A lined solution trench around the toe of the TSF to reduce seepage losses of drain water into the ground. Indigenous vegetation is used on the outer slopes of the TSF as an ongoing rehabilitation measure concurrent with the operation of the TSF. This is considered to be good practice from an environmental perspective by reducing potential dust pollution, reducing visual pollution through the rising green vegetation screen, reducing potential stormwater erosion of the TSF slopes, and improving sustainability through the use of indigenous grass, bushes and trees.
Grant Macfarlane, Partner and Principal Engineering Geologist, SRK Consulting
Chloe Bolton, Civil Engineer, SRK Consulting
Ashleigh Maritz, Senior Environmental Scientist, SRK Consulting
Climate change is not a new phenomenon, and climatic conditions are not static. For the past few decades, and more so during the past decade, conversations have been taking place regarding the science and engineering related to climate change. While much is required to be done in this regard, environmental considerations have always been an integral aspect of how tailings storage facilities (TSFs) are designed and operated. The growing impact of climate change is demanding innovative thinking to understand and address the additional risks imposed on these structures.
The urgency of this task was brought into focus with the recent publication of the Global Industry Standard on Tailings Management (GISTM), which refers, inter alia, specifically to climate change as an important factor to consider.
In particular, the GISTM’s requirements cover the need to develop and update the knowledge base related to each tailings facility – using approaches aligned with international best practices. This knowledge, according to the GISTM, should ‘capture uncertainties due to climate change’. The other key element required is a ‘robust design’ which integrates the knowledge base and minimises the risk of failure. The standards also expect that the principal of adaptive management is to be closely observed, to ensure that necessary modifications are applied when conditions change.
All phases of the TSF life cycle must be addressed in this way, including not only during its operational years but its closure and its post-closure stages as well. This means that the future and longer-term implications of changing climatic conditions have to be predicted as scientifically as possible.
This, of course, is no simple task. The scientific and engineering responses to climate change implications have long been evolving, with public awareness growing for many years. There is still much to be learnt and applied in the fields of science and engineering regarding TSFs, as traditional wisdom needs constant augmentation.
Certainly, there have already been important shifts in the way that tailings facilities are designed and managed – mainly in terms of changing rainfall patterns, flood diversion structures, the storage capacity of return water dams and the freeboard considerations of the TSFs. There have been longer and hotter periods of drought, extended periods of high rainfall, and an increased occurrence of high intensity rainfall events and extreme weather occurrences such as tropical cyclones. The intensity of storms invariably leads to increased erosion and flooding, and calls for greater armouring of berms, benches, slopes and ramps.
Changes in factors like ambient daily temperatures, wind conditions and cloud cover will affect evaporation. Important considerations that, amongst others, are considered in the water balance studies on TSFs. Our recent experience confirms that multi-day rainfall events, with high intensity over a short duration, are becoming more frequent and resulting in unwanted events that are not accounted for in the traditional design criteria. At several mining operations during this past rainy season in southern Africa for example, the return water dams servicing the respective TSFs have experienced repeated spills in recent months – even though there was no single rainfall event greater than the 1:2-year return interval 24-hour event. What is happening is that two to three of these rainfall events are occurring every few days, with low but continuous rainfall in the intervening periods.
There have also been more dramatic events such as Cyclone Idai in March 2019, which have led some mines in Zimbabwe to adapt their TSF management measures regarding rainfall and flooding. This tropical cyclone brought over 600 mm of rain to the country’s eastern regions in just three days, exceeding the Probable Maximum Precipitation (PMP) level for a 24-hour rainfall event for that area. It would be irresponsible not to consider the possibility of a similar occurrences at the sites of other mines.
More recently, Cyclone Eloise caused a cumulative rainfall that was greater than the 1:200-year return interval, 24-hour duration event in the areas of the Eastern Limb of the Bushveld Complex. Further north in Zimbabwe, a mine recorded over 200 mm of rain in 10 days.
In line with the necessary adaptive management principles, daily visual inspections of TSFs on some Zimbabwean and South African mines, especially those on the Eastern Limb, have been implemented on account of the higher-than-normal rainfall this past rainy season. This ensures that early warning signs such as pending sloughing or instability are detected as soon as possible. Such inspections also detect damage of re-vegetated areas due to erosion on the side slopes. A potentially rising phreatic surface within the TSF can be assessed with data from vibrating wire (VW) piezometers installed in the TSF, as well as the visual inspections looking for signs of seepage and ponding of surface water.
Although many of the early warning systems, monitoring and trigger action response plans (TARPs) in place today pre-date the GISTM, many of the elements that are being monitored have climate-related triggers. These interventions continually enhance the adaptive capacity of mines and improve the overall resilience of tailings facilities.
After Cyclone Idai and a rainfall event two years ago in which more than 125 mm fell in three hours at a TSF, additional surface mitigation measures have been designed for implementation. These improve drainage and prevent ponding and subsequent seepage into the outer wall zone of the TSF as well as limit erosion damage to berms, benches and access ramps. Armouring of these structures, using suitable materials, also forms part of the longer-term solutions.
Higher rainfall events have also been addressed by applying the principle of redundancy – allowing for double the required decanting capacity of the penstock towers and outfall pipelines. Pools can therefore still be responsibly decanted in the event of heavy rainfall, eliminating the concerns related to freeboard or over-topping, as well as excessive siltation in the decant water with subsequent siltation of the silt traps and return water dam basins.
In terms of addressing the risk of dam breaching, the application of probabilistic analysis presents an opportunity to incorporate climate change models into various breach scenarios. It is likely that such analyses will form an integral part of dam breach studies before too long.
The rainfall variability caused by climate change is also leading mine operators and consultants to investigate opportunities of reducing and optimising water consumption in plants and tailings production. This can involve the broader application of dewatering technologies to produce thickened tailings and paste – which behave in a different manner to conventional slurry tailings. This in turn requires new deposition methods, altered TSF designs and modified water management strategies.
Optimising the use of already treated water is also a focus, with channels and trenches adjacent to TSFs being lined to minimise seepage and potential pollution – and thereby maximise water recovery and the potential to re-use on-site water resources. Where operations either have multiple TSFs or adequate capacity, deposition can be optimised to minimise evaporation from beaches and supernatant pools, making more water available for mine use.
The hotter and drier periods that demand better stewardship of water also result in more dust being generated off the TSF beaches. To address this, mines have adopted deposition strategies that reduce the cycle-time of deposition. Responsible mines also monitor their dust fallout and have implemented strategies for affected communities to lodge complaints about dust so that action can be taken. Ongoing rehabilitation and vegetation of the outer slopes remains an economical and efficient strategy to minimise dust being generated from TSF beaches and slopes. There is increased use of indigenous plants in this application, with some mines now harvesting wild grass seeds for use on TSF slopes. It is not uncommon for the TSF operators to have a nursery on site where thousands of young seedlings are prepared for planting on the TSFs.
In conclusion, the management of water has always been central to the responsible design and operation of tailings dams. Climate change has become an important variable in this process, adding a level of uncertainty that will occupy the minds of scientists, engineers and TSF owners for the foreseeable future. For now, though, there is considerable expertise being invested in the adaptive management of these facilities, and the GISTM provides a valuable tool for measuring and auditing the ongoing interventions and improvements.