Olivier Salvi

Anders Lönnermark

Haukur Ingason

Benjamin Truchot

Roland Leucker

Félix Amberg

Dirk-Jan Molenaar

Horst Hejny

the grouping made of L-Surf Services

New energy carriers in vehicles and their impact on confined infrastructures -Overview of previous research and research needs

Keywords: safety, security, new energy, underground, integrated risk management

The global warming debate forces the vehicle industry to come up with new environmentally friendly solutions. In 10 years time, or even faster depending on the pressure from different governments in particular in Europe, vehicles will not only use gasoline, diesel and LPG, but also CNG, Hydrogen, ethanol, DME and other bio-fuels, as well as batteries and fuel cells. This quick development and the diversity of new energy carriers can jeopardize the safety in underground infrastructures such as tunnels or car parks. This can cause a major drawback in the adoption of new energy carriers as regulators or operators may prohibit use of these vehicles in underground systems if no new relevant measures will be taken. Unclear situation will also affect the implementation of international policies aiming at reducing the environmental footprint and especially CO 2 emission in road traffic.

The problem became clear after a workshop with the vehicle industry, tunnel operators, authorities, and safety experts organised in November 2008 by L-surF Services with the support of ITA-COSUF, ECTP and HYSAFE. This workshop demonstrated that the construction sector lacks appropriate design data and tools as well as knowledge to build safe underground infrastructure compatible with a diversity of new and alternative energy carriers. Vehicle industry, infrastructure operators and regulators have not yet addressed this problem.

In a first part, an overview of the regulatory situation regarding safety and security of the admission of new energy carriers for vehicles in underground infrastructures is presented. Then, a detailed review of previous relevant research projects performed makes it possible to formulate recommendations in terms of a strategic research & development agenda. The overview shows that it is necessary to develop an integrated risk assessment and management method specific for underground transport systems, metros and hubs in confined spaces taking into account the "emerging risk" aspects.

INTRODUCTION

Improving safety and security in urban underground infrastructures is multi-disciplinary by nature because it deals at the same time with:

• New energy carriers for transport,

• Design and construction of the transport infrastructures,

• Safety and security of the whole systems.

Several recent or on-going international, European and national projects have been identified as a starting point. The paper presents a review of the main results gained in the following fields:

• Risk assessment and management in tunnels, underground, metros and confined spaces,

• Security management in transport systems,

• Integrated risk management of new and emerging risks,

• Innovative energy carriers for vehicles and their safety.

The review of the state-of-the-art in these field shows that often safety and security are analysed separately. Therefore, a combined risk analysis can contribute to find more cost-effective solutions.

In addition, an integrated approach considering the whole components of the transport stations and terminals and their interactions help to find the optimum to improve the safety and security by design, to include also emerging risks connected to new energy carriers.

OVERVIEW OF THE REGULATORY SITUATION

The regulations related to the use of new energy carriers in vehicles and the adaptation of the underground infrastructures has not yet been developed.

Informal discussions with several authorities from France, Germany, The Netherlands, Italy and Spain have lead to the following statements:

• The topic of the compatibility of the existing underground transport infrastructures with the diversity of energy carriers in the green vehicles of the future is not yet on the agenda.

• An approach combining technical requirements with political targets needs appropriate investment in R&D.

• Being pro-active, anticipating the problems at an early stage to develop a scientific based regulation, seems to be difficult (several authorities found that the question comes too early). UPTUN has achieved a lot of innovations in the field of fire safety and related fields [START_REF] Khoury | Global approach to tunnel safety (part of Workpackage 5 -Evaluation of safety levels and upgrading of existing tunnels -D56)[END_REF][START_REF]UPTUN -Upgrading existing tunnels for fire safety -Final Technical Report[END_REF]. They comprise detection and monitoring devices as well as mitigations systems. Tools assisting in evacuation and methods to protect structural elements have been developed as well. The risk based modelling activities lead to the establishment of a model capable of assessing the current fire safety level in a particular tunnel and also determining the improvements of the fire safety as well as the economic impact based on upgrading measures. The concept developed in UPTUN is of great value when doing future studies on new energy carriers as in many cases there is a need to upgrade the tunnels taking into consideration new energy carriers.

RISK ASSESSMENT AND MANAGEMENT IN TUNNELS, UNDERGROUND, METROS AND CONFINED SPACES

Further development needed

Reference scenarios were established to assess risks in existing tunnels, mainly road tunnels. The definition of reference scenarios has to be generalised to other underground infrastructures such as transport terminals and stations including the connections with metro tunnels, with the car parks and other underground depots to park the vehicles when they are not in use.

In addition, the scenarios considered were dealing with fires i.e. the impact of the fire load and of traditional toxic gas generated by the combustion (carbon monoxide, carbon dioxide). In further initiatives, new scenarios including explosion and releases of various types of toxic gas have to be developed to be representative of the real situations that can happen in transport terminals and stations, in case of accidents or attacks.

L-surF (Large Scale Underground

Research Facility on Safety and Security), http://www.lsurf.org/

Summary of the project

Research on safety and security in enclosed underground spaces is of outstanding importance as current incidents (tunnel fires, terror attacks in metros etc.) have shown. However currently the EU competence related to safety and security is largely unstructured, fragmented and mostly national oriented. Especially missing is a large scale research facility and the coordination and synergy of existing facilities. The necessity for a European wide initiative in these respects also was identified and clearly expressed during the 1st International Symposium on safe and reliable tunnels, held in Prague in February 2004. Within the Design Study for L-SURF all relevant aspects for such a facility were elaborated to a level that the facility could be established at least as a legal entity with the necessary structures and activities and that preliminary concepts and plans for the physical construction are laid out. The study describes the constructional lay-out of the facility, based on an entirely new concept for easily creating any contours, shapes and sizes of enclosed spaces needed, but also all other aspects like installations, environmental impacts etc. The concept:

will allow novel approaches to R&D work describes innovative measuring sensors, based on the latest technologies (e.g. nanotechnology) available evaluates the research needs and outlines the R&D -activities develops an integration process for the existing and projected national facilities with their competences and researchers, thus restructuring and improving the relevant EU competence while simultaneously showing ways for using R&D funds more economically shows ways and means to raise the necessary funds for the different stages in the setting up of the facility and networking of existing facilities includes a business plan (e.g. referring to models like CERN) for a new legal entity dedicated to the establishing of L-SURF. The partners of the consortium have to set-up this legal entity based on the business plans developed within the frame of the Design Study [START_REF] Amberg | Harmonising safety and security research for underground infrastructure in Europe[END_REF].

Further development needed

From the project, the need for integration of expertise to find systemic solutions to global emerging risks was demonstrated.

EGSISTES: Global evaluation of intrinsic safety & security for underground transport systems (French project funded by the National Research Agency (ANR))

Summary of the project EGSISTES is a 3-year project dedicated to the evaluation of the global security in underground infrastructures (from January 2007 to January 2010). Coordinated by INERIS, the consortium included E.S.E. (Egis tunnels), LME University of Valenciennes, RATP and CEA. This project was split into three phases described underneath. The project is organized in 3 work packages:

1. Vulnerability analysis • Risk analysis • Accidental risk and threat 2. Knowledge improvement and model development for consequences evaluation

• Experimental approach (fire, explosion, gas dispersion)

• Numerical simulation (1D and 3D numerical tools) 3. Existing tools capability evaluation The first part of the project was dedicated to the evaluation of the global risk in underground infrastructures. The risk analysis was performed considering both accidental risks and intentional threats in order to demonstrate the linked between the two in terms of physical phenomena. Fire, toxic gas dispersion and explosion are the three physical phenomena that can be encountered in underground infrastructures for both risk situations. The main difference relates to the hazardous substances to consider: in case of attack, the substance can be any substance while in case of accident, the substance involved is part of the system under consideration.

The second part of the project was focused on the improvement of the physical phenomena understanding using both the experimental and numerical approaches. Experiments were carried out for fire, heavy gas dispersion and explosion. The first two phenomena were studied in the 1/3 rd scale INERIS fire gallery in order to study the interaction between the stratification and the ventilation. The impacts of disturbances on the smoke layer were studied using scaled vehicles and fan, photography's from experiments are reproduced on Figure 1. Explosion was experimentally studied in the INERIS explosion gallery. Two series of experiments were achieved for studying both solid explosion and flame propagation in an explosive cloud. For those different situations, the 3D simulation tools were used to enlarge the experimental results. Finally, the third part of the project consists in validating existing simulation tools and achieving real cases to show the capability of existing tools to model the physical phenomena for both intentional threats and accidental risks. Several real configurations were simulated split into two main topics: the first is underground station for mass transport and the second is road tunnels.

