Tunnel air quality modeling: A case study of the Souk Sagheer Traffic Tunnel, Makkah, Saudi Arabia.

摘要

Traffic tunnels have become increasingly popular in modem cities as a way to ease traffic congestion and overcome natural barriers. However, traffic tunnels present significant environmental and health issues due to the elevated levels of pollutants inside the tunnels, poor visibility, and smoke caused by accidents. In this research, a critical review of the recent literature on air pollution modeling in traffic tunnels and on the ventilation systems used in tunnels is presented. In addition, an air quality modeling concept that has been applied to the Souk Sagheer Traffic Tunnel in Makkah, Saudi Arabia, is also presented. This tunnel is bidirectional and has a forced ventilation system. The level of air pollution inside the tunnel, especially the carbon monoxide (CO) concentration, has been reported to exceed the permissible limits. The tunnel is particularly congested with traffic during the pilgrimage season and has different modes of operation at different times of the year. In the present work, the current status of the tunnel is simulated using a one-dimensional model that takes into consideration the effects of the forced ventilation and the piston action of vehicles. The developed model that validated with measured data, and the Mann-Whitney test shows that the means values of measured and predicted results are equal at a 7% significance level. The measured results show that during peak traffic times, high concentrations of CO, nitrogen dioxide (NO2), sulfur dioxide (SO2), and fine particulate matter often exceed the regulatory limits. SO2 has the highest ratio of measured to recommended concentration of all of the pollutants considered. In this study, several solution scenarios are simulated, such as improving the current longitudinal ventilation, utilizing a transverse ventilation system, or building a wall to separate the tunnel into two smaller tubes. The simulation results show that building a separation wall between the two directions of traffic will significantly reduce the pollution inside the tunnel. For example, the mean value of CO inside the tunnel is reduced from 43.8 mg/m3 to 12.1 mg/m3 when a wall barrier is introduced. A wall barrier will increase the wind speed and enhance the piston action, thus improving the longitudinal ventilation. Finally, a risk assessment chapter calculates the ratio of exposure and maximum allowable limits by World Health Organization. The ratios are calculated for short exposure level.;This study is important because it shows that bidirectional tunnels are inefficient to ventilate. Moreover, it shows that for the case of the Souk Sagheer Tunnel, additional rows of jet fan does not seem to solve the air quality problem inside the tunnel. Finally, this paper highlights the necessity to investigate SOx emissions because they seem to be the most polluting inside the tunnel.