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.03/07/95 09 :41 p5U3 2Z4 1 ZU 11UUL11AHL —LLYLt •ravvv- v._ <br /> • <br /> Stormwater Filters <br /> There are generally four types of stormwater filters for which guidance information is <br /> a'aiieble: - <br /> • Vegetated swales - vegetated channels with a slope similar to that of standard <br /> storm drain channels (less than 6 percent), but wider and shallower to <br /> maximize flow residence time and promote pollutant removal. These facilities <br /> are often referred to as "grassed waterways" by the agricultural community. <br /> • Vegetated filter strips — vegetated sloped strips in which flow is distributed <br /> broadly along the length of the vegetated area as overland sheet flow. <br /> • Sand filters — consist of an inlet structure, sedimentation chamber, bed, <br /> underdrain piping, and liner. These structures filter runoff through the sand <br /> layer into an underdrain system that conveys treated runoff to a detention <br /> facility or to the point of ultimate discharge. There are a variety of sand filter <br /> types, including sand filter inlets and sand filtration basins. <br /> • Compost filters - structures designed to collect runoff and filter it through <br /> organic media prior to discharge to the storm drain system via perforated pipes. <br /> The most widely used guidance for design, installation and maintenance of vegetated <br /> swales and filter strips is Biofiltration Swale Performance, Recommendations and <br /> Design Considerations (October 1992) by Dr. Richard Horner of the University of <br /> Washington in Seattle. This guidance is recommended for use in Eugene because it is <br /> based on a field test of a prototype swale designed and constructed by Dr. Horner in <br /> 1988. Performance studies have been ongoing since 1988, and Dr. Horner published <br /> updated data and revised specifications based on his findings. The City of Portland <br /> and other local agencies like the document because it is straightforward and provides <br /> example calculations. It also provides background information about vegetated <br /> biofilters which is essential to develop a good understanding of how the facilities work <br /> and how /where they should be applied. Dr. Horner uses the 6- month, 24 -hour storm <br /> event as the criteria for sizing swales and filters; the City may need to adapt this to <br /> Eugene based on whatever design storm recommendation/standard is selected. <br /> Currently, the best information available on sand filter technology to improve <br /> stormwater runoff quality comes from the City of Austin, Texas. According to Tom <br /> Schueler in Techniques (Summer 1994), Austin first pioneered the use of sand filters in <br /> the early 1980's because dry conditions ruled out the use of ponds/marshes and <br /> S1945042na \task02001onsite controls\techmem.ck 7 <br />