The results from the EGSISTES project are directly linked to the splitting into three parts.

The first part has enlightened the physical link that exists between accidental events and threats for critical infrastructure.

The second part has enabled to improve the understanding about smoke behaviour in case of fire in tunnel for the congested tunnel configuration. The smoke stratification was evaluated for both backlayering and downstream smoke layer showing that if the backlayering layer remains quite stratified, the downstream smoke layer stratification is weak, with and without vehicles. The second main result of the experimental campaign is the description of the toxic gas behaviour in case of leak in tunnel. The experiment has shown that the behaviour is not only governed by the ventilation system but by different external parameter such as atmospheric parameters, leak direction, etc. Consequently, it is quite impossible to have a precise prediction of the gas behaviour in tunnel because of the impossibility to have a great description of the source term. Finally, in the third part addressing the simulations tools, it has been shown in the project that CFD codes are able to give an acceptable description of the physical phenomena. The comparison for the fire experiments were achieved with the FDS and Phoenics CFD codes while the toxic gas dispersion was modelled using only FDS. On top of that, a specific explosion model was developed to predict the pressure wave propagation specifically for tunnel geometry.

Further development needed

As described above, the EGSISTES project provides results concerning the physics of generic dangerous phenomena and a first overview of the numerical tools capability to predict the consequences. This first approach has to be used as input data for new projects considering that new energy carriers will not generate fundamentally different phenomena but will induce an important modification of it. Considering for example the case of batteries, they will produce highly toxic gases (HCN, NO 2 , HCl…). The smoke composition has to be characterized in order to predict consequences in case of fire in confined spaces. Furthermore, the physic composition of batteries generally induces a rapid heat release rate rise that can modify substantially the fire curve.

To put it in a nutshell, EGSISTES offers a global risk study that use experimental and numerical approach for current risks in underground infrastructures. It is important to provide an extension of this approach to the future risks in these infrastructures in relation with the evolution of the energy sources for transport.

Further development needed

The philosophy behind this is perfectly suitable to work on the improvement of safety and security in urban underground infrastructures using innovative energy carriers. All information about the life cycle of an underground facility is put together in a data base management system. All kinds of risks are included in The STEEM programme, Maximised Energy Efficiency Tramway System, is an initiative developed by RATP, Alstom and INRETS with Predit funds, the French programme for transport research and Ademe, the body in charge of environment and energy management. A total of 48 ultracapacitors installed on the tram roofs can ensure a catenary-free operation, the supply taking only 20 seconds between stops. For this purpose, a fast supply station was installed at Lucotte depot (near Paris).

Biogas train

In Sweden, an Y1 Diesel train has been rebuilt to instead run on biogas. It has been in regular traffic since 2006 with good experience. This might be a possible solution also for other non-electrified train routes, where Diesel trains are most often used today.

HySafe Network and other related projects

A lot of other examples can be found on the development of the technology; however, regarding the safety aspects , the research is less prolific.

One the most important project at European level is the HySafe network of excellence (http://www.hysafe.org) and the internal projects such as HyTunnel (http://www.hysafe.org/HyTunnel) and InsHyde (http://www.hysafe.org/InsHyde) which were directly addressing safety of hydrogen vehicles in confined spaces.

In addition, the HyApproval (http://www.hyapproval.org/) project aimed to make a "handbook for approval of Hydrogen refuelling stations" which will be used to certify public hydrogen filling stations in Europe.

Among the project mentioned, HyTunnel and InsHyde are the one producing preliminary results useful for further development on the topic.

The objectives of the HyTunnel project were to:

review tunnel regulations, standards and practice in respect to the management of hazards and emergencies, e.g. EC directive identify appropriate accident scenarios for further investigation review previously published experimental and modelling work of relevance extend the understanding of hydrogen hazards inside tunnels by physical experiments and numerical modelling. suggest guidelines for the safe introduction of hydrogen powered vehicles into tunnels During the project, 10 experiments were performed with hydrogen and compressed natural gas, as well as a benchmark exercise for the numerical simulation. The tests, small and large scales, have shown the various combustion regimes according to the size of the cloud (air-hydrogen) and the concentration of hydrogen in the mixture.

In addition to the results gained, HyTunnel project has pointed out needs for further research in particular on the following topics:

-Realistic scenarios in tunnels (release downwards under the car) with delayed ignition of nonuniform mixtures -Scientifically grounded requirements to location and parameters of PRD -Impinging jet fires and conjugate heat transfer in conditions of blow down -Releases into congested space with DDT -Develop hydrogen safety engineering methodology and apply it to a tunnel scenario (long term).

Another internal project from the HySafe NOE is the InsHyde project aiming at investigating realistic small-medium indoor leaks, based on experimentation and CFD modelling and providing recommendations for the safe use/storage of indoor hydrogen systems. The performed experimental and modelling activities covered hydrogen/helium dispersion, hydrogen combustion and hydrogen sensors evaluation. The project in general enabled to improve the modeling of small releases and to better understand the hydrogen dispersion and combustion phenomena.

The project delivered a 90 pages document entitled "Initial guidance for using hydrogen in confined spaces -Results from InsHyde", which can currently be publicly accessed from the InsHyde webpage (www.hysafe.org/Inshyde) [START_REF] Venetsanos | Initial Guidance for Using Hydrogen in Confined Spaces -Results from InsHyde[END_REF].

At the end of the project, the project team suggested further work in particular pre-normative activities in order to formulate the requirements (at EC and global level) for permitting the use of hydrogen vehicles (cars and commercial vehicles) in confined spaces. This pre-normative work should extensively be based on experimentations and simulations.

Further development needed

These two projects described the preliminary research work performed to understand hazardous phenomena related to the use of hydrogen in tunnels and confined spaces, and have clearly shown the need for further work to improve safety and have it capitalised in normative documents.

RESEARCH PERSPECTIVES

Improving safety and security by design in transport infrastructures using innovative energy carriers is a key issue for sustainable surface transport and mobility, especially in urban environment.

The review of the recent projects presented in this paper demonstrates that addressing this challenge requires a multi-disciplinary approach. Therefore, based on the state-of-the art, it appear necessary to develop an integrated risk assessment and management method specific for underground transport systems, metros and hubs in confined spaces taking into account the "emerging risk" aspects (lack of knowledge on the socio-technical system using innovative energy carriers, different believes in the public, lack of regulations).

CONCLUSIONS

The progress beyond the state-of-the art which is expected in future research should address the following aspects:

• Develop a scenario-based methodology to analyse and assess risks in transport infrastructures using innovative energy carriers The innovation of the approach should come from the combination of safety and security by implementing the "bow-tie" model and therefore optimise the choice of risk reducing measures for both aspects (safety and security), in particular for phenomena not yet well covered in the technical literature for designers, i.e. the explosion and the release of toxic gases.

The risk assessment should be quantitative regarding the prediction of the consequences of accidents or attacks and also regarding the probabilities of failure of the active mitigation devices and the human behaviour. Therefore, test campaigns should be performed to improve advanced simulation models.

• Build an integrated approach to improve safety and security by design addressing with a holistic approach the whole system composed by: a) the vehicles (using innovative energy carrier), b) the infrastructure including the lay-out and the mitigation devices, and c) the human behaviours This objective requires a multi-disciplinary and well coordinated research investment in several disciplines such as civil engineering, modelling and fluid dynamics, safety and security technologies as well as in human factors. The advanced modelling of the effects of accidents or of an attack will consider all aspects of the system.

All the development expected to make the existing infrastructures compatible with the innovative energy carriers of the vehicles of the future will have to be applied very quickly and in a homogenous way in Europe. Therefore, the development will need to pay attention to the usefulness and practicality of the results and the large dissemination among the end-users.

Figure 1 :

 1 Figure 1: Photography's of the EGSISTES experiments. Left: Backlayering reference case, i.e. without disturbances. Right: Impact of vehicles.

3. 4 .

