November 2004 (PDF 11.6 MB) - Barrick Gold Corporation

Table E2. Step procedure for determining inversion parameters to be used inthe assessmentStepProcedure1. Sort the night-time (period between 1 h before sunset to 1 h after sunrise taken to be 6 pm to7 am) temperature gradients with associated wind speed in ascending order.2. Convert the temperature gradients into their corresponding stability categories according toTable E1.3. If F or G stability categories occur for a period of 30 per cent of the total night-time or more,either separately or in combination, then temperature inversions are considered to be asignificant feature of the area and need to be assessed.4. Determine the ninetieth percentile temperature gradient value from the full data set created atStep 2 (that is, the highest tenth per cent value). This may be done automatically using thepercentile function available in many spreadsheet programs; or the value may be determinedmanually by sorting the data in ascending order by temperature gradient and choosing thehighest tenth per cent value. The wind speed associated with the chosen temperature gradientis the one to be used in the assessment.Once all data have been collected, the percentageoccurrence of each stability category may be determined.From the measured data, the temperaturegradient to be used in predicting increased noiselevels due to inversions is the ninetieth percentilevalue (that is, the highest tenth per cent value) oftemperature gradients that occur during the nighttime in winter. The wind speed to be used in theassessment is that associated with the ninetiethpercentile inversion strength. A step-by-step guideto the analysis procedure is given in Table E2.E3Classifications of stabilitycategory based on cloud coverThe most widely used stability classificationscheme is that developed by Pasquill (1961). This isbased on observations of cloud cover, wind speedand solar elevation. This scheme has been modifiedby Turner (1964) to create an alternative scheme thatis more amenable to application with computerbaseddatabases. Both schemes are discussed below.Table E3. Key to Pasquill stability categories aHourly averagewind speed at10 m(m/s)DaytimestabilitycategoriesStability categories based on night cloud cover (b)(Night = 6 pm to 7 am)Thinly overcast or≥ 4/8 low cloud≤ 3/8 cloud< 2 G G2–3 E F3–5 A—DD E5–6 D D> 6DDSource: Adapted from Pasquill (1961)a. In dispersion modelling, stability class is used to categorise the rate at which a plume will disperse. In thePasquill-Gifford stability class assignment scheme (as used in this study) there are six stability classes, Athrough to F. Class A relates to unstable conditions, such as might be found on a sunny day with light winds.Class F relates to stable conditions, such as those that occur when the sky is clear, the winds are light and aninversion is present. The intermediate classes B, C, D and E relate to intermediate dispersion conditions. Aseventh class, G, has also been defined to accommodate extremely stable conditions such as might be found inarid rural areas.b. The neutral category D should be used for overcast conditions regardless of wind speed.NSW industrial noise policy79