Case Study: Using large-scale field plots to monitor ...

Case Study: Using large-scale field plots to monitor erosion of waste dump batters at Birla Nifty Copper Operation Mike Robinson – Environment Manager, Birla Nifty Copper Operation Evan Howard – Principal Environmental Consultant, Landloch Pty Ltd Peter Berghofer – Environmental Technician, Landloch Pty Ltd

Abstract Five large-scale plots were installed on a waste dump at the Birla Nifty Copper Operation in August 2012. Plots were established with different surface treatments including different surface materials, and different depths of soil over the oxide waste. The erosion plots have been designed to collect bedload sediment in a trench at the toe of the slope, and suspended sediment laden runoff is routed through a tipping bucket and splitter to collect runoff volumes and suspended sediment loads.

The time-series runoff and erosion data collected is being used to validate the Water Erosion Prediction Project (WEPP) erosion model that has been used previously to assess erosion potential of slopes on the site. The data will support the development of a rehabilitated landform design that is suitable for the materials and climate at Nifty.

This case study details the plot design and construction method, and the process of collecting the data after rainfall events. Preliminary runoff and erosion data are provided.

Introduction Birla Nifty Copper Operation (Nifty) is located in the Great Sandy Desert Region of the East Pilbara, approximately 1,250km north of Perth and 350km east of Port Hedland (ABM 2014). The site features an historic open pit operation which ceased in 2006 and is currently mining underground using conventional methods (ABM 2014).

Climate Nifty is located in an arid zone with an annual average rainfall of approximately 250mm. The majority of rainfall occurs between November and March. The site is influenced by rainfall from infrequent tropical depressions and cyclones. Rainfall data for Nifty (1996 to present, not continuous) show cyclone related rain events affecting the site, on average, every three years (Figure 1). Temperatures vary between 35 and 45 degrees Celsius in the summer and 25 to 30 degrees Celsius in the winter.

Cyclones Daryl, Emma, Clare, Glenda

Cyclone Fay

Cyclone Monty

Cyclone Graham

Figure 1. Cyclone related rain events recorded at the Bureau of Meteorology’s Nifty Copper Mine weather station between 2003 and 2006.

Soils and wastes The topsoil consists of red dune sand with high infiltration capacity. Although this reduces runoff potential and hence can act to reduce erosion potential, erosion remains a significant issue when runoff is allowed to concentrate. Erosion potential is also increased by the placement of the permeable topsoil over the less permeable oxide. This causes the sand to become saturated, lose cohesion and slip from the surface.

Wastes include a hardsetting non-acid forming oxide and a potentially acid forming (PAF) material. The oxide waste is almost impermeable and has a high clay/silt content. Despite the material typically having a non-dispersive clay fraction, tunnel erosion does occur due to the presence of a high proportion of fine sand/silt sized particles.

Extensive erosion damage has occurred on the batters of the existing waste dumps. A combination of excessive slope gradient and length, presence of features that concentrate flow, and material properties have contributed to the current eroded condition. The climate is also a significant factor in erosion of waste dumps with much of the annual rainfall often occurring in very few events.

Objective of erosion plots Erodibility parameters of the materials used for rehabilitation works (dune sand and oxide) were developed in laboratory studies, and used to model erosion using the Water Erosion Prediction Project (WEPP) model (Flanagan and Livingston 1995). Erosion plots were constructed on the East Tip at Nifty to collect time series runoff, and erosion data that can be used to validate the parameters and provide more detailed information with which to model erosion at Nifty. Runoff and erosion data have been collected from these plots since their construction in August 2012. This case study provides details of the construction, maintenance, and data collection requirements of the erosion plots and outlines the WEPP modelling features important in the final landform design.

Historical applications of erosion plots in a mine setting The value of erosion plots is recognised in the mining industry around the world. Landloch (2008) noted that plots have been implemented in a wide range of environments including: •

Forested areas of Queensland;

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Cropping research in India;

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Cropping research in Kenya;

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Steep cropland studies in the Philippines, Malaysia and Thailand;

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Pineapple production in Queensland;

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Open cut coal mine rehabilitation in Queensland;

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Forestry road studies in Victoria;

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Mine-site rehabilitation in Papua New Guinea and

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Mine-site rehabilitation in Guinea, West Africa.

In the last 20 years, significant advances have been made in the methodology used for runoff/erosion plot studies in Australia. Consistent and distinct methodology development has been driven by the need to: •

Streamline plot management;

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More easily consider single events; and

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Obtain more detailed data on runoff and erosion.

Erosion Erosion processes on the constructed landforms at Nifty are a function of rainfall and the resulting runoff. Four forms of soil erosion have been observed on site: interrill, rill, gully, and tunnel erosion.