 4 TUNCONSTRUCT, http://www.tunconstruct.org/ Summary of the project TUNCONSTRUCT was a multi-disciplinary research project that promoted the development and implementation of European technological innovation in underground construction under participation of some of Europe´s best recognized names in the underground engineering industry. The 41 project partners, from 11 European countries, integrated not only the on-the-field engineering experience and technical know-how of the industry, but also the research capabilities and conceptual innovation of the academic sector. TUNCONSTRUCT was fully committed to contributing to an increased quality of life for Europeans by reducing construction time and cost of planned and future underground infrastructures[START_REF] Leucker | D.4.2.2. Report for Service Life Cost Model[END_REF]. The project was co-financed by the European Commission, under its 6th framework program, which provides 54% of its 26 million Euro quadrennial budget. Coordination rested on the Graz University of Technology, Austria. An innovation developed within the project -the Integrated Optimization Platform (IOPT) -resulted in a breakthrough in the design optimization, reducing cost, time and risk in underground construction.

3.1. UPTUN, http://www.uptun.net/ Summary of the project UPTUN

  Development of innovative technologies for the assessment of existing technologies in tunnel applications. Focus was on technologies in the areas of detection and monitoring, mitigating measures, influencing human response, and protection against structural damage. The main output was a set of innovative cost-effective technologies.-Development, demonstration and promotion of procedures for rational safety level evaluation, including decision support models; and knowledge transfer. The main output was a risk based evaluating and upgrading model.

dealt with issues and technology to improve existing tunnels in terms of fire safety in a holistic manner. The project consortium comprised 41 partners from 18 European countries. The UPTUN project main objectives were:

-

Source: http://rinsider.club-feroviar.ro/en/afiseaza_stire.php?id=4437, and press release RATP & ALSTHOM dated July 1st,

this life cycle analysis. This makes the approach suitable for design optimization for safety and security. As the focus is on confined spaces, the risk assessment tool from TUNCONSTRUCT can be used as a blueprint for further development.

SECURITY MANAGEMENT IN TRANSPORT SYSTEMS

Progress on security management can benefit from a recent project performed in the USA by the FTA and two European projects.

FTA Transit Security Design Considerations (US project, 2004)

Summary of the project

The most interesting reference dealing with security management in transport terminal and stations and summarizing the state of the art is the report from the US Department of Transportation [START_REF] Volpe | FTA Transit Security Design Considerations[END_REF].

The report provides security design guidance on three major transit system components -bus vehicles, rail vehicles, and transit infrastructure. It provides a resource for transit agency decision makers, members of design, construction and operations departments, security and law enforcement personnel and consultants and contractors, in developing an effective and affordable security strategy following the completion of a threat and vulnerability assessment and development of a comprehensive plan. Developed by the Federal Transit Administration in collaboration with transit industry public and private sector stakeholders, these design considerations provide actionable steps that transit agency staff can select from to create a security strategy. However, the report focuses exclusively on security assessment, and does not address risks related to innovative energy carriers. The approach recommended in the FTA report is similar to the method classically used in Europe, i.e. scenario and consequence analyses.

Further development needed

Concerning the behavior of mitigation devices, the report does not mention specific tests to better understand the dangerous phenomena that can take place in underground terminal and stations. It does not mention any simulation tools for the accidental situations or attacks. It is therefore necessary to further develop models and simulation tools that will help refining the design of the infrastructure under consideration.

At European level, the two projects of significant importance are COUNTERACT and RAILPROTECT. The main objective is to improve security against terrorist attacks aimed at public passenger transport, intermodal freight transport and energy production and transmission infrastructure. When the security of people and the integrity of critical infrastructures are involved, reducing the risk of terrorist attack and its consequences is paramount. The project will review existing security policies, procedures, methodologies and technologies to identify the best practices which in turn will be promoted throughout the relevant security community in the EU. The project is divided in three industry clusters covering public passenger transport, intermodal freight transport and energy. In addition to their sector specific aims, the clusters will also exchange experiences and views between them.

COUNTERACT

4.3.

RAILPROTECT, http://elsa.jrc.ec.europa.eu/showproject.php?id=13 Innovative Technologies for Safer and more Secure Land Mass-Transport Infrastructure under Terrorist Attacks

Summary of the project

The project aims at reducing the vulnerability of rail station infrastructures and rolling stock to explosion loads. The assessment of this vulnerability is achieved via numerical simulations of the blast effects. Thus, engineering design simulation tools are being developed based on structural and fluid mechanics and appropriate geometry mapping techniques for large structures. Case studies of typical stations and coaches are also elaborated for assessing the efficiency of these tools in predicting the performance of structures and the injury risk levels of the occupants. The project has been motivated by the need to mitigate shortcomings identified subsequent to the most recent terrorist attacks that have targeted the urban transport systems of two major European capitals. The need to take initiatives in this area is reinforced by the current climate, which indicates that such attacks will not abate in the foreseeable future. The nature of land mass passenger transport, with its open security architecture and widely dispersed assets, would suggest that measures comparable to those applicable to civil aviation cannot, and will not, be put into place in the near future. Nevertheless, a number of security measures have been introduced following conventional lines and centering on increased surveillance and patrols. If it is not possible to completely eliminate the hazard, and it is not possible to secure all the assets that make up the land mass passenger transport system, then the best option available is to reduce the overall risk by securing specific vulnerable system elements. One such area is the resilience of moveable and unmovable parts of the land mass passenger transport infrastructure.

Further development needed

COUNTERACT and RAILPROTECT provide valuable inputs for the risk assessment and the choice of security scenarios, for the modeling of the hazardous phenomena (RAILPROTECT provides important inputs on explosion modeling) and the most recent technologies to reduce risks.

INTEGRATED RISK MANAGEMENT OF NEW AND EMERGING RISKS

http://www.integrisk.eu-vri.eu/ Summary of the project iNTeg-Risk is a large-scale integrating project aimed at improving the management of emerging risks in the innovative industry. This will be achieved by building a new risk management paradigm for emerging risks, which is a set of principles supported by a common language, commonly agreed tools & methods and Key Performance Indicators integrated into a single framework. As main impact, it will reduce time-tomarket for the lead market EU technologies and promote safety, security, environmental friendliness and social responsibility as a trade-mark of the advanced EU technologies. The project has started in December 2008.

Within the project, there is a specific case study called "Safety and Security of Underground hubs with interconnected transportation services and shopping centers" involving INERIS, SP, STUVA and VSH.

The project has identified so far the problem framing which is summarized hereunder. Urban public mass transportation systems rely to a large extend on efficient and effective underground transportation system which are well connected to all other transport means of public traffic. The result of these developments in the use of the underground is twofold: On one side new metro tubes must be built deeper and deeper and on the other side they are interconnected with the other transport media in large underground constructions (hubs), including stations, parking lots and shopping areas on different underground levels. This results in longer access and escape routes, complex and hard to overlook structures split in several levels which are used by ten to hundred thousands of people every day, implicating new risks for constructors, operators, passengers and users. In case of emergency there are entirely new boundary conditions for the evacuation of passengers, for example: Longer distances to reach the surface or safety structures underground, more demanding ventilation systems which must be adapted to the geometrical conditions as well as to the bigger amounts of air to be managed, higher interaction between different transport means (cut of traffic, emergency stops, evacuation of people etc). Therefore, underground hubs require innovative concepts for ventilation and evacuation.

Further development needed

This project will neither develop detailed technical solutions such as new simulations tools for accidents or attacks, nor recommendation on the design of infrastructures. This is a work to be done in future specific project.

INNOVATIVE ENERGY CARRIERS FOR VEHICLES AND THEIR SAFETY

Several projects especially co-financed by the European Commission in the transport activities address development of innovative energy carriers for vehicles. In this paragraph a brief review is made to show the diversity of energy which are under consideration and the quick development which is expected in the public transport system mainly concerned by safety and security in transport stations and terminals. The bus projects in particular have provided the developers and the operators with excellent data and experience because the vehicles are generally operated by professional drivers and often under especially hard conditions, such as uninterrupted operation for more than 12 hours under some very harsh, extremes of climatic conditions. The HyFLEET:CUTE project sees the operation of the world's largest fleet of hydrogen powered buses. It builds on the learning and developments from many of these previous projects. The HyFLEET:CUTE bus fleet is supported by a hydrogen infrastructure which will produce, refine, distribute and dispense hydrogen in many different ways. New advanced prototypes of hydrogen Fuel Cell and Internal Combustion Engine buses will also be developed and trialled. An advanced hydrogen infrastructure is being established in Berlin which is capable of refuelling a fleet of up to 20 buses. This is aimed to achieve reliability and availability performance as good as current CNG (Compressed Natural Gas) infrastructure, and to be highly energy efficient. A very active and extensive dissemination and information programme -the Global Hydrogen Bus Platform -combined with the inclusion of Beijing, China, aims to extend the impact of this technology world-wide.

