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Block Captain News

Manzanita Frequently Asked Questions (FAQ)

Updated: Dec 21, 2022

Why is there flooding in Manzanita and what can be done about it?

The Manzanita interchange is Tam Valley’s most obvious indicator of current and future sea level rise threats in our community. The Tam Valley Sea Level Rise Task Force has met with key County and State officials regarding the history of this flooding and plans to deal with it in the future. The answer is: it’s complicated, both technically and beaurocratically.

This FAQ was designed to provide quick answers (in italics) to key questions, background information to support those quick answers, and links to resources for those who wish to delve deeper into the topics. We hope you find it useful and will join us in the conversation.

Why does Highway 1 under the Richardson Bay Bridge flood so often?

  • The roadway and sidewalks under the bridge are subsiding, sea level is rising, and the culverts that enable outflow of tidal and rainwater are increasingly blocked with mud and debris. The excess water flows into the roadway and parking lot.

  • History: The Richardson Bay Bridge was built in 1957 and seismic retrofits were completed in 1995, six years after the Loma Prieta earthquake. The roadway under the bridge, which was built on marshland and bay mud, was not raised at that time. Since then, there have been pavement overlays at the Park & Ride and improvements to curbs and sidewalks.

  • Sources of flooding: One of the biggest sources is roadway subsidence. The roadbed underneath the freeway has subsided 4 -12 inches since 1995 due to the weight of the road compressing the mud underneath. Fortunately, Caltrans reports that bridge inspections indicate the foundations of the bridge itself are not subsiding. “The bridge foundation consists of spread footings sitting on competent soil/rock and piles embedded in bedrock.” They note that the roadway and sidewalk subsidence “has no impact on the bridge’s structural integrity”.

If we do nothing about it, will flooding get worse?

  • Dangerous high tides are increasing in frequency and magnitude (NOAA). Flooding will increase not just at the Highway 1 underpass but also in the Tam Junction business district and the Birdland and Kay Park neighborhoods.

  • Current Condition

    • It’s helpful to know what the land elevations in the vicinity of Manzanita are so we can predict the implications of sea level rise in the future. Elevations (relative to 0’ NAVD-88) at the underpass range from under 6’ to 8‘. On the Bike/Multiuse path, elevations range from under 7’ to nearly 8’. In the parking area around the Holiday Inn Express, the elevation ranges from 6’ to over 9’. The entrance to the Park and Ride is under 6’.

    • The elevation of much of the Tam Junction business district is 7-8’; most of Birdland and Kay Park is under 10’ as is the Gateway Shopping Center in Marin City, the Tam High Athletic Fields, and the Redwoods Senior Citizen complex across from Tam.

    • In July, 2021, a 7.14’ high tide completely inundated and closed the underpass and the multi-use/bike path. It was a “sunny day” tide, meaning there was no storm surge and no water flowing from the hills. The maximum tide ever observed in San Francisco Bay was 8.72 in January of 1983. There will be seven 7.0+ high tides in the next 12 months including five during December and January.


  • Realistic predictions for additional Sea-Level-Rise (SLR) range from 1 foot by 2035 and 2 feet by 2045 to 4 to 6.5 feet by 2100. Caltrans uses the State of California’s Sea-Level Guidance (2018 Update) which calls for agencies to plan for a 3.5 foot rise by 2050 and 7.6 foot rise by 2100. A predicted extended El Niño effect and accelerated melting of West Antarctic ice shelves would exacerbate the challenges. A five-year storm surge would increase tide levels by two feet. Add these numbers to the elevation figures above and you get a sense of what the future holds.

  • Marin County published Richardson Bay Resilience to describe visually what the County is doing to prepare for climate-induced SLR. Check it out. It is likely to be very bad. We also suggest you spend some time with the Bay Shoreline Flood Explorer and try different SLR and storm surge scenarios to better understand how Tam Valley will be impacted.

Who is responsible for fixing the problem? Is it a priority for Caltrans and other responsible agencies?

  • A complex network of federal, state, regional, and county agencies have jurisdiction over the Manzanita Interchange.

  • Relevant factors: There are many aspects to the present and future SLR-related flooding problem, not just the subsidence of the roadway. Tidal influence in creeks and culverts, lack of sediment flowing into Bothin Marsh, levees and sea walls diverting tidal flow – these are just a few relevant factors. A good source for understanding the breadth of the problem is the Manzanita Flood Reduction Study Report (2021).

  • Responsible parties: Caltrans; the Transportation Authority of Marin (TAM); the Marin County Flood Control District; and the Board of Supervisors. Other agencies with some authority include Marin County Parks; Army Corps of Engineers; Environmental Protection Agency; San Francisco Regional Water Quality Control Board; California Department of Fish and Wildlife; Bay Conservation and Development Commission; and private property owners since solutions generally require addressing larger sections of the shoreline.

