Resolution 3528 - Laserfiche WebLink

SECTION 10: DESIGN FOR FLOOD EVENTS Findings: • Surface water quality in Powder Mill Creek is addressed by standards that are different from those used to manage surface water quantity. • The surface water quality treatment system (sand peat filters) is designed to bring water quality to acceptable levels for up to 8 cfs, supported by a 11.9 acre-foot sedimentation pond which can receive up to 90 cfs. This design will generally accommodate the first flush of water from a storm. This first portion of the runoff should contain the majority of pollutants accumulated on the ground. • The design of the surface water treatment system is not based on the 25-year event. Several times each year the inlet to the sand peat filter system portion of the surface water treatment system will be bypassed by excess runoff being diverted to the main branch, Powder Mill Gulch detention facility. • Design and operational controls for portions of the site involved with jet fuel loaded are required on page 29 of the Decision Document, Area 3 (1)i. through iv. • Design and operational controls for storage, handling and disposal of hazardous materials and hazardous wastes are regulated by federal, state and local laws identified on pages 4 and 5 of the Decision Document. These requirements will substantially reduce the possibility that any hazardous materials or hazardous wastes will enter the storm water system. • The Powder Mill Gulch main branch detention facility is designed to limit peak discharge to 49 cfs or less for all storms up to and including a 25 year storm event. • A peak flow of 49 cfs is consistent with the Powder Mill Gulch Drainage Basin Plan Update adopted by City Council in 1988. • The spillway of the detention facility is designed to safely discharge excess runoff from the detention facility resulting from a 100,000 year event. • Public Works Department design standards for detention require a 25-year design storm for all basins. Other affected sub-basins of Powder Mill Creek and those areas discharging to Japanese Creek or Edgewater Creek must provide detention for the 25- year, 24-hour storm event, per Condition V.A.4.a. (page 14) of the Decision Document. • A surface water report submitted to the record by Appellant Freudenberg establishes that the pre-development peak discharge rate from the Boeing site was 72 cubic feet per second (cfs). (Landau Associates Study dated May 10, 194, pg 4, para. 2) 17