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4. The amount of water quality treatment volume credit given will be based upon the three year average of the volume irrigated, determined by the average of three years of monitoring records. c. Biofiltration and Filtration BMPs. Biofiltration and filtration BMPs must be designed to provide: 1. Adequate pretreatment measures to remove sediment before runoff enters the primary bilfiltration area; 2. Drawdown within 48 -hours or 72 -hours from the end of the storm event for surface or sub -surface features, respectively; 3. A minimum of 12 -inches of organic material or sand above the rock trench or draintile system; and 4. Drain tile system must be designed above the Seasonal High Water Table. d. Stormwater Ponds. Stormwater ponds must be designed to provide: 1. Water quality features consistent with NURP criteria and city standard plate; 2. Permanent wet pool with dead storage at least equal to the runoff from a 2.5 -inch rainfall over the area tributary to the pond; and 3. An outlet structure capable of preventing migration of floating debris and oils for at least the one-year storm. 4. An outlet structure to control the two-year, ten-year and 100 -year frequency events to existing peak runoff sites; and 5. An identified overflow spill way sufficiently stabilized to convey flows greater than the 100 -year critical storm event. e. Outfalls. An outfall structure discharging to a wetland, public water or public water wetland must incorporate a stilling -basin, surge -basin, energy dissipater, placement of ungrouted natural rock riprap or other feature to minimize disturbance and erosion of natural shoreline and bed resulting from stormwater discharges. f. Freeboard Requirements. All new residential, commercial, industrial and other habitable or non -habitable structures, and all stormwater basins, must be constructed so that the lowest floor and lowest entry elevations of structures comply with the following g. Table 6: draft 06-15 ECS Ordinance.docx 22/29