How to Build a Living Seawall
Simple fixes bring marine life back to urban coastlines
In the coming decades, coastal communities will build or reinforce hundreds of miles of seawalls, breakwaters, and other coastal defenses to protect themselves against rising sea levels and increased storms, as well as to accommodate growing populations.
Already, more than half the shoreline in parts of Japan, Europe, the U.S., and Australia is artificial. And that puts the squeeze on intertidal creatures. Seawalls and similar structures truncate the gradual slope of natural intertidal zones and sharply reduce the area available for species that depend on this habitat.
But lately, researchers are making the case that these structures can be compatible with a healthy measure of marine life—and at very little extra expense, says Mark Browne, a marine ecologist at University College Dublin.
Several years ago, Browne—then a postdoctoral fellow at the University of Sydney in Australia—was part of a team of ecologists who bolted ten-liter flowerpots made of cast concrete to the seawall in Sydney Harbour. The vessels retained water at low tide, mimicking the tide pools found on natural rocky shores.
Seven months later, the researchers counted 25 species in the pots that weren’t present on adjacent sections of the vertical seawall, an increase in species richness of 64 percent.1 “We were getting universal increases across the different phyla that we were looking at,” says Browne—large crabs, sea stars, sponges, tube worms, snails that thrive in sheltered habitat, a diversity of seaweeds. A previous experiment in Sydney Harbour was even cheaper. Engineers simply left out a few of the sandstone blocks from a new seawall. This created cavities that soon became home to urchins, sea slugs—even octopus.
“Even quite small modifications to the seawall surface will increase the diversity” of species, adds Richard Thompson, professor of marine science and engineering at Plymouth University in the U.K. Thompson was part of a team that drilled small holes—about the diameter and depth of a coin—into concrete panels and then attached them to a nearby seawall. Where only one or two species of intertidal organisms typically colonized a section of standard seawall, up to 16 might be present on the textured surface, the researchers found. The experimental panels “were a poor comparison” to natural rocky shoreline, Thompson cautions, “but they were far better than the smooth concrete.”
Those results call for a shift in thinking on the part of engineers and designers, who tend to default to smooth, rectilinear structures. “As human beings we want things that are sleek and straight and orthogonal, and that does not help ecology,” says Cristina Bump, a Boston-based architect who has studied efforts to enhance seawall habitat in both Sydney, Australia, and Seattle, Washington—where a multimillion-dollar, twenty-block seawall reconstruction project is getting underway.
For the Seattle study, a team from the University of Washington bolted a number of five-feet-by-seven-and-a-half-feet concrete panels to the seawall. Some of the panels had “steps” or “fins” projecting a foot or two off the vertical surface. Some had an all-over bumpy texture, achieved by casting the concrete in a mold used to make faux-cobblestone patios. “We just used something that was already out there,” says Maureen Goff, who worked on the project as a master’s student. “We looked at trying to place real rocks into concrete, but it was structurally not great, and more expensive.” There’s a learning curve for ecologists in this field, too.
The researchers installed the panels at three sites along Seattle’s downtown waterfront and monitored the marine life that colonized the surfaces over a period of two years. “The overall community was pretty similar to a vertical seawall, but we discovered that certain features are good for certain organisms,” says research ecologist Jason Toft. Mussels found purchase in the spaces between the “cobbles.” Exuberant stands of Fucus, a greenish-brown seaweed that provides habitat for other intertidal species, grew on the horizontal surfaces. Copepods and fly larvae called chironomids congregated above and below the fins and steps, perhaps finding that these spots were sheltered from the waves.
Similar features and textures may be incorporated into the rebuild of Seattle’s seawall. The city would especially like to have more copepods and chironomids around, because these animals provide food for young endangered salmon.
“There’s a lot that lives down there,” Toft says. Even in a highly urbanized stretch of waterfront, a surprising number of species can thrive with just a few, relatively inexpensive tweaks to their habitat. A cobble, a divot, a cranny—sometimes, what nature needs from us is just a little bump. ❧
1. Browne, M. and M. Chapman. 2011. Environmental Science & Technology doi:10.1021/es201924b.
Graphic courtesy of Cristina Bump, Associate AIA, LEED AP (www.cristinabump.carbonmade.com)