Massachusetts aims at resiliency for road infrastructure by assessing risks to the Central Artery/Tunnel and protecting it from potential future flooding.

Although Massachusetts was spared the strongest impacts of Superstorm Sandy, the hurricane served as a wakeup call about the risks of future impacts from climate change on the Common-wealth’s coastal infrastructure. Like elsewhere, Massachusetts’s climate is changing and will continue to do so over the course of this century.

The Massachusetts Department of Transportation (MassDOT) is faced, as other State DOTs are, with the challenge of adapting infrastructure to the changing climate and extreme weather events. To address this challenge proactively, MassDOT is developing a program that is focused on evaluating the vulnerability of statewide transportation assets to climate-related hazards and creating tools to facilitate resilient highway designs.

Current projects in this resiliency program include a coastal and a statewide assessment of transportation vulnerability and a tool for mapping that vulnerability. In addition, MassDOT published a report in 2015 assessing the risks to Boston’s Central Artery/Tunnel. The agency is now creating additional assessment tools and reviewing the adaptation options proposed in the report. This data-driven project has been time and labor intensive, but the results have provided MassDOT with assurances about the Central Artery/Tunnel’s stability in the short term and ideas on how to improve its resiliency in the long term.

The Big Dig’s Vulnerability

The Central Artery/Tunnel, known to locals as the Big Dig, is a critical link in the regional transportation network and a vitally important asset to Boston and the surrounding communities. In the event of a disaster, the Central Artery is an indispensable route for evacuation, emergency responses, and recovery services.

As evidenced by the impacts of Superstorm Sandy on New York City’s tunnel system, infrastructure located near the ocean, as is the case with the Central Artery/Tunnel in Boston, is vulnerable to storm-driven flooding. This vulnerability puts the Boston tunnel at a greater risk of shutting down entirely during extreme weather events. The Central Artery system is especially important because alternative routes, such as surface streets, cannot accommodate the same high traffic volumes, and there are only two additional sets of tunnels that connect downtown Boston to points outside the city.

To protect this essential asset, MassDOT, in partnership with the Federal Highway Administration, conducted an assessment to determine the tunnel’s vulnerability. The assessment had three objectives: (1) develop an inventory of all assets in the Central Artery/Tunnel network, (2) assess the vulnerability of the network to sea level rise and potential flooding due to coastal storm surge and wave action resulting from extreme storm events, and (3) investigate adaptation options to reduce identified vulnerabilities and develop plans to mitigate or prevent damage from future storm events. The MassDOT study used FHWA’s Climate Change & Extreme Weather Vulnerability Assessment Framework (FHWA-HEP-13-005) to guide its approach.

Collecting Asset Data

MassDOT began the vulnerability assessment by collecting information on the location and condition of the assets that make up the Central Artery/Tunnel system. The agency compiled asset data from a variety of internal divisional and departmental databases and then reviewed and refined the initial asset list with MassDOT operations and maintenance staff who are familiar with the selected facilities.

During these discussions, staff indicated that the system consists of a number of interdependent components. Considered together, these components make the entire system a critical asset that is worth assessing. Based on the staff’s input, the assessment team decided that the system, as a whole, would be included in the vulnerability assessment.

During the initial phase of data collection, the members of the assessment team quickly realized that they needed to confirm some of the existing information, such as elevation data. To collect more detailed information, the team conducted field visits to take photographs and measure the height of selected assets. To ground-truth (verify in person) the existing elevation information, the team conducted targeted elevation surveys at locations identified as potential flood pathways.

“The field observations and ground-truthing played a large role in our project,” says Joseph Rigney, P.E., tunnel engineer with MassDOT. “Through the site visits, we gathered information that wasn’t available in our digital data, and in some instances we identified new structures that were not part of our existing asset databases. The information we collected by going out in the field turned out to be essential in assessing vulnerability and later in developing adaptation solutions.”

