By Craig Cramer
A new 15-page booklet by CALS researchers, Using Porous Asphalt and CU-Structural Soil®, details how combining these two technologies:
- Reduces runoff from parking lots and other hard surfaces.
- Increases groundwater recharge.
- Improves water quality in lakes, rivers and streams.
- Helps trees thrive in otherwise tough environments.
Developed 30 years ago, porous asphalt resembles regular asphalt except that the mix contains no fine particles. Water seeps through tiny holes in the asphalt into a bed of crushed gravel below. From there it slowly filters into groundwater.
Photo Ted Haffner
A reservoir of CU-Soil™ below porous asphalt catches water that would otherwise runoff into surface water, and it allows tree roots to penetrate and thrive without heaving the surface.
Ted Haffner, a Department of Horticulture graduate student, researched the increased effectiveness of porous asphalt when the gravel was replaced with CU-Soil™, a patented mix of soil and 3/4- to 1 1/2- inch, angular crushed stone.
CU-Soil™ forms a weight-bearing lattice that can support sidewalks, patios and parking lots, yet it allows tree roots to penetrate and forage for water and nutrients without heaving the hard surface above. It was developed 15 years ago by one of Haffner's advisors, Dr. Nina Bassuk, director of the Urban Horticulture Institute.
By allowing water and air to penetrate, CU-Soil™ and porous asphalt promote healthy tree root growth, which is especially critical in urban areas where tree-friendly soil is limited, Bassuk points out. As the tree roots penetrate and fill the CU-Soil™ beneath the asphalt, they help reduce runoff even more by taking up some of the water that penetrates the asphalt and transpiring it through leaves.
For best tree growth, Bassuk recommends a 24- to 36-inch layer of CU-Soil™. That large a reservoir will soak up about 6 to 9 inches of rain over a 24 hour period. In many cases, reduced drainage infrastructure can offset the cost of installing the CU-Soil™, she adds.
Haffner, who will receive his M.S. degree in January 2008, worked with the City of Ithaca in 2005 to install a 14-space lot to test the porous asphalt/CU-Soil™ combination adjacent to Cayuga Lake inlet. Additional support for his work came from the U.S. Forest Service and the International Society of Arboriculture.
New guidelines designed to reduce runoff have led to the construction of retention ponds and similar structures adjacent to parking lots. But the space saved by using porous asphalt and CU-Soil™ can free up space for additional parking or other uses. One of those other uses is to plant more trees, adds Bassuk. The combination also makes it possible to plant trees right in the lots where they can improve aesthetics, shade cars and help cool overheated urban environments.
The booklet is available online in the outreach section of the Urban Horticulture Institute website: www.hort.cornell.edu/uhi/outreach/