One of 14 hydrogen ICE Buses at the refueling station in Berlin See more at: http://www.greencarcongress.com/hsub2sub/index.html HyFLEET:CUTE project provides information on the typical technology for hydrogen buses:

• Typical specification:


B Truchot

M Oucherfi

F Quezel-Ambrunaz

S Duplantier

L Fournier

F Waymel

Are the tunnel ventilation systems adapted for the different risk situations?

Keywords: Tunnel, ventilation gas dispersion, toxic release, experimental study, numerical simulation

The ventilation design criteria for both road and rail tunnel is based on the design fire defined by the standards and the general knowledge about smoke propagation. The problem of such an approach is that it considers only the impact on the safety ventilation of the smoke propagation and dispersion inside the tunnel excluding other possible accident. However some other situations, such as toxic gas release, are possible and even if the aim is not to design the ventilation on other dangerous phenomena with a lower occurrence frequency, it must be ensure that the ventilation system does not increase the consequences of the accident. Mainly, the problem of toxic gas dispersion is pointed out in this paper. Because of the large variety of dangerous materials that can transit in tunnel, the probability of an accident that impacts a toxic transport cannot be neglected. In the worst case scenario, such as a massive release of high toxic gases, the ventilation is useless because of the toxic quantity that induces a large number of deaths inside the tunnel. However, when the toxic release is lower and ventilation can be used, having in mind that toxic gas is generally heavy gas or a cold gas, the behaviour will of course be different than the one of smoke and the ventilation system may not be adapted for such a situation. This case has scarcely been studied yet. In this study, both experimental approach and numerical tools were used to improve the global understanding of dense gas dispersion in underground infrastructure such as road tunnels. The experimental work was achieved in the INERIS fire gallery which represents a 50 m long 1/3 rd scale tunnel using Argon. It was achieved for different leaks conditions in order to appreciate the dense gas natural behaviour. This work has also enabled the comparison between experimental work and CFD calculation with FDS code for the particular application of dense gas dispersion. . The work was extended to some other configurations and geometry in order to simulate real scale situation with different kind of gases : a highly toxic dense gas such as Chlorine, a light gas stored as a liquid at a very low temperature such as Ammonia, and a gas which remains liquid at ambient temperature and pressure and is drained into an evaporating pool such as Acrolein. This work will consider the natural behaviour of the gases and the influence of longitudinal ventilation both inside and outside of the tunnel.

Because of the confined geometry, accidents in an underground infrastructure have a high potential of risk. The public underground infrastructure can be split into three main categories: road tunnel, rail tunnel and underground mass transport system. The risk analysis and the consequences evaluation varies as a function of the categories. The two main factors generating this differentiation are: first the intrinsic potential of risk of the infrastructure and second the relation between people and the infrastructure. This can be illustrated considering first a road tunnel in which hazardous goods transportation is allowed. In such an infrastructure, people drive around trucks that may contain hazardous goods such as flammable, explosive or toxic products, keeping in mind that people are free of their displacement in case of accident. On the opposite, in rail tunnel, people are confined in the trains but they do not circulate in the infrastructure at the same time as hazardous goods. Finally, in underground mass transport system, the intrinsic risk potential is low considering the lack of hazardous goods. This illustrates the importance of having a safety system for underground infrastructure in accordance with the risk level.

This paper is mainly focused on the road tunnel problematic. However, the discussion can be easily extended to other infrastructures. Concerning road, an important reflexion was achieved during the past years to obtain some references scenarios in order to design the safety system for tunnels. In most of the reference guides [START_REF] Cetu | Guide to road tunnel safety documentation: booklet 3. Risks analyses relating to dangerous goods transport[END_REF] and in the QRA model [START_REF] Ligeron | Interface de calcul du Risque Intrinsèque -Guide utilisateur -Version E, 2006 3. AIPCR, Systems and equipment for fire and smoke control in road tunnel[END_REF] that is commonly used, thirteen scenarios were identified and correspond to three typologies of dangerous phenomena:

• Fire,

• Explosion,

• Toxic release. The safety in tunnel, and mainly the ventilation system, is generally designed considering the fire risk. According to accidents reported in the past twenty years [3], fire appears to be the phenomena with the highest occurrence frequency and of course this approach seems relevant. However, the occurrence of other phenomena cannot be considered null and, even if the objective is not to design safety system on these scenarios, it is important to investigate how to manage such incident with the available safety system. In most of the cases, ventilation in road tunnel is designed with a reference fire. This one is characterized by its heat release rate and its kinetic. The reference fire is a way to quantify the risk acceptance level. For tunnels allowed to hazardous goods transportation, this reference fire is between 100 MW as in Germany and 300 MW as recommended in Netherlands.

Considering fire as relevant scenario for the ventilation design sounds reasonable when considering first the occurrence frequency as discussed above and then the ability of the ventilation system to reduce the fire damages and casualties,. However, even if the aim is not to manage each accidental situation that can occur in tunnel, it seems important to have a global idea on the consequences of the different accidental scenarios and the possibility to manage it using an existing fire ventilation system. Because of the pressure wave propagation velocity close to the sound velocity, explosion is not a risk easily manageable. Consequently, this situation is only dealt with when important consequences are estimated a priori, such as a building above the tunnel. In such a case, the tunnel structure is designed to resist to the overpressure generated by the explosion. This paper offers a discussion about toxic gas management in tunnel using fire designed ventilation system. Because only few works exist on this topic, this work is a global approach. Three steps were achieved and are detailed in this paper. The first important point was the understanding of the physical phenomena relative to gas dispersion in tunnel. In this purpose, an experimental campaign was carried out. It was the inlet of the second phase that aims to determine the CFD code ability to model the dispersion of heavy gases in such a configuration. Finally, using the CFD code, several toxic releases in tunnel were modeled to apprehend consequences. Consequences were computed not only inside the tunnel but outside too considering atmospheric dispersion at the tunnel heads.

GAS BEHAVIOUR IN TUNNELS, AN EXPERIMENTAL APPROACH

Experimental apparatus

In order to improve the behaviour understanding of an heavy gas cloud in the tunnel with different ventilation configurations, an experimental campaign was performed using the INERIS fire gallery. This fire gallery is 50 m long and its section was modified to represent a tunnel section with a 1/3 scale with 3 m large and 1.8 m high. This gallery is represented on Figure 1. The duct in the upper part of the gallery enables to simulate transverse ventilation system. Longitudinal ventilation system is modeled using the main extraction fan of the fire gallery. Indeed, the gallery is equipped with a fan that controls the air flow in the tunnel. Performing experiment at a given scale imposes to define the scale's impact on the dimensionless numbers. The two phenomena are the longitudinal flow dynamic represented by the Reynolds number and the buoyancy effect due to the density difference between the two gases that is represented by the Froude number. As demonstrated in previous papers about the fire part of this experimental campaign ( [START_REF] Boehm | Smoke stratification stability: presentation of experiments[END_REF] and [START_REF] Truchot | Numerical analysis of smoke layer stability[END_REF]), this configuration was designed to respect the Froude number while keeping the Reynolds in a correct range. Because using toxic gases for such experiment is impossible, a representative gas was used. At this stage. it appears important to use a gas with a molar weight higher than the one of air, in order to maintain a constant air relative density. In addition to prevent thermal effect, the gas injection should be performed at surrounding temperature. Experiments were performed using Argon, a neutral gas with a molar weight of 40 g/mol which induces a relative density of 1.4 with ambient air and using a specific injection device heating the gas after its decompression at the tank exit. Several questions were a priori asked about gas behavior in tunnel:

• Is there any stratification of heavy gas dispersing in tunnel?

• Is there a backlayering such as the one observed in case of fire?

• Consequently, is it possible to define a critical velocity for such a flow?

