Open Cut » Environment
There are community and statutory expectations that the design of open cut coal mines in New South Wales will include control strategies that enable the mining operations to stay within target noise limits at sensitive receiver locations adjacent to the mining operations. Probabilistic analysis of noise modelling results is being used as a tool to assess the benefit of long, medium and short term control strategies on the management of noise levels at these sensitive receiver locations.
Probabilistic analysis requires a detailed set of meteorological parameters that reflect the meteorological conditions expected during the mine's operational life. The modelling technique forecasts the percentage occurrence of a noise level based on the frequency of the meteorological descriptors characterised by the wind speed, wind direction and atmospheric stability condition. The two methods typically used to determine the atmospheric stability conditions are:
- the Pasquill-Gifford Stability Category method based on the standard deviation of the horizontal fluctuation in wind direction corrected for wind speed;
- the measured vertical temperature profile or lapse rate.
The objective of this project was to use long term datasets of continuous noise and meteorological data to determine which method is the most appropriate for the assessment of noise enhancing conditions, design of noise control strategies and the establishment of performance-based noise licence conditions for the New South Wales coal mining industry.
The experimental study identified high variability in the measured noise levels at far-field locations when grouped by meteorological conditions derived from either Pasquill-Gifford Stability Category or lapse rate. To minimise the influence of the temporal and spatial variability in the mining activity on the far-field noise levels, near-field noise monitors were used in the analysis as a surrogate for the variation of the mining activity over time. The results of the study on the long term datasets showed that it was not possible to confidently establish a best method for determining atmospheric stability as a correlate for enhanced noise propagation when using the meteorological descriptors available from onsite meteorological stations.
To better understand what might be driving this outcome a series of aerial temperature profiling trials were completed to further investigate the relationship between Pasquill-Gifford Stability Category, the temperature lapse rate and measured noise levels at a far-field location. Using a drone to measure the vertical temperature profile to a flight ceiling of 120m above ground level, observations confirmed the presence of strong lapse rate conditions and/or stratified layering. At times these observations were consistent with the observations of the meteorological stations and at other times in conflict with the observations of the meteorological stations. From the small observation set recorded, it was found that the onsite meteorological stations are limited to providing a generalised interpretation of the prevailing meteorological conditions, this in turn results in high variance in the aggregated noise level outcomes.
From this study, it is hypothesised that a significant part of the variance of the observed noise level outcomes is related to the imperfect articulation of the prevailing atmospheric stability by the onsite meteorological stations. For a mining operation to manage with this uncertainty, it was shown that a combination of both the Pasquill-Gifford Stability Category and the lapse rate can be used to improve confidence in understanding the range of plausible noise level outcomes for a given meteorological condition. However, the analysis indicates a greater understanding of the vertical temperature profile and vertical wind profile in the local environment is required in order to achieve a more significant reduction in uncertainty related to the prevailing meteorological conditions described by the 10m towers.