ENVE5103 Lecture 3a

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ENVE5103 Lecture 3a

• Gaussian Dispersion Modelling for a Single Stack
• Downwind maximum ground level concentration
• Ground reflection
• Mixing height limitation
• Plume rise
• Stack tip downwash
• Effects Requiring Special Techniques
• Building downwash
• Cavity and wake effects
• Elevated terrain
• Fumigation

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Gaussian Dispersion Modelling for a Single Stack
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Gaussian Dispersion Modelling for a Single Stack

• Downwind maximum ground level concentration
• Ground reflection
• Mixing height limitation
• Plume rise
• Stack tip downwash

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2-D STEADY DISPERSION MODEL

• Solution for windspeed of u m/s and continuous
  release of Q g/s of pollutant at x y 0
  (stack location) and z H (the effective
  stack height)
• H h ??h
• h physical stack height, ?
• ?h plume rise

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GROUND LEVEL CONCENTRATION ALONG CENTER LINE

• We are most interested in ground level, z0,
  concentrations (where humans and other life forms
  reside),
• On the center line, y0, (where concentrations
  are at their maximum

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2-D STEADY DISPERSION MODEL GROUND
REFLECTION

• From the release height of H above ground,
  dispersion can progress upward towards the mixing
  height. In the downward direction the ground acts
  as a mirror unless the pollutant gets deposited.
• The effect of the ground can be handled
  mathematically by treating the reflection as
  another point source located below ground (at - H)

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Figure 4-3 Wark, Warner Davis

• Use of an imaginary sourceto describe reflection
  at the ground

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Figure 4-4 Wark, Warner Davis

• Effect of ground reflection on pollutant
  concentration

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MAXIMUM GROUND LEVEL CONCENTRATION

• At z 0 (cwith reflection ) 2(cwithout
  reflection )
• Not as simple at other z.
• C(x,0,0) first increases with x due to ground
  reflection but horizontal dispersion (y
  direction) eventually decreases it. (Fig 4-5)
• The location and magnitude of the maximum
  concentration can be determined from the
  equations above. Fig 4-8 provides a convenient
  tool. Other empirical methods are also available

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Figure 4-5 Wark, Warner Davis

• Concentration profiles along the center line of a
  stack plume

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Figure 4-8 Wark, Warner Davis

• Maximum Cu/Q value as a function of stability
  class and downwind distance

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MIXING HEIGHT LIMITATION

• As the ground represents a lower limit to the
  vertical dispersion, the mixing height represents
  an upper limit. Multiple reflections from the
  ground and the stable layer above need to be
  considered giving rise to
• Approximation
• No effect of mixing height for xltxL
• Completely mixed in the z direction for xgt2 xL
• Interpolate (on log-log plot) in between xL
  and 2 xL
• xL corresponds to ?z0.47(L-H)

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ONE DIMENSIONAL SPREADING MIXING HEIGHT
LIMITATION

• After a sufficient distance downstream (say ?z
  mixing height) ?the plume can only disperse
  horizontally.
• If we consider the plume well mixed in the
  vertical direction, we can obtain
• where L mixing height

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PLUME RISE

• H h ??h
• h physical stack height, ?
• ?h plume rise due to thermal buoyancy and
  momentum
• Correlations of various complexity exist between
  plume rise, stack temperature, stack velocity,
  atmospheric conditions etc. (e.g. Hollands,
  equation 6.35 de Nevers)

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PLUME RISE - HOLLANDS EQUATION
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PLUME RISE - BUOYANCY AND MOMENTUM FLUXES
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Table 4-6 Wark, Warner Davis

• Equations for calculating final plume rise

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THE U.S. EPA SCREEN(3) MODEL Buoyancy Induced
Dispersion (BID)

• Entrainment due to shear between plume and
  outside air increases dispersion in the plume.
  SCREEN model uses dispersion parameters that are
  larger than those previously reported from
  Prairie experiments.
• More noticeable for concentrations near plume
  level, than for ground-level concentrations

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STACK TIP DOWNWASH

• For Vs lt 1.5 us
• (Vs stack gas velocity, us wind velocity at stack
  height)
• hs physical stack height
• hs physical stack height, corrected for stack
  downwash
• Note that maximum downwash correction is 3 stack
  diameters
• As before, we have H hs ??h

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Effects Requiring Special Techniques

• SCREEN3 will automate all the basic Gaussian
  dispersion calculations mentioned above for a
  single source (point, area, flare, volume).
• In addition, SCREEN3 incorporates special
  techniques for dealing with
• Building downwash
• Cavity and wake effects
• Elevated terrain
• Fumigation