  • Priorities: Caltrans, to their credit, in their Adaptation Priorities Report (December 2020), listed the Richardson Bay Bridge and Hwy 1 underpass as one of 63 Priority 1 bridges to address in District 4 (which includes the entire SF Bay Area). The Hwy 1 bridge over Coyote Creek is on the priority 1 list as is the Golden Gate Bridge.

    • Follow-up: Caltrans’ follow-up to the Adaptation Priorities Report is to undertake detailed adaptation assessments of each of the Priority 1 bridges, including a close look at the exposure to more localized climate projections and more detailed engineering analyses. Caltrans has initiated these assessments. If their findings warrant, they will develop adaptation options to ensure the bridge is able to withstand future climate changes. They want multi-agency coordination and involvement of the private sector because the impacts of climate change cross jurisdictional and ownership boundaries. They want to see communities engage in best practice land management in adjacent drainage areas to reduce stormwater and debris flows.

Is there a plan to improve the flooding situation? Does it take into account SLR?

  • Multiple agencies are currently in “study” phases, trying to figure out how to respond to current flooding. Planning for sea level rise is inconsistent but there does appear to be an increasing focus on it, especially at the State level.

  • Flood gates and sandbags: Within the last few years, Caltrans replaced some of their flood gates and the Marin County Flood Control District (Zone 3) installed the sandbag wall that reduces the amount of water flooding into the intersection. These measures have helped the situation but obviously haven’t resolved the problem.

  • Caltrans’ Adaptation Priorities Report: Caltrans has initiated studies required in the Adaptation Priorities Report, as described previously.

  • Flood reduction study: Published in January, 2021, the Manzanita Area Flood Reduction Study (co-sponsored by the County of Marin and Caltrans) studied small-scale and generally lower cost solutions that can be implemented in the next five to 10 years to reduce impacts of flooding during King Tide events. The recommendations did not account for longer-term sea level rise. Caltrans engineers are concerned the estimated costs for the different fixes are insufficient given the scope of the work.

  • Recommendations from the study included continued coordination between Marin County and Caltrans; a more comprehensive evaluation of flooding to include not just tidal flooding but that which is caused by runoff from Coyote Creek and its tributaries during peak storm events in all phases of the tide; and a plan for implementation of short-term improvements called for in the study.

  • The short-term improvements include replacing tidal gates with Tideflex valves which block tidal water from flooding into culverts and drains but allow water to flow into the bay from the land.

  • There is currently no funding for these projects. Some of the improvements called for in the Flood Reduction Study need to occur on private land. The Marin County Flood District would be key to implementing the work. Caltrans could partner with Marin County to fund portions of the project. Community lobbying of elected officials, the Transportation Authority of Marin, and the Flood District is needed.

Caltrans has scheduled a pavement rehabilitation project for Hwy 1 in 2027. They could probably add some of the recommended actions to that project.

  • Multiuse Path: The Evolving Shorelines project (sponsored by Marin County Parks, Golden Gate National Parks Conservancy, and OneTam) is developing a plan for moving the Multiuse/bike path westward to enhance ecological functions of Bothin Marsh and Coyote Creek and to remove that part of the trail from the tidal flood zone. The changes being studied would likely reduce the impacts of increased tidal flows and improve ecosystem services. For an update of the status of the project, see the Adaptation Concepts report (August, 2021).

  • Clearance requirements: Clearance under the overpass has to be at least 15’ at the center of the roadway and 14.5’ over the shoulders. Currently, the center clearance is 16’8”. If the roadbed was raised to a height that solved the problem of current and future tidal flows, then the vertical clearance limitations would be exceeded.

  • Raise the bridge: There is discussion within Caltrans to raise the freeway bridge. At this point, we have no more information than that.

Other information

  • On average, between November and March and Caltrans closes the intersection about 32x per year at an average cost of $138,000 per year.

  • That big bump in the intersection that was recently repaired was a culvert that crossed the highway. The highway subsided adjacent to the culvert, but the culvert itself had not, resulting in the bump. The culvert is still there but is sufficiently deeply embedded in the mud that Caltrans maintenance crews were able to flatten the roadway without moving it.

How can I help?

  • Understand that there is no simple answer! Solutions will be expensive, inconvenient, and take years to implement. We are not going to stop sea level rise, we will have to adapt to it.

  • Start by learning more. Click on some of the hyperlinks in this FAQ and explore. Visit our website and read some of the research. Then ask us questions and tell us what you think. We will do our best to get answers.

  • At a minimum, get on our email list. We are working hard and don’t send out much, but we want our neighbors to be informed. Shoot us an email to and we’ll put you on our list.

  • Get involved! Join one of the Tam Valley Sea Level Rise Task Force’s working groups in either Finance, Adaptation Science/Engineering, Jurisdictional Issues, or Public Relations/Education. We need your help. Send us an email to learn more.

  • Our goal is by December 2022, to produce a prioritized framework for Sea-Level-Rise adaptation in Tam Valley that is science-based, realistic, and reflects the will of residents and businesses. This framework can then be used to guide decision-making at the local, state, and federal levels.


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