Keywords: Species and food webs, Fishes, Salmonids, Shoreline armoring, Salish Sea Currents magazine

Design innovations at the new seawall along Seattle's waterfront could inspire improvements for other shoreline structures around Puget Sound. They may even encourage broader regulatory changes that enhance habitat for migrating salmon and other species.


Nobody with an understanding of marine life would describe Seattle’s downtown shoreline as a thriving ecosystem. More than a hundred years of city development have left its tidelands covered in fill, flanked with concrete and overshadowed by industrial piers.

Yet salmon habitat seems to be improving there, scientists say, thanks to new features installed during replacement of the downtown seawall.

The enhanced seawall, which has been called the largest eco-engineering project of its kind, may be boosting the fitness and chances of survival for young salmon as they migrate through a treacherous section of waterfront on their way to the ocean.

Instead of encountering a barren slab of concrete, salmon can now swim across a “bench” in front of the seawall. The bench forms a narrow strip of intertidal habitat, effectively providing a shallow-water pathway for juvenile salmon. Glass blocks in the sidewalk above the seawall allow light to penetrate to the water below, and a rough surface on the wall itself has horizontal shelves to encourage the growth of algae and invertebrates.

Juvenile chum salmon swim along the new Seattle seawall. Photo: Mike Caputo/UW

Juvenile chum salmon swimming along the new Seattle seawall. Photo: Mike Caputo/UW

New scientific findings about marine organisms growing on or near the seawall plus behavioral changes in young salmon swimming through the area suggest a real payoff from these enhancements, which added about 2 percent to the cost of the $410-million seawall.

“I am very satisfied with the result,” said Jeff Cordell, a researcher with the University of Washington’s School of Aquatic and Fishery Sciences who has been involved with the project from the start. “What we are encouraged about is that fish seem to be using habitat where they didn’t before.”

Some people say these new findings could lead to discussions, strategies and possibly regulations to reduce the ongoing damage in other locations where shoreline armoring remains a necessity. The Washington Legislature has commissioned an investigation to determine if future shoreline construction should be required to enhance, not just protect, habitat — thus going beyond the current “no-net-loss” standard to create “net ecological gain.”

More than a vertical wall

For close to a century, the seawall along Seattle’s sprawling waterfront has protected waterfront buildings and other structures from the pounding waves of Elliott Bay. For all practical purposes, the seawall did its job well. But after the 2001 Nisqually earthquake, engineers examined the integrity of the structure. They found, among other problems, that wood-boring invertebrates had weakened wooden components of the seawall, raising concerns that the wall could collapse in the next big earthquake.

Coastal geologists estimate that wood, rock or concrete structures have displaced natural habitat along 29 percent of the Sound’s 2,500 miles of shoreline.

Until it came time to replace the seawall, few people gave much thought to the tiny migrating salmon working their way among the pilings, searching for food and doing their best to avoid predators along Seattle’s waterfront.

Throughout Puget Sound, marine biologists have long described the environmental damage caused by seawalls, bulkheads and other shoreline armoring. Coastal geologists estimate that wood, rock or concrete structures have displaced natural habitat along 29 percent of the Sound’s 2,500 miles of shoreline.

In designing the new Seattle seawall, a variety of experts, including Cordell, began asking questions: Could something be built to offer fish better protection from predators and perhaps a little more food to eat while still holding back the land? In other words, could a functioning structure provide improved habitat with less hazard?

Under natural conditions, juvenile salmon tend to stay in shallow waters along the shoreline to avoid larger predators as they search for food. One of the major problems caused by shoreline armoring is that shallow water disappears when the tide comes in. In fact, where tidelands have been filled in — such as in older downtown and industrial areas — the water may never leave the wall.

In building the new 3,100-foot section of Seattle seawall from the Colman Dock ferry terminal north to the Seattle Aquarium, construction crews created a platform, or bench, in the front of the wall. The top of the bench was intentionally placed at an intertidal elevation — meaning the bench is covered with water at high tide and exposed at low tide, forming a shallow-water habitat during a portion of each day.

The bench was built by stacking mesh bags of quarry rocks, nicknamed rock mattresses, between the new wall and a sheet-pile barrier. The sheet pile, a series of interlocking steel plates, was installed initially to keep water out of the construction site. It was later cut down to form the front of the bench.

Marine mattresses, plastic mesh bags filled with rock, were installed to provide shallow habitat for migrating salmon. Photo: SDOT

Marine mattresses, plastic mesh bags filled with rock, were installed to provide shallow habitat for migrating salmon. Photo: SDOT (CC BY-NC 2.0)

To facilitate construction, the new seawall was located 10 to 15 feet farther back from the water than the old one. The sidewalk along the waterfront was then built in cantilever fashion over the wall and above the new intertidal bench. To counteract the effects of the sidewalk shadow, glass blocks were installed in the walkway to transmit light to the bench and water below.

The new seawall also features a series of horizontal shelves attached at various locations, along with lots of crevices, to encourage the growth of seaweeds, invertebrates and other marine organisms.

