Simulating the Impacts and Assessing the Vulnerability of the Central Artery/Tunnel System to Sea Level Rise and Increased Coastal Flooding

Abstract

Highway tunnels along the coast are critical infrastructure and represent one of the more sensitive components of a transportation network to coastal flooding. By their very nature, large portions of these tunnels are beneath present sea level and the tunnel entrances and the equipment necessary to operate the tunnels such as vent buildings may or may not be above present flood thresholds. The Central Artery/Tunnel (CA/T) system in coastal Boston, Massachusetts USA is a vital link in the regional transportation network and is comprised of more than 160 lane-miles (more than half of them in tunnels), 6 interchanges and 200 bridges. While not examined here, the CA/T system relies upon other infrastructure systems to fully function, such as the electrical grid. The ADvanced CIRCulation model (ADCIRC) coupled with the Simulating WAves Nearshore (SWAN) Model was used for storm surge modeling. SLR scenarios were selected for four distinct time periods (2013, 2030, 2070, and 2100) to bracket the potential future sea level rise outcomes for the Boston Harbor area and a Monte Carlo statistical approach was utilized to estimate the probability of flooding throughout the Boston Harbor region. The vulnerability analysis was based solely upon exposure, defined as flooding experienced at CA/T systems when at the thresholds of the design flood standards that governed the original design of the CA/T. The extent of flood vulnerability under current climatic conditions is fairly limited with low exceedance probabilities. By late 21st century, however, there is considerable flooding at Tunnel Portals and other buildings and structures. Both local adaptation options such as flood walls protecting individual assets or gates protecting tunnel portals and regional plans blocking major flood pathways were evaluated. In complex systems such as the CA/T, the number and spatial extent of vulnerable areas increase over time as sea level rises and the intensity of storms increase, suggesting that local adaptations may be most applicable in the near-term and regional based adaptations (safeguarding multiple areas for multiple stakeholders) will become more cost effective and necessary solutions in the long-term. Focusing first on local actions also means that the CA/T owner is less reliant on other organizations and agencies to manage the CA/T adaptation as it will own the land necessary for any changes and will only have to manage its own efforts. The regional plan has the co-benefits that it will protect more assets in the region than just the CA/T and there is the possibility that the cost can be shared among more agencies and organizations than just the owner.