For more than 80 years, polluted runoff from Seattle’s Aurora Bridge has been discharged untreated to Seattle’s Lake Washington Ship Canal, impacting migrating salmon and other aquatic life. That inspired environmentally innovative developer Mark Grey to join forces with Salmon-Safe to convene a multiple partnership to treat runoff through rain gardens, including at his Salmon-Safe certified Data 1 development project adjacent to the bridge.
Building on the success of the Aurora Bridge project, Salmon-Safe has expanded work to other bridges on the Lake Washington Ship Canal. In the Fall of 2017, following a presentation by The Nature Conservancy regarding the research conducted for the Aurora Bridge, a private anonymous donor offered to fund a brief study to determine if the other 5 bridges which impact the Ship Canal had the potential for green infrastructure to mitigate stormwater runoff from the bridge deck spans. Salmon-Safe retained KPFF Engineers to conduct the feasibility study and calculate the runoff. The runoff calculations are based on Seattle’s annual rainfall of 38 inches. In addition, KPFF identified a composite bridge deck material which could be used to replace the grating on four of the draw bridges and collect additional contaminated runoff that may have normally fallen through the grates. The product, Fiber Span, has been used in other parts of the country and has a good track record of life safety as well as noise reduction.
Our Green Bridges Pilot Study included the Ballard Bridge, Fremont Bridge, I-5 Bridge, University Bridge, and Montlake Bridge. With green infrastructure in mind, the scope of work was to determine the functionality of the existing runoff collection system, to quantify the extents of the collection basins, to develop new low impact development runoff collection and treatment strategies, and to locate adequate treatment sites. City of Seattle utility maps and record drawings were the key sources used to gather information about each bridge and provided the means to create feasible runoff mitigation solutions. Once the initial background information was obtained for each bridge, an approximate ratio of the bioretention area required to treat subsequent basin areas was used to size the treatment facilities. In all, our team determined that with this commitment to bioretention we could collect and treat 98 million gallons of runoff per year and reduce the detrimental impacts that bridge runoff has on this important salmon migration corridor.