February 15, 2001August 2, 2026

Comments on Stormwater, Hydrology, and Hydraulics Aspects of Proposed 3rd Runway and Related Development Actions at Seattle-Tacoma International Airport

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Northwest Hydraulic Consultants submitted technical review comments to the U.S. Army Corps of Engineers and Washington State Department of Ecology on behalf of the Airport Communities Coalition regarding the December 2000 Comprehensive Stormwater Management Plan and Natural Resource Mitigation Plan for the proposed third runway at Sea-Tac Airport. The letter raises concerns about dam safety at proposed detention facilities and vaults, violations of King County stormwater design manual requirements for vault depth, inappropriate methodologies for assessing open water durations, and deficiencies in erosion and sediment control plans. The review identifies specific structural and safety risks with proposed vaults SDS7 and G1, and flags that construction had already begun without independent regulatory review.

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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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