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ADMS 5 User Group Meeting 2017
DMUG
19th April 2018
London
Catheryn Price
ADMS 5 User Group Meeting 2017
Introduction
 This talk is based on a project that CERC carried out, with
Graham Tickle (GT Science & Software Ltd) for the
Atmospheric Dispersion Modelling Liaison Committee
(ADMLC)
▪ ‘High Level Review of the Sensitivity of Dispersion Model
Predictions to Individual Source Term Parameters’
 The project was made up of:
▪ a literature review: main issues and a review of
sensitivity testing
▪ sensitivity tests, carried out for specific models and
source types
ADMS 5 User Group Meeting 2017
Introduction
 Sensitivity tests source terms:
▪ Evaporating pools
▪ Pressurised catastrophic failures (flashing)
▪ Jet releases (high momentum and directional)
▪ Spray releases (atomisation of liquids from pressurised
storage)
▪ Warehouse fires
▪ Pool fires
 These are very specific emergency release source types
  This talk is mainly based on the broader issues
explored in the project
Definitions and key concepts
ADMS 5 User Group Meeting 2017
Definitions
 First step of the review was to address exactly what is
meant by ‘source term’ and ‘model sensitivity’
 Commonly-used words and concepts, but not necessarily
easy to give definitive, universal descriptions
 Once defined, the key concepts involved in source terms
and sensitivity could be explored
ADMS 5 User Group Meeting 2017
Definitions of ‘source term’
 Describes the magnitude, timing, dimensions and
behaviour of a release of a substance into its environment
 HSE guidance: “‘how much’, ‘for how long’ and in ‘what
form’?”
 Initial redistribution of the substance
 Starting point for atmospheric dispersion calculations
ADMS 5 User Group Meeting 2017
Source terms
 Perhaps the most straightforward source term is a
gaseous release from a stack
▪ Parameters are well-defined and understood
 More complex source terms can involve interrelated
processes such as changes in momentum and turbulence,
heat transfer and phase changes
 Questions arise, e.g:
▪ What is the event that leads to the release?
▪ Is the substance stored under high pressure / low
temperature?
▪ Are there dynamic interactions between different parts of
the source term?
ADMS 5 User Group Meeting 2017
Source terms
ADMS 5 User Group Meeting 2017
Definitions of ‘sensitivity’
 Model predictions are ‘sensitive’ to a parameter if
changes in that parameter result in significant changes in
the outcomes
 Outcomes could be:
▪ changes in an absolute output of the model (e.g. the
concentrations), or
▪ the corresponding impacts (e.g. the number of people
exposed)
 The parameters to which the predictions are most
sensitive are the most important parameters to ‘get right’
ADMS 5 User Group Meeting 2017
Sensitivity tests/analysis
 Investigations into the sensitivity of model outputs:
‘sensitivity tests’ or ‘sensitivity analyses’
 These are tests that measure the effect of model input
parameter values on the output of the model
 Can be qualitative or quantitative
 Range from simple to highly complex, sophisticated setup
 Individual parameters can be varied (‘local’ sensitivity
analysis) or several parameters at once (‘global’
sensitivity)
 Recent studies have used statistical methods to deal with
the very large amount of scenarios that would otherwise
result from varying many parameters simultaneously
ADMS 5 User Group Meeting 2017
Sensitivity and uncertainty
 Many different reasons for carrying out a sensitivity
analysis
 A common reason is to investigate uncertainty
 Although the terms ‘sensitivity’ and ‘uncertainty’ have
distinct meanings, it is often useful to consider the two
concepts together
 A sensitivity analysis can be used:
▪ to reduce uncertainties
▪ to help modellers to refine key inputs
▪ to direct modellers to where effort would be best placed
when collating and processing model input data
ADMS 5 User Group Meeting 2017
Sensitivity and uncertainty
High sensitivity
High
uncertainty
Low
uncertainty
Low sensitivity
Time and
resources best
spent here
ADMS 5 User Group Meeting 2017
Designing sensitivity tests
ADMS 5 User Group Meeting 2017
Designing sensitivity tests
 Designing even simple sensitivity test schemes can be
challenging
 For a given source type, need to decide on the overall
scope:
▪ scenarios
▪ sub-scenarios (cases)
▪ parameters
▪ range of parameter values
ADMS 5 User Group Meeting 2017
Designing sensitivity tests
Source type
Scenario 2
Scenario 4
Scenario 3
Case A
Base case
Parameter 1Scenario 1
Parameter 2
Parameter 3
Parameter 4
Parameter 5
Case C
Case B
Value A
Value C
Value B
Value D
ADMS 5 User Group Meeting 2017
Designing sensitivity tests
 What range of parameter values to use?
