Reimagining Coastal Cities as Sponges to Help Protect Them From the Ravages of Climate Change

Infrastructure experts in the San Francisco Bay Area have begun replacing impermeable roads and stormwater drains with water gardens and restored marshlands.

A view of a building flooded due to a week long rainfall in San Francisco, California, on Jan. 11, 2017. Credit: Tayfun Coskun/Anadolu Agency/Getty Images
A view of a building flooded due to a week long rainfall in San Francisco, California, on Jan. 11, 2017. Credit: Tayfun Coskun/Anadolu Agency/Getty Images

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As an environmental officer in Samoa, Violet Wulf-Saena worked with the Lano and Saoluafata Indigenous peoples to restore coastline mangrove ecosystems that could slow incoming waves and protect communities from storm and flood damage. 

Two decades later, in California’s San Francisco Bay Area, she’s the director of a nonprofit called Climate Resilient Communities that works on the same issue: restoring marshlands and wetlands to better protect vulnerable neighborhoods in low-lying areas from sea level rise.

Some areas of the Pacific Islands, where Wulf-Saena grew up, are projected by conservative estimates to see the sea level rise 10 inches by mid-century. By then, East Palo Alto, about 30 miles south of San Francisco, where Wulf-Saena works now, may also be frequently underwater during high tide events. 

“Nature is the best protection to sea level rise, and if we restore these ecosystems we can mimic a lot of that protection,” she said. “It can be like a sponge.”


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Most aspects of the built environment in the modern city are designed to drain away water as quickly as possible. Rain slides off of roofs, over concrete and asphalt and down into sewers, where it’s then redirected to the sea, lakes or rivers. The traditional approach to large water events like floods and storm surges has been to engineer the water out of the way, using seawalls, levees and flood barriers. 

This means that cities like San Francisco could face billions of dollars in flood and storm damage as climate change worsens and overwhelms that infrastructure, all without capturing and reusing a lot of that water, which could ease some of California’s periods of drought. 

Now, infrastructure experts are pushing for urban spaces to be reimagined as sponges—not just by restoring marshlands, but also with more parks and gardens soaking up stormwater, pebbles underneath surfaces acting as natural filtering systems and a more porous type of concrete absorbing water and slowing it down. 

“There’s this idea that a flood is essentially water out of place,” said Zachary Lamb, a city and regional planning professor at the University of California, Berkeley. “What have we done to the surfaces of our environment to change the way water would naturally flow?” 

Over the last two centuries, 85 percent of the San Francisco Bay’s natural wetlands have been displaced by buildings, port infrastructure and agricultural development, creating what Lamb calls “a very human-shaped edge” between the bay and its surrounding urban landscapes. Analysts predict that vast swaths of the Bay Area’s coasts could experience between 5 and 20 inches of sea level rise by mid-century

The best defense, Lamb said, will require reimagining the bay’s edge—starting with its natural wetlands. In San Francisco, several environmental organizations like Save the Bay and the San Francisco Estuary Institute have been working to restore the inter-tidal zones, wet-footed forests, soft-shouldered creeks and mudflats that once encircled the edge of the bay and acted as a natural buffer to floods. These areas naturally absorb stormwater and slowly release runoff as tides and storms subside. 

Shoreline restoration is the low-hanging fruit when it comes to making a coastal city spongier, experts say. But, cities should also look at greening its gray infrastructure. Asphalt and concrete streets are impervious to water, allowing it to accumulate and cascade into homes and businesses. Cities could instead install bioswales, or vegetated channels that dip into the landscape, as an alternative to storm gutters and sewers. Bioswales, also known as rain gardens, have the added benefit of capturing common water pollutants and recharging groundwater. 

San Francisco has also started to use more permeable paving that’s more porous and better at absorbing water. Crane Cove Park, a seven-acre urban expanse along the city’s east-facing waterfront, uses ECOncrete, which mimics tidepools and allows the growth of marine organisms like oysters, corals and barnacles.

“When we do need to build along our coastlines, the materials we use should not only perform for the structure, but also for marine life,” said ECOncrete CEO Ido Sella. “Soft” and “living” shorelines—including wetlands, mangroves and reefs—have flood mitigation and storm buffering abilities while also providing habitat for wetland creatures.