Runoff initiation occurs when the soils infiltration capacity is exceeded. Shallow, un-concentrated runoff is responsible for interrill erosion where particles are transported by sheet flow. Rill erosion is the result of concentrated runoff which is formed by surface features that laterally redirect flow on the surface. Rip lines and berms are examples of features that cause flow concentration. Gully erosion has the most significant visual impact on the batters of waste dumps at Nifty. Gullies are an extension of rill erosion, where the rill has widened and increased in depth. Some gullies exceed 2 metres in width and depth. Tunnel erosion has been observed in the oxide. Movement of silt and fine sand sized particles in the oxide weakens its structure, and creates areas of subsidence that progress into observable tunnels. Subsidence and tunnel erosion is evident particularly on the dump tops where water has not been able to readily drain away.

Erosion plot design The erosion plots were designed to enable these erosion processes to occur within a bounded area that is able to be measured. The plot areas incorporated: •

A dump batter section approximately 20m wide extending from the toe to the crest of the batter (100 - 150m slope length) covered in a specific surface treatment;

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An engineered crest bund and shaped land surface in the vicinity of the dump top crest to limit tunnel erosion on the plots; and

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Boundary edges to stop water discharging from or into the plot area being measured.

Data was to be collected for three potential surface materials: oxide, dune sand, and a 1:1 mixture of oxide and dune sand (Landloch 2011). Five plots were installed on the east face of the East Tip (Figure 2). The surfaces developed were: •

0.2m dune sand overlying oxide, ripped, no vegetation to be established,

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0.1m dune sand overlying oxide, ripped, no vegetation to be established;

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0.1m dune sand overlying oxide, ripped, vegetation to be established;

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0.1m of 1:1 sand/oxide mix overlying oxide, ripped, no vegetation to be established; and

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In situ oxide, ripped, no vegetation to be established.

Heap Leach Pad Pit

Tailings Dam

East Tip

Erosion Plots Figure 2: Erosion plots located on the east face of the East Tip.

Erosion processes such as rill and gully formation occur at a large scale and plots were constructed to cover the entire length of the slope and a sufficient width of batter to ensure that these processes could occur. Use of large plots also reduces the need for replications of surface treatments.

Surfaces were established using techniques that would be employed in full scale rehabilitation works. This enabled observations of runoff and erosion to include any potential weakness in rehabilitation techniques such as off-contour rip-lines and the quality of mixing and spreading of the required surface materials. An assessment of the capabilities of the on-site equipment to produce the surface treatments to specification was made possible by using large field scale plots.

Plot components The erosion plots contain the following components: •

Boundary fencing (Figure 3 i) runs the length of the plots on both sides and consists of an impermeable canvas suspended on a wire. The fence contains runoff within the plot area as well as excluding runoff from outside the plot area;

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A crest bund is positioned across the top of the dump to prevent runoff from the dump top flowing into the plot area;

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A trench at the base of each plot (Figure 3 ii) is lined with an impermeable rubber liner and shaped to direct flow to the centre of the trench. The trench directs runoff into a manifold while at the same time storing deposited bedload sediment;

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A manifold is attached to the liner at the centre of the trench. The manifold directs flow into a tipping bucket located beneath it (Figure 3 iii);

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A tipping bucket and counter records the tips and enables calculation of runoff; and

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A flow splitter attached to the tipping bucket frame enables collection of suspended sediment samples as well as providing an additional measure of total runoff.

i)

Boundary fencing on plot sides

iii)

Manifold and tipping bucket

ii)

Sediment trench

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Surface treatment – 0.1m topsoil over 0.1m oxide

Figure 3: Plot components including example of surface treatment.

Construction and maintenance method Batter re-profiling The eastern batter of the East Tip was selected for installation of the plots. The batter was reshaped to an angle of 18 degrees. Excess material from the bottom was removed to allow drainage of runoff from the toe of the slope.

Spreading surface materials Topsoil and oxide was spread to the specified depth on the relevant plots using a dozer. Plot 4 had a surface layer of 0.1m depth of topsoil/oxide mix. The material was mixed at a 1:1 ratio with materials being mixed by paddock dumping on the batter with an excavator and pushing the piles down the slope with a dozer. The thickness of the profile was measured to ensure consistency across the plot.

Final rip of plots During initial cross-slope ripping, it was found that the topsoil was too unstable to be ripped successfully. Rip lines were observed to collapse as soon as they were created. To overcome this, the plot surfaces were wet with a water truck prior to ripping with the dozer. All plots were ripped using a triple tyne attachment with tynes inserted to a depth of 0.3m.

Constructing boundaries and sediment trench Plot boundaries were measured and marked with survey pegs. A crest bund was installed on the upper boundary with a dozer. Parallel trenches on the sides of each erosion plot were constructed using a specially designed trenching tool attached to the front bucket of a backhoe. Side fences were installed in the trench. Once the fence was constructed, the underlying trench was then backfilled with a spade ensuring rip-lines are reinstated to the boundary fence.