▪ Realistic?
 Typical of values generally used by modellers?
▪ Appropriate?
 Are they physically possible/compatible with other parameter
values?
▪ Constraints?
 Do some models have limits on input values?
▪ Consistency between models
 Do different models express inputs in different ways?
▪ Consideration of additional options/settings involved
Parameters: e.g. GASTAR – jet sources
Gas or liquid jet
Isothemal Thermal Aerosol
Flash
calculation
Storage
temperature
Atmospheric
pressure
Temperature
Diameter
Mass flux
(kg/s)
Aerosol fraction
(kg/kg)
Hazardous
fraction
Height
Azimuthal
angle
Elevation
angle
An option (discrete) or description of groups of parameters
A parameter (numerical)
not to be considered
ADMS 5 User Group Meeting 2017
Designing sensitivity tests
 What should be set as the base case for each sub-
scenario?
▪ Although we’re only testing source terms, other
parameters/settings also have to be specified
▪ The choice of these parameters could have great
implications on the results
▪ For example, might test in a variety of meteorological
conditions
ADMS 5 User Group Meeting 2017
Designing sensitivity tests
 Which outputs to consider?
 Modelling for different purposes:
▪ Flammable gases, toxic gases, human health impacts,
habitat impacts, etc
 Near-field or far-field?
▪ Near-field results generally more sensitive
 Averaged or peak concentrations?
▪ Flammable limit thresholds vs. toxic thresholds
 Plume centreline or ground level concentrations?
ADMS 5 User Group Meeting 2017
Designing sensitivity tests
 Quantifying the sensitivity
 When the results are analysed, the quantification of the
sensitivity is an important issue.
 Can present the overall results qualitatively, particularly
when parameters are varied one at a time.
 Other, often detailed, sensitivity analyses include some
quantification of the outputs, by applying rankings or
other benchmarks
 Difficult to compare results from different types of
models
▪ e.g. some models have a deliberate tendency to provide
conservative estimates - screening
ADMS 5 User Group Meeting 2017
Further considerations
ADMS 5 User Group Meeting 2017
Source term or dispersion?
 When does the ‘source term’ end and ‘dispersion begin’?
 Possible distinction:
▪ dispersion starts when the mixing of the release with
ambient air is dominated by the atmospheric turbulence
rather than turbulence derived from the storage conditions
and the release process
 Some models are purely source term models, with no
consideration of atmospheric dispersion
 At the other end of the scale are those that are essentially
solely dispersion models, with little in the way of source
term calculations
ADMS 5 User Group Meeting 2017
Internal and external parameters
 Some source parameters are ‘hard-coded’ within
models (sometimes known as internal parameters)
 A sensitivity analysis that is aimed at informing model
users would tend to focus on external parameters
 A sensitivity analysis that focused on internal
parameters would be useful to model developers
ADMS 5 User Group Meeting 2017
Transparency
 How are source parameters are used by the model?
 Is the ‘meaning’ of the parameter value clear?
▪ e.g. diameter in a jet model – is an orifice or a ‘pseudo’
diameter expected?
▪ Many models, when running jet sources require the
expanded jet diameter as input, not the diameter of the
orifice
▪ Can be important for high pressure releases
ADMS 5 User Group Meeting 2017
Discontinuities
 The presence of any discontinuities in the model
calculations can be very important
 Models can suddenly ‘switch’ from one regime to another
▪ e.g. the transition between boiling and evaporation regimes
in some pool source models
 If the model user is not aware of them, these effects
might detrimentally affect the interpretation of the
sensitivity analysis
ADMS 5 User Group Meeting 2017
Different types of models
 Comparing results from models that are intended for
different purposes
 Some models have a deliberate tendency to provide
conservative estimates and to be deliberately sensitive to
certain parameters
▪ Screening vs. detailed modelling
▪ Hazard assessment: land use vs. emergency planning
ADMS 5 User Group Meeting 2017
Communication
 It’s easy to take understanding of sensitivity for granted
 Experience tells us that some parameters are very
important - can sometimes seem obvious
 But this might not be at all obvious to someone
unfamiliar with dispersion modelling (or dispersion in
general)
 Communicating this can help discussions between
modellers and those who measure and collect
information to be used as input data
ADMS 5 User Group Meeting 2017
Conclusions
 Understanding and defining source terms is an essential
part of dispersion modelling
 For complex releases, the exact, detailed, nature of the
source term can often not be completely simulated
 Source terms for dispersion models are, inevitably,
pseudo-sources: a simplification to some degree
 Designing sensitivity tests can be challenging
 Understanding and communicating the issues of