Theoretical add

Before discussing about heavy gas behaviour in tunnels, some points must be highlighted mainly considering the experimental choices. Argon that was used in these experiments for safety reasons has a molecular weight of 40 g/mol that induces a density relative to air of 1.4. Considering that stratification is mainly governed by the density gradient between the gas and the air, it appears that for heavier gases the weak stratification that was observed should be stronger. As described by [START_REF] Davis | An analysis of some replicated wind tunnel experiments on instantaneously released heavy gas clouds dispersion over solid and crenellated fences[END_REF], the main scaling parameters that can be used to evaluate the stratification is the bulk Richardson number defined as:

² U L g Ri air air gas ρ ρ ρ - = (1)
This means that, for a given ventilation velocity, the higher ρ gas is, the more stratified the flow is. This is illustrated on Figure 2. This curve indicated that, for low flow velocity as those used in the experiments, flow should be stratified. This was observed in an "ideal" case: leak directed to the ground, no perturbation inside the tunnel like vehicles and no atmospheric disturbance. In such an ideal case, the experimental flow is similar to the theoretical one. This curve also shows that the stratification of heavier gas should be stronger than the stratification of argon because of their important molar weight. This induces that the influence of external parameters will be lower.

Experimental design

In order to answer questions mentioned above, the main tool is numerical visualization using several video cameras in the fire gallery. Ammonium salt (NH 4 Cl) was used to seed the invisible Argon gas and then enable to visualize the flow. Because this visualization equipment cannot provide measurement values, a catharometer coupled with a scrutator was used to obtain a vertical concentration profile. Several configurations were experimentally performed to understand impact of different parameters. Experimental configurations are listed in Table 1. In this table, the modified parameter between the current case and the reference configuration is highlighted. Considering the 1/3 length scale factor of the experimental apparatus, the leak scale factor is (1/3) 5/2 , this means that the experimental leak rate of 1 000 l/min corresponds to a 15 600 l/min (or 0.43 kg/s) flow rate. This mass flow rate is representative of a chlorine or ammoniac release through a 10 mm hole for tank at ambient temperature and saturated vapor pressure. This hole size is lower than the value given by the QRA model that will be discussed later but is closed to the accidental experience.

Reference configuration

To evaluate the impact of the different parameters on the gas behavior in the tunnel, a first reference case was defined as follow. The leak is vertical and directed to the ground with a 40 cm distance between the hole and the ground, 15 m downstream the tunnel entrance. The volume flow rate of the leak is 1000 l/min and the tunnel is free of vehicles. The tunnel is longitudinally ventilated with at a velocity of 0.4 m/s. In this configuration a stratified gas layer appears downstream the leak as shown on Figure 3. It must be taken into account that only the lower part of the tunnel section were instrumented with 7 probes from the ground to 90 cm 15 m downstream the leak. To obtain this concentration profile, measured concentrations were averaged in time. It is important to highlight that the instantaneous concentration presents fluctuations. Furthermore, external perturbations could influence the stratification.

Reference case: Numerical approach

On the basis of above described experiments and to go further in the leak impact investigation, a simulation software has been used. The well known CFD code FDS (Fire Dynamic Simulator) was used. Using a CFD code implies to be aware about its capability and limit. Therefore, comparison between its results and the experiments enables to have a good overview of the CFD code capability. The objective of these simulations is not to validate FDS for gas dispersion modeling in tunnel but to evaluate its capability for such an application. Knowing the limit of the model, we will have a tool for extending the experimental approach to real cases. The model was built in order to simulate the reference case using FDS. The fire gallery was modelled using a mesh with 25 cm cells in length and 10 cm in height and width. Comparison between numerical prediction and experiment is shown on Figure 4. This figure shows that, for the given reference configuration, FDS gives a quite good prediction of the gas behaviour. Of course, the two curves are not identical but the flow regime predicted is the good one: the flow presents stratification with a concentration higher in the lower part of the tunnel than in the upper one.

Impact of the leak direction

To evaluate the impact of the leak direction on the gas behavior, the jet orientation was modified from the reference case. Two directions were studied: jet directed counter flow and jet directed to the ceiling. The second case is detailed in this paper. For this configuration, the leak is simulated to be on the upper part of the truck and consequently the jet is vertical to the ceiling. The ventilation flow, position of the leak and the other parameters are identical to the one described for the reference case. The concentration profile obtained then is reproduced on Figure 5. The reference case is also plotted on this graph for comparison reasons. On this graph, concentration measurements were distributed all along the gallery height to have an overview of the whole concentration profile. Unlike experimental configuration number 1, the stratification phenomenon does not occur for configuration number 3. This figure clearly shows that the concentration is quite homogeneous along the tunnel height. This is confirmed by video. Photography from experiment is reproduced on Figure 6.

Consequences outside the tunnel: an illustration of the experimental configuration

As discussed above, the experimental release of 0.43 kg/s, reported to the tunnel scale, represents a 10 mm hole in a tank containing liquid ammoniac or liquid chlorine stored under vapor pressure at 20°C. Before going any further, it appears relevant to wonder about the consequences of such a leak outside the tunnel, for an estimated real configuration. Considering a 48 m² cross-section tunnel with a ventilation velocity of 0.7 m/s, corresponding to the experimental velocity converted to real scale, the dispersion was modeled using the integral model provided by PHAST. Doing this implies to assume a homogeneous concentration at tunnel head. The mass air flow is then around 40 kg/s, which induces a toxic gas mass concentration of 1%. Computations were achieved for those two cases and results are plotted on Figure 7. Reader must be particularly attentive to curve scale that differs because the difference between the two gases threshold. Considering the significant effects outside the tunnel of the free dispersion of chlorine inside the tunnel, it is relevant to wonder about the impacts of longitudinal ventilation outside the tunnel. Inside the tunnel, a higher ventilation velocity enables to maintain the upstream part of the tunnel free of toxic gases and introduces dilution which should to diminish the external effects. The atmospheric dispersion was then carried out with a 3 m/s ventilation induced air flow velocity. Dispersion at the tunnel head is plotted on Figure 8. This comparison clearly shows the ventilation flow rate impact on the outside gas dispersion: the cloud expends a little further widthwise but the footprint is shorter downstream (600m instead of 1050), which is beneficial.

Experimental campaign conclusion

As mentioned in the introduction of this section, experimental campaign aims to answer three questions:

• Is there any stratification of heavy gas dispersing in tunnel?

• Is there a backlayering such as the one observed in case of a fire?

• Consequently, is it possible to define a critical velocity for such a flow?

Concerning the first item, the answer is more complex than in fire cases. When dealing with fire cases, hot smokes are emitted along a vertical direction from ground to ceiling. Because of the temperature difference, smokes are lighter than the ambient air producing a stratification layer. In such case, it is possible to define well known stratification criteria [START_REF] Newman | Experimental evaluation of fire-induced stratification[END_REF]. Considering dense gas, such a criterion should be based the equation not on temperature but using directly the concentration difference. However the pertinence of this parameter appears limited. Indeed, in case of fire, the main is the fire heat release rate.

The flow in the combustion zone is driven by the free convection phenomena due to the high density of heat released. In case of toxic leak, the flow due to the leak is a jet for which the direction cannot be predicted. This induces that it appears difficult to have a precise description of toxic cloud dispersion in tunnel. It also appears that without considering stratification, using CFD may help to adjust ventilation velocity to minimize the effects of toxic gases both inside and outside the tunnel. These conclusions are not opposite to the possibility to define a critical ventilation regime. Such a regime can be used to maintain the upstream part of the tunnel safe. Experiments show that a gas layer in the lower part of the tunnel can be observed for some configurations. This phenomenon of a gas layer that goes counter flow can be called low-backlayering by similarity with the fire backlayering phenomenon.

In experiments, it appears that this phenomenon is not as stable as smokes stratification in a fire case. The gas dispersion was highly depending on gallery surrounding conditions, such as external wind. However, it appears clear that, all other parameters fixed, raising the ventilation rate induce a low-backlayering length reduction.

This section also shows that the CFD code FDS was able to predict the flow regime in one case of heavy gas dispersion in tunnel. This results shows that numerical results is good enough to use the code in order to go further and using it to simulate real scale configurations.

Finally, the comparison of the conditions outside the tunnel in cases with and without ventilation was performed, using models based on the experimental results. This shows that ventilation system design for fire fighting can be used not only in order to push the cloud downstream the leak in order to prevent the toxic gas to invade the whole tunnel but also in order to dilute toxic gas prior to its dispersion in the atmosphere.

To summarize, experimental campaign shows that stratification may exist for heavy gas leak in tunnel. However this stratification depends on leak configuration and external parameters such as atmospheric conditions. Second, experiment shows the possibility to obtain a low-backlayering layer and also the possibility to define a critical regime for such dispersion.

