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EXH 273: Low Streamflow Analysis — STIA Master Plan Update Improvements

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Sea-Tac Airport
Third Runway
PCHB
Box 08

This technical report analyzes low streamflow conditions in three creeks near Seattle-Tacoma International Airport (Miller, Walker, and Des Moines Creeks) to assess the potential impacts of airport construction projects on summer water flows. Using a 47-year record of streamflow data and computer modeling tools, analysts determined target streamflow levels for each creek and evaluated how proposed runway and infrastructure improvements might affect seasonal low-flow periods critical to salmon habitat. The study found existing summer low streamflow targets of 0.33 cfs for Des Moines Creek, 0.77 cfs for Walker Creek, and 0.73 cfs for Miller Creek, which were used to guide mitigation planning.

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Exhibit 273 (AR 019513–019515), December 2001, presents the low streamflow analysis for the Seattle-Tacoma International Airport (STIA) Master Plan Update third runway EIS. Covers determination of summer low-streamflow periods and target streamflows for Miller Creek, Walker Creek, and Des Moines Creek using calibrated HSPF (Hydrologic Simulation Program – FORTRAN) models under 1994 land-use conditions. Establishes 7-day, 2-year-frequency low-flow targets: 0.33 cfs (Des Moines Creek), 0.77 cfs (Walker Creek), 0.73 cfs (Miller Creek). Includes HSPF model calibration review for water-years 1991–1996 with observed vs. simulated flow tables at gage stations near creek mouths and upstream locations including Miller Creek detention facility. Also references Hydrus and Slice embankment modeling, septic tank discharge removal, and water withdrawal cessation impacts. Prepared under project number 556-2912-001(28B); references Parametrix 2000a/2001a SMP and Pacific Groundwater Group 2001 embankment report.

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