Marin Voice: Groundwater Data Matters As Sea Level Rises

Marin’s sea-level-rise planning will be stronger if groundwater, land-motion monitoring and other flood factors are integrated into existing flood-risk planning before major public investments are designed, funded or built.

Marin has spent years discussing sea-level-rise risk through reports, maps and adaptation studies. One additional data layer deserves more consistent attention in public planning discussions: groundwater measurements.

When most people think about sea-level rise, they picture bay water overtopping shorelines, flooding roads and inundating neighborhoods. Those risks are real, but they may not be the first impacts many communities experience. In some areas, groundwater can rise before the shoreline is overtopped; flooding may begin when groundwater emerges at the surface or enters buried infrastructure.

This distinction changes both the timeline and the solutions. Many sea-level-rise discussions focus on 2100 and beyond. Yet roads, sewer systems, storm drains, pump stations, public buildings and municipal bonds are often planned around 20- to 50-year horizons.

Local governments should ask not only, “What happens in 2100?” but “What measurable flood risks are emerging within the lifespan of infrastructure being built today?”

Groundwater monitoring can help answer that question. Real-time data matter because groundwater, tides, rainfall and stormwater systems interact over hours and days, not just decades, and those short-term responses during extreme events reveal when and where flooding risks are emerging.

Recent groundwater monitoring in the area around the Manzanita parking lot, near the intersection of highways 101 and 1 in southern Marin, shows that flood conditions are becoming more frequent and disruptive.

Manzanita illustrates the importance of measuring multiple flood drivers together, including groundwater elevation, tides, rainfall, stormwater backflow, drainage capacity, land elevation and salinity. Much of the area was built on artificial fill over former wetlands and tidal flats. Groundwater levels respond to king tides, atmospheric rivers and seasonal conditions, and can vary significantly over short distances. Buried utility trenches, permeable fill and other subsurface features can amplify these responses, moving water inland and upward toward the surface.

As Marin evaluates major adaptation investments, public attention understandably centers on seawalls, levees, raised roads, pump systems, wetland restoration, tidal barriers or managed retreat. These measures may help address surface flooding, but they should be evaluated with a clear understanding of local groundwater conditions. The same is true for stormwater systems and backflow-control devices, including tide gates, duckbill valves, flapper gates and backflow preventers.

These systems can help, but they require ongoing inspection and maintenance to perform as intended. A stuck-open valve can allow bay water to backflow into storm drains; a stuck-closed valve can trap stormwater during heavy rainfall.

Flood risk reflects several interacting factors, including rising bay water, land movement, shoreline erosion, tidal pumping, storm surge, rainfall, drainage capacity and groundwater conditions. Even small amounts of land subsidence can increase vulnerability. Marin should expand existing monitoring into a countywide real-time database and public dashboard showing groundwater, rainfall, tides, flood levels, shoreline change, land movement and other key indicators.

Real-time monitoring would complement, not replace, Marin’s existing sea-level-rise work. Current projections, maps and planning studies provide the framework for understanding future risk. Long-term monitoring provides the evidence base for testing, updating and improving that framework over time, using locally derived groundwater, tidal, rainfall, flood, erosion and land-elevation data.

Public funding for sea-level-rise adaptation is already emerging through state climate bonds, transportation sales-tax programs, competitive grants, flood-control assessments and project-specific funding. Marin leaders should ensure some are dedicated to measurements needed for sound decisions. Public money should support planning and capital projects, but also the real-time data collection needed to determine which projects are necessary, where they should be built, how they should be designed and when. Financial and insurance considerations will increasingly depend on accurate local risk data.

Groundwater-related flooding often develops in low-lying areas and can be tracked over time. Communities can monitor trends, establish thresholds and adapt before problems become chronic. Marin County has an opportunity to become a leader in groundwater-informed climate adaptation. The future of Marin’s shoreline will be shaped not only by the water we can see, but also by the water moving beneath our feet. The next step is to integrate groundwater conditions, land movement, tides, rainfall, erosion, surface flooding and public funding into adaptation decisions across the county.

Marin County resident James A. Jacobs is a hydrogeologist and research fellow at the University of California, Santa Cruz. He has served with the Tamalpais Community Services District and Southern Marin Sewer Agency since 2003. The views expressed are solely his own.