# ISCST3 Tech Guide

## 6.2.4 The Short-Term Open Pit Source Model

The ISC open pit source model is used to estimate impacts for particulate emissions originating from a below-grade open pit, such as a surface coal mine or a stone quarry. The ISC models allow the open pit source to be characterized by a rectangular shape with an aspect ratio (length/width) of up to 10 to 1. The rectangular pit may also be rotated relative to a north-south and east-west orientation. Since the open pit model does not apply to receptors located within the boundary of the pit, the concentration at those receptors will be set to zero by the ISC models.

The model accounts for
partial retention of emissions within the pit by calculating an
escape fraction for each particle size category. The variations in
escape fractions across particle sizes result in a modified
distribution of mass escaping from the pit. Fluid modeling has shown
that within-pit emissions have a tendency to escape from the __upwind__
side of the pit. The open pit algorithm simulates the escaping pit
emissions by using an effective rectangular area source using the ISC
area source algorithm described in Section 1.2.3. The shape, size and
location of the effective area source varies with the wind direction
and the relative depth of the pit. Because the shape and location of
the effective area source varies with wind direction, a single open
pit source should not be subdivided into multiple pit sources.

The escape fraction for each particle size category, g_{i}, is calculated as
follows:

where:

v_{g} = is the gravitational
settling velocity (m/s),

U_{r} = is the approach wind
speed at 10m (m/s),

" = is the proportionality constant in the relationship between flux
from the pit and the product of U_{r }and the
concentration in the pit (Thompson, 1994).

The gravitational settling velocity, v_{g},
is computed as described in Section 1.3.2 for each particle size
category. Thompson (1994) used laboratory measurements of pollutant
residence times in a variety of pit shapes typical of actual mines
and determined that a single value of " = 0.029 worked well for
all pits studied.

The adjusted emission rate (Q_{i})
for each particle size category is then computed as:

Where Q is the total
emission rate (for all particles) within the pit, N_{i} is
the original mass fraction for the given size category, and g is the
escape fraction calculated from Equation (1-68). The adjusted total
emission rate (for all particles escaping the pit), Q_{a}, is
the sum of the Q_{i} for all particle categories calculated
from Equation 1-69. The mass fractions (of particles escaping the
pit), N_{ai}, for each category is:

Because of particle settling within
the pit, the distribution of mass escaping the pit is different than
that emitted within the pit. The adjusted total particulate emission
rate, Q_{a}, and the adjusted mass fractions, _{Nai},
reflect this change, and it is these adjusted values that are used
for modeling the open pit emissions.

The following describes the specification of the location, dimensions and adjusted emissions for the effective area source used for modeling open pit emissions. Consider an arbitrary rectangular-shaped pit with an arbitrary wind direction as shown in Figure 1-10. The steps that the model uses for determining the effective area source are as follows:

1. Determine the upwind sides of the pit based on the wind direction.

2. Compute the along wind length of the pit (R) based on the wind direction and the pit geometry . R varies between the lengths of the two sides of the rectangular pit as follows:

where L is the long axis and W is the short axis of the pit, and 2 is the wind direction relative to the long axis (L) of the pit (therefore 2 varies between 0E and 90E). Note that with this formulation and a square pit, the value of R will remain constant with wind direction at R = L = W. The along wind dimension, R, is the scaling factor used to normalize the depth of the pit.

3. The user specifies
the average height of emissions from the floor of the pit (H) and the
pit volume (V). The effective pit depth (d_{e}) and the
relative pit depth (D_{r}) are then calculated as follows:

4. Based on
observations and measurements in a wind tunnel study (Perry, et al.,
1994), it is clear that the emissions within the pit are not
uniformly released from the pit opening. Rather, the emissions show a
tendency to be emitted primarily from an upwind sub-area of the pit
opening. Therefore an effective area source (with A_{e} being
the fractional size relative to the entire pit opening) is used to
simulate the pit emissions. A_{e} represents a single area
source whose dimensions and location depend on the effective depth of
the pit and the wind direction. Based on wind tunnel results, if D_{r}$0.2,
then the effective area is about 8% of the total opening of the mine
(i.e. A_{e}=0.08). If D_{r}<0.2, then the
fractional area increases as follows:

When D_{r} = 0, which means
that the height of emissions above the floor equals the effective
depth of the pit, the effective area is equal to the total area of
the mine opening (i.e. A_{e}=1.0).

Having determined the effective area from which the model will simulate the pit emissions, the specific dimensions of this effective rectangular area are calculated as a function of 2 such that (see Figure 1-10):

and

Note that in equations 1-75 and 1-76, W is defined as the short dimension of the pit and L is the long dimension; AW is the dimension of the effective area aligned with the short side of the pit and AL is the dimension of the effective area aligned with the long side of the pit (see Figure 1-10). The dimensions AW and AL are used by the model to define the shape of the effective area for input to the area source algorithm described in Section 1.2.3.

The emission rate, Q_{e}, for
the effective area is such that:

where Q_{a} is the emission
rate per unit area (from the pit after adjustment for escape
fraction) if the emissions were uniformly released from the actual
pit opening (with an area of L@W). That is, if the effective area is
one-third of the total area, then the emission rate (per unit area)
used for the effective area is three times that from the full area.

Because of the high level of
turbulence in the mine, the pollutant is initially mixed prior to
exiting the pit. Therefore some initial vertical dispersion is
included to represent this in the effective area source. Using the
effective pit depth, d_{e}, as the representative dimension
over which the pollutant is vertically mixed in the pit, the initial
vertical dispersion value, F_{zo}, is equal to d_{e}/4.3.
Note that 4.3@F_{zo} represents about 90% of a Gaussian plume
(in the vertical), so that the mixing in the pit is assumed to
approximately equal the mixing in a plume.

Therefore, for the effective area source representing the pit emissions, the initial dispersion is included with ambient dispersion as:

For receptors close to the pit, the initial dispersion value can be particularly important.

Once the model has determined the characteristics of the effective area used to model pit emissions for a particular hour, the area source algorithm described in Section 1.2.3 is used to calculate the concentration or depsosition flux values at the receptors being modeled.