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Report Details Marin's Vulnerability to Rising Sea Level

Flooding from sea-level rise could inundate California county.

(TNS) - In just 15 years, flooding due to sea-level rise could inundate some 700 buildings across 5,000 acres, affecting the lives of tens of thousands of Marin, Calif., residents, according to a new county report.

And the draft Marin Shoreline Sea Level Rise Vulnerability Assessment, to be presented Tuesday to the Marin Board of Supervisors, says that is only the beginning.

The report was prepared by the county’s Department of Public Works and Community Development Agency with a $250,000 grant from the California State Coastal Conservancy.

“The assessment is the first step in coordinating the local cities, towns, agencies and districts to understand what happens to Marin County with sea-level rise,” said Chris Choo, a principal planner in public works. “It will help us share information about our risk and resiliency, develop partnerships and work together to find and fund solutions.”

The report presents six vulnerability assessment scenarios ranging from 10 inches of sea-level rise in the next 15 years to 60 inches by the end of the century. Some of the scenarios also pair different sea-level rise projections with 100-year storm surges.

For example, couple a 10-inch rise in sea level with a 100-year storm surge and, the report projects, there will be flooding of 8,000 acres, 2,500 parcels and 3,800 buildings in Marin.

By the end of the century, the expected 60 inches of sea level rise paired with a 100-year storm would result in flooding of nearly 13,500 acres hosting 12,600 parcels with 12,000 buildings.

Estimates of building damage in the year 2100 worst-case scenario range from $60 million to $6 billion. The majority of the buildings that would be damaged are on residential parcels. If that land could not be reclaimed for development, another $9 billion in losses would be sustained.

The report states that several public facilities, including three schools, the Tiburon Fire Station, San Rafael Fire Station 54, Larkspur Ferry and emergency fuel tanks, and the Sewerage Agency of Southern Marin wastewater treatment plant all could be vulnerable in the near- to medium-term.

And in the long-term, sea-level rise could threaten the Belvedere/Tiburon Post Office, San Rafael Main Post Office, San Rafael Transit Center and seven schools — as well as several lower-income retirement communities.

‘Sea-level rise is here’

Supervisor Kate Sears, who serves on the executive committee overseeing the assessment effort, noted that during high tides and king tides parts of Marin are already experiencing increased flooding. This winter, for example, Highway 37 was closed for a record 27 days because of flooding. The report states that areas affected now or during king tides could flood almost daily in the near-term.

“Sea-level rise is here,” Sears said. “And that is a benefit because we can all see what it is like. Climate change isn’t an abstraction.”

“This project did two things that are critically important,” Sears said. “One of them is creating this platform of information. The other crucial piece is the collaboration between the different jurisdictions.”

The assessment was coordinated with every municipality in the county as well as local, regional and state agencies. Sears said it is crucial that people recognize that sea-level rise is a threat that must be addressed collectively. She also said data from the report can be used to determine which projects to address sea-level rise will produce the best effects at the lowest cost.

A majority of Marin’s low-lying areas, even those protected by levees, could see additional tidal impacts after just 3 feet of sea-level rise, according to the report. Many homes and their surroundings are built on filled bay mud and could sink, or subside, as the ground below saturates with water.

San Rafael and small shoreline unincorporated communities in Southern Marin are expected to be the first to see significant tidal flooding in the near-term.

Some of the most vulnerable areas, such as San Rafael’s Canal neighborhood, are occupied by those with the fewest resources; by the end of the century the entire Canal area could flood daily from the shoreline to Interstate 580 and Highway 101.

Inland implications

The report concludes, however, that it won’t just be Marin residents living in low-lying areas that will feel the effects of sea-level rise. Disruptions or damage to utility and transportation networks could affect hundreds of thousands of people living and working in Marin in the not too distant future.

The most vulnerable high-capacity roads include: Highway 101 and I-580; Shoreline Highway from the Manzanita Park and Ride to Tam Junction; Miller Avenue and Camino Alto in Mill Valley; Tiburon Boulevard in Tiburon; San Clemente, Paradise, and Lucky drives in Corte Madera; several arterials and local streets downtown around the Highway 101 corridor and in the Canal neighborhood in San Rafael; Rowland Way and Bel Marin Keys Boulevard in Novato.

The report states that few alternative route options are politically or physically viable.

Utility managers told the report’s creators that the earliest threats of flooding and subsidence may be to non-structural building components, such as utility and mechanical systems at or below grade. Most habitable buildings depend on several utility systems including water, electricity and septic. Pacific Gas and Electric Co.’s electricity transmission lines and natural gas lines could be vulnerable. Buildings untouched by rising tides may become isolated and cut off from essential services.

Three public meetings have been scheduled to give the public the opportunity to discuss and learn more about the report: 7 p.m. April 19 at Pickleweed Park in San Rafael; 7 p.m. April 25 at Mill Valley Community Center; and 10 a.m. April 29 at Novato City Hall.


©2017 The Marin Independent Journal (Novato, Calif.)

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