Meteorological Inputs to Tailings Storage Facility Assessments and Risk Assessments – Experience and Lessons Learned

December 16, 2024

As a result of the issuance of the Global Industry Standard on Tailings Management (GISTM) in 2020, the International Council on Mining and Metals Tailings Management Good Practice Guide (ICMM GPG), and ICOLD’s Bulletin 194, Tailings Dam Safety, there is a drive to designing tailings storage facilities (TSFs) to handle the Probable Maximum Flood (PMF) during closure and, in many cases, during operations. Quantifying the effects of extreme precipitation and changing meteorological conditions on the safety of tailings storage facilities during operations and closure is critically important. This paper provides guidance with respect to the PMP and how climate change should be accounted for in the PMP estimate. Additionally, guidance is provided on how to couple wind speeds during extreme precipitation events to determine suitable freeboard.

Applied Weather Associates (AWA) has completed hundreds of site-specific PMP, precipitation frequency, and climate change assessments around the world; many for large dams and complex mining operations. These studies have included both deterministic and probabilistic evaluations. For TSFs with ponds, the results have been used to determine spillway adequacy and freeboard requirements, to understand water balance, to evaluate extreme precipitation depths and seasonality, and to provide input for evaluation of hydrologic runoff responses. These studies are also applicable for cover designs and designing erosion protection systems.

AWA develops the PMP following the storm-based approach, which provides deterministic values that are commonly used for evaluation of spillway adequacy and design. Until recently, these deterministically derived values did not have a probability associated with them, and therefore quantifying where those values fell within the risk spectrum was not possible. For nearly ten years now, AWA has applied a regional Lmoments process along with a stochastic storm transposition process to quantify the probability of the PMP depths. This also provides unique outputs that include several recurrence intervals, including the 10,000-year event. This combination of deterministic and probabilistic outcomes is very valuable for assessing risks associated with a TSF.

Over the last few years, AWA has also included climate change projections specifically related to extreme rainfall and PMP. This presentation will provide details on PMP development, the application of probability assessments, and how climate change assessments are applied to those outcomes. These will be discussed with specific examples related to tailings storage facility safety. Several locations around the world will be considered. This analysis is very important for informing likelihoods in risk assessments.

AWA also has provided input to estimating the required freeboard that is required to protect from overtopping of tailings dams during extreme precipitation events. Proper consideration of extreme meteorological events (precipitation and wind) are key data inputs to assist designers and owners on how to best allocate funding for ongoing safe operations and closure.

The GISTM and ICMM GPG introduced the concept of credible failure modes, a key component of which is understanding if a potential failure mode is physically possible. This paper will provide input to physically impossible thresholds related to extreme precipitation events.

Kappel, B.,  D. Hulstrand and A. Small. 2024. “Meteorological Inputs to Tailings Storage Facility Assessments and Risk Assessments – Experience and Lessons Learned,” in Proceedings of the Tailings and Mine Waste Conference, 10-13 November 2024. Colorado, USA.