Open Cut » Environment
The first part of the study investigates the use of meteorological forecast models (the MM5 model) to provide short term (24 to 36 hour) predictions of a relative dust risk index (see later). The study area covers those parts of the Hunter Valley where open cut mining takes place. A second part of the study has used meteorological models to predict the transport of dust emissions for short periods (24 to 36 hours) into the future.
The dust risk index is a function of space and time. It is approximately proportional to the rate at which wind erosion dust is expected to be generated. It is provided as a series of coloured contour figures showing the forecast index at a grid of 500 m by 500 m cells covering the study domain. The predictions are provided on an hourly basis for the upcoming forecast period (currently 24 hours).
The study provides a review of the behaviour of particles in the atmosphere and the physical processes involved in wind erosion and various models for estimating the rate of dust generation due to wind erosion. The purpose of the review is to identify an appropriate method for estimating the dust risk index and to ensure that the meteorological and other parameters required to estimate it are available from the metrological model and other data available to us.
The primary cause of dust emissions is not the direct action of the wind on small particles (sub-60 µm). The cohesive forces binding particles in this size range to the surface are too strong to be overcome by the aerodynamic forces that the wind can exert on these particles. During wind erosion, particles are liberated from the surface to the atmosphere by collisions of larger particles with sizes of the order of 800 µm. These sand-sized particles have sufficient cross-sectional areas projecting into the wind and have sufficiently small masses to be able to be mobilised by the wind. Thus the key to estimating the rate at which wind erosion dust is generated is to quantify the horizontal flux of soil. This is proportional to the rate at which dust is generated by the wind.
The basic equation selected to estimate soil flux is based on work by Draxler (WRAP, 2006). This relates the soil flux to the friction velocity associated with wind gusts and threshold friction velocity required to mobilise particles on particular surfaces (e.g. exposed mining land, forest/woodland, grazing area etc.). Most wind erosion is associated with brief periods of high wind speed (i.e. with the speeds associated with wind gusts). Developing a method of estimating the strength of wind gusts and associated friction velocity was an important part of the study.
The final part of the study has involved work by SKM to predict dust concentrations by modelling the dispersion of dust. The approach adopted makes use of the CSIRO's TAPM model, which is used in conjunction with the CSIRO's Cubic Conformal Atmospheric Model (CCAM). CCAM serves the same role as MM5 and is used in place of the MM5 to provide a forecast of the upcoming meteorological conditions. The model uses estimated dust emissions based on published data from Environmental Impact Statements (EISs). The results are published to a web site as hourly contour plots showing forecast PM10 concentration over the study domain.
A brief comparison between the predicted 24-hour average PM10 concentration at Muswellbrook and Singleton shows if the concentrations are categorised into six categories then in the majority of cases the predicted value and the observed value will lie within one category.
Overall both components of the study provide additional tools that should enable the management of dust from mining in the Hunter Valley to be improved. The full usefulness of the tools will depend on the ability to proactively make use of the forecasts to reduce emissions when required. This will not be a simple task.
An e-newsletter has also been published for this project, highlighting its significance for the industry.