By Christopher Dunagan, Encyclopedia of Puget Sound

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.

Mike Caputo/UW

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.

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.

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.

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.”