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Prindple No. 10: Parking Lot Runoff treated runoff is collected at the bottom and returned to the storm sewer or discharged to a receiving stream. Other types of sand filters include surface, underground, and organic sand filters. Filter strips rely on vegetation to slow runoff velocities and filter out sediments and other pollutants from stormwater runoff (Figure 10.4). To be effective, the runoff must flow as sheetftow across the filter strip. Once flow concentrates to form a channel, it effectively "short-circuits" the filter strip. . Further, a significant amount of land is required (equivalent to 100% of the impervious drainage area). For these reasons, filter strips are only recommended for very small parking lots or parts of larger parking lots. The parking lot should be adjacent to stream buffers or open space. Additional storm water management options i neLude porous pavements (see Principle No.8) and infiltration trenches. Porous pavement is a pervious asphalt or concrete that allows rainfall to infiltrate into the subsurface. Infiltration trenches are stone-filled reservoirs. Pollutants are removed from the stormwater as the runoff flows downward through the soils beneath the reservoir. Infiltration trenches are typically located along the outer edges of parking lots. In comparison to bioretention facilities, infiltration trenches may requ'ire greater care in design, maintenance, and operation (Horner et aL., 1994) unless the bioretention is used as a recharge BMP. PERCEPTIONS AND REALITIES ABOUT PARKING LOT STORMWATER MANAGEMENT ON SITE Communities may be reluctant to require stormwater management at parking lots (Table 10.2). Although there is data on some BMPs, others are relatively new and their effectiveness has not been extensively documented. Unless BMPs are explicitly required, developers may be reluctant to provide stormwater management due to the cost. Maintenance requirements are a consideration for landowners. It should be noted that bioretention facilities, open channels, sand filters, and fiLter strips provide LittLe quantity control. (Quantity controls such as detention ponds are used to minimize the chance of onsite flooding.) These techniques, however, can often reduce the volume and velocity of runoff from parking Filter Strips Figure 10.4: Filter Strips (Claytor and Schueler, 1996) J'Xl P. 1ftaI. -- ~ t.ar ~ =- ~ ~. ) -~ -.-. ~t.e . .-/ - 87 -