PASTE & TAILINGS 2021
Considering climate change in tailings dam design
SRK Consulting * discusses how climate change is demanding innovative thinking when considering additional risks to TSF structures going forward 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 .
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 postclosure 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 .
Changing rainfall
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
Adaptive management
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-thannormal 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 longerterm 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
P8 International Mining SUPPLEMENT | JULY 2021