Transportation corridors through Elkhorn Slough will face significant challenges under conditions of climate change and sea level rise. The draft Central Coast Highway 1 Elkhorn Slough Resiliency Study (see web attachment) provides a comprehensive assessment of the threats and options available to respond to those threats including the transportation, environmental and economic dimensions.
The study presents a detailed understanding of the risks and options for adaptation in this critical stretch of Highway 1 and the railway corridor, as well as an adaptation management strategy to ensure updated scientific information is integrated into the project planning process. The eight-mile stretch of Highway 1 near Elkhorn Slough is already constrained and will be increasingly impacted by coastal storm flooding and sea level rise.
To adapt the transportation infrastructure to rising sea levels, the existing highway and railway could be elevated via two methods: 1) raised fill embankment and 2) piles. An embankment entails placing and compacting a volume of earthen material (fill) in order to raise the grade of a roadway above adjacent ground surface. Embankments typically have steeper side slope. Where space allows, traditional engineered side slopes can be graded to a much gentler slope to allow for additional habitat area creation. Piles (pylons) refer to structures that support bridge or highway overpasses, typically elevating them over water.
The roadway and railway adaptation scenarios were evaluated using best available modeling tools to investigate systemic changes to transportation, hydrology and ecology triggered by certain adaptation actions. The results of the transportation modeling indicate that the four-lane elevated Highway 1 would best suit the needs of the corridor, allowing for increased capacity on a road that is already overburdened by demand. The benefit-cost analysis finds that a four-lane elevated Highway 1 would be economically justified, since the value of reduction in traffic delays would be greater than the costs associated with transportation and ecological improvements.
Since its construction in the late 1800s, the railway through Elkhorn Slough has operated on tracks elevated on fill embankment. The present railway floods under king tide conditions, with disruptions to service until water levels return to normal. To avoid more frequent disruptions to this transportation function as sea levels rise, the grade of the fill embankment could be raised higher to keep pace with water levels. Alternatively, the railway could be raised on trestle, which is an open cross-braced framework used to support an elevated structure. The results of the study indicate that a new single-track railway facility could be constructed and elevated on a trestle adjacent to the existing railway fill embankment. The existing railway fill embankment could be left in place, to aid in sediment retention for the inboard marsh complexes.
The study emphasizes the importance of planning for Highway 1 and railway adaptation in the early to mid-2030s and implementing a course of action well before sea levels are predicted to follow the exponential part of the curve in mid-to late-21st century. The no action scenario would result in widespread loss of habitat and biodiversity through the slough and worsen an existing transportation function problem, to the detriment of the community, region, and visitors to Monterey Bay.
The draft study was released for review on May 12, 2020. The close of the public review period is June 11, 2020. Heather Adamson, AMBAG Director of Planning, will present an update on the study and seek feedback from the Committee. Ms. Adamson presented the project to this Committee at its November 4, 2019 meeting.