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03/07/95 09:42 $503 224 7820 IYVUUYYAxL -1 LYllC Ctl �� Infiltration Facilities Guidance is available for seven main types of infiltration facilities: • Infiltration trenches — shallow (two to ten feet deep) trenches in relatively permeable soils that are backfilled with a sand filter, coarse stone, and lined with filter fabric. The trench surface can be covered with grating and/or consist of stone, gabion, sand,. or a grassed covered area with a surface inlet. Depending on the design, trenches allow for the partial or total infiltration of stormwater runoff into the underlying soil. • Infiltration basins — depressions created by excavation, berms, or small dams to provide for temporary ponding of surface runoff while it percolates into the soil. • French drains - designs suited for small drainage areas no greater than 5 acres in size. Drainage is pre - treated through overland flow over grassed areas with contributing drainage area slopes being less than five percent. Typically broader in width than depth. • Roof downspout systems — variations of infiltration trenches that are designed specifically to accept and infiltrate roof drainage only. Because roof runoff is considered clean, no pretreatment is required. • Porous pavement - appears the same as standard asphalt or concrete pavement but the material is designed to be permeable rather than impervious. Porous pavement includes an infiltration zone below the pavement surface. Most appropriate for low -use portions of parking lots and driveways with expected low sediment and pollutant loads. • Concrete grid pavement system — strong structural materials having regularly interspersed void areas that are filled with pervious materials such as sandy loam and grassed turf. Underlain by soils capable of allowing infiltration. • Dry wells, or sumps - vertical, perforated cylinders below the ground surface designed to infiltrate water into the soil. They can vary in depth from four feet to thirty feet, which is depth commonly used in some urban and suburban areas in the Portland metropolitan area. These facilities are not designed to provide treatment; of stormwater- -their main purpose is to remove water from the site as quickly as possible. Recommendations made below for infiltration facilities are based on the most recent information available and lcnowledge of techniques currently used by agencies in the Northwest. Unlike stormwater ponds and marshes, for which reports and papers have been published compiling and summarizing results of studies ongoing around the country, there is limited guidance available on infiltration facilities. S \945042na\task02001onsitc controls\techmem.ck 9 •