EXH1114041907

TO: Ms. Barbara Hinkle, Port of Seattle FROM: Michael Bailey P.E.,and Michael Kenrick, P.E. Hart Crowser, Inc. RE: Summary of Hydrologic Issues Borrow Areas 1, 3, and 4 Third Runway Project Seattle-Tacoma International Airport J-4978-03 In response to your request this memorandum discusses potential hydrologic effects related to proposed development of Borrow Areas I, 3, and 4. Borrow Areas I, 3, and 4 consist of about 200 acres of forested land, immediately south of the airport. Area I includes 0.34 acre of wetlands north of South 208th Street. Area 3 includes about 2.3 acres of wetlands. There are no wetlands in Area 4. Locally, shallow perched water zones are likely to be encountered in excavating surficial soils, but the borrow excavations would not extend into the underlying Shallow Regional Aquifer. Summary You asked Hart Crowser to discuss 1) how will borrow area operations impact base flows to Des Moines Creek and riparian wetlands?, and 2) what will the long-term effect be on local hydrology after reclamation of the borrow areas? In general, excavation of Borrow Areas 1, 3, and 4 will expose relatively more permeable, sandy soils which underlie the existing silty, glacial till surface soils. During operations, on-site runoff would be controlled in accordance with an approved storm water pollution prevention plan. Infiltration, groundwater recharge, and baseflow to Des Moines Creek would increase. After reclamation, on-site runoff would be reduced, and the increased infiltration, groundwater recharge, and baseflow would remain as long-term benefits. Seattle • Anchorage • Portland •…

Open full document

Upcoming Events

PSCAA Advisory Council Meeting Agenda
Add to CalendarMWednesday Mar 11, 9:00am - 11:35am
1904 3rd Avenue, Suite 105, Seattle, WA 98101
Call to order – Roll Call – Quorum Established – Land Acknowledgement We would like to acknowledge that our Agency operates in four present-day counties, which are the traditional lands of the Coast Salish people, past and present. This acknowledgement does not take the place of authentic relationships with indigenous communities in our jurisdiction and [...]