Admin Order 58-14-01-F - Laserfiche WebLink

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1.9 FLOW CONTROL Flow control is intended to protect downstream properties, infrastructure, and natural resources from the increases in stormwater runoff peak flow rates and volumes resulting from development. The City's policy is to ensure that runoff leaving the post- development site: • Does not exceed the capacity of the receiving conveyance facility • Does not increase the potential for stream bank and channel erosion • Does not create or increase any flooding problems Flow controls are required in the Headwaters Area of Eugene (see the Headwaters Streams Map in Appendix J). For construction of new or replaced impervious surface in this area, on -site infiltration or on -site detention is required to control stormwater volumes and flow rates. Regardless of the method used, flow control shall be sufficient to maintain peak flow rates at or below their pre - development levels for storms larger than the Water Quality Design Storm and smaller than the Flood Control Design Storm. Flow control is also required for development in areas where the downstream capacity of an open or closed stormwater system is not sufficient to convey the post development flows. To meet the flow control requirements, surface infiltration and filtration facilities are required to the maximum extent feasible. Impervious area reduction techniques may also be used to reduce runoff. Structural systems can be used as a last option to provide storage capacity. Flow control facilities include detention and retention: DETENTION FACILITIES Detention facilities temporarily store stormwater runoff in a pond, tank, vault, or pipe. The water is slowly released from the facility, typically over a number of hours. RETENTION FACILITIES Retention facilities also store stormwater runoff. Rather than storing and releasing the entire runoff volume, however, the facility permanently retains a portion of the water on- site, where it infiltrates and recharges the groundwater aquifer, and in the case of surface retention facilities, evaporates or is absorbed and used by surrounding vegetation. Retention facilities reduce the total volume of water released downstream. Stormwater Management Manual 1 -19 Eugene 2014