The following is an excerpt from City Parks, Clean Water, a report by the Trust for Public Land’s Center for City Park Excellence that examines the role of urban parks in managing stormwater. This is the eleventh installment in a series of 19 posts.
A widely suggested solution for managing runoff is to use pervious (or permeable) pavement. Permeable pavements are engineered surfaces designed to function like standard construction materials while still allowing stormwater to pass into the ground. Materials can include asphalt, concrete, and spaced paver blocks. In urban parks, pervious pavement can make a difference in the form of anything from walkways to basketball courts. In theory, replacing all the asphalt and concrete in the United States with pervious forms would make a huge dent in the runoff problem. The reality, however, is more complex.
First, not every location benefits from porosity; an impervious asphalt trail through a field is not worth replacing, since the water flowing off its edges goes into the ground just a few feet from where it would drop through if it were porous. Perviousness makes a difference only if it keeps water from running into a gutter or a pipe.
The second issue is cost. Pervious pavement is about 20 percent more expensive than its conventional counterparts, but even more significant is the cost of its management. Fine sediment in runoff clogs the pavement’s pores and the spaces between paving blogs, gradually rendering it less effective, so permeable materials require sediment removal once or twice a year with sweepers or special vacuuming devices. (This also means that in snowy areas crews cannot use sand on porous surfaces. As for road salt, it works on pervious asphalt but damages pervious concrete.) Moreover, since some structural integrity is sacrificed to enhance infiltration, permeable asphalt and concrete must be laid very carefully if they are to support an equivalent amount of weight. An employee of the Seattle Parks and Recreation Department says, “We don’t have a single park path where we don’t drive maintenance vehicles, and some of our porous paving hasn’t held up to the weight of those trucks.”
Planning for winter weather is also something that must be considered, although that is a bigger issue on roadways than in parks. Sanding permeable pavement clogs the pores and necessitates sweeping or vacuuming. On the upside, porous pavements often require much less treatment than traditional surfaces because daytime snowmelt drains through before it can refreeze into ice at night. According to the University of New Hampshire Stormwater Center, pervious pavement can succeed in the winter with as little as one-quarter the standard volume of salt.
Well-designed, -swept, and –maintained permeable pavement can last for upwards of 30 years and can make a big difference in reducing runoff. Moreover, with greater public use, its cost should decline due to competitiveness. Having a water-smart park does not have to mean sacrificing paved pathways or wide expanses of playing fields, it just means that proper design choices must be made in regards to soil and pavement.
Filed under: facilities, green infrastructure, maintenance/management, planning Tagged: | Center for City Park Excellence, City Parks Clean Water, pavement, permeable pavement, pervious pavement, stormwater, stormwater management, Trust for Public Land, urban parks