Notes
AR024628. Meeting discussion document prepared by Parametrix presenting a site-specific approach for stormwater mixing zone analysis for proposed stormwater outfalls around Seattle-Tacoma International Airport (STIA) in connection with the Third Runway EIS. Covers dilution analysis for the 3RWN outfall on Miller Creek using HSPF simulation results (47-year hourly discharge data), finding probability of stormwater discharge during 7Q10 and 30Q5 streamflow conditions is less than P<0.000. Proposes alternate critical condition definition based on higher-probability discharge combinations during rainfall events, yielding 14- to 24-fold dilution at 15 to 20 m downstream of 3RWN. Notes that complete mixing would exceed the 25% of width and percent-of-flow criteria (25% chronic, 2.5% acute) under WAC 173-201A-100, and invokes WAC 173-201A-100 paragraphs (3), (7)(a), (8)(a), (10)(b), (12), and (13) as basis for site-specific exceptions. Maximum allowable downstream distance for chronic mixing zone is approximately 91 m.