 years then we would have 4,600 dam years, and the rate would thus be slightly lower (11%) than the theoretical frequency. We understand the panel needed to aim for precision, but we wonder if the extra effort was worth for the sake of the conclusions.

Based on their data (7; 4,095) the panel derives a frequency of 1 .7*10-3 for B.C. Dams following the declared selection criteria and failure definition. Let’s note that 4 failures occurred before 1990, two in the ’90s, and Mount Polley in 2014.

The panel then proceeds to compare that value with US “comparable” water dams for which they declare a frequency of 6.2-7.6*10-4 failures/dam year. There are many assumptions behind this number, but that’s unavoidable due to the need to build “comparable” population. We will note that US dam performance was notoriously not among the best in the world, so using that value as a benchmark is questionable.

Anyways, the conclusion is that B.C. tailings dams failure frequency is about twice the benchmark value of 7 .6×10-4 for western U.S. water dams.

In the 2013 paper quoted above we derived tailings dams major failures rates as displayed in the table below.

Mount Polley Dam Breach: discussing Tailings Dam Failure Frequency and Portfolio Risk

Factual and Foreseeable Reliability of Tailings Dams and Nuclear Reactors-a Societal Acceptability Perspective

Factual and Foreseeable Reliability of Tailings Dams and Nuclear Reactors-a Societal Acceptability Perspective

If we were to evaluate approximate comparable rates for BC based on the data gathered by the panel, we would have 2*10-3 for the 1974-1984 (’79) decade, 2*10-3 for the 1984-1994 decade, 1* 10-3 for the 1994-2004 (’99) decade, and the same value for the decade which ended last year, with the Mount Polley breach. This leads to confirm that:

  • the rate of failure of B.C. Tailings dams was double than the world-wide rate in the decade around ’79.
  • the rate of failure of B.C. Tailings dams has not followed the world-wide trend of reduction and was five time higher than the world-wide rate in the decade aroud ’99.
  • the rate of failure of B.C. Tailings dams of the decades around ’99 and ’08 is not significantly different from the historic rates in the world (’79) and US (’79 & ’99).

TO BE CONTINUED…

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Category: Consequences, Hazard, Probabilities, Risk analysis, Risk management

5 responses to “Mount Polley Dam Breach: discussing Tailings Dam Failure Frequency and Portfolio Risk”

  1. Annika Bjelkevik says:

    Hi,
    Just want to add my first thought about looking at this kind of statistics – What about all the failures we do not know about? Looking worldwide there is no register of tailings dam – so we do not know the number of active dams over the years AND we do not have information on all the failures that have happened. Now a day’s most of the failures make it to the media, but not a decade ago or later… And then we have all the tailings dams not in operation that can still fail. So, my point is that I think it is impossible to derive any reliable statistics in probability numbers and that we must not overestimate the value of statistics in this case. Still it is very important to learn from the accidents that happen and that it is our responsibility to do that. Mount Polley has received allot of publicity as Canada has been open about it. Shortly after there was a failure in Brazil as well, but not much information from there reach the English speaking community (and I do not know about the Portuguese).

    • riskope says:

      We see your point, and that’s why in our 2013 paper we limited ourselves to “major” failures, pointing out a number of limitations. There has been a consensus on the number of active dams in the world and the Panel had precise data available to them.
      At Riskope we are not “stats-maniacs”, but refuse to cross our arms and say that deriving a numerical understanding of failure likelihood is impossible….as that would be obscurantism.
      We believe a point of equilibrium between “stats-mania” and obscurantism has to be found.
      We have heard very reputable academics quote ridiculously low numbers for tailings dams failure frequency/rates because no one had ever taken the time to derive data based orders of magnitude. Our research and the results of the Panel bring some rational approach to the table and will allow better protection for the environment and the public.

      • I like this conversation. The mathematics as an exercise, are intriguing. The manner in which the sites were/are selected, informative and ? provoking.

        The real value in-hand lies in the opportunity to have the discussion at all. The more facets of consideration we can openly share and discuss, the better opportunity for learning and manner in which to “do a better job” today and tomorrow.

        We are host to numerous structures of the MP tailings variety. Subtle differences in some, greater in others, but in the same all potential candidates for an event …and at the same time not an infinite number, but a manageable one to address with better practices.

        This is our dilemma, do we analyze and hypothesize the what ifs and what nots to death or do we change our actions (site operations and facility management) where we have these at-risk infrastructure and say..we need to keep a better eye on the use, state, and management of these to avert the opportunity for risk. Seems to me this is actually a very valuable investment (practice).

        If there exist means to monitor sites at-risk from natural, use, or event actions, and they are affordable (THEY ARE), why not employ them comprehensively, include them in our cost of operations — there-by off-setting a large percentage of RISK valuation during the operational cycle and set-asides needed for disaster reserve (which obviously in the MP case were ridiculously small) ?

        Lessening the actual potential of unknown and un-valued risk and events is a part of the business, and can really benefit all stakeholders in the mining environment. Isn’t that what our mandate is–even in the context of the REPORT?

        Let’s just get to it and create safety aware and transparent conditions on these sites where risk mitigation significantly adds to production, and likely additional profits and benefits to the community and shareholders.

  2. Doug Turnbull says:

    Regardless of the statistical data or baseline comparison – the mining industry in the 21st century – should be starting to think about the report bottom line – the elimination of tailings dam and another way of fine waste disposal – like the report dry tails stacking recommendation as an example. Just because it was ok for tailings dams as a waste solution up to 1960 before the mega projects started coming on stream does not mean we should continue with something that is known to fail at whatever frequency you want to calculate against whatever baseline for comparison – but once is too often if we are serious about risk management. Thank you Riskope for opening the blog – nice work – we need a discussion on this.

  3. It is so sad that such an event occured. In a long term view the responsible are the current Greenies legislations of Environment which does not allow use of natural confinement facility like a Natural lake. Why there is a Lake? it is because water can not escape since it is surrounded by impermeable hard rock walls, most of the time.
    Environmental regulators obliged industrial to put containment facilities up hill away from creeks and lakes and material is generally made of soil.

    I think it is time the Environment people start thinking long term as within the next 50 to 75 years or more a bunch of other dams will spill in lakes and rivers as it is a natural process of erosion over time. Just think of our famous beavers and how they can affect the course a water body. Maybe Safety Factors should be significantly increase transforming all projects into uneconomical projects and Create a Huge National Parc all accross Canada…Zero Risk?

    In my opinion it could make more sense to move living species from the lake to another location and isolate the submerged talings from the rest to control its way of contamination and suspended matters to at the end of the day have it stabilized. The worst that can happen is that lake over time becomes a wetland peat bogs filtering and capturing CO2…capping the tailings. No No No you have to put these tailings storage up hill so if they fail they end-up in the lakes and rivers without control, such a better way to preserve the environment.
    May Be I should start a Non Profit Organization where people could make donations to save the dams, I could make a business out of it giving presentation at 100K each time…
    My 2 cents

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