sensitivity is an important part of dispersion modelling

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Source Terms and Model Sensitivity

  • 1. ADMS 5 User Group Meeting 2017 DMUG 19th April 2018 London Catheryn Price
  • 2. ADMS 5 User Group Meeting 2017 Introduction  This talk is based on a project that CERC carried out, with Graham Tickle (GT Science & Software Ltd) for the Atmospheric Dispersion Modelling Liaison Committee (ADMLC) ▪ ‘High Level Review of the Sensitivity of Dispersion Model Predictions to Individual Source Term Parameters’  The project was made up of: ▪ a literature review: main issues and a review of sensitivity testing ▪ sensitivity tests, carried out for specific models and source types
  • 3. ADMS 5 User Group Meeting 2017 Introduction  Sensitivity tests source terms: ▪ Evaporating pools ▪ Pressurised catastrophic failures (flashing) ▪ Jet releases (high momentum and directional) ▪ Spray releases (atomisation of liquids from pressurised storage) ▪ Warehouse fires ▪ Pool fires  These are very specific emergency release source types   This talk is mainly based on the broader issues explored in the project
  • 5. ADMS 5 User Group Meeting 2017 Definitions  First step of the review was to address exactly what is meant by ‘source term’ and ‘model sensitivity’  Commonly-used words and concepts, but not necessarily easy to give definitive, universal descriptions  Once defined, the key concepts involved in source terms and sensitivity could be explored
  • 6. ADMS 5 User Group Meeting 2017 Definitions of ‘source term’  Describes the magnitude, timing, dimensions and behaviour of a release of a substance into its environment  HSE guidance: “‘how much’, ‘for how long’ and in ‘what form’?”  Initial redistribution of the substance  Starting point for atmospheric dispersion calculations
  • 7. ADMS 5 User Group Meeting 2017 Source terms  Perhaps the most straightforward source term is a gaseous release from a stack ▪ Parameters are well-defined and understood  More complex source terms can involve interrelated processes such as changes in momentum and turbulence, heat transfer and phase changes  Questions arise, e.g: ▪ What is the event that leads to the release? ▪ Is the substance stored under high pressure / low temperature? ▪ Are there dynamic interactions between different parts of the source term?
  • 8. ADMS 5 User Group Meeting 2017 Source terms
  • 9. ADMS 5 User Group Meeting 2017 Definitions of ‘sensitivity’  Model predictions are ‘sensitive’ to a parameter if changes in that parameter result in significant changes in the outcomes  Outcomes could be: ▪ changes in an absolute output of the model (e.g. the concentrations), or ▪ the corresponding impacts (e.g. the number of people exposed)  The parameters to which the predictions are most sensitive are the most important parameters to ‘get right’
  • 10. ADMS 5 User Group Meeting 2017 Sensitivity tests/analysis  Investigations into the sensitivity of model outputs: ‘sensitivity tests’ or ‘sensitivity analyses’  These are tests that measure the effect of model input parameter values on the output of the model  Can be qualitative or quantitative  Range from simple to highly complex, sophisticated setup  Individual parameters can be varied (‘local’ sensitivity analysis) or several parameters at once (‘global’ sensitivity)  Recent studies have used statistical methods to deal with the very large amount of scenarios that would otherwise result from varying many parameters simultaneously
  • 11. ADMS 5 User Group Meeting 2017 Sensitivity and uncertainty  Many different reasons for carrying out a sensitivity analysis  A common reason is to investigate uncertainty  Although the terms ‘sensitivity’ and ‘uncertainty’ have distinct meanings, it is often useful to consider the two concepts together  A sensitivity analysis can be used: ▪ to reduce uncertainties ▪ to help modellers to refine key inputs ▪ to direct modellers to where effort would be best placed when collating and processing model input data
  • 12. ADMS 5 User Group Meeting 2017 Sensitivity and uncertainty High sensitivity High uncertainty Low uncertainty Low sensitivity Time and resources best spent here
  • 13. ADMS 5 User Group Meeting 2017 Designing sensitivity tests
  • 14. ADMS 5 User Group Meeting 2017 Designing sensitivity tests  Designing even simple sensitivity test schemes can be challenging  For a given source type, need to decide on the overall scope: ▪ scenarios ▪ sub-scenarios (cases) ▪ parameters ▪ range of parameter values
  • 15. ADMS 5 User Group Meeting 2017 Designing sensitivity tests Source type Scenario 2 Scenario 4 Scenario 3 Case A Base case Parameter 1Scenario 1 Parameter 2 Parameter 3 Parameter 4 Parameter 5 Case C Case B Value A Value C Value B Value D
  • 16. ADMS 5 User Group Meeting 2017 Designing sensitivity tests  What range of parameter values to use? ▪ Realistic?  Typical of values generally used by modellers? ▪ Appropriate?  Are they physically possible/compatible with other parameter values? ▪ Constraints?  Do some models have limits on input values? ▪ Consistency between models  Do different models express inputs in different ways? ▪ Consideration of additional options/settings involved