CONSEQUENCES OF GAS LEACKAGE IN TUNNEL

The toxic release scenario in tunnel is rarely considered. If such a leak is not frequent, such an accident would have catastrophic consequences. In the QRA model, several configurations were proposed:

• 50 mm in diameter hole in a chlorine tank that induces a 45 kg/s liquid leak,

• 50 mm in diameter hole in a ammoniac tank that induces a 36 kg/s liquid leak,

• 100 mm in diameter hole in an acrolein tank that induces a 24.8 kg/s liquid leak,

• 4 mm in diameter hole in an acrolein tank that induces a 0.02 kg/s liquid leak. It appears that those values are prudent for evaluating a hazardous line transport as a global view but are over estimated for consequences prediction in tunnels. Accidental experience shows that the hole diameter in case of leakage on a hazardous truck is lower than 25 mm. For this reason, a 4kg/s chlorine release was chosen to represent the highly toxic gaseous case and a 100 m² pool evaporation was used for toxic liquid. Of course, other toxic products can be released and those scenarios just aim to be representative. According to these elements, we wonder about the impact of a toxic release in tunnel considering existing system. Gas dispersion was modeled in a 400 m long tunnel with a 78 m² section equipped with a ventilation system that is able to ensure a longitudinal velocity of 3 m/s. Simulation were achieved using the CFD code FDS.

Chlorine leakage: Consequences of a highly toxic gas

Chlorine was chosen in order to model the consequences of a gas release in tunnel because it is a highly toxic heavy gas. Its molar weight is 70 g/mol which induces a density of 2.4 relative to air. Because of the high toxicity of this gas, consequences have to be studied not only in the tunnel but outside too. The toxic effect thresholds for chlorine are reminded in Table 2 Simulations were achieved to provide a first description of the gas concentration inside and outside the tunnel with and without ventilation. In the ventilated case, a 3 m/s ventilation velocity was used. The mass release rate of chlorine for these simulations is 4 kg/s. The simulation results of these two configurations are plotted on Figure 10 and Figure 11. The maximum concentration on these pictures is set to 0.07% that corresponds to the lethal effect for 10 minutes exposure. These two results show that, for a leak 10 times smaller than the one defined in the QRA model, the consequences inside the tunnel would be catastrophic. These results also shows that the concentration at the tunnel head is higher than threshold and of course, consequences will propagate outside the tunnel. This can in urban configuration cause dramatic consequences. To estimate these consequences, the toxic gas dispersion was modeled considering the tunnel head as a release with a concentration equal to the average concentration at the outlet. Using these averaged concentration and the computed velocity at the tunnel end, an integral dispersion model was used to predict the atmospheric dispersion of the toxic cloud. The dispersion results are given underneath on Figure 12 for the ventilated tunnel case. Consequences of the unventilated case cannot be modeled using an integral model since the velocity at the head is out of the scope of such a model. These results show that, in case of an highly toxic gas release in tunnel, consequences would be catastrophic both inside and outside the tunnel, generating consequences up to several kilometers away from the tunnel's head.

Liquid leakage: Pool evaporation consequences

Considering hazardous goods transportation on road, it is important to also consider toxic products which remain liquid at ambient temperature and pressure and create a pool which evaporates. Acrolein is considered in this paper to provide an overview of such product leak consequences. The toxic thresholds of 10 -4 kg/kg 10 -4 kg/kg this product, given in Table 3 To model the toxic effect of such an event, a 100 m² pool was assumed and evaporation was computed based on ventilation velocity and pool surface, taking into account the physical characteristics of the products. These simulations shows that, toxic thresholds being particularly low for such a product, lethal concentrations are easily reached inside the tunnel, as shown on Figure 13. This figure indicates that the toxic gas does not stay stratified downstream the leak but shows that a 3 m/s ventilation velocity enables to maintain the upstream part in fresh air. This case has to be compared with the situation without velocity represented underneath on Figure 14. In this configuration, the conditions seem better but acrolein will sooner or later mix with fresh air, get warmer and lighter and reach men height. So in both cases, the consequences in tunnel are dramatic but at least using fire design ventilation system enables to keep part of the tunnel free of toxic gases. This solution however induces consequences outside the tunnel because of the release of a large quantity of toxic product. The outside effects for a ventilated tunnel were computed using an integral dispersion model with PHAST. Dispersion results are plotted on the figure 15 underneath. These computations show again the difficulty to manage a toxic gas release in confined geometry such as 10 -4 kg/kg 10 -4 kg/kg tunnel. Even if the fire design ventilation system enables to keep the upstream part of the tunnel free of gas and then make the evacuation safe for driver blocked in that part, this solution which is good for uncongested tunnel, has consequences outside the tunnel that can be worse than for the unventilated case.

CONCLUSIONS

The paper is dedicated to the toxic gas dispersion problem in semi confined infrastructure like road tunnels. In these tunnels, the ventilation safety system is commonly based on the design fire curve in order to be able to manage smoke. This approach seems relevant considering the occurrence frequency for a fire in tunnels as shown in the accidental experiment knowledge. However, in tunnels where hazardous goods transportation is allowed, some other accidental scenarios are possible such as explosion or toxic gas dispersion. If using the ventilation system for explosion case is not relevant, for the gas dispersion cases the ventilation system can be a mean to prevent dramatic consequences. However, we should keep in mind that designing the ventilation system for managing dense gas is not realistic, considering the low probality of this kind of accident to occur. The objective of this paper was then to propose a reflexion on the fire design ventilation system impact on the gas dispersion in tunnels. First of all, an experimental campaign is described. This campaign was achieved using Argon in a 1/3rd scale model. This campaign enabled us to understand the gas behaviour in the tunnel and mainly the influence of several external parameters that directly impact the stratification of the cloud. This campaign also enables to evaluate the FDS code capability to predict gas dispersion inside the tunnel. The results have shown that using this code for enlarging the scope of the experimental campaign is coherent. Finally, on the basis of this experimental campaign, it was possible to have a first discussion on the fire design ventilation system impact on the toxic gas dispersion both inside and outside the tunnel.

In the last part of this paper, some real scale configurations were modelled. This aims to enlarge the scope of the experimental campaign as described above. Two cases were discussed: a toxic gas release and an evaporating toxic liquid. For both cases, calculations show that consequences inside the tunnel could quickly become dramatic. They also show that ventilation system can be used to prevent from having toxic gases upstream the leak and then, in uncongested tunnels, to enable drivers protection in that part of the tunnel. However, for those two cases, atmospheric dispersion has to be considered in order to have a global view of the toxic impact. Using the ventilation system induces a toxic source term from one tunnel entrance and the ventilation procedure should be designed in order to minimize the toxic impact not only inside the tunnel but also outside. This paper is a first reflexion of the available possibilities to manage ventilation system in case of toxic release in tunnel. Both the experimental campaign and real cases modelling show that consequences must be considered not only inside but also outside the tunnel.

Figure 1 :

 1 Figure 1: Fire gallery characteristics representation.

Figure 2 :

 2 Figure 2: Theoretical curve of flow regime as a function of velocity for different gases.

Figure 3 :

 3 Figure 3: Vertical distribution of Argon concentration 15 m downstream the leak.

Figure 4 :

 4 Figure 4: Comparison between the FDS prediction and measurement results.

Figure 5 :

 5 Figure 5: Vertical distribution of Argon concentration 15 m downstream the leak.

Figure 6 :

 6 Figure 6: Photography of gas dispersion. Jet to the ceiling case, experimental case number 3.

Figure 7 :

 7 Figure 7: Experimental configuration -toxic gas dispersion outside the tunnel assuming ammoniac leak (left) and chlorine leak (right). Non reversible (blue), lethal (green) and significant lethal (yellow) for 10 minutes exposure threshold are plotted.

Figure 8 :

 8 Figure 8: 3 m/s ventilation velocity chlorine dispersion outside the tunnel assuming ammoniac. Non reversible (blue), lethal (yellow) and significant lethal (red) for 10 minutes exposure threshold are plotted.

Figure 9 :

 9 Figure 9: Observation of a gas layer in the lower part of the tunnel.

Figure 10 :

 10 Figure 10 : Mass concentration distribution in a side view centered on the leak five minutes after the leak beginning in an unventilated tunnel.

Figure 11 :

 11 Figure 11 : Mass concentration distribution in a side view centered on the leak 75 s after the leak beginning with 3 m/s ventilation velocity.

