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Better Site Design " Provides groundwater recharge when the runoff is allowed to infiltrate into the subsurface; and ., Enhances the appearance of parking lots. Dry Swales Dry swales are essentially "engineered" grass channels that provide full treatment of stormwater pollutants (see Principle No.5 for additional information). The dry swale design includes a layer of prepared sandy loam soil topped by dense turf. Runoff flows into the swale, depositing some of its sediment load as it flows through the dense vegetation. Water quality treatment is provided as the runoff infiltrates through the sandy loam layer. The treated runoff is collected in an underdrain pipe system and discharged into the downstream receiving waters or into a storm water BMP for further treatment or attenuation. Because the swale is designed to dewater within a few hours after a storm, standing water and its attendant nuisance problems are generally not a concern. The feasibility of dry swales at parki ng lots is determined by a number of factors, including drainage area, slape, and length. The amount of stormwater runoff generated at parking lots could overwhelm a dry swale system. In general, dry swales are most appropriate for smaller parki ng lots (or drai nage areas) or larger parking lots subdivided into smaller subdrainage areas and mildly sloping topography. Wet swales can also be used in parking lots, under some conditions. Wet swales are similar to dry swales, but do not have an underlying filter bed. The wet swale occurs when the water table is located very close to the surface. As a result, the swale is often fully saturated or filled with standing water during the greater part of the year. Concerns regarding the standing water may limit the usefulness of wet swales. lFigure iO.3: Perimeter SaJl11J~ fH"'er ((UaJ~or aJlJ1)d SchueQer, 1996) f'AlfI;1NG Lor SHt!l!rF/.OW I _ I I ~ OIRQ 51D?S ~ W'2Ui5J $AND ~ IWOPILE """"""'" (J~'. . '.' .. -~:~~. d' 0IAt1!T - FIL1~t:: PAlJItlC 6' nFrCAL. SecT10N AD/4PPf!f} : ~m/~tr;:V1 PERIAAcrE~ oSANORl.rER WJ~ri!mete!F Salli'Dd fUtelli"s Perimeter sand filters (Figure 10.3) are a more engineered approach to treating parking lot runoff at the source. These devices are usually placed along the downstream edge of parking Lots. Perimeter sand fiLters are particularLy suited for parking lots because they are placed underground and consume Little usable Land. J~ Runoff flows over the surface of the Lot into a grated sedimentation chamber where -coarse sediments are trapped. The runoff is then spread over a filter bed and polLutants are captured as the runoff flows downward through the fitter. The -_.... _......,..,~~~. - 86-