Notes
Northwest Hydraulic Consultants (NHC), retained by Airport Communities Coalition (ACC), submits technical review comments to U.S. Army Corps of Engineers (Colonel Ralph H. Graves, Ms. Muffy Walker, Ms. Gail Terzi), Corps Ref. No. 1996-4-02325, regarding the December 2001 “Low Streamflow Analysis and Summer Low Flow Impact Offset Facility Proposal” prepared by Parametrix, Inc., for Port Of Seattle. Letter identifies specific errors in the Miller Creek HSPF model including double-counting of groundwater discharges from third runway embankment areas due to incorrect PERLND 80 precipitation multiplier; critiques Pacific Groundwater Group (PGG) embankment seepage methodology for examining only post-construction conditions without baseline current-conditions model; notes failure to incorporate Hart Crowser November 2, 2001 geotechnical report findings on wetland soil removal and engineered subgrade drain layers; and identifies overestimation by two to three times of airport embankment seepage reaching Miller Creek at SR509 compliance point. Also flags unresolved issues for Walker Creek (IWS Industrial Wastewater System expansion reducing headwater baseflows, post-1991 recorded data showing 30%+ low-flow reduction) and Des Moines Creek (100% discrepancy between recorded and simulated low flows, IWS lagoon seepage, stream losing reach). Exhibit AR019091–AR019093.