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EXH AR024556: Mixing Zone Analysis — 3rd Runway North Discharge Miller Creek (Basin SDW-1)

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Sea-Tac Airport
Third Runway
PCHB
Box 10

This technical document describes a mixing zone analysis for stormwater discharge from the 3rd Runway North at Sea-Tac Airport into Miller Creek, designated as Basin SDW-1. The analysis uses design storm modeling with various rainfall scenarios (2-year/2-hour, 2-year/72-hour, and 3-year/72-hour events) to calculate effluent flow rates for both acute and chronic water quality criteria. Stormwater from the basin's approximately 128 acres of pervious and impervious area is routed through a detention pond before discharge, with outflow rates determined by a storage-discharge rating curve.

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Notes

Mixing zone analysis for the 3rd Runway North discharge to Miller Creek, Basin SDW-1, using Waterworks design storm modeling. Documents hyetograph and design precipitation methodology referencing DSS Tech Note 3 (Figures A2, A4, Appendix C Tables C3/C3a) and Washington State Characteristics of Extreme Precipitation Events (Figure 8f). Design flows calculated for acute criterion (maximum of average flow from 2yr/2hr event or peak flow from 2yr/72hr event) and chronic criterion (average flow from 3yr/72hr event). Basin SDW-1 parameters: pervious area 90.9 acres, impervious area 37.0 acres, CN pervious = 86, Tc = 7.6 min, routed through detention pond. Includes FTABLE 47 storage-discharge rating curve for SDW-1 Miller Creek Level 2, with depth, area, volume (up to 14.28 acre-ft), and outflow (up to 42.79 cfs) values. Volume conversion noted as V × 2,323,200 / 53.33. AR numbers 024556–024558.

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