A sediment trench was installed across the entire width at the base of each plot. An excavator was used to roughly create the trench and final shaping by hand shovel ensured that the lowest point was in the centre of the plot where the manifold was positioned. A rubber liner was positioned in the trench with the edges buried within the plot surface to prevent runoff from escaping under the liner.

Installation of erosion monitoring equipment At the base of each plot, a retaining wall and cement pad was constructed to provide a foundation for erosion monitoring equipment. A manifold supported by a frame at the rear, was placed at the top of the retaining wall and was connected to the lowest point of the sediment trench. The manifold has a slot in its base that directs the runoff into the tipping bucket. Beneath the opening in the manifold, a tipping bucket is fitted. A mechanical counter and flow splitter is attached to the tipping bucket frame.

Drainage Runoff discharged from the tipping buckets and other runoff from adjacent areas is stored within a drain built at the base of the plot area. The drain was shaped using a dozer and a wall ~4m high was built to prevent overflow into the surrounding undisturbed environment.

Maintenance Erosion plots are obviously designed to erode. Therefore, regular maintenance is required to ensure that the erosion monitoring equipment remains functional. Maintenance became crucial to the

continued collection of accurate data when sediment build up in the sediment trench reduced the capacity of the trench, increasing the risk of runoff and sediment not being collected and measured. The plot sides often sustained damage from wind (ripping the buried edge from the ground) and erosion from rilling. Regular checks of all edges were needed to ensure no runoff escaped the plot and no runoff entered the plot from outside. General structural checks were also carried out on the manifold and tipping bucket, ensuring there was no leak in any joins where runoff could escape.

Data collection The duration for which the plots need to be operated is dependent on the amount of data collected during each year. Where possible, the following data are collected after each rainfall event: •

Sub-daily rainfall data (15 minute in this case);

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Bedload sediment load (corrected for moisture content);

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Suspended sediment load;

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Tip counts from the mechanical counters; and

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Observations of surfaces including changes in rip line shape and rill network development.

Rainfall data Since the installation of plots in August 2012, there have been 2 very large rainfall events and several smaller events. In February 2013, rainfall from Cyclone Rusty was recorded at 236mm over a 3 day period. Based on historical climate data, this rain event had an average return interval of 50 years. Two low pressure systems caused substantial rainfall in the 2013-14 wet season. In January 2014, 85mm was recorded in a day. These events have substantially changed the appearance of the surface treatments since commencement of the plots.

Bedload sediment During the 2012-13 wet season, bedload sediment was consistently measured in the sediment trenches. Rill development on the topsoil over oxide plots (plots 1-3) increased rapidly until Cyclone Rusty in February 2013. During the cyclone, the topsoil material became saturated and moved as a mass downslope across the underlying oxide. Bedload sediment swamped the sediment trenches and buried the erosion monitoring equipment. For the same event, a significant amount of bedload sediment was recorded on the topsoil/oxide mix plot (plot 4), as well as accelerating rill development. The measurement equipment remained intact. The oxide plot (plot 5) eroded the least during this event, with only a minor amount of bedload sediment deposited in the trench.

After Cyclone Rusty, the plots were inspected for damage. It was decided, based on the data recorded prior to this event and the current state of the topsoil over oxide plots, that plots 1-3 could be closed. The surface treatment on plot 4 was repaired. Plot 5 remained largely untouched. Sediment trenches on plots 4 and 5 were cleared of bedload sediment and these plots remain operational for further data collection.

Plot 5 has a noticeable increase in rilling activity in the 2013-14 wet season. Consequently, higher bedload sediment volumes have been measured, even though the rain events were less severe than that of Cyclone Rusty. In comparison to plot 4, plot 5 is currently still producing less bedload.

Suspended sediment Very little suspended sediment was recorded on plots 1-3 before their closure in February 2013. Like bedload, suspended sediment loads appear to be increasing for plot 5 as the oxide treatment begins to rill.

Preliminary data Data for the 2012-13 wet season have been processed. A total of 7 rainfall events occurred in the year. Reasonable correlation exists between observed and measured rain and runoff except for plot 1. High variability in the runoff data was measured for this plot. For sand plots (plots 1-3) collectively, ~1% of measured rain was recorded as runoff. The same proportion was measured for the topsoil/oxide mix plot (plot 4). Approximately 5% of rainfall was recorded as runoff for plot 5. WEPP predicts similar values in the long term: 0.5% for the topsoil plots, 1% for the topsoil/oxide plot, and 10% for the oxide plot.