Figure 12 :

 12 Figure 12: Toxic cloud footprint of lethal effect for 10 minutes exposure for atmospheric class F3 (blue line) and D5 (green line) on the left and foot print for F3 class with significant lethal effect for 1 (red), 10 (yellow) and 20 (green) minutes of exposure on the right.

Figure 13 :

 13 Figure 13: Acrolein dispersion in real scale tunnel considering a 3 m/s ventilation velocity. The mximum value that is represented is the toxic threshold for 10 minute exposure (73 ppm (mol fraction) = 1.4e-4 kg/kg (mass fraction)).

Figure 14 :

 14 Figure 14: Acrolein dispersion in real scale tunnel without ventilation. The maximum value that is represented is the toxic threshold for 10 minute exposure (73 ppm (mol fraction) = 1.4e-4 kg/kg (mass fraction)).

Figure 15 :

 15 Figure 15: Acrolein dispersion outside the tunnel, foot print for F3 class with significant lethal effect for 1 (red), 10 (yellow) and 20 (green) minutes of exposure duration.

Table 1 :

 1 Experimental configuration and their characteristics.

				Leak distance
			Jet direction	from tunnel	Volume flow rate	Vehicles
				entrance
	{1}	Reference configuration	Vertical to the ground -40 cm height	15 m	1 000 l/min	None
	{2} Counter Flow	Horizontal counter flow -40 cm height	15 m	1 000 l/min	None
	{3}	Ceiling direction	Vertical to the ceiling -80 cm height	15 m	1 000 l/min	None
	{4}	Higher rate	Vertical to the ground -40 cm height	15 m	1 500 l/min	None
	{5}	Reference configuration	Vertical to the ground -40 cm height	10 m	1 000 l/min	Simulated 1/3 rd scale cars

  .

		1 minute exposure	10 minutes exposure	20 minutes exposure
	Non reversible toxic effect	110 / 0.03%	41 / 0.01%	30 / 0,007%
	Lethal toxic effect	910 / 0.22%	280 / 0.07%	200 / 0.05%
	Significant lethal effect	1082 / 0.26%	324 / 0.08%	226 / 0.05%

Table 2 :

 2

Chlorine toxic effect threshold concentration in ppm and in mass fraction (ppm / mass fraction).

  are very low.

		1 minute exposure	10 minutes exposure	20 minutes exposure
	Non reversible toxic effect	62	8	4
	Lethal toxic effect	557	73	40
	Significant lethal effect	650	85	46

Table 3 :

 3 Acrolein toxic effect threshold concentration in ppm.


Laure Malherbe

André Wroblewskf'

Laurent Létinois

Laurence Rouïl

EVALUATION OF NUMERICAL MODELS USED TO SIMULATE ATMOSPHERIC POLLUTION NEAR ROADWAYS

Keywords: Traffic-related pollution, street canyon, dispersion modelling, model évaluation

Numerical modelling of traffic-related pollution can be useful to check compliance with regulatory thresholds, compare the atmospheric impact of various traffic scénarios or represent roadside concentrations in air quality maps. Because of operational constraints, local actors usually make use of parametrised models which are easy to implement but require caution and rigour in their application. Since 2007 an Internet information base including data sets, technical information and référence méthodologies, lias been built up by the French Central Laboratory for Air Quality Monitoring (LCSQA) to help local actors in evaluating their modelling tools. In addition, simulations hâve been performed for différent types of streets to provide examples of model évaluation studies and draw some indications about the application framework of a few commonly used models. The results of those tests are presented and discussed for two street canyons and one semi-open street located in the French city of Nantes.

INTRODUCTION

Numerical models used to assess atmospheric concentrations near road traffic hâve received growing interest as support to air quality monitoring. Indeed many monitoring stations for which exceedances of regulatory limit values (in particular of those related to NO2 and PM10) hâve been observed are traffic stations. In compliance with the European Directive on ambient air quality and cleaner air for Europe (2008/50/EC), Member States hâve to report on those exceedances and in particular give an estimate of their geographical extent. More generally, population exposure not only to average background pollution but also to traffic-related concentrations has raised serious concerns, eliciting the need for small-scale air quality assessment.

In addition to properly designed monitoring surveys, simulation tools may prove very useful in that context, providing their reliability and application framework are well known. This study was mainly aimed towards local actors involved in air quality monitoring. Its objective was to gather expérimental data and develop référence méthodologies which can help them to evaluate the available models and optimize their use.

A comprehensive inventory of measuring campaigns conducted in France and Europe near streets or roads was fïrst undertaken. Given the campaigns characteristics and potentially retrievable data, a large panel of sites covering différent configurations (street canyons, crossroads, open roads...) was selected as suitable for modelling évaluation purposes. Then common tools (ADMS-Urban, CALINE4, OSPM, SIRANE, STREET) were applied to some of those cases to ensure that the proposed data and méthodologies were consistent and supply examples of comparison between model output and measurements. Ail processed data, modelling results, évaluation tools and technical information hâve been made available to the French air quality monitoring agencies on a website.

In section 1 and 2, the implemented models and three case studies (two street canyons and one semi-open street in the French city of Nantes) are briefly described. The main conclusions of a preliminary sensitivity analysis are then provided (section 3). In section 4, simulation results are presented and discussed for the three test cases. The content of the web site is outlined in section 5. The most significant outcomes and some perspectives are given as conclusion.

MODELS

ADMS-Urban is an advanced Gaussian dispersion model mainly intended to assess air quality in urban areas (CERC, UK, version 2.2, 2006). It can simulate the atmospheric dispersion of pollutants released from industrial, domestic and road traffic sources. It also includes a street canyon model based on OSPM formulation to represent the dynamics and dispersion features expected in roads with street canyon characteristics.

The Operational Street Pollution Model (OSPM) is a parameterised model for flow and dispersion conditions in street canyons (NERI, Denmark, version 5.1.90, 2007). The concentration of a pollutant is described as the sum of two components: a Gaussian plume model for the direct contribution from street traffic, and a box model for the recirculating part of pollutants in the street.

SIRANE is an urban dispersion model developed to simulate pollutant exchanges occurring between interconnected streets and between the streets and the overlying atmosphère (LMFA, France, version 1.16, 2008). In a street section, concentrations are calculated as the resuit from a mass balance between the incoming and outgoing flows (direct émissions from traffic, exchange at the intersections and at roof level). Dispersion above roofs or in open streets is modelled by a Gaussian plume.

STREET is a parametric model reserved for assessing annual average concentrations in various types of street layouts (TUV, Oxalis Mobilité, KTT, version 5.2, 2008). It does not simulate dispersion by itself but utilizes a library of simulation results produced by the 3D Eulerian MISKAM model (Institut fur Physik der Atmosphàre, Mainz, Germany). Concentration values computed by STREET dépend on three main inputs which are the street geometry and orientation, the prevailing wind conditions and the annual average émission rate.

Note that CALINE4 was deliberately not used in the case studies presented here. This Gaussian line source model was mainly designed to predict atmospheric concentrations within a few hundreds of meters from open roadways (California Department of Transportation, version 1.31, 2005). Our decision was motivated by a first series of tests in which application of CALINE4 to street canyons lead to high underestimation (Wroblewski et ah, 2008).

TEST CASES Streets

Six streets were studied: three street canyons of Berlin, Hanover and Copenhagen instrumented during the European TRAPOS program (1994-1995) and three streets located in the city of Nantes: a deep street canyon (height/width ~2) a classical street canyon (height/width ~1) a semi-open street. In each of those last three sites, a monitoring campaign was carried out by AIR Pays-de-Loire for a period of several months to two years. The presentation of the results will be limited to those cases (Table 1 andFigure 1).

Input data

Since no information about CO background levels was available, only NO X , NO2 and PMio concentrations were modelled. The following data were used as input:

Hourly NO X and PMio background measurement data. They were respectively taken from two urban background monitoring stations located in the surroundings of the streets and selected by AIR Pays-de-Loire. O 3 background measurements were also introduced into ADMS-Urban and SIRANE (for modelling chemistry).

Hourly meteorological recordings of wind, cloud cover (for computing stability in ADMS-Urban and SIRANE) and global solar radiation (needed by OSPM simplified chemistry module) (Meteo-France, Nantes airport).

Information about traffic and fleet composition. Data were provided by AIR Pays-de-Loire and used to compute NO X and PMio hourly emissions in the three streets. COPERT IV emission factors (EEA, 2007) were applied.

