Slide 31 of 39
Notes:
A typical profile of porous pavement might look like this schematic. Some basic features should be incorporated into all porous pavement practices, including: pretreatment, treatment, and conveyance.
In porous pavement designs, the pavement itself acts as pretreatment to the stone reservoir below. Because the surface serves this purpose, frequent maintenance of the surface is critical to prevent clogging. Another pretreatment item can be the incorporation of a fine gravel layer above the coarse gravel treatment reservoir. Both of these pretreatment measures are marginal, which is one reason that these systems have a high failure rate.
The stone reservoir below the pavement surface should be composed of layers of small stone directly below the pavement surface, and The stone bed below the permeable surface should be sized to attenuate storm flows for the storm event to be treated. Typically, porous pavement is sized to treat a small event, such as the water quality storm (i.e., the storm that will be treated for pollutant removal) which can range from 0.5" to 1.5". Like infiltration trenches, water can only be stored in the void spaces of the stone reservoir.
Water is conveyed to the stone reservoir through the surface of the pavement, and infiltrates into the ground through the bottom of this stone reservoir. A geosynthetic liner and sand layer should be placed below the stone reservoir to prevent preferential flow paths and to maintain a flat bottom. Designs also need some method to convey larger storms to the storm drain system. One option is to use storm drain inlets set slightly above the elevation of the pavement. This would allow for some ponding above the surface, but bypass flows that are too large to be treated by the system, or if the surface clogs.