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03/07/95 09 :42 $503 224 7820 WOODWARD -CLYDE (6011/022 <br /> • <br /> Design of infiltration facilities (trenches, basins, french drains, roof downspout <br /> systems) is typically accomplished using the Darcy Law approach. Basic design <br /> guidance is presented in the Stormwater Management Manual for the Phu et Sound <br /> Basin (February 1992) - Chapter III -3. The design information uses the 6- month, 24- <br /> hour storm event as the criteria for sizing infiltration facilities; the City may need to <br /> adapt this to Eugene based on whatever design storm recommendation/standard is <br /> selected. It is highly recommended that the City require permit applicants to <br /> demonstrate that their site can infiltrate readily; facilities should drawdown completely <br /> within 24 hours or less, depending on the design parameters. Standard single -ring or <br /> double -ring infiltration tests can be performed as specified in ASTM standards, or <br /> another method can be specified by the City. This information can be provided in the <br /> interim guidance notebook. <br /> Very few studies have been done on removal efficiencies of infiltration facilities <br /> (Schueler, 1993). However, some pollutant removal effectiveness data, along with cost <br /> information, is contained in EPA's Guidance for Specifying Management Measures for <br /> Sources of Nonpoint Pollution in the Coastal Zone (1993) - Chapter 4 -II, Table 4 - 7. <br /> Portland's new document, Stormwater Quality Facilities Design Manual (March <br /> 1995)- Chapter 5 incorporates recent study information about poor performance of <br /> infiltration trenches due to lack of maintenance. <br /> The use of drywells or sumps for disposing of on -site runoff is currently encouraged <br /> through the City's stormwater fee discount program. Many commercial/industrial <br /> properties are installing drywells, since this land use type derives the most savings <br /> from the discount program. However, drywells alone are not intended to treat <br /> stormwater runoff from areas with the potential to discharge pollutants; pre - treatment <br /> must be provided. General guidance for drywells and french drains is presented in <br /> 10% Rule Compliance: Urban Stormwater Quality Guidance for the Maryland <br /> Chesapeake Bay Critical Area in Intensely Developed Areas (May 1993) - Part III. <br /> It is recommended that Eugene refer to the Stormwater Management Manual for the <br /> Puget Sound Basin (February 1992) - Chapters III -3.6.7 and 3.6.9 for applicability and <br /> design guidance for porous pavement and concrete grid pavement systems. This <br /> appears to be the most extensive information available. Porous pavement has met with <br /> some controversy due to its tendency to clog in field tests and its high maintenance <br /> requirements. Concrete grid systems have also seen limited use since the systems are <br /> only suitable for low traffic areas such as parking lots, and are prone to clogging <br /> problems. Pollutant removal efficiencies for porous pavement are shown in 10% Rule <br /> Compliance: Urban Stormwater Quality Guidance for the Maryland Chesapeake Bay <br /> Critical Area in Intensely Developed Areas (May 1993) - Part III. <br /> S1945042na \task02001onsite controls\techmem.ck 10 <br />