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Wearing laver • For all surface types, a minimum initial infiltration rate of 20 inches per hour is necessary. To improve the probability of long-term performance, significantly higher initial infiltration rates are desirable. • Porous Asphalt: Products must have adequate void spaces through which water can infiltrate. A void space within the range of 16 — 25% is typical. • Pervious Concrete: Products must have adequate void spaces through which water can infiltrate. A void space within the range of 15 — 35% is typical.. • Grid/lattice systems filled with gravel, sand, or a soil of finer particles with or without grass: The fill material must be at least a minimum of 2 inches of sand, gravel, or soil. • Permeable Interlocking Concrete Pavement and Aggregate Pavers: Pavement joints should be filled with No. 8, 89 or 9 stone. Drainage conveyance Roads should still be designed with adequate drainage conveyance facilities as if the road surface was impermeable. Roads with base courses that extend below the surrounding grade should have a designed drainage flow path to safely move water away from the road prism and into the roadside drainage facilities. Use of perforated storm drains to collect and transport infiltrated water from under the road surface will result in less effective designs and less flow reduction benefit. Underdrains Note that if an underdrain is. placed at or near the bottom of the aggregate base in a permeable pavement design, the permeable pavement is no longer considered an LID BMP and cannot be used to satisfy Minimum Requirement #5. However, designs utilizing an underdrain that is elevated within the aggregate base course to protect the pavement wearing course from saturation is considered an LID BMP and can be used to satisfy Minimum Requirement #5. See Appendix III-C and the WWHM Users Manual for guidance in modeling permeable pavements with underdrains. Acceptance test • Driveways can be tested by simply throwing a bucket of water on the surface. If anything other than a scant amount puddles or runs off the surface, additional testing is necessary prior to accepting the construction. Roads may be initially tested with the bucket test. In addition, test the initial infiltration with a 6-inch ring, sealed at the base to the road surface, or with a sprinkler infiltrometer. Wet the road surface continuously for 10 minutes. Begin test to determine compliance with 20 inches per hour minimum rate. Use of ASTM C 1701 is also recommended. Volume V — Runoff Treatment BMPs — December 2014 5-24