Stormwater Mgmt: When the Rain Hits the Road, What’s the Best Option?

Before “they paved paradise to put up a parking lot,” as Joni Mitchell crooned in Big Yellow Taxi, rainwater would fall and naturally soak into the earth. Now, however, civil engineers of all levels have to consider the potentially dramatic impacts of replacing natural swaths of land with impervious surfaces like asphalt or traditional concrete. Those striving for green solutions and LEED-certification can instead investigate the options for permeable pavement as a means of managing and, in some cases, even harvesting rainwater.

The goal of such solutions, according to the LEED BD+C Reference Guide is to “limit disruption of natural hydrology.” This includes creating minimal barriers to existing water channels and mitigating pollution and contaminants to such channels. By leveraging pervious pavement, or weight-bearing surfaces that still allow for water penetration, designers can minimize the need for stormwater infrastructure without compromising the integrity of the existing natural system.

Typically, permeable pavement is constructed through a four-layered system. At the base is the standard soil subgrade. Above that two stone bases, the first of which is a sub-base created with larger stones and typically 2-4 times as thick as the upper open-graded base. At the upper-most, surface level is a thin layer of aggregate bedding on which interlocking concrete pavers sit. The gaps between each of these pavers is also filled with the same aggregate.

If one were to take a cross-section of such a permeable system, they’d find a natural filtration system from top to bottom. While this is a general overview of a permeable system, specific variations would depend on variables including average rainfall, area of collection surface, and volume of captured runoff. Other optional configurations include the incorporation of geotextile around the sub-base.

When applicable, it’s wise to incorporate actual water harvesting in such planned stormwater management. The reasons are three-fold:

1) The most cost-effective time to build a ground level rainwater catchment is before constructing a permeable pavement system.
2) Rainwater harvesting compliments permeable pavement systems and can achieve LEED credits associated with Water Efficient Landscaping (1 credit) or Water Use Reduction (3 credits).
3) Harvesting systems can make a property more valuable.

Keep in mind, such systems are ideated early in the design process and in such a way that they work collaboratively with surrounding plant life. Leverage software and manual methods alike to determine appropriate runoff rates, and always, always, always check with your state regulations as to exact design requirements!