EXH0423022156

alternatives, could affect the hydrology of the quality and have no history of detectable levels of Airportareaaswell as the downstreamsystems, pollution. Alternatives 2, 3, and 4 (the "With Project" alternatives) would include earthwork and the Although pollutant loading would increase addition of impervious land surface area. These somewhat because of greater amounts of factors would decrease the amount of rainfall stormwater runoff associated with the "With infiltrating the soil and increase stormwater Project" alternatives, compliance with mitigation runoff flow rates and volumes. Unmitigated, requirements would be expected to prevent these changes in hydrology could cause significant pollution or degradation of surface downstream flooding, channel erosion, and and groundwater resources. degraded in-stream habitat. Detailed hydrologic modeling of the Airport and its surrounding (1) METHODOLOGY watersheds was performed to quantify the magnitude of downstream impacts and to The objectives of this analysis were to determine appropriate mitigation strategies, characterize existing hydrologic conditions in downstream systems, to evaluate hydrologic Preliminary esfimams indicated that 61 acre-feet impacts, and to determine appropriate mitigntion. of new on-sit_ detention storage volume would HSP-F-t' Version 10.0, a continuous simulation be required for proposed developed areas hydrologic model, was used to model the draining to Miller Creel and 31 acre-feet of hydrology of the Airport, Miller Creel and Des storage would be required for areas C_atiningto Moines Creek.2' Data included in this document Des Moines Creek. These detenfioi_volumes weregeneratedaspartof the modelinganalysis would attenuatepeak runoffratesfrom the containedinAppendixG. Airporttoprovideprotectionfrom downstream floodingfor stormshavingup to a 100-year The HSP-F modelforMillerCreekwas basedon return period.…

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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 [...]