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EXH AR022933: Review Comments – Low Flow Impact Analysis & Offset Facility Proposal, Jul 2001

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Sea-Tac Airport
Third Runway
PCHB
Box 09

This August 2001 King County review evaluates the Port of Seattle's Low Flow Impact Analysis and proposed Flow Impact Offset Facility, which aims to mitigate reductions in stream flows in local creeks—including Des Moines, Miller, and Walker creeks—caused by future airport development. The review finds that while the proposed water storage and augmentation measures substantially offset low-flow impacts during the critical late-summer period, there are significant gaps in documentation, biological assessment, and design details that need to be addressed. Recommendations include professional engineering certification, enhanced biological monitoring, improved calibration documentation, and refinements to the reserve storage facility designs.

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Notes

King County Department of Natural Resources review comments by M. Green on the July 2001 Low Flow Impact Analysis Flow Impact Offset Facility Proposal submitted by Parametrix Inc. for Port Of Seattle, supplementing the Comprehensive Stormwater Management Plan (SMP). Covers HSPF hydrologic modeling calibration accuracy, biological conclusions for salmonids, Des Moines Creek low flow statistics at S 200th Street Tyee Golf Course weir, Miller Creek and Walker Creek basin non-hydrologic changes (septic system removal, water withdrawal relinquishment), reserve storage vault design, flow augmentation period July 24–October 24 (91 days), 12.2 acre-feet reserve storage volume, and 2-year 7-day low flow targets (0.35 cfs existing, 0.25 cfs future). Exhibit AR022933 in the PCHB Third Runway EIS administrative record.

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