  • 17. Parameters: e.g. GASTAR – jet sources Gas or liquid jet Isothemal Thermal Aerosol Flash calculation Storage temperature Atmospheric pressure Temperature Diameter Mass flux (kg/s) Aerosol fraction (kg/kg) Hazardous fraction Height Azimuthal angle Elevation angle An option (discrete) or description of groups of parameters A parameter (numerical) not to be considered
  • 18. ADMS 5 User Group Meeting 2017 Designing sensitivity tests  What should be set as the base case for each sub- scenario? ▪ Although we’re only testing source terms, other parameters/settings also have to be specified ▪ The choice of these parameters could have great implications on the results ▪ For example, might test in a variety of meteorological conditions
  • 19. ADMS 5 User Group Meeting 2017 Designing sensitivity tests  Which outputs to consider?  Modelling for different purposes: ▪ Flammable gases, toxic gases, human health impacts, habitat impacts, etc  Near-field or far-field? ▪ Near-field results generally more sensitive  Averaged or peak concentrations? ▪ Flammable limit thresholds vs. toxic thresholds  Plume centreline or ground level concentrations?
  • 20. ADMS 5 User Group Meeting 2017 Designing sensitivity tests  Quantifying the sensitivity  When the results are analysed, the quantification of the sensitivity is an important issue.  Can present the overall results qualitatively, particularly when parameters are varied one at a time.  Other, often detailed, sensitivity analyses include some quantification of the outputs, by applying rankings or other benchmarks  Difficult to compare results from different types of models ▪ e.g. some models have a deliberate tendency to provide conservative estimates - screening
  • 21. ADMS 5 User Group Meeting 2017 Further considerations
  • 22. ADMS 5 User Group Meeting 2017 Source term or dispersion?  When does the ‘source term’ end and ‘dispersion begin’?  Possible distinction: ▪ dispersion starts when the mixing of the release with ambient air is dominated by the atmospheric turbulence rather than turbulence derived from the storage conditions and the release process  Some models are purely source term models, with no consideration of atmospheric dispersion  At the other end of the scale are those that are essentially solely dispersion models, with little in the way of source term calculations
  • 23. ADMS 5 User Group Meeting 2017 Internal and external parameters  Some source parameters are ‘hard-coded’ within models (sometimes known as internal parameters)  A sensitivity analysis that is aimed at informing model users would tend to focus on external parameters  A sensitivity analysis that focused on internal parameters would be useful to model developers
  • 24. ADMS 5 User Group Meeting 2017 Transparency  How are source parameters are used by the model?  Is the ‘meaning’ of the parameter value clear? ▪ e.g. diameter in a jet model – is an orifice or a ‘pseudo’ diameter expected? ▪ Many models, when running jet sources require the expanded jet diameter as input, not the diameter of the orifice ▪ Can be important for high pressure releases
  • 25. ADMS 5 User Group Meeting 2017 Discontinuities  The presence of any discontinuities in the model calculations can be very important  Models can suddenly ‘switch’ from one regime to another ▪ e.g. the transition between boiling and evaporation regimes in some pool source models  If the model user is not aware of them, these effects might detrimentally affect the interpretation of the sensitivity analysis
  • 26. ADMS 5 User Group Meeting 2017 Different types of models  Comparing results from models that are intended for different purposes  Some models have a deliberate tendency to provide conservative estimates and to be deliberately sensitive to certain parameters ▪ Screening vs. detailed modelling ▪ Hazard assessment: land use vs. emergency planning
  • 27. ADMS 5 User Group Meeting 2017 Communication  It’s easy to take understanding of sensitivity for granted  Experience tells us that some parameters are very important - can sometimes seem obvious  But this might not be at all obvious to someone unfamiliar with dispersion modelling (or dispersion in general)  Communicating this can help discussions between modellers and those who measure and collect information to be used as input data
  • 28. ADMS 5 User Group Meeting 2017 Conclusions  Understanding and defining source terms is an essential part of dispersion modelling  For complex releases, the exact, detailed, nature of the source term can often not be completely simulated  Source terms for dispersion models are, inevitably, pseudo-sources: a simplification to some degree  Designing sensitivity tests can be challenging  Understanding and communicating the issues of sensitivity is an important part of dispersion modelling