A well-constructed conventional asphalt parking lot does a great job of sealing water off at the surface, preventing expansion and heaving from water trapped below the surface when it freezes. This characteristic is the same one that makes large parking lots and roadways the substantial environmental threat that they are!

Asphalt, concrete, compacted dirt, and even shingle or metal roofs are examples of impervious surfaces - often large areas that don't allow water to soak through. Impervious surfaces have two common qualities: they generate larger amounts of runoff (often contaminated with petroleum and salt from vehicles) compared to native soil AND they radiate and reflect much more heat than vegetated or forested land (called the heat island effect).

All porous paving types virtually elliminate the issue of runoff. We've installed three types of porous paving as demonstrations: Pervious Asphalt, Pervious Paver Blocks, and Stabilized Sod Cells. Each of these strategies allow water to flow through the surface to accumulate in a deep basin below, filled with nearly 3 feet of crushed rock. These basins provide a place for the water to go and hopefully settle through the subsoil, being filtered first by the gravel, then by the soil particles themselves before recharging groundwater. Since our region is characterized by dense, compacted soil layers this settling of water (called infiltration) happens very slowly, so our pavement would have a tendency to overflow. We solved this problem by installing overflow pipes which drain to planted detention bays to store, slow, and purify excess water from the parking lot.

Only one porous paving product demonstrated by the WISE Project reduces the heat island effect. This is the stabilized sod cells, which use a reinforcing pvc grid which the grass grows through to prevent compaction (maintaining porosity) and minimize rutting. While this product provides the "best of both worlds" in terms of ecological benefits, it isn't suitable for high traffic applications (the grass would be shaded, pulverized, and die). It is ideal for seasonal, intermittent, or overflow parking. We chose to employ it for the latter application.

While each of the three products are very stable and provide substantial ecological benefit, they are also prone to clogging with sediment and fine organic matter and must be periodically maintained by vacuum sweeping which can be costly. Care must also be taken when plowing not to damage the surface. Otherwise, when thoughfully applied, these products are an ideal way to mitigate the negative effects of parking lots.