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Principle No.8: Parkinq Lot Size · Fine silt particles pass through the pavement and settle in the underlying bed, reducing infiltration capability over time; · Damage by snow plows (plow blades may catch the edge of individual blocks); · Placement of alternative pavement on impermeable layer; and · Poor geotechnical testing or engineering design (improper soils/ infiltration rate). Issues related to cost and the relative effectiveness in meeting water quality goals are summarized in the Table 8.2. Table 8.2: Summary of Issues Related to Various Types of Alternative Pavements, based on BASMAA (1997) Material InitiatCost .. .. Maintenance Water Quality . - Cost Effectiveness:* . Conventional Asphalt / Concrete Medium Low Low Pervious Concrete High High. High Porous Asphalt High High High Turf Block Mediui11 High High Brick High Medium Medium Natural Stone High Medium Medium Concrete Unit Pavers Medium Medium Medium Gravel Low Medium High- Wood Mulch Low Medium High Cobbles Low Medium Medium · Relative effectiveness in meeting storm water quality goals ECONOMIC BENEFITS Construction costs for pervious pavements are generalLy greater than those for conventional pavements (see Table 8.3). Construction cost savings due to reduced curb and gutter and reduced storm water management requirements may offset this initial cost difference. Similarly, reduced storm sewer and stormwater management facility maintenance requirements may offset the generally greater maintenance requirements associated with pervious pavement. For example, the City of Olympia "paved" an overflow parking lot at Olympia High School with Geoweb (a geotextile usually planted with grass). The Geoweb cost $60.50/yd2; conventional paving would have cost approximately $48/yd2. The Geoweb cost, however, included the cost of constructing an infiltration trench, in lieu of a retention pond (Runoff Report, 1997). - 77 -