Notes
Memorandum dated March 9, 2001 from Dave Garland, Washington Department of Ecology NWRO Water Quality Nonpoint Unit, to Kevin Fitzpatrick and Ann Kenny, reviewing Earth Tech’s December 2000 ‘Sea-Tac Airport Master Plan Update Low Streamflow Analysis’ (Exhibit 2099, AR 033695–033697). The memo evaluates HSPF streamflow modeling for Miller Creek, Walker Creek, and Des Moines Creek in relation to the Port Of Seattle’s 401 Permit application for the Sea-Tac Airport Third Runway. Key findings include post-project 7-day low flows slightly reduced in all three streams versus 1994 pre-project baseline; Miller Creek average August flow decreased 0.17 cfs; fill embankment drainage predicted to increase Miller Creek baseflow by 0.108 cfs (August) and 0.065 cfs (September); 0.10 cfs supplemental reserved stormwater release needed in Miller Creek and 0.08 cfs in Des Moines Creek to maintain 1994 conditions. Reviews secondary impervious recharge, non-hydrologic residential conversion impacts (25,000 gpd reduction), Pacific Groundwater Group slice model, Hart Crowser infiltration study, and Parametrix HSPF modeling. Recommendations include real-time managed stormwater releases, stream gages, and more detailed PGG slice model integration over 5,400-foot embankment.