Insufficient data exists to undertake an assessment of erosion. However, the following cumulative erosion rates were recorded for 2012-13: •

Dune sand plots: 0.4-0.8t/ha (not including erosion caused by Cyclone Rusty)

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Topsoil/oxide mix plot: 11.8t/ha

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Oxide plot: 3.4t/ha

It is important to note that the erosion potential of the plots may change as consolidation continues and as roughness changes.

Discussion The erosion plots installed at Nifty continue to provide useful data. Methodologies for data collection and maintenance ensure that the plots can continue to collect accurate measures of runoff and erosion for the life of the project. In the 2 years that this project has been running, significant conclusions have already been made for some of the surface treatments constructed. A surface treatment with topsoil over oxide will not withstand the intense rainfall events that occur at Nifty. This is consistent with observations of other previously rehabilitated batters on site. Addition of oxide to dune sand slightly increases erosion resistance. However the measured erosion for this treatment remains quite high. The oxide without any addition of soil has eroded the least to date. At present, the rip lines have maintained their structure on this plot. However, some tunnelling has been observed on the upper section of the plot. As the surface continues to be subjected to runoff, some rills are also developing.

The rip lines of the sand plots (plots 1-3) had disappeared due to wind erosion shortly after the rip lines were created (prior to the wet season starting). Once the wet season commenced, rainfall infiltrated through the permeable topsoil layer and flowed along the interface between the topsoil and oxide. This sub-surface flow caused failure of these three plots.

Data collected thus far from this study at Nifty have been important in directing rehabilitation planning and closure considerations. To date, insufficient data exist to validate the WEPP model output. Another year of data is likely required, assuming a similar number of runoff events occur as have occurred in the previous two years. The validation process will increase the confidence in the output produced by WEPP.

To date, performance of the WEPP model appears to be reasonable. Measured runoff/rainfall ratios align reasonably well with those predicted by WEPP using long term climate data. The measured erosion aligns with observations, with the oxide plot eroding the least, followed by the topsoil/oxide mix plot, with the topsoil plots eroding the most (had the data for Cyclone Rusty been able to be measured).

Site personnel have benefited from the implementation of this project at Nifty. During the construction process, surface auxiliary staff and machinery were used from site. This project was a valuable way for the rehabilitation and closure team and surface auxiliary team to collaborate. The surface auxiliary team were able to provide practical advice in the planning stage and also were able to trial equipment and operators to assess the capabilities of the machinery and staff for the rehabilitation that will be required on site in the near future. As a result of successful cooperation from multiple departments on site, ongoing maintenance and data collection work has been met with positivity and has promoted awareness of rehabilitation efforts on site.

In addition to site personnel, erosion plots can also benefit regulators. By constructing different surface treatments utilising the materials available for rehabilitation, a practical demonstration of erosion performance is provided. Providing objective measurements and visual evidence of successful and unsuccessful surface treatments demonstrates to regulators that Nifty is committed to achieving the best possible rehabilitation outcomes for their existing waste dumps.

Conclusion Erosion plots perform several functions for rehabilitation planning in a mine setting. Validation of the WEPP model is a useful step in defining suitable surface treatments for rehabilitating waste dumps. The erosion plot planning and construction process facilitates collaboration between mining and environment departments and enables assessment of the practical challenges related to rehabilitation. The usefulness of the data is highly dependent on the methodology for data collection and maintenance. Diligent data collection ensures the maximum potential of the plots is reached. Challenges to effective data collection arise when accessibility to the erosion plot site is limited. Large

events during the wet season can delay data collection and maintenance. Without bedload sediment and suspended sediment being cleaned out after significant rainfall events, plot capacity to collect sediment is reduced, which may lead to missed data.

Plot design is dependent on the materials available for rehabilitation and the climate where the plots will be located. Erosion plots are widely accepted in the international community as an effective tool in erosion model validation. Historical applications illustrate that this process can be implemented from tropical to arid climates. With Western Australian mine regulators continually looking for evidence to support landform designs and rehabilitation plans, erosion plots are a useful means of collecting data to support the designs mining companies are seeking to use in order to meet their closure obligations.

References Aditya Birla Minerals, 2014, Birla Nifty: Copper Production Operations, company website, viewed 22/04/2014, http://www.adityabirlaminerals.com.au/page/2_2_1

Flanagan, D.C., and Livingston, S.J. (1995).Water Erosion Prediction Project (WEPP) Version 95.7 User Summary. In (Flanagan and Livingston, editors) ‘WEPP user summary’, NSERL Report No 11, July 1995.

Landloch Pty Ltd 2008, Supply of tipping buckets and troughs for erosion plots, Simandou. Report prepared for Rio Tinto.

Landloch Pty Ltd 2011, Design of stable batter slopes for the Nifty Waste Dump. Report prepared for Birla Nifty Copper Operation.