Once the asset data was compiled, the assessment team began developing information on potential flooding due to sea level rise, coastal storm surge, and wave action.

Hydrodynamic Analysis And Mathematical Modeling

Previous vulnerability studies for the Boston area relied primarily on a “bathtub” approach or on simplified empirical or statistical models for assessing the impacts of sea level rise and storm surge on populations and property. The bathtub method applies sea level rise scenarios at constant elevations to model the impacts of coastal flooding on infrastructure, but it does not include storm surge, wave dynamics, or landform responses.

Mapping Our Vulnerable Infrastructure Tool (MOVIT)
Although MassDOT has found complex modeling tools highly valuable for accurately assessing the vulnerability of transportation assets, the agency also recognizes the important role that institutional knowledge plays in identifying vulnerable assets. In addition to modeling and mapping potentially vulnerable transportation assets, MassDOT also wanted to translate staff’s on-the-ground experience into useful data. Following the pilot study, MassDOT developed the Mapping Our Vulnerable Infrastructure Tool (MOVIT), a Web-based application that compiles and displays information on the locations and assets that have experienced weather-related problems. To develop MOVIT, the agency’s staff conducted interviews and staff surveys to collect information on known flood areas along the highway system. MassDOT plans to train staff to use the tool to capture additional data on weather-related vulnerabilities.
This screenshot of the MOVIT application shows an aerial view of a road (highlighted). Shown in the pullout is the metadata for the area, including information on the location, the climatic event, and the asset’s response. Through a Web-based map interface, MOVIT enables users to place a point or an area on the map and enter data to describe weather-related infrastructure vulnerabilities associated with that location.

Although this approach provides a useful way initially to identify areas that might become vulnerable to sea level rise, it cannot represent the dynamic nature of storm events and tide cycles, combined with sea level rise, which can create short-term, high-impact flooding. Adding sea level rise to storm surge, wave dynamics, and tides in a dynamic model can result in higher water levels than those that result from the sum of these variables. For the new assessment, MassDOT wanted to simulate important coastal storm processes and impacts at a more detailed level than the traditional bathtub approach could provide.

As Kevin Walsh, director of the MassDOT Highway Division’s Environmental Services Section, notes, “Because of the importance of the Central Artery/Tunnel infrastructure and its impact on a large population in the heart of Boston, we needed a very sophisticated approach that could capture all of the elements needed to address the complexity of the terrain and bathymetry.” (Bathymetry is the measurement of depth at various places in a body of water.)

To identify specific locations that might require adaptations, the MassDOT assessment team used a hydrodynamic model that employs mathematical representations of tides, waves, winds, storm surge, sea level rise, and wave setup--the increase in water level caused by wave run-up. The team used the ADvanced CIRCulation (ADCIRC) model to simulate storm formation in the Atlantic Ocean. The team coupled that information with the Simulating WAves Nearshore (SWAN) software to simulate storm-induced waves in concert with the hydrodynamics data. The coupled model, called the Boston Harbor Flood Risk Model, is capable of simulating anticipated coastal storm processes and their potential impacts from storm surge flooding.

To maintain consistency with other local work related to climate change, the assessment team selected four time horizons--2013, 2030, 2070, and 2100--and developed scenarios that simulate the projected sea level rise and impacts of hurricanes and Nor’easters for each of those times. Modeling both types of storms is important because they can cause the same severe impacts but differ in size, geography, and characteristics. The team modeled a statistically robust sample of storms under different climatic circumstances to determine the probability of flooding throughout the Boston Harbor region.

For each of the four time horizons, the assessment team generated maps indicating the risk of flood inundation and showing the associated water depths throughout the Central Artery/Tunnel network. The maps identify locations, structures, and assets that lie within various flood risk levels. The team also used the maps to assess flood entry points and pathways.

