The Oro Loma horizontal levee, a constructed wetland built to address future sea level rise.

Alameda County to use horizontal levees to counteract flooding

on September 24, 2018

Alameda County’s shoreline may be vulnerable to significant flooding by 2045 due to rising sea levels. One solution: constructed wetlands, also known as horizontal levees.

Traditional levees, including vertical seawalls made of concrete, have lined the bay since the late 1800s. Today, more than a century later, Bay Area communities are experimenting with more natural, horizontal levee systems made of mud and plants, such as San Lorenzo’s Oro Loma levee, which is less than fifteen miles south of the Port of Oakland. Horizontal levees have a high price point, costing millions to build. But supporters say they are critical for protecting the Bay Area from inevitable sea level rise and flood damage—and that such efforts require cooperation between cities.

“If we don’t do something very different about planning, all of our options are going to be gored by sea level rise,” said Marc Holmes, a retired program director of The Bay Institute, an environmental organization focused on protecting the San Francisco Bay. “None of the solutions are business as usual.”

Horizontal levees protect against flooding and flood damage in several ways, Holmes said. Native plants on the levees help filter polluted or contaminated waters. The plants also serve as a natural barrier against floodwaters threatening land. The slope of the planted levee acts as a buffer for waves. While seawalls can accelerate erosion of the surrounding area, horizontal levees are self-maintaining, because plant roots keep them stable over time.

Horizontal levees also host abundant plant and animal species. At the Oro Loma levee, bulrushes and cattails thrive in what was once a mudflat covered in pickleweed.

“The levee mimics what used to be here 500 plus years ago,” said Jason Warner, general manager of the Oro Loma Sanitary District.

With almost 70,000 plants in 17 raised beds, the levee is a research site for scientists studying how well different East Bay plant habitats remove nutrients from wastewater. Oro Loma will also serve as a model site for future natural levees in the Bay Area.

In Oakland, where much of the shoreline is developed, the need for coastal protections is great, said Sam Schuchat, executive officer of the Coastal Conservancy, a state agency that protects California’s coast.

“Oakland’s waterfront is also home to some of the city’s low-income communities,” said Schuchat. If displaced by rising waters, such communities might have few places to go.

The Oro Loma levee, fully completed in 2017, acts as a bulwark against future floodwaters heading towards Oakland.

“The water will come up from the south, unless you plan on walling in the entire bay,” said Holmes. “The most effective near-term solution is to accelerate restoration of the marsh systems.”

More than 5,000 volunteers, over the course of two years, worked on the construction of the Oro Loma levee. The total cost, $9.1 million, was funded by Measure AA, a regional initiative passed in 2016 by all nine Bay Area counties. Over the next 20 years, Measure AA will raise $500 million for Bay Restoration projects through an annual parcel tax of $12.

To design potential projects that could be funded by Measure AA, in addition to other funding opportunities, Oakland participated in the Bay Area: Resilient by Design Challenge, through which 10 Bay Area research teams partnered with local communities to devise solutions to environmental challenges. One solution, an “estuary commons,” proposed a network of streams, ponds and land to protect Alameda, San Leandro and East Oakland from flooding.

Warner said he is hopeful that horizontal levees will help the Bay Area adapt to rising sea levels. “There are a lot of thoughtful people working hard to solve this problem,” he said.

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