Researchers at the University of New Hampshire Stormwater Center have been working on stormwater problems since 2004. They believe the traditional techniques (detention ponds and swales) that collect water and rechannel it don?EUR??,,????'?????<

A recent experiment at UNH had a mixing truck dump 1,500 gallons of water onto the campus porous concrete parking lot and onto the ground next to the lot. The porous concrete soaked the water up like a sponge and outperformed the infiltration of the natural soil.

Rob Roseen, director of UNH?EUR??,,????'?????<

The porous hardscape needs cleaning once or twice a year to prevent clogging, Roseen notes, but even if gets clogged, it works better than soil and far better than the basic asphalt or concrete surface.

The upfront cost are higher, but you save from not building detention basins or laying storm pipes. Conventional concrete costs around 90 dollars a yard; pervious concrete costs about 20-percent more.

The Stormwater Center explains the basics to creating porous media infiltration beds (from top to bottom):

1. A 4-inch thick layer of choker course of crushed stone
2. A 12-inch minimum thickness layer of filter course of poorly graded sand (a.k.a. bankrun gravel).
3. A 3-inch minimum thickness filter blanket that is an intermediate setting bed (pea gravel). The filter blanket is placed to prevent downward migration of material into the reservoir course.
4. A reservoir course of crushed stone (thickness depends on required storage and underlying native materials). The fine gradation of the filter course is for enhanced filtration and delayed infiltration. The high air void content of the uniformly graded crushed stone reservoir course maximizes storage of infiltrated water and creates a capillary barrier to winter freeze-thaw.
5. (Optional) An underdrain in the reservoir course.
6. Nonwoven geotextile filter fabric (geotextile) to stabilize the sloping sides of the porous asphalt excavation.