SENSITIVITY TESTS

Preliminary sensitivity tests were first performed with ADMS-Urban, OSPM and SIRANE to better characterize the response of those models as a function of the input data. Crébillon street and all related input variables and parameters were taken as the reference case. In all the tests, a 12 month long modelling period (1 May 2004/30 April 2005) was considered. Given a dispersion model and a pollutant (NO X or NO 2 ), the model sensitivity to each variation of an input (all other parameters being fixed) was quantified by the coefficient:

G = -c rPf )ic re

1=1, 2...,N=total number of variations for parameter p AC|%J is the relative variation of NO X or NO2 mean concentration (average over the period) due to a relative change Ap|%J in the parameter value. Q^l means that a 100% variation of the parameter entails a 100% variation of the average concentration.

All models appear to be most sensitive to: NO X traffic emission rates, with more pronounced effect on NO X concentrations; background concentrations, with more pronounced influence on NO2 concentrations; the street geometry (Table 2).

However, the three models do not behave identically and reveal specificities: ADMS-Urban is highly sensitive to background levels, especially as regards NO2. SIRANE is particularly sensitive to the street width and to the height of the meteorological measurements. Model results regarding NO2 are much more sensitive to background concentrations than those related to NO X . Though representation of canyon effects in ADMS-Urban is based on OSPM formulation, both models have different responses. In particular, OSPM is less sensitive to a variation in background concentrations whereas it is more sensitive to a variation in NO X traffic emissions. This result could be attributed to the coupling made by ADMS-Urban between a street model like OSPM and an advanced Gaussian dispersion model.

MODEL EVALUATION RESULTS

Except for SIRANE and STREET, which calculate a unique value for the whole street, concentrations have been simulated on the same side as the measurements. With ADMS-Urban, it was possible to define two simulation points respectively located on the edge of the pavement (point 1) and slightly more inside the street (point 2). For each street and each model, simulated and measured concentrations were compared by computing and plotting a wide range of statistical scores and graphs. Former simulations (TRAPOS cases, [START_REF] Wroblewski | Evaluation of numerical models used to simulate atmospheric pollution near roadways[END_REF] highlighted how a good control of the input data, especially a precise knowledge of the hourly emissions and of the wind and stability conditions, determines the quality of the results. The recent simulations (Nantes cases, [START_REF] Létinois | Evaluation de modèles pour la simulation de la pollution à proximité des axes routiers[END_REF], which benefitted from a better field expertise and more detailed input data, confirm that conclusion.

On average over one year, the relative differences between model outputs and observations are generally less than 30% for NO 2 and 50% for PMJO (Table 3), in compliance with the regulatory quality objectives (Directive 2008/50/EC, Annex I). From a temporal point of view, scatter plots and time series show a rather large dispersion between hourly simulated and measured concentrations (Figure 2). Correlation is higher for Strasbourg street, namely the most academic street according to the canyon geometry. Furthermore, the models do not respond the same way to an hourly variation of the input data (background concentrations, emissions, meteorology), each of those variables having a different contribution depending on the model. It should be noted that results are more scattered for NO X than for NO2. As for PM10, model performance is highly variable: unexpectedly, OSPM shows poor agreement with the measured values whereas ADMS-Urban and SIRANE almost systematically underestimate them; those results could be partly due to a lack of knowledge about PMJO emissions.

WEB SITE

With a view to transparency and better exchange of experience, all results and relevant information have been made accessible to French local actors through an Internet information bank (http://www.lcsqa.org/pollution-de-proximite) organized in the following way: technical sheets of commonly used models; detailed descriptive list of monitoring campaigns, with possible uploading of the corresponding input data files whenever available; output modelling data; statistical tools for comparing models and measurements (Excel calculation sheet); technical reports. Those online resources are intended to be regularly updated with information coming from recent monitoring surveys, modelling studies and bibliographical reviews.

CONCLUSION

Modelling roadside concentrations for regulatory purposes or exposure assessment requires that the user be well aware of the model reliability and application domain. Measurement surveys conducted in street canyons or near roadways by local air quality monitoring agencies make a large and interesting renewable source of data sets to support modelling activities. The aim of our study was to take advantage of this information and put technical and numerical data at the user's disposal through Internet. To enrich the website with simulation results and furnish examples of comparison between models and measurements, several common modelling tools were applied to some selected cases (five street canyons and one semi-open street). On average over long periods, providing that favourable conditions are gathered: good knowledge of the sites, precise and temporally consistent input data (hourly variations of traffic emissions, background pollution, meteorology), rigorous definition of the parameters required by each model, the models yield satisfactory results with respect to regulatory quality objectives. The precision of the results is all the better as the models are used in situations for which they have been specially designed (classical street canyons for all models, open streets for ADMS-Urban and SIRANE). At short time steps, because of the parametric nature of the models, differences between simulated and observed concentrations can still be high. Note that the hourly variations of concentrations tend to be better reproduced when the hourly variations of the stability conditions are taken into account and background pollution weighs heavier on the results. Additional tests could be performed to see whether a finer adjustment of some parameters, such as the percentage of primary N0 2 in NO X emissions (EMEP/EEA, 2009;[START_REF] Inrets | Emission factor modelling and database for light vehicles[END_REF], may improve the agreement between model outputs and measurements. Further simulations will be carried out for two situations representing borderline cases for most models (open ways with intersections). Those tests will end off the practical part of the study which will then focus on experience sharing (enrichment of the website, meetings with the French local agencies in charge of air quality monitoring).

Figure 2 .

 2 Figure 2. Scatter plots between modelled (y-axis) and measured (x-axis) hourly concentrations. NO2 [p:g/m 3 ]. Strasbourg street, Nantes. With ADMS-Urban, lower concentrations are calculated at point 1, i.e. the most exterior point compared to the centre of the street.

Table 1 .

 1 Characteristics of the three studied streets.

	Name	Height	Width	H/L	Annual average traffic	Measurement period	Measured pollutants
	Rue de Crébillon	21.0	9.3	2.3	10 650veh./day	1 May 2004 to 30	NO X , PM 10 , CO
						April 2005
	Rue de Strasbourg	18.0	14.5	1.2	27 090 veh./day	1 January 2004 to 30	NO X , CO
						April 2005
	Quai de la Fosse	15.9	30.0	0.5	43 810 veh./day	1 October 2004 to 31	NO X , PM 10 , CO
	(built side)					January 2005
	Quai de la Fosse		30.0	/	43 810 veh./day	16 December 2004 to	NO X , PM 10 , CO
	(open side)					31 January 2005
				Figure 3. Cross sections of the streets.

Table 2

 2 Mean/maximum sensitivity coefficients for the most influent parameters.

			NOx			NO 2
		ADMS-Urb.	OSPM	SIRANE	ADMS-Urb.	OSPM	SIRANE
	Background	0.443 / 0.443	0.164 / 0;314	0.100/0.100	0.877 / 0.879	0.316 / 0.610	0.586 / 0.596
	concentrations
	NO X annual mean	0.505 / 0.551	0.572 / 0.758	0.900 / 0.900	0.299 / 0.375	0.252 / 0.509	0.576 / 0.779
	emission rate
	% of primary NO2	No effect	No effect	No effect	0,082 / 0,082	On-going tests	0,384 / 0,386
	in NO X emissions
	Canyon height	0.221 / 0.324	0.402 / 0.627	0.237 / 0.484	0.278 / 0.318	0.297 / 0.523	0.131 / 0.261
	Canyon width	0.360 / 0.687	0.441 / 0.539	1.009 / 2.412	0.211 / 0.369	0.121 / 1.155	0.568 / 0.778
	Height of wind	0.088 / 0.109	-	0.747 / 1.086	0.069 / 0.088	-	0.444 / 0.656
	measurements

Table 3 .

 3 Examples of statistical comparison between models and measurements.

ACKNOWLEDGMENTS

This project was funded by the Ministry in charge of the Ecology and Sustainable Development and carried out within the framework of the French Central Laboratory for Air Quality Monitoring (LCSQA). The data used in the case studies were provided by the local air quality monitoring associations AIR Pays-de-Loire and ATMO Poitou-Charentes who conducted the monitoring campaigns. The authors wish to thank them, in particular Arnaud Rebours from AIR Pays-de-Loire and Agnès Hulin from ATMO Poitou-Charentes, for the interesting discussions they had together during the study.