Notes

This document presents a preliminary dilution analysis for proposed stormwater outfalls around Sea-Tac Airport (STIA), noting that standard 7Q10 and 30Q5 flow conditions are inappropriate critical conditions because stormwater discharge is extremely unlikely during those low-flow periods. A site-specific approach is proposed using higher-probability discharge combinations derived from 47 years of HSPF simulation data, estimating 14- to 24-fold dilution within 15–20 meters downstream for the 3RWN outfall. The document also references WAC 173-201A-100 provisions for exceptions to standard numerical mixing zone criteria.

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As the source for federal transportation grants, the Puget Sound Regional Council was charged with developing a system to meet the growing need for commercial aviation. When the search for a second airport failed, they authorized the Port of Seattle to build the Third Runway, with a mandate to develop a noise abatement and mitigation program. In their effort to stop the Third Runway, the ACC argued over every detail of the Port's efforts--including property buyouts and sound insulation. The dispute was meant to be settled by a three member Expert Arbitration Panel. This is their final report. It finds 2-1, that the Port's program was insufficient in several respects. Despite that, funding for the Third Runway was approved by the PSRC, and the 'Port Package' program, proceeded largely unchanged. Expert Arbitration Panel's final decision finding that the Port of Seattle had not shown sufficient reduction in real on-the-ground noise impacts to satisfy noise reduction conditions required for approval of a third runway at Sea-Tac International Airport. The majority decision concluded the Port's noise abatement programs were insufficient despite being impressive in scope.

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