Part 2 - Eskom

ENVIRONMENTAL SCOPING REPORT FOR THE PROPOSED 40MW OPEN CYCLE GAS TURBINE POWER
PLANT IN THE AMERSFOORT AREA, MPUMALANGA
Bohlweki-SSI Environmental
Figure 7.7: Frequency distribution derived from modelled data (South
African Weather Services: 2006 to 2007)
The average wind speed for the Amersfoort area is 3.24 m/s, with the highest
recorded wind speeds (between 8 and 11 m/s) coming from the west. Of the annual
modelled hourly data from the weather services, approximately 1.79% of that hourly
data is recorded as calm winds, representing periods of little dispersion. Information
pertaining to calm periods, average wind speeds and wind direction all play a
significant role with regards to dispersion effects and will play a fundamental role
during the modelling undertaken in the EIA phase of the project.
7.1.3.1. Atmospheric Stability
Atmospheric stability is commonly categorised into one of six stability classes. These
are briefly described in Table 7.1. The atmospheric boundary layer is usually
unstable during the day due to turbulence caused by the sun's heating effect on the
earth's surface. The depth of this mixing layer depends mainly on the amount of
solar radiation, increasing in size gradually from sunrise to reach a maximum at
about 5-6 hours after sunrise. The degree of thermal turbulence is increased on
clear warm days with light winds. During the night a stable layer, with limited vertical
mixing, exists. During windy and/or cloudy conditions, the atmosphere is normally
neutral.
E02.JNB.000308
ESKOM HOLDINGS LIMITED
01
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08/10/2009