Notes
Northwest Hydraulic Consultants (nhc), retained by the Airport Communities Coalition (ACC), submits follow-up technical comments dated May 3, 2000, to U.S. Army Corps of Engineers (Jonathan Freedman) and Washington State Department of Ecology (Tom R. Luster) regarding the Stormwater Management Plan (SMP) for the Sea-Tac Airport third runway Master Plan Update Improvements. Letter identifies major discrepancies in HSPF hydrologic model input files for Miller Creek and Walker Creek basins, including inconsistent soil type assumptions (till vs. outwash soils) between pre-development flow targets and detention facility performance models, and four separate sets of runoff parameters for airport fill areas. NHC concludes the Miller Creek streamflow analysis is seriously flawed, flow targets for Level 1 and Level 2 (1994 existing development) are unsupported, and a complete overhaul of SMP hydrologic modeling is required including calibration to recorded streamflow data. Also raises concerns about King County Surface Water Design Manual (KCSWDM) 1990 vs. 1998 version applicability under the 09/04/97 Interlocal Agreement (ILA) between Port of Seattle and City of SeaTac, lack of local regulatory oversight, Master Drainage Plan (MDP) procedural compliance, and direct discharge impacts to Puget Sound. References Regional Detention Facility (RDF), Parametrix, Pacific Groundwater Group, and King County independent review.