Facts about worldwide tailings dam performances
Oct 23rd, 2019
Reliable facts about worldwide tailings dam performances are hard to come by. Emotions or politically driven approaches often lead to fake news in mining which feed media eager to sell copies.
Indeed, “wannabe” comprehensive reviews of tailings dam failures oftentimes also draw conclusion that may at first feel right. However, under closer scrutiny, they are far from being realistic for a variety of reasons we will endeavor to discuss today.
Taxonomy of accidents
A reasonable approach is to first create a taxonomy of accidents: this has been done in a few approaches. Are we looking at Major Accidents, that is accidents with higher consequences? Or are we looking at the full “statistical population” of accidents?
When we came out with the first estimate of the rate of failures of tailings dams in the world, we looked at Major accidents. Major accidents are those which have major consequences, as also stated later by Bowker and Chandler. Bowker et al. in their tailings failure database define “very serious” and “serious” tailings dams failures as follows.
- Very Serious: multiple loss of life (~20) and/or release of ≥ 1,000,000 m3 total discharge, and/or release travel of 20 km or more
- Serious: loss of life and/or release of ≥ 100,000 m3 semi-solids discharge, and or runout >.9km
However, even with this type of apparently sound approach one has to be cautious.
Consequences dimensions and geo-ethics
Indeed, a dam break has always multidimensional consequences (health and safety, environmental, etc..), and looking only at:
- traveled distance,
- discharge volume, and
- number of casualties
is not be sufficient when trying to prioritize a large portfolio.
When looking to prioritize risks within an inventory of dams, all accidents of the same category based on the three dimensions will be treated equally. This may not be correct and in some cases even profoundly ethically flawed.
Size of the dam: height, volume
In a recent paper titled
“A Comprehensive Review on Reasons for Tailings Dam Failures Based on Case History” we can read
…it can be concluded 39% of accidents occurred in tailings dams less than 15 meters, 34% of the cases occurred in tailings dams between 15 and 30 m in height, and only 27% of incidents in tailings dams over 30 m. This distribution is generally similar for many countries although there are some differences.
The height distribution of world tailings dams inventory is unknown
Please, note this information is not valuable unless we know the distribution of the dams’ height around the world. Further down in the article we read: The graph (“Fig. 7”, below) illustrates that dam breakages occurred mostly with dams less than 45 m. Is it possible these are by far the most frequent dams on the planet?
Nb: We believe there was a mistake in the title of the axes of the graph and they should be exchanged in order for the figure to make sense.
We think the statement is correct simply because smaller dams are most common. ICOLD uses a precise definition for large dams. ICOLD define a large dam as : “A dam with a height of 15 meters or greater from lowest foundation to crest or a dam between 5 meters and 15 meters impounding more than 3 million cubic metres.” (ICOLD Constitution, Page 3).
Figure source:” A Comprehensive Review on Reasons for Tailings Dam
Failures Based on Case History, Zongjie Lyu, Junrui Chai, Zengguang Xu, Yuan Qin, and Jing Cao” https://doi.org/10.1155/2019/4159306
Incomplete statements are hazardous
Therefore we consider the statement made in the paper as incomplete and possibly significantly misleading. The difference between the number of failures in the “>60m” dams (10) and the “0-15m” dams (66) is less than one order of magnitude. Now we do not know the height distribution of the world tailings dam inventory but we suspect the number of dams “0-15m” is way larger than ten times than the number of dams >60. If this last assumption is correct, then the conclusion of the paper is simply wrong!
The truth is we do not have any idea (for now) of the tailings dam height distribution. Therefore we cannot responsibly and formally provide an answer.
A few more caveats
A bit further down in the same article we read:
The height of the tailings dam can increase the storage of tailings, but it also increases the safety risks.
This statement is not necessarily correct as we have shown in our papers, volumes of tailings (or height of dams) have no correlation to the number of casualties, for example. Transportation mechanisms, eg. presence of a river downstream, can greatly influence environmental damages. Again, this statement is incomplete and potentially very misleading.
The highest probability (58%) of tailings dam breakages built using the upstream construction method. Water retention dam type accounts for a higher proportion (18%) of tailings dam accidents. (We assume this means dams with supernatant ponds).
Upstream dams have a high probability (58%) for damage. Downstream and centerline dams have good stability and less dam failure events. Downstream or centerline dam construction method is recommended to build the tailings dam. For the established upstream tailings dam, it is necessary to give sufficient supervision and management to care about the operation of the tailings dam, thereby reducing the risk of tailings dam breakage and ensuring project safety and property safety. (This last paragraph contains a number of incomplete and misleading statements again. We will get back to this later on).
The above statements, once duly clarified and formulated, correct glossary applied, are likely reasonable. However, without knowing the number of Upstream dams, Downstream dams, and Centerline dams in the world the conclusion are again “gut-feeling” right at best.
This discussion is too important to be unclearly framed. It is paramount that statistics on worldwide tailings dam failures are researched and scientifically sound. The mining industry Social License to Operate (SLO) as a whole is at stake, as demonstrated by events like this.
Tagged with: fake news, Tailings Dams, tailings dams failures, tailings performances
Category: Consequences, Hazard, Mitigations, Probabilities, Risk analysis, Risk management