Notes
Memorandum dated March 9, 2001, from Dave Garland, Ecology NWRO Water Quality Nonpoint Unit, to Kevin Fitzpatrick and Ann Kenny, reviewing the Earth Tech December 2000 report ‘Sea-Tac Airport Master Plan Update Low Streamflow Analysis.’ The memo evaluates HSPF streamflow modeling and supplemental analyses for Miller Creek, Walker Creek, and Des Moines Creek in connection with the Port Of Seattle’s 401 Permit application for Sea-Tac Airport Third Runway construction. Key findings include post-project 7-day low flows slightly reduced in all three streams compared to 1994 pre-project baseline; estimated 0.10 cfs deficit in Miller Creek and 0.08 cfs deficit in Des Moines Creek requiring supplemental stormwater releases; and analysis of secondary impervious recharge, non-hydrologic residential conversion impacts, and delayed embankment fill drainage. Recommendations include construction of stormwater storage facilities, real-time managed releases, and more detailed integration of the Pacific Groundwater Group slice model over the 5,400-foot embankment along Miller Creek.