Admin Order 58-14-06 - Laserfiche WebLink

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1.7 STURMWATER QUALITY 1.7.1 The Purpose of Stormwater Quality The purpose of stormwater quality is to reduce runoff pollution and mitigate the volume, duration, time of concentration and rate of stormwater runoff from development by implementing low impact development practices and green infrastructure while capturing and treating 80% of the annual average rainfall. This runoff collects and transports pollutants to downstream receiving waters. Pollutants include: • Suspended solids (sediment) • Heavy metals (dissolved and particulate, such as lea_ d, copper, zinc, and cadmium) • Nutrients [such as nitrogen and phosphorus] • Bacteria and viruses • Organics (such as oil, grease, hydrocarbons, pesticides, and fertilizers) • Floatable debris • Increased thermal load (temperature) 1.7.2 Stormwater Quality Design Methodologies Stormwater quality facilities shall be designed, at a minimum, to treat the Water Quality Design Storm Stormwater quality facilities which are designed using an approach other than the Simplified Approach must be sized using the following design values: Flow -rate based design: Swales, of/water separators, and proprietary treatment systems shall be sized to treat a rate of flow draining through them. • A rainfall intensity of 0.13 inches per hour shall be used to design the off -line conveyance type facilities. • A rainfall intensity of 0.22 inches per hour shall be used to. design the on -line conveyance type facilities. Combination rate/volume based design: Other stormwater quality facilities, such as stormwater planters, rain gardens, sand filters, drywells and soakage trenches - shall be sized to treat a volume of runoff. + When using the SBUH (see Appendix M), facilities shall be designed to treat runoff generated by 1.4 inches of rainfall over 24 hours (with NRCS Type 1A rainfall distribution) - See Appendix L for more detailed information regarding the formulation of Eugene's stormwater quality standards and Water Quality Design Storm. Stormwater Management Manual 1 -17 Eugene 2014