Update from Kate - Flooding at Manzanita and Marin City

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Update from Kate

Flooding at Manzanita and Marin City:

What’s Been Done and What’s To Be Done?


With the arrival of winter high tides, we leave home wondering if we’re going to get through those known flood “hot spots.” We keep you informed as best we can, including recently sending you the link to the Caltrans-maintained camera at Manzanita. In addition, residents write inquiring what we’re doing to ultimately fix the problem.

Reality Check: Shoreline Highway - A State Highway

Subscribers are well aware from prior updates that Route One/Shoreline Highway is owned and managed by the California Department of Transportation (Caltrans). With miles of our own roads to maintain, the County of Marin is unable to spend our roadway funds there. However, we serve as a liaison to convey issues to Caltrans staff. As a result, my office spends time in close communication with our County Public Works staff who have established ongoing relationships with their counterparts at Caltrans and who help refer questions and problems to the proper person for assistance and resolution. 

However, just because we don’t own the roadway doesn’t mean we don’t do anything. In fact, there’s a lot we do.

Ongoing Flooding: Marin County Flood Control District Has A Role

On our local tax bills, those of us living in southern Marin in Flood Zone 3 fund an agency called the Marin County Flood Control District on which I serve along with the other Marin County Supervisors. Zone 3 was formed in 1956 to address creek and tidal flooding adjacent to Richardson's Bay and is overseen by a 7-member Advisory Board.

Zone 3, known as Richardson's Bay, covers over 13 square miles, including the City of Mill Valley and numerous unincorporated communities including Marin City, Alto-Sutton Manor, Almonte, Tamalpais Valley, Homestead Valley, and portions of Strawberry Point. There are 12,560 parcels in the Zone.

With Flood Control Zones established throughout the County of Marin, we use proceeds from this pool of money to fund local flood control projects. There are established Flood Control Zone Advisory boards for each one of the zones that help advise the District on how this funding should be spent. Flood Control District staff here at the County of Marin help design, build and manage these flood control facilities (like pump stations and flood gates) and projects at various locations.

Marin County Flood Control Work in Manzanita

In late 2016, Marin County Flood Control District staff took the lead in constructing a temporary sand bag wall along the west side of the Richardson Bay Bridge, next to the Caltrans yard (the western side drainage) to reduce direct tidal flooding along Shoreline Highway. In 2017, District staff rebuilt the wall in time for the first round of King Tide flooding in November and December.

In late 2017, Caltrans installed over $1M of system improvements to drainage facilities in the Manzanita area to reduce flooding, including a new tide gate on the drainage inlet from Shoreline Highway, as well as two new tide gates along the east side of the Richardson Bay Bridge (the eastern side drainage channels). Both of these drainage systems regularly overtop during the big King Tides resulting in roadway flooding even under sunny day conditions.

Although Caltrans is still working on sealing the tide gates, the flooding on the roadway and in the Manzanita Park and Ride lot this December was much less than previous years and no roadway closures were required. This provides some initial evidence that these measures have successfully reduced tidal flooding of the road and parking area. However, this being a dry winter with no storm events during these high tides, it’s too early to know how successful these improvements are under more challenging conditions.

Marin City Flood Study Is Out For Review: Comments Are Due December 24, 2017


The Marin County Flood Control District released a study in late October that evaluates the causes of flooding and identifies a series of solutions to reduce flooding in Marin City with a focus on reduction of roadway flooding. This study can be found on the Marin Watershed website here under the Southern Marin page.


Marin City Flood Study Public comments are due December 24, 2017. Please take a look and let us know what you think by sending your comments to Roger Leventhal, Senior Engineer, at rleventhal@marincounty.org. The goal is to complete the study by January or February 2018.


Bothin Marsh Planning Study


As discussed in a previous Update, Marin County Parks is leading a study of the Bothin Marsh area and the Mill Valley-Sausalito multiuse path to better understand and plan for opportunities and risks under current and future sea level rise conditions. While the parameters of the study and an accompanying community process are being developed, a motivating factor in undertaking marsh restoration efforts is the helpful flood control benefits that can result. For further information and to add your name to the e-mail contact list for information and future meetings, please contact Veronica Pearson at vpearson@marincounty.org.


The Road Ahead Toward Long-Term Solutions


We are working collaboratively across public agencies to identify measures we can take now and in the near term to alleviate flooding at these known “hot spots,” where frequent flooding occurs. The reality is that the longer term fixes are complex, capital-intensive infrastructure projects. These kinds of projects take years to design, present for public input, fund, obtain state and local agency permits, and then build.


We don’t yet have all the answers for Manzanita or Marin City but we’re taking all the short and medium term steps we can to better understand the local context and how these smaller projects work and work together. It’s by taking these important incremental steps that we both learn how best to address local flooding and also build strong institutional relationships. We are building relationships with Caltrans, elected leaders in local agencies, the business community and local residents so we will be ready to pursue the challenges of accomplishing the more complex, longer term solutions we need.


If you have thoughts, I’m always happy to receive them at ksears@marincounty.org.