Item 3. Developing EPOC’s PWB related to mitigation for 2025-26
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?
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
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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