Preliminary Results

The MassDOT team defined an asset as exposed if the depth of flooding in the model data exceeded the storm-designed standards that governed the original design. For example, the designers of the Central Artery/Tunnel had established the design standard for tunnel entrances to meet the 1,000-year flood event (that is, the flood elevation that has a 0.1 percent probability of being equaled or exceeded in any given year) plus a minimum wave height of 1.5 feet (0.46 meter). The original planners had designed the tunnel’s structures (for example, vent buildings) to the city’s building code for the 100-year flood event, including wave action.

The assessment team labeled assets as exposed if the projected flood elevation exceeded these standards. Using the high-resolution flood elevation maps generated for the Central Artery/Tunnel system, the team identified all assets currently exposed to flooding, as well as those that will be exposed in the future.

The results showed that, under current conditions, the extent of flooding in the Central Artery/Tunnel system is fairly limited. Only a few individual structures are considered vulnerable. As sea levels rise and storm surges increase in future years, the number of assets that will experience flooding, as well as the depth of flooding, will increase. By 2070, the number of vulnerable structures requiring major adaptation will more than triple, compared with the current number.

Next Steps

Using this information about current and future climate-related vulnerabilities, MassDOT is investigating various adaptation options to make the system more resilient to anticipated flooding. First, the agency is looking at local adaptation options to protect individual structures and portals. Local-, structure-, and portal-focused adaptation solutions currently under consideration include temporary flood barriers and a robust program of tide gate repairs and installations for stormwater outfalls (the points where a stormwater system discharges into a body of water).

The agency also is beginning to collaborate with stakeholders regarding regional adaptation options that focus on addressing flood pathways. In contrast to solutions that focus on improving the resiliency of individual structures, regional adaptation solutions focus on addressing flood entry points to protect larger areas from the risk of flooding. The regional solutions under consideration could be more cost effective than the collective local solutions within the same flood path, but the regional approaches will require coordination and investment by multiple stakeholders.

In addition, MassDOT is conducting analyses to identify flood pathways and flood duration timelines to further refine the adaptation options.

Although the assessment team initially developed the Boston Harbor Flood Risk Model to assess the tunnel’s vulnerability, the model’s usefulness has extended far beyond the initial project. Currently, MassDOT is using the model to develop an assessment examining the impacts of sea level rise and increased tidal and storm surge flooding on Federal and State coastal transportation infrastructure. As part of this study, MassDOT is applying the methods used to develop the Boston Harbor model to create the Massachusetts Coastline Flood Risk Model to account for present and future climate change impacts along the entire coastline of Massachusetts, including the islands of Martha’s Vineyard and Nantucket.

MassDOT has also shared the Boston Harbor Flood Risk Model with its State and local partners, including Boston, which is using the model as part of its climate-ready initiative, and Cambridge, which used it to model sea level rise and storm surge scenarios for its vulnerability assessment.

Agency officials are mindful that they will need to redevelop and rerun the Boston Harbor Flood Risk Model within the next 10 years to account for changes in climate conditions and available technology to model storms, coastlines, and city landscapes. Today, MassDOT is therefore focusing on resiliency options for the present and 2030 while keeping 2070 in mind.

Gina Filosa is an operations research analyst at the Volpe Center. She holds an M.A in urban and environmental policy and planning from Tufts University and a B.A. in environmental studies from Providence College.

Leslie Stahl is a community planner at the Volpe Center. She holds an M.S. in urban and regional policy from Northeastern University and a B.S. in marketing and a B.A. in public relations from Penn State University.

Steven Miller is a MassDOT supervisor, lead for its climate change resilience program, and project manager for a MassDOT and FHWA pilot project. He holds a B.S. in geology from Northeastern University.

Katherin McArthur is an environmental analyst at MassDOT. She holds a master’s degree in environmental management from Yale University and a B.A. in ecology and evolutionary biology from Princeton University.

For more information, see www.massdot.state.ma.us/Portals/8/docs/environmental/SustainabilityEMS/Pilot_Project_Report_MassDOT_FHWA.pdf or contact Steven Miller at 857–368–8809 or steven.j.miller@dot.state.ma.us.