Parking lot owners often face the problem of needing to squeeze as many cars as possible into a limited area. Porous asphalt lots are providing solutions by maximizing parking through eliminating the need for detention basins.
Luther Park, an assisted care facility in Des Moines, Iowa, faced this situation. The facility needed additional parking spaces, but the only area available was wedged between its existing dry bottom detention basin and neighborhood residences. Sonny Lande, property manager of Luther Care Services, approached Grimes Asphalt & Paving Corp., Grimes, Iowa, with the idea of squeezing a porous asphalt lot into the space.
Since Grimes had built the first porous asphalt parking lot in Iowa in 2004, the company was familiar with the process and began construction. Grimes completed the Luther Park project in 2005 and has built porous lots at Carlisle High School in Carlisle, Iowa, and Carney Marsh in Ankeny, Iowa, in 2006.
“Luther Park’s existing parking area was a traditional lot with a traditional storm water management system which consisted of a dry bottom detention basin,” said Steve Moyna of Grimes Asphalt. “The porous lot that we built filled out the newly purchased land and included an overflow which allowed water from large rainstorms to drain into the adjoining detention basin. This meant that the owner would not lose parking stalls, which enlargement of the existing conventional basin would have required.”
Expansion of the dry bottom basin would not be needed, because porous lots are constructed so that rain drains through the interconnected voids in the surface into a stone recharge bed below the surface. This bed of large, clean stone retains the water briefly until the water can percolate into the soil underneath and is absorbed into the aquifer.
Besides the small space, the lot had an added challenge. “Shortly after excavation began, the field superintendent notified us regarding an existing power cable that supplied electricity to the entire facility,” said Moyna. “Luckily we were able to work with our design firm and reconfigure the stone recharge bed bottom to allow the cable to stay in position while still maintaining the storage capacity needed by ’benching’ over the cable and going deeper within the same overall footprint.”
Grimes laid a geotextile material underneath the stone bed that would allow water to soak downward, but prevent fine material from migrating upward into the stone bed and clogging the voids. The company used about 45 tons (40.8 t) of porous asphalt to create a 4-in. (10 cm) porous surface for vehicle parking. To date, the lot has not exhibited any frost-heave issues.
One concern many owners in northern climates have about porous lots is that the pores can become plugged with sand used for traction control in the winter. This problem can be solved by using liquid de-icing chemicals on the lot, while a yearly vacuuming will remove any sand that migrates from adjoining municipal street systems.
“On a per ton basis, the porous asphalt is more expensive than conventional paving,” said Moyna. “But when considered as part of the complete project cost, it can be 20 to 50 percent less than other types of underground storm water storage.”
Sonny Lande, the property manager for Luther Care services, had begun the project by working with the Des Moines Permit and Development Center for solutions in how to manage storm water on the property.
“I am pleased with how the lot worked out, and would consider building more if we expand,” Lande said. “The porous lot allowed us to maximize the number of parking spaces we could get in, and still do what is right for the environment.”
Pennsylvania Porous Lot
Like the lot in Iowa, the porous asphalt lot built at Riverbend Environmental Center in Gladwyne, Pa., (near Philadelphia) has proved to be an excellent means of managing storm water. Riverbend Environmental Education Center sees approximately 12,000 visitors per year, including students visiting during the school year and those who enroll in summer programs. But the parking lot was hardly environmentally friendly. The lot was located at the bottom of a steep hill, adjacent to a stream that drains into the Schuylkill River.
“The parking lot was a real mess,” admitted Laurie Bachman, Riverbend’s executive director. “If it rained, water and mud would flow down into the parking lot where people would have to walk through before getting to the center. We needed a new lot, that would work, both from an aesthetic as well as from an environmental standpoint.”
Riverbend worked with Susan McDaniels of Cahill Associates, West Chester, Pa., for proper design of the lot. Riverbend received financial assistance from a number of organizations for the project. The porous lot was installed in September 2006.
The project had a number of challenges for the designer. The lot was surrounded by rocks and boulders with woods nearby. This location could be a barrier to water infiltration. Also because of the lay of the surrounding land, the lot had to be built with a slope of 6 percent.
The design of the lot addressed the slope by building earthen berms underneath the pavement (see diagram). A 12-in. (30.5 cm) tall berm would serve to retain water in the stone bed, with overflow pouring into a second bed contained by a 6-in. (16.2 cm) tall berm. This “terraced” arrangement would allow the water to infiltrate at a controlled rate, rather than immediately flooding to the section of the bed at the end of the slope.
Another feature of the lot also addressed the infiltration into the woods surrounding the lot. Although the lot was designed to handle most rainstorms, an overflow system was constructed for more intense storms. McDaniels explained the system as hydraulic control structures.
“If rain would fill up the stone recharge bed, before it would flood the parking lot, the water would flow over stainless steel weirs that would redirect the water into bio-retention areas in the surrounding woods, which are low-lying areas where native vegetation would absorb the water.”
This back-up system enables the porous lot to handle both normal rainfall and near-flood conditions.
Now when students and other visitors come to Riverbend Environmental Center, they will see not just exhibits, but a parking lot that is a picture of environmental friendliness.
“We are very pleased with the lot,” said Executive Director Bachman. “It looks great and handles water in a way that we can be proud of.”
This story was reprinted with permission from the National Asphalt Pavement Association from its HMAT magazine, May/June 2008.
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