“The goals,” said Cordell, “were to create an intertidal migratory corridor for juvenile salmon and to increase the nearshore ecosystem.”

New highway for salmon

The Seattle seawall lies on the migratory pathway used by juvenile Chinook, chum, pink and coho salmon making their way to the Pacific Ocean from the Duwamish River and its more-productive upstream reaches bearing the name Green River. Young salmon from more distant rivers to the south, such as the Puyallup and Nisqually, may also find themselves in the clutter of piers and pilings along the urban waterfront.

Before the seawall reconstruction, researchers in snorkeling gear watched for salmon during their migration from March through August. They found much higher densities of young salmon between the piers than in the shadows under the piers.

Scientists have long observed that salmon are hesitant to go under docks, floats and other overwater structures during daytime hours, often stopping at the sharp lines of shadow.

“They don’t like shade lines,” Cordell explained, noting that juvenile salmon tend to stay in the sunlight because their success in finding food is dependent on their ability to see. To avoid the shade of the Seattle piers, some fish even travel through more treacherous deep water, going out and around the piers.

Glass blocks in the sidewalk above the seawall, allow light to penetrate to the water below. Photo: SDOT (CC BY-NC 2.0)

Glass blocks in the sidewalk above the seawall, allow light to penetrate to the water below. Photo: SDOT (CC BY-NC 2.0)

The new seawall with its cantilevered sidewalk embedded with glass blocks altered the nearshore area in some critical ways. Light passing through the glass blocks increased visibility under the piers, where previously light levels were very low. At the same time, moving the seawall farther back and under the sidewalk resulted in a loss of light at the water’s edge that was only partially compensated with the glass blocks.

After construction, researchers observed a notable increase in feeding behaviors under the piers, where the fish had never been known to feed before, according to Cordell. Between the piers, the fish continued to forage much as before, resulting in a more consistent feeding pattern as the fish swam from pier to pier during their migration along the shoreline.

Sonar reveals nighttime behavior

In a separate study, UW graduate student Kerry Accola observed the behavior of young salmon at night as well as during the day by using high-frequency sonar equipment mounted on the bottom of a kayak. Video captured with a hydro-acoustic camera shows individual shadowy fish up to 26 feet in front of the kayak. Sometimes hundreds of fish could be seen at one time, swimming together in synchrony.

UW graduate student Kerry Accola uses high-frequency sonar equipment mounted on the bottom of a kayak to observed the behavior of young salmon. Photo: UW

UW graduate student Kerry Accola uses high-frequency sonar equipment mounted on a kayak to observed the behavior of young salmon. Photo: UW

Accola, a research fellow at Washington Sea Grant, learned that juvenile salmon were even more plentiful at night than during the day, which could mean that the fish like to travel at a time when they cannot feed as well.

She also confirmed what the daytime snorkel team had discovered: Where glass blocks were imbedded in the sidewalk, the young salmon seemed more willing to swim under the piers. In fact, along the enhanced section of seawall, the sonar camera showed little difference in the numbers of salmon under the piers versus between the piers.

As Accola paddled along the old seawall north of the Seattle Aquarium — a section of wall that has not yet been rebuilt — she noticed that the fish still seemed reluctant to go under the piers. The sonar, which could spot the young salmon even in murky water, often showed them grouping up at the edge of a pier but rarely hanging out underneath.

“When they came to bad habitat, they seemed to school together,” she said, speculating that the fish would then move forward as a group. Meanwhile, along the new seawall, the young salmon seemed to swim with less hesitation through the corridor containing more-inviting habitat.

Despite the seawall improvements, Accola found that some salmon were still swimming out and around the piers in both the new and old sections of seawall before returning closer to shore.

The sonar images did not allow her to distinguish whether the fish were darting about — an indication of feeding behavior — but Accola found their traveling patterns to be more consistent, more natural, along the rebuilt seawall when compared to the old section of seawall to the north.

Salmon offered a better menu

The shelves and textured walls installed on the upper portion of the new seawall has encouraged vegetative growth, which in turn supports tiny invertebrates, such as copepods, eaten by juvenile salmon. 

Researchers found nine species of seaweed growing on the new intertidal bench at between-pier study sites during a survey in 2018, according to a report by Anchor QEA consultants. Two species of seaweed were found in darker under-pier locations. Sea lettuce was the most common species.

In the same study sites, researchers found 11 invertebrate species growing on the wall, including four species of barnacles, as well as sea squirts, mussels, limpets, snails and chiton.

Patches of bull kelp, which provide fish habitat, declined from 2010 to 2018 along the Seattle waterfront — including locations along both the new and old sections of seawall, according to surveys conducted by boat and from shore. Though unfortunate, those findings are fairly consistent with kelp surveys elsewhere in Puget Sound. Causes of widespread kelp decline are not fully understood, but they may be related to higher temperatures and other factors that impede spawning and growth of kelp.

