How to Manage Stormwater Runoff in a Parking Lot

Posted on August 02, 2018

Today's parking lot projects