EXH0725026496

In accordance with the Permit Writers Manual, WERs must be measured three times during critical conditions and once during non-critical conditions. Conditions are considered critical when the physical, chemical, and biological characteristics of a receiving water interactwiththe effluent to produce the greatest potentialadverse impact. The process of determining critical conditionsfor the third runway treated stormwater discharge may be somewhat unique because both the receivingwater (Miller Creek) and the discharge are significantly affected by the same rainfall events. This memo summarizes several points to consider in deciding which dilution conditions (i.e.-flows, seasons) should be considered critical. For streams, the 7Q10 and 30Q5 flows are often considered critical. Last year, we started evaluating dilution scenarios for the discharge of treated stormwater into Miller Creek. Realizing that both the Miller Creek flow and the stormwater discharge rates might be significantly affected by the same rainfall events, we developed a statistical approach for evaluating dilution. Based on HSPF hindcasting usingdata for the trailing year time interval, we prepared hourly estimates of the Miller Creek and third runway stormwater discharge rates. We determined that there was essentiallyzero orobabilitvof any stormwater discharge durinq stream stages that would ordinarily be considered cn_al (i.e.-_'TETTOT30QSJ.We predicted there would never be third runway stormwater _uring stream flows below the 10= percentile flowrate, which is substantially higher than the 7Q10. In fact, the only scenarios with probability greater than 0.001 occurred when stream flow and effluent discharge were both at rates higher than their respective 80thpercentile rates. That was consistentwith our…

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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 [...]