Both before and after seawall construction, researchers found the density of free-swimming invertebrates, such as copepods, to be higher in open areas between the piers than under the piers. But after construction of the habitat bench, the variety of prey species available to feed the young salmon had increased all along the seawall and especially between the piers.

Krill and copepods under a microscope. Photo: Jeff Napp, NOAA/NMFS/AFSC (CC BY 2.0)

Krill and copepods under a microscope. Photo: Jeff Napp, NOAA/NMFS/AFSC (CC BY 2.0)

Notably, during the peak of the juvenile pink and chum salmon migration in April, two kinds of copepods favored by these salmon were found in much higher ratios along the seawall after construction versus before.

Researchers speculate that the increased number of juvenile salmon observed under the piers may, in part, be attributed to an improved food web. Perhaps increased light levels under the piers increased biological productivity or, alternatively, perhaps the extra light increased visibility enough for the fish to find their prey.

Further studies are planned to determine exactly what the young fish are eating and whether the increased invertebrate populations are meeting their nutritional needs.

“The new Elliott Bay seawall represents a novel experiment in re-engineering ecological functions into a profoundly altered shoreline…,” states the latest study by UW fisheries biologists. “Through continued monitoring of nearshore community response to eco-engineering, we can better understand both the opportunities for and limitations to success in urban shoreline restoration.”

Seawall inspires new thinking

The new Seattle seawall has been touted as proof that salmon habitat can be improved, even in the most degraded urban environment. For many, the project offers new prospects for improving salmon habitat on just about any shoreline in Puget Sound.

So far, the big push by cities and counties has been to get people to remove old bulkheads on shorelines where the wave energy is low and hard armoring is not really needed. So-called “soft shore” protections — including logs and rocks anchored to the beach in a natural configuration — can help reduce shoreline erosion without upsetting the ecosystem.

For publicly owned shorelines, local governments have been using state and federal grants along with their own resources to replace bulkheads with soft-shore structures throughout Puget Sound. For private property owners, so-called “Shore Friendly” programs offer encouragement and sometimes financial and technical support to remove old bulkheads.

Seahurst Park before removal of seawall (left) and after restoration of beach and wetland (right). Photos: EcologyWA (CC BY-NC 2.0)

Seahurst Park before removal of seawall (left) and after restoration of beach and wetland (right). Photos: Ecology (CC BY-NC 2.0)

The primary message of Shore Friendly is that most shoreline properties in Puget Sound do not need the protection of bulkheads. The program acknowledges that armoring may be necessary where homes were built right up to the shoreline years ago before modern regulations outlawed such construction.

Under the Shoreline Management Act, rules call on property owners to consider the least harmful alternatives, yet many local governments have been allowing the replacement of bulkheads without questioning the environmental consequences, according to Tim Trohimovich of Futurewise, an environmental group that has studied the role of government in protecting shorelines.

A key provision of the Washington Administrative Code states, “The replacement structure should be designed, located, sized, and constructed to assure no net loss of ecological functions.”

The no-net-loss standard allows for environmental mitigation elsewhere along the shore to make up for ecological damage caused by construction.

Trohimovich says many local governments could do a better job of exercising their authorities to ensure “no net loss” — either by strengthening local regulations or by enforcing current rules. The Seattle seawall project provides an example of finding ways to improve habitat even at difficult sites, he said.

Meanwhile, the governor’s Southern Resident Killer Whale Task Force has concluded that the no-net-loss standard may not be strong enough to bring about significant environmental restoration. The task force, charged with finding ways to keep Puget Sound’s orcas from going extinct, says the state needs to adopt a policy of “net ecological gain” to improve salmon habitat and save the whales.

Legislation that would have required a builder or developer to improve ecological functions as part of any construction project never made it out of committee this year. Instead, the Legislature approved a $256,000 study to consider the burdens and implications of a net-ecological-gain requirement.

The bottom line is that habitat improvements are coming too slowly to help salmon and killer whales, said Mindy Roberts, a member of the orca task force and director of Puget Sound programs at Washington Environmental Council.

“That Seattle seawall project was what we had in mind behind the idea of net ecological gain,” Roberts said. “The goal is not to restore a site to pristine condition but to leave it in better shape for future generations.”

Opponents of the bill that would have required net ecological gain argue that this very concept runs counter to individual property rights, as prescribed by state law and the U.S. Constitution. To require mitigation, they argue, government must show a clear connection, or nexus, to the environmental damage caused by the development. Builders should not have to pay for environmental damage caused by past practices that were legal at the time, they say.

In funding the new study, the Legislature called for an assessment of the legal issues surrounding net ecological gain, a description of how to measure such gain, and an analysis of how the new standard might apply to the Shoreline Management Act, the Growth Management Act and other environmental laws.

The Washington Academy of Sciences was assigned the task of determining what kinds of environmental protections may be missing from current state law and where the no-net-loss standard may be falling short. A consultant is being hired to review legal and policy matters.

A report to the Legislature is due by Dec. 1.

About the author: Christopher Dunagan is a senior writer at the Puget Sound Institute.

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