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<h3>Design Guidance</h3>
<h3>Design Guidance</h3>
<h4>Geometry and Site Layout </h4>
<h4>Geometry and Site Layout </h4>
Permeable pavement systems can be used for entire parking lot areas or driveways or can be designed to receive runoff from adjacent impervious paved surfaces. For example, the parking spaces of a parking lot can be permeable pavers while the drive lanes are impervious asphalt or vice versa depending on the drainage pattern. In general, the impervious area should not exceed 1.2 times the area of the permeable pavement which receives the runoff. A hybrid permeable pavement/soakaway design can feature connection of a roof downspout directly to the stone reservoir of the permeable pavement system, which is sized to store runoff from both the pavement surface and the roof drainage area.
<h4>Pretreatment</h4>
In most permeable pavement designs, the surface acts as pretreatment to the stone reservoir below. Periodic vacuum sweeping and preventative measures like not storing snow or other materials on the pavement are critical to prevent clogging (see Maintenance Section). Another pretreatment element is a pea gravel choking layer above the coarse gravel storage reservoir.
<h4>Conveyance and Overflow </h4>
All pavement designs require an overflow outlet connected to a storm sewer with capacity to convey larger storms. One option is to set storm drain inlets slightly above the surface elevation of the pavement, which allows for temporary shallow ponding above the surface. If the surface is overloaded or clogged, then flows that are too large to be treated by the system can be bypassed through the storm drain inlets. Another design option intended as a backup water removal mechanism is an overflow edge (Figure 4.7.5). An overflow edge is a gravel trench along the downgradient edge of the pavement surface that drains to the stone reservoir below. If the pavement surface is overloaded or clogs, stormwater will flow over the surface and into the overflow edge and underlying stone reservoir, where infiltration and treatment can still occur. On smaller sites, overflow can simply sheet flow onto the traditional paving and drain into the storm sewer system.
Pavements designed for full infiltration, where native soil infiltration rate is 15 mm/hr or greater, do not require incorporation of a perforated pipe underdrain. Pavements designed for partial infiltration, where native soil infiltration rate is less than 15 mm/hr (i.e., hydraulic conductivity less than 1x10-6 cm/s) should incorporate a perforated pipe underdrain placed near the top of the granular stone reservoir. Partial infiltration designs can also include a flow restrictor assembly on the underdrain to optimize infiltration with desired drawdown time between storm events (Figure ).