These days, the conversation around climate change so often focuses on matters of soaring temperatures and extreme weather events. While they no longer dominate the discourse, rising sea levels will nonetheless still be a major issue to face as global average temperatures continue to rise.

This poses unique challenges in coastal areas. Municipalities must figure out how to defend their shorelines, or decide which areas they’re willing to lose. The City of Palo Alto is facing just this challenge, and is building a natural kind of seawall to keep the rising tides at bay.

Seawalls That Breathe

The traditional way to fight back against the sea is with seawalls. These typically consist of steep slopes constructed on the shoreline, which are designed to reflect wave energy back to the sea and stop it from eating away at the land. They are normally built using rocks, steel, or concrete walls to dissipate the energy of incoming waves. They are typically simple to design and construct, and prove relatively effective at staving off erosion. However, they can also be quite imposing and unsightly, and often do very little to support native fauna and flora.

The City of Palo Alto is taking an altogether different approach by building a horizontal levee to protect the shore of Harbor Marsh. It eschews the usual steeply sloped seawall concept entirely. Instead, the coast is to be given a gentle gradient constructed of earth, creating a so-called “ecotone slope”—a long, sloping habitat down to the water line. Where the tide meets the shore, native plantings will support a tidal marsh, transitioning to a freshwater marsh with different plants farther up the slope, with volunteers planting 35 species in all. It’s hoped that restoring these habitats in the area will provide support to species like the Ridgway’s rail and the salt marsh harvest mouse.

Furthermore, from the top of the horizontal levee, wastewater will be fed in to support the growth of native plants, which will work with the soil to filter out pollutants as it makes its way to the sea in a process referred to as “polishing treatment”.  It’s not intended to remove heavy pollutants from the water; this work is handled at existing municipal water treatment facilities. What the levee can handle is soaking up some of the nitrogen and phosphorous content to support plants on the slope. This reduces the amount of these nutrients that gets released out into the bay, which can cause fish die-offs, algal blooms, and other undesirable consequences.

Due to its limited size, the horizontal levee will only handle 100,000 gallons of wastewater per day, which isn’t much against the 20 million gallons that currently flows out into the bay.  Ultimately, that’s because the work at Harbor Marsh is a pilot project for the City of Palo Alto. Ideally, it will prove effective in both limiting coastal erosion as well as supporting native plants and animals. If it proves successful, it could become a strategy used elsewhere along the San Francisco coastline and beyond. The Bay Area as a whole needs to be protected against rising sea levels, as the name implies, so projects like this are a key focus as authorities plan for the future.

As it stands, large artificial seawalls probably aren’t going anywhere. It’s very straightforward to build massive concrete and steel structures to defend a piece of coastline. The engineering involved is well understood, and the construction process does not require particular finesse in the selection of plants or the maintenance of native habitats. However, in areas where it’s desirable to slow erosion in a greener fashion, horizontal levees could become popular. After all, it’s a lot nicer to stroll on a path alongside a burgeoning native marshland than it is to feel the sun bouncing off acres of harsh concrete. If the Harbor Marsh experiment works, expect to see similar projects take off in coastal areas around the world.