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Building downwash
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BUILDING DOWNWASH AND WAKE EFFECTS

• Figs. 3-19 and 3-20 demonstrate these. Special
  treatments are included in models.
• BUILDING DOWNWASH - Simple rule of thumb
•   downwash unlikely to be a problem if
• hs ? hb 1.5 Lb
•  hs stack height
• hb building height
• Lb the lesser of either building height or
  maximum projected building width.
• Good Engineering Practice (GEP) rule for stack
  design.
•  

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BUILDING DOWNWASH AND WAKE EFFECTS

• Downwash procedures documented in ISC Users
  Guide.
• SCREEN also implements these procedures.
• CAVITY
• Calculations based on
• minimum
• maximum
• horizontal distances alongwind
•    
• WAKE
• Near Wake downwind distance lt 10 Lb
• Far wake downwind distance gt 10 Lb
•  

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SCREEN3 User Guide
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• Structure Influence Zone (SIZ) For downwash
  analyses with direction-specific building
  dimensions, wake effects are assumed to occur if
  the stack is within a rectangle composed of two
  lines perpendicular to the wind direction, one at
  5L downwind of the building and the other at 2L
  upwind of the building, and by two lines parallel
  to the wind direction, each at 0.5L away from
  each side of the building, as shown below. L is
  the lesser of the height or projected width. This
  rectangular area has been termed a Structure
  Influence Zone (SIZ). Any stack within the SIZ
  for any wind direction is potentially affected by
  GEP wake effects for some wind direction or range
  of wind directions, and shall be included in the
  modelling project. Please see Figure 4.5 and
  Figure 4.6.

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Figure 4.5 GEP 5L and Structure Influence Zone
(SIZ) Areas of Influence (after U.S. EPA(19)).
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• Figure 4.6 GEP 360 5L and Structure Influence
  Zone (SIZ) Areas of Influence (after U.S.
  EPA(24)).

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• Building downwash for point sources that are
  within the Area of Influence of a building shall
  be considered. For US EPA regulatory
  applications, a building is considered
  sufficiently close to a stack to cause wake
  effects when the distance between the stack and
  the nearest part of the building is less than or
  equal to five (5) times the lesser of the
  building height or the projected width of the
  building.
• Distancestack-bldg lt 5L

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• For point sources within the Area of Influence,
  building downwash information (direction-specific
  building heights and widths) shall be included in
  the modelling project.
• The Building Profile Input Program (BPIP or
  BPIP-PRIME) can compute the direction-specific
  building heights and widths once the basic
  building data is entered.

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Elevated Terrain Analysis

• Complex Terrain as illustrated in Figure 5.1,
  where terrain elevations for the surrounding
  area, are above the top of the stack being
  evaluated in the air modelling analysis.
• Simple Terrain where terrain elevations for the
  surrounding area are not above the top of the
  stack being evaluated in the air modelling
  analysis. The Simple terrain can be divided
  into two categories

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Simple Terrain

• Simple Flat Terrain is used where terrain
  elevations are assumed not to exceed stack base
  elevation. If this option is used, then terrain
  height is considered to be 0.0 m.
• Simple Elevated Terrain, as illustrated in Figure
  5.2 is used where terrain elevations exceed stack
  base but are below stack height.

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Regulatory (MOE) wording for terrain
considerations

• Terrain data
• 16. (1) If an approved dispersion model is used
  for the purposes of this Part with respect to any
  point of impingement that has an elevation higher
  than the lowest point from which the relevant
  contaminant is discharged from a source of
  contaminant, the model shall be used in a manner
  that employs terrain data.

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Fumigation

• Fumigation occurs when a plume that was
  originally emitted into a stable layer is mixed
  rapidly to ground-level when unstable air below
  the plume reaches plume level. Fumigation can
  cause very high ground-level concentrations.
• Typical situations in which fumigation occurs
  are
• 1. Breaking up of the nocturnal radiation
  inversion by solar warming of the ground surface
  (Fig 5.15 deNevers)
• 2. Shoreline fumigation caused by advection of
  pollutants from a stable marine environment to an
  unstable inland environment and
• Advection of pollutants from a stable rural
  environment to a turbulent urban environment.

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Figure 5.15 de Nevers

• Fumigation

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Shoreline fumigation

• A stable onshore breeze carries a shoreline
  source inland
• Breeze encounters air from solar heated soil,
  mixing from below
• The mixing from below pulls plume to ground
• The end result is the same as in fumigation due
  to inversion break-up (Fig. 5.15 deNevers)
  although the effects leading up to it are
  different
• SCREEN3 addresses both of these situations.