Posted on May 6, 2024
If you live around the San Francisco Bay, you’re probably familiar with cement sea walls and sturdy levees.
But, increasingly, a nature-based design is providing an alternative — one with significant benefits in the face of sea level rise.
When we first met Jessie Olson, she was in the middle of a multiyear project, to create what’s known as a horizontal levee, alongside a newly opened tidal marsh in Menlo Park.
Joined by volunteers and colleagues from Save the Bay, the team installed hundreds of plants that will help clean the bay waters as the tides surge in and out.
“A horizontal levee is not what you think of when you think of a traditional levee, something created out of concrete or riprap or human-made structures. A horizontal levee is a green space. It’s a gently sloping levee that comes out into the bay. It provides green habitat, it provides wave attenuation, shoreline protection, erosion control, many more benefits than what you would get from a traditional hardened levee structure,” Olson said.
The new tidal marsh was created from a former salt pond several months ago, when crews broke open a traditional levee to let Bay waters in. And its green, horizontal edge could be critical to keeping it healthy in the face of sea level rise. That’s because the sloping strip acts like a step ladder, giving the marshy area room to expand as water levels rises.
“It’s possible for a tidal marsh to get drowned out if it can’t keep up pace with the rate of sea level rise. And providing these horizontal levee spaces gives them somewhere to grow upward and continue to provide that important habitat,” Olson said.
And the list of opportunities is growing, including sections of the San Francisco Bay shoreline that serve a critical function, housing many of the Bay Area’s biggest wastewater treatment plants. We toured several facilities that are now preparing to spend millions of dollars on upgrades to meet tighter state water quality regulations for what’s discharged into San Francisco Bay. Projects also set to include horizontal levees.
In Palo Alto, Senior Engineer Samantha Engelage, P.E., showed us a marsh area next to the Water Quality Control Plant. That’s where the district is planning a horizontal levee prototype designed to fight erosion and potentially help add to the wastewater purification cycle.
“We are going to embrace the tide and use it to build up sediments within that habitat, while providing habitat and home for four species and treat wastewater a little bit more and remove more nutrients and remove more contaminants of emerging concern,” English said.
Across the Bay at the Oro Loma Wastewater Treatment Plant in San Lorenzo, volunteers from Save the Bay and other groups have helped to green light yet another experimental levee project. Again, it’s designed to leverage marsh plants to help absorb nutrient materials from the wastewater.
Lorien Fono is executive director of the Bay Area Clean Water Agencies.
“Our long term vision for the region is when we do nutrient reduction we get multiple benefits. And we also enhance habitat and provide our communities shoreline recreational benefits. So if we’re going to spend these funds, we want to get the most bang for the buck,” Fono said.
For Jessie Olson, the adoption of horizontal levees is also a force multiplier. Adding new opportunities as the Bay area races against the clock to restore thousands of acres of tidal marsh around San Francisco Bay, in the upcoming decades.
“So yes, nature-based solutions horizontal levees, these tidal marsh restoration projects, they’re major answer to how the bay is combating climate change and sea level rise,” she said.
Supporters also argue the horizontal levees are a comparative bargain. With San Francisco Bay projected to rise by several feet in the next few decades, cost estimates to protect the shoreline already run into the billions.
