Notes
GeoSyntec Consultants letter dated 7 February 2002 to U.S. Army Corps of Engineers Regulatory Branch (Muffy Walker/Gail Terzi), submitted on behalf of Airport Communities Coalition (ACC), providing technical comments on the Port of Seattle Sea-Tac Third Runway embankment fill low streamflow analysis. Reviews the November 27, 2001 Pacific Groundwater Group (PGG) report ‘Port of Seattle – Sea-Tac Third Runway – Embankment Fill Modeling in Support of Low Streamflow Analysis.’ Key findings include: failure to account for initial years’ water lag (1–6 years depending on embankment height of 20 ft, 110 ft, or 150 ft); unreliable long-term hydraulic conductivity estimates; unvalidated model assumptions; errors introduced by using 1D Hydrus converted to SLICE spreadsheet; and potential under-design of storage vaults. Uses Hydrus 2D independent analysis. Embankment sections totaling over 8,000 ft analyzed including 1,600 ft at 150 ft height, 3,700 ft at 110 ft, and 3,200 ft at 20 ft (per PGG Table 5-7). AR 024316–024318.