RE:RE:RE:RE:RE:RE:RE:RE:RE:RE:Dry climate should reduce the issue?Well, I guess that demands some response. I have similar experiences in the desert, including measurements and analysis of extreme events in Arizona and New Mexico.
Hydrologists define the risk of extreme rainfall as the Probable Maximum Precipitation (PMP), based on regional weather records, and a knowledge of the weather conditions that generate these events. In NW Nevada (and the broader region) a convergence of large scale storms leads to potentially high PMP. In the NOAA regional report, the PMP for 24 hours is about 8.5 inches of water. See the map and discussion that leads to it on page 22 in
NOAA PMP
What this means, is based on available data, the PMP over a 10 square mile area is over 8 inches in a day in January. It varies by month, and declines in the summer. Most people cannot fathom what this would be like, but it is a design standard that would need to be met by water structures, including tailings impoundments. The PMP for a smaller area are logically higher, but this is rarely done as exemplified by the standard diagrams on p 51, where you see the opposite for larger scales. Finer scale analysis is prone to bias, so predicting the PMP over say, a mine site or city, is fraught with issues.
It is an older document, but the more current design manuals I have are not public linked. There is no material difference in the results with a longer climate analysis.
This kind of stuff is nothing unusual, and why flood analysis and structure design depends on dealing with the low probability, high magnitude events that stress the systems. For a mine, in general terms, the risk is water management on site, and in particular the tailings management system where saturation is operational norm, and where there is usually no means for the operations department to bleed off excess water in an emergency (to avoid routine water pollution issues and to save construction capital).
To give you an idea of how surprising rainfall events can be, the event that flooded Calgary in June 2013 had local precipitation in the mountains that almost certainly exceeded 1000 mm in 24 hours, and I have seen reasoned estimates approaching 1500 mm. This is in a part of the world where the annual total is typically more like 500 mm, 70% as snow. You would be hard pressed to find anyone alive that had seen an event of this magnitude, and this is often the case with extreme events. It probably had a recurrence of 1000+ years. This is why anecdotal evidence of flood records is problematic for decision making and is usually not considered.
As an investor in NCU, very little of this matters, other than to realize that the people who design mines have to think of a lot of different issues and this adds cost. It is all normal, and NCU would be no different. The prudent design and operations of a tailings facility needs to be ready for the PMP and the catastrophic event of overtopping the impoundment. This usually results in erosion and some kind of failure.
The wet climate at Mt. Polley, which is actually not especially wet but rather semi-arid, is not a reasonable factor to attribute to the failure, and the preliminary expert panel agrees. It is clearly geotechnical, and likely linked to operational issues that stressed the foundational materials.