Notes
Technical memorandum by Norm Crawford of Hydrocomp, Inc. (received 2001-10-30) providing recommendations for HSPF modeling of Miller Creek, Walker Creek, and Des Moines Creek at Sea-Tac Airport third runway construction. Document AR023792–AR023794 covers linkages between HSPF, Hydrus, and Slice models for pervious land segments (PERLAND), impervious land segments (IMPLAND), outwash grass, Vashon till seepage, AGWI percolation, DEEPFR parameter, SURO surface runoff, and active groundwater flowpaths. Recommends 8-step modeling workflow including HSPF IMPLAND runoff calculation, PERLAND infiltration, Hydrus vertical flow attenuation, Slice lateral flow to stream toe, till seepage reduction by factor 0.33/multiply by 0.67, and direct stream return without additional routing (INFLOW IVOL). Also identifies area discrepancies (128 acres Hydrus/Slice vs. 124.27 acres HSPF; Miller 111.67/116.22 acres, Walker 16.33/8.05 acres) and missing SDWIA/SDWIB baseflows in Reach 47.