Background on Seattle's Shoreline and Seawall

Coastal shorelines are transformed as coastal populations grow and demands for commercial, transportation, residential, and tourist infrastructure increase. Shoreline alterations such as seawalls are necessary in urban and industrialized areas to protect shorelines from erosion and wave energy, support piers and docks, and prevent flooding. As human populations increase, transformation of shorelines will accelerate. Also, increased demand for shoreline protection is expected if predictions associated with climate change such as increase in storms and sea level rise are realized.

Sloping beaches originally characterized the shoreline of Seattle, Washington.  In 1911-1934 the City built a concrete vertical seawall at the outer edges of the intertidal beach, transforming the shoreline into a deep-water port and filling the intertidal areas. There are now over three kilometers of seawall along the Seattle central waterfront with few remaining sloping beaches.  The transformation of natural beaches into vertical hard surfaces consequently affected aquatic organisms—compared to natural shores, vertical seawalls support fewer species, probably because they lack habitat complexity and provide less space and refuge opportunities. Shortly after a 2001 earthquake, a section of surface street adjacent Seattle's shoreline settled, raising concerns about the condition of the seawall.  Further investigations indicated that it was in bad condition, and the seawall continues to deteriorate despite regular maintenance.

The need to replace the seawall prompted the City to form a team to focus on habitat enhancements in the marine nearshore along the seawall.  Seawall replacement presented an opportunity to improve the habitat conditions of the structure.  The importance and advantages of including ecological criteria in the design and management of such structures has recently been recognized, and one way to make relatively featureless seawalls more environmentally friendly is by adding habitat complexity—complex substrata are known to enhance the diversity and density of intertidal organisms. The City provided resources to design, build, and deploy habitat enhancement panels along the seawall. With combined City and Sea Grant support, scientific testing of these panels was conducted.


For more information on the history of the seawall and plans for replacement and repairs, visit:
http://www.cityofseattle.net/transportation/seawall.htm




Despite the highly altered shoreline, Elliott Bay and downtown Seattle still serve as a migratory corridor and rearing habitat for juvenile salmon, including Chinook salmon that have been listed as threatened.
Studies have show that juvenile salmon, including Chinook, stay close to Seattle’s shore in shallow areas during this early life stage. In addition, studies of the nearby Olympic Sculpture Park show that their diets are closely linked to organisms in the intertidal zone.









Juvenile Chum Salmon
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