Caltrans evaluates options to manage erosion & flooding
California is among a handful of coastal states where the department of transportation is actively seeking adaptive strategies for shoreline infrastructure threatened by sea level rise and increased storm surge. One highway in particular, State Route 37 (SR37) in the San Francisco Bay Area, has led the pack of highways under consideration by the California Department of Transportation (Caltrans) and the regional Metropolitan Transportation Commission because it is chronically congested and periodically flooded and closed, in part due to climate change.
SR37 and its surrounding landscape have been the subject of in-depth studies by the Road Ecology
Center – University of California, Davis, with support from the SHRP2 program, Caltrans, the National Center for Sustainable Transportation, and the Center for Transportation Environment and Community Health. Stretches of SR37 are exposed to flood risk and erosion due to poorly maintained or low-elevation levees and berms and low elevation of the roadbed. This critical linkage in the Bay Area highway network was flooded and closed for five weeks in early 2017 due to berm over-topping. Estimates for elevating the highway to avoid 2,100 sea elevations and storms top $4 billion for a pier-causeway.
Recently, options for adapting SR37 have expanded to include contrasting options, including a bridge across the marsh or the bay, which is likely to cost as much as the pier-causeway studied several years ago, and moving the highway out of the tidal marsh to higher ground.
Sea level rise of 8 in. along the U.S. coastline over the past century is predicted to increase by another 12 to
24 in. by mid-century, though the actual degree of increase is unknown and no corresponding adaptive management process has been proposed. The impacts of sea level rise likely are to be both chronic as water levels rise and stochastic during extreme events or failure of shoreline flood management structures. The impacts likely are to be a combination of increased flooding and erosion loss of the shoreline. Sea level rise poses a critical threat to shoreline ecosystems, communities, infrastructure and economic activities. Adapting to sea level rise and its associated impacts will be best accomplished if it is supported by communities and integrates natural and human systems. However, there is uncertainty in the timing and magnitude of flooding and erosion due to natural shoreline processes, the timing and nature of potential adaptive responses to those hazards, and who will be responsible for potentially costly fixes. There also is uncertainty associated with whether shoreline armoring will be used to reduce short-term impacts and what long-term negative impacts could result from increased armoring.
Scientists have estimated that as many as 13 million people in the U.S. could be displaced by increased inundation due to 1 meter of sea level rise, coastal erosion and coastal flooding. Estimates for the total property value exposed to the effects of storms, sea level rise and nuisance flooding include $7.2 billion for Long Island, N.Y., projected by 2080; more than $100 billion for California with 1-meter sea level rise; and $17 trillion for the entire U.S. coastline. These property values and economic activities do not account for the value of natural and constructed shoreline features that provide free or subsidized services to coastal communities and state economies. Though sea level rise impacts on shoreline communities and infrastructure will have some of the greatest economic and social effects the U.S. has faced, there has been little high-resolution study of these impacts, or national and state-scale acceptance of these future liabilities.
Shoreline flooding often is a result of the combined effects of storm water runoff during extreme events and elevated sea levels. This means adapting to flooding on the shore should involve planning and action inland and uphill of the impacted areas. Because of the intimate relationship between green and built infrastructure on the shoreline, co-adaptation of both systems will be most effective in the long term.
Agencies usually go through arduous analyses and cost estimates to determine how much it will cost to raise shoreline roadways in place. This was true for the SR37 process, where raising-in-place and widening cost estimates ranged from more than $1 billion for a berm-based highway to more than $4 billion for a pier-causeway. In many ways, this is an attempt to plan for stasis and may be comforting because the resulting structure would be in the same place, connect with existing infrastructure and temporarily would reduce congestion. However, the best alternatives may be either those thought the least politically feasible or those not yet considered.
An essential quality of the assessment, planning and stakeholder process that transportation agencies have had to embrace with SR37 is that it can be as dynamic as tidal action on the shoreline. Caltrans’ first proposal for the corridor in 2011 was a raised and widened berm for the roadway roughly following the same alignment. Within the next five years, additional design proposals were brought forward through a stakeholder process, including moving the traffic to parallel alignments off the marsh, using a tunnel or bridge to cross the bay, raising the highway on a pier-causeway, and using a floating bridge similar to the Interstate 90 bridge across Lake Washington near Seattle. By 2016, local and regional transportation planners were primarily focused on two options: a raised berm or a pier-causeway parallel to the existing alignment. Because this approach would have created a new approximately 10-mile road through the tidal marsh, there was considerable pushback from environmental and restoration interests, resulting in a stalemate.
The Resilient by Design challenge provided an escape hatch from the increasingly frustrated adaptive planning process. During late 2017 and early 2018, a team of consultants and firms called Common Ground engaged local stakeholders and experts to discuss the values at stake, previous research and the various alternative approaches that had been proposed. Their aim was not only to incorporate the objectives of conservationists and transportation planners, but also to advocate for a larger constituency of North Bay communities who could benefit from increased equitable public access to the baylands adjacent to SR37.
To this end, Common Ground proposed designs for two adaptation strategies that were both consistent with these goals for mobility, conservation, access and adaptation:
- Elevate the highway on to a pier-causeway; and
- Move the highway to higher ground that is unlikely to flood or erode during this century under even extreme projections for sea level rise and storm surge.
Both options would limit erosion, reduce flood risk, expand vehicle capacity, offer additional opportunities for public access to the baylands, and provide unconstrained ecological and hydrological connectivity. The first option would preserve access to existing facilities along the current alignment and similar travel times, but could cost an order of magnitude more than the higher ground alignment based on cost estimates for similar structures in the earlier Road Ecology Center study. While the second option would significantly lower costs and impacts to the tidal marsh, it could require major improvements to other highways in the network and convincing the public to accept the new highway alignment and changes to travel times would be this option’s largest hurdle.
More Than a Road
Regardless of the design or alignment ultimately selected, Common Ground advocated approaching potential design alternatives as more than just instrumental solutions to a highway project. Common Ground envisions the elevated scenic highway as an iconic front door to a vast ecological commons that would be more responsive to landscape features and ecological processes. The highway also could make the adjacent baylands more accessible to rapidly expanding North Bay communities. Either highway alignment could be combined with expanded regional rail and other regional intermodal transit options. By linking the fate of the highway to the landscape, both could become more adaptive by sharing funding sources and a more cohesive regional identity that could garner greater public interest and support. Contextualizing highway adaptation as part of a larger strategy for multimodal transportation, ecosystem resiliency, and community access allows the project to move beyond just mitigating the threat of sea level rise. The project is an opportunity to foster more nuanced and resilient relationships between shoreline communities and infrastructure and the changing landscapes in which they are situated.