The elegant part about using more green and blue infrastructure to mitigate storm and flood risk is that it almost always has co-benefits, said Amy Chester, the managing director of Rebuild By Design, a nonprofit that focuses on making communities and infrastructure more resilient to climate change. These changes can improve water quality, rebuild habitats for wildlife and provide recreational and aesthetic amenities that make coastal cities a more pleasant place to live. 

More parks and urban forests can also enhance public health. The “heat island” effect that leads metropolitan areas to be significantly warmer than their surroundings is due in large part to their lack of parks and green spaces, and recent research has found that that heat exposure disproportionately harms people of color and households living below the poverty line. 

In drought-stricken areas, the same infrastructure used in a sponge city to protect it from too much water, in addition to residential rain barrels and underground collection basins, could also help when they have too little. 

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Drought conditions have worsened in California, where some residents are already starting to face mandatory water restrictions as cities see historically-low levels of rainfall and anticipate a dry winter. San Jose Water Co., a private company that serves more than 1 million people in San Jose and surrounding areas, plans to require its customers to cut water use by 15 percent from 2019 levels. 

“In places like California that face periods of drought, when there is finally rain after that drought, the ground is so dry that it’s no longer porous, and the water just travels right off of it,” Chester said.  

California’s Sierra Nevada snowpack, which provides nearly a third of the state’s water supply and replenishes the Hetch Hetchy reservoir where most of the Bay Area gets its water, dried up completely this year, according to the latest data from the California Department of Water Resources. 

And, it takes a tremendous amount of energy to get that water to cities, said Hadley Arnold, executive director of the Arid Lands Institute, a Los Angeles-based design company. Currently, 20 percent of California’s electricity is used to capture, clean, distribute and treat water, which means “we’re using a resource that’s unreliable now with the shorter snow season and earlier snow melt, and we’re warming the atmosphere while we’re doing it.” 

“If your snow isn’t there to support your drinking water, you might want to have a different relationship to rain,” she added.

On a small scale, that might look like homeowners installing rain barrels to collect rainwater to tap for irrigation during periods of drought. Residential rain barrels usually range from 15 to 100 gallons, and a number of Bay Area cities have begun to encourage residents to install rain barrels through rebate programs

But Arnold said the sponge city has the potential to take the concept of distributed rainwater capture and scale it up—collect rainwater and store it not in a rain barrel, but in the earth, most likely in groundwater basins. This water could be used to irrigate gardens and flush toilets, but cities also could process it to make it clean enough to drink. 

In an effort to make climate resilience more of a priority, President Biden said in May he would double the funding of a Federal Emergency Management Agency program that gives states, local communities and tribal governments money for disaster preparedness projects, if they can meet a 25 percent local match requirement. 

The Building Resilient Infrastructure and Communities program, or BRIC, could receive an additional $1 billion from Biden’s major infrastructure bill that passed the Senate with bipartisan support in August. 

Some cities are using the funding opportunity to pursue “sponge” projects that involve marshland ecosystem protection, including the Bay Area’s Menlo Park, which applied for $50 million in BRIC funding to revitalize areas of the shoreline and protect critical power and transportation infrastructure. The city received over $17 million in support from Pacific Gas and Electric Company and Facebook to meet the local match requirement.

However, some environmental planners say that Menlo Park, a city in the heart of Silicon Valley, is an exceptional case. Many local governments do not have the institutional capacity, bandwidth or tax base to apply for something as sizable as a BRIC grant, even when it’s necessary, said Margaret Bruce, executive director of the San Francisquito Creek Joint Powers Authority. The SFCJPA helped lead Menlo Park’s grant application process and other flood mitigation projects in the area. 

Bruce pointed specifically to East Palo Alto, a city surrounded by water on three sides from the San Francisco Bay and the San Francisquito Creek, making it very vulnerable to sea level rise and floods that will exacerbate inequities in the region

East Palo Alto is home to mostly lower-income neighborhoods and communities of color, including one of the largest populations of Pacific Islanders outside of Hawaii. 

“In East Palo Alto, there’s no big, vulnerable piece of infrastructure that some private entity owns and wants to protect,” Bruce said. “And there are communities like East Palo Alto all over the country who don’t have a PG&E or a Facebook who will come to the table with cash.”