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2.1 INTRODUCTION Stormwater facilities presented in this chapter satisfy the standards established for stormwater quality, flow control, flood control, and in some cases a combination of the three. Two sizing methodologies are included in this chapter: the simplified and presumptive approaches. Each sizing approach has limitations on application, particularly as related to the soil types on -site. Soil formations are classified by NRCS soil types A, B, C, or D. For the purposes of this manual, NRCS soild types A & B are presumed to have infiltration rates of 2" per hour and greater. NRCS soils tptes C & D are assumed to have infiltration rates less than 2" per hour. 2.2 STORMWATER FACILITIES This section identifies techniques to reduce impervious surface treatment areas and the three classifications of stormwater quality facilities. 2.2.1 Impervious Area Reduction Techniques Contained Planters, Eco roofs, Permeable Pavements, and Tree Credits are impervious area reduction techniques. These techniques reduce the effective area of imperious surface and reduce the size of the resulting stormwater quality facility. Impervious area reduction techniques reduce runoff pollution and mitigate the volume, duration, time of concentration and rate of runoff. Eco roofs, permeable pavements, and trees intercept rainfall directly and are not allowed to receive stormwater runoff from other impervious surface areas. Contained Planters may either intercept the rainfall or receive stormwater runoff from impervious surfaces areas if sized using the Presumptive Approach. All impervious area reduction techniques may be used for private facilities. Impervious area reduction shall not be used for public facilities. 2.2.2 Infiltration Treatment Facilities Infiltration treatment facilities require soils that drain well and infiltrate 2 inches per hour or greater. Infiltration treatment facilities require an overflow to an approved point of discharge unless they are sized to fully infiltrate the Flood Control Design Storm event. When sized for flood control, infiltration treatment facilities must infiltrate the Flood Control Design Strom Event within 30 hours. 2.2.3 Filtration Treatment Facilities Filtration treatment facilities are appropriate for sites with soils that infiltrate less than 2 inches per hour, have bedrock less than S feet below the surface, groundwater elevations less than 6 feet or slopes greater than 10 %. They include a surface overflow and an underdrain in the gravel layer where treated flow is routed to an approved discharge point. Lined filtration treatment facilities may be approved on sites with steep slopes, high groundwater, or contamination and when located next to structures or property lines to protect foundations, basements, and adjacent properties. Otherwise the use of liners is discouraged to maximize what infiltration is availible from native soils. 2.2.4 Mechanical Treatment Devices Manufactured treatment devices must be selected from the list of approved proprietary treatment technologies and sized using the Presumptive Approach. Proprietary treatment Stormwater Management Manual 2 -3 Eugene 2014