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), commenting on the Port of Seattle Third Runway Embankment 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.’ Identifies four key deficiencies: (1) failure to account for initial years lag time before water passes through embankment to drainage layer — estimated 1 year for 20 ft cross-section, 4 years for 110 ft, 6 years for 150 ft high embankment; (2) long-term flow overestimation through embankment; (3) unvalidated hydraulic conductivity assumptions; (4) errors introduced by one-dimensional Hydrus model conversion to SLICE spreadsheet. Concludes storage vault volumes may be substantially under-designed and insufficient water will be available to mitigate low stream flow impacts during initial years of operation. Administrative Record document AR022838, Exhibit 453.