Frequency Analysis and Consequence Analysis
The tunnel risk analysis model is used to analyse the risk for tunnel users, i. e. a statistically expected risk value for the group “tunnel user” is determined (statistically expected fatalities per year). The individual risk portions, namely the risk related to mechanical damage, fire and dangerous goods, are presented separately.
The methodology includes the following two core elements:
; A quantitative frequency analysis
(event tree analysis)
; A quantitative consequence analysis
(statistical approach and simulations)
By multiplying the frequencies of the individual incident scenarios with the related damage extent values, partial risks (statistically expected fatalities/year for each individual event type) are determined. By adding the values of all possible incident paths, the value of the expected risk (statistically expected fatalities/year) for the tunnel under examination is determined.
One of the core elements of the tunnel risk analysis model is a standardised event tree. The frequencies of a set of defined damage scenarios are calculated using the event tree analysis method. Starting from an initial event (the frequency of which is known) in several stages, the various possible incident paths are developed, which lead to different damage scenarios. These damage scenarios differ from each other in terms of event type, vehicle involvement, consequences due to damage, etc. The probabilities related to the individual damage scenarios are determined by means of the event tree, whereas the values for the extent of damage (median expected damage value) related to the individual damage scenarios are determined by a consequence analysis.
Factors that influence the frequency of individual damage scenarios are taken into consideration in the model in the form of changes in the relative frequencies, presented via the branching of the event tree.
A consequence analysis serves to estimate the expected medium extent of damage for each individual damage scenario identified in the event tree. The extent of damage has to be calculated for all three risk portions.
Mechanical damage impacts of collisions
The mechanical damage impacts of collisions are estimated on the basis of a statistical evaluation of tunnel collisions involving bodily injury. In carrying out this estimation, the collisions are differentiated with respect to the following categories:
- Type of traffic (e. g. bidirectional traffic, unidirectional traffic)
- Type of collisions (e. g. single vehicle collisions, rear-end collisions, and front-end collisions)
- Vehicles involved (e. g. passenger car, HGV, bus)
Damage impacts of fires
For calculating the damage impacts resulting from fires, TuRisMo provides two possible approaches for different levels of application:
- a detailed risk analysis model providing a method for carrying out damage extent simulation calculations for each individual tunnel
- a standard risk analysis model providing the possibility to use predefined damage extent values
The damage impacts of fires are estimated using a complex calculation model that is made up of several modules. The calculation model combines both a one-dimensional unsteady flow model and a three-dimensional flow model with an evacuation simulation. The global parameters are represented in the one-dimensional flow model and local parameters and smoke spread effects are represented in the three-dimensional flow model. The time-dependent distributions of both temperature and pollutant concentrations in the tunnel are included in a one-dimensional evacuation model which serves to model the impacts of a fire on the tunnel users involved as well as their self-rescue and escape efforts, taking into account several factors such as vehicle positions, infrastructural conditions, etc.
There are, however, many cases involving tunnels that fulfil certain requirements, and therefore do not require detailed risk analyses. For these cases, a standard risk analysis model has been developed that allows including pre-calculated fire damage impact values in the event tree for a wide range of defined model tunnels.
The calculation of the model values to be used was carried out in the course of the development of the model using the model approach referred to above for determining the fire impact values for unidirectional tunnels, while those for bidirectional tunnels were determined using a one-dimensional smoke spread model. These calculations were carried out using standardised parameters and varying the most important influencing factors such as ventilation system, tunnel length, longitudinal slope, tunnel cross section and escape route length within a range that corresponds with real conditions frequently encountered in practice.
The calculated fire damage impact values are thus only valid for a pre-defined range of tunnel parameters. However, the standard risk analysis model can be adapted for specific advanced applications, to the extent that appropriate basic data are available.
Damage impacts of accidents involving dangerous goods
Damage incidents involving dangerous goods are considered in the model using a simplified approach that is based on the determination of the fire risk. For a specific analysis of the risks associated with dangerous goods transports in tunnels, RVS 09.03.12 is applicable.
A separate and much more detailed approach for the assessment of dangerous goods in compliance with the European Agreement concerning the International Carriage of Dangerous Goods by Road (commonly known as ADR) is described under “Dangerous Goods”.