High concentrations of air pollution can build up around tunnel entrances. In order to limit these and to predict the impact of measures, VITO has developed a unique air quality model.
Overview of the design parameters that affect the impact of a tunnel exit on the local air qualiy in a built environment
In the past few decades, the air quality in Flanders has improved considerably. Much of that progress can be attributed to road transport – stricter emission standards, cleaner combustion engines and the introduction of low-emission zones having led to a sharp fall in emissions of pollutants such as nitrogen oxides. Nevertheless, overview maps of air quality in Flanders still show too many hotspots and these don't only occur at busy traffic junctions. Large quantities of air pollution are also being released at the exits of (motorway) tunnels, for example.
The build-up of air pollution at tunnel exits is, of course, a consequence of the traffic passing through the tunnel itself and of tunnel ventilation. Why aren't the exhaust gases simply collected and filtered at the top of the tunnel, instead of being blown out into the open air? ‘Filtration systems are available for that purpose, but their efficiency is limited and they also use a lot of energy,’ says Stijn Vranckx from VITO. ‘It's important to realise that inside a tunnel, you're dealing with a strong airflow that moves about a third as fast as the traffic itself, so simply extracting that airflow and filtering it isn't an option.'
Unique generic model
To limit the impact on the environment, the exits of existing tunnels are therefore often equipped with protective infrastructure such as screens (which also suppress traffic noise) and raised verges. In the case of new tunnels, deep cuttings can be chosen. However, this is always done on an ad hoc basis: the impact on local air quality at the exit of each tunnel is calculated and charted separately. However, the way this is done – most often by means of complex 3D construction models – is a time-consuming and not very cost-effective approach.
During the past year, VITO has therefore developed a unique generic model, commissioned by the Flemish government's Department of the Environment, that can be used as a means of quickly calculating the impact of tunnels on local air quality. ‘Our model provides an estimate of the pollution to be expected at the exit of the tunnel, based on data such as the dimensions of the tunnel and the traffic flow,’ says Vranckx. The outcomes from the planning model can be used to inform the project owner whether a problem is likely to arise. If so, he can take timely action. ‘And we can also accurately assess the impact of those measures, even if they still only exist on paper.' When developing the model, the air quality experts from VITO worked together with tunnel engineers from Tractebel Engineering.
The future users of the model will primarily be EIA experts and consultancy firms commissioned by project developers. The predictive model will certainly be very useful for the construction site of the Oosterweel link in Antwerp, which is the largest construction site in the country. 'Not only will that project involve the construction of new tunnels and covering structures, but the built-up areas surrounding the project will also undergo some significant changes. Our model lends itself perfectly to the task of predicting the changing impact on air quality at both current and future tunnel exits on and along the Antwerp Ring,’ says Vranckx.
