By Russell Mills
Dispersion models calculate how an airborne CBRN / HazMat contaminant may travel geographically and what impacts it may cause, providing users – from civil emergency services to military commanders to commercial safety analysts – with essential situational awareness about the likely next steps of a release. This enables critical groups to effectively plan and train for response strategies in advance, as well as implement, evaluate and adapt their response actions during a real-time incident.
The ultimate aim of these modelling tools is to help users control and mitigate the impacts of a CBRN / HazMat incident to the greatest extent possible, in order to protect people’s health and safety.
Why we need indoor and outdoor models
Riskaware has developed both outdoor and indoor dispersion models, under projects funded by Dstl, in order to understand how an airborne contaminant may move within enclosed spaces, such as critical national infrastructure, and outdoor urban environments, such as a city block.
Our dispersion models use many gaussian puffs to represent the contaminant plume as a whole. We model how these individual puffs interact with one another in order to see how the overall mass will behave with greater accuracy. Indoor and outdoor environment will, however, experience different characteristics and parameters and therefore require different models in order to understand how a contaminant will behave in each zone.
It’s equally important to understand how a contaminant will move between these two environments, how the interaction of the two environments will influence the movement of the plume, and their relative risks. There will be many scenarios where an outdoor contaminant can infiltrate a building and it’s necessary to model the transition itself as well as the behaviour of the contaminant in either environment individually.
Riskaware modelling capability
Riskaware has further developed a capability for the US DoD DTRA’s HPAC to integrate the indoor models with the outdoor models, giving end-to-end situational awareness of all the potential areas of exposure, the threats of a release, and the most effective strategies for mitigating risk.
Outdoor modelling (Urban dispersion model (UDM))
Our UDM model simulates how airborne contaminants move and interact with surrounding buildings or terrains in an urban environment. Based on detailed wind flow data, including how wind behaves when encountering buildings, groups of buildings, and uneven terrains, UDM models where a plume is most likely to travel and how it will spread.
UDM can intricately account for different buildings shapes and sizes, and complex combinations, using multi-polygon parts which affect the gaussian puffs, or group similar buildings together to simulate their combined airflow.
Indoor modelling (Rapid Indoor Dispersion Tool (RIDT))
Our indoor modelling capabilities model how airborne contaminants infiltrate and travel through indoor spaces. This can be done through a single zone model or a complex, multiple zone model.
Single zone models implement a single entrance and exit per space. These models show the infiltration of air, how much air is drawn in and how the air travels between different rooms with different environments. It further determines the exfiltration of air from an indoor space into an outdoor space to produce an output about the levels of contamination that exist inside.
Multiple zone models create a model with multiple points of entrance and/or exit, such as windows, air cons, ventilation, and doors. Such models can account for in-depth infrastructures, modelling how the air travels through the multiple exit and entrances, and into interconnected corridors and multiple rooms within a building. In addition, the movement of individuals in the room and how this affects air flow can be considered. These models are generally used for critical national infrastructure or parliament buildings.
The Rapid Indoor Dispersion Tool (RIDT) was developed by Riskaware under contract to the Defence Science Technology Laboratory (Dstl) in the UK, as a fast-executing computer modelling tool for simulating the dispersion of airborne pollutants within indoor spaces. It is also available via Riskaware’s open-source repository under the open-source MIT License (Expat).
The application of eddy diffusion, the modelling methodology used in RIDT, was recently published by Dstl in the scientific literature, and the Dstl scientific team was closely involved during the software development of the model. It is particularly useful for modelling larger indoor spaces where the contaminant is not initially fully mixed within the space.
Integrated modelling (Urban Sub-system (USS))
The indoor and outdoor models can exchange data, allowing the output from both models to be combined. This means the exchange of materials between these different paradigms can be easily modelled, which not only simplifies the user experience but also provides the essential integrated view of a scenario.
Our USS tool performs this task in HPAC to control the whole building-aware modelling environment. This allows in-depth interrogation of the indoor model hazard and the integrated outdoor challenge, all within one unified system.
Types of models and scenarios
Both of these models can be used individually or in combination, to analyse ‘what-if’ scenarios for the purpose of planning and training emergency services and civil or military response. Combining these models gives greater insight into more complex scenarios, so users can determine the most effective, safe action for people to take.
Here are some example scenarios:
Indoor source term:
This would be a scenario where the release originates inside a building. Indoor models can be used to model how the contaminant moves through the buildings, and therefore how it may affect the individuals inside, and whether any of this would be extracted to the general atmosphere.
This would be a scenario where the release originates outside and whether this will infiltrate a building, how it would do this, and what action the people indoors should take i.e., whether they should evacuate the building or remain indoors, and in this case, whether they should evacuate into another room or just close all entrance points.
Combined models will consider indoor and outdoor environments, and the relationship between the two, as a whole. Riskaware has developed the ability to run these models at the same time, allowing mass to be transferred from one environment into the other.
We are currently working with the Lawrence Berkeley National Laboratory to develop these capabilities and introduce the integrated modelling capability into the modelling infrastructure of the latest HPAC system.
To find out more about our work with indoor and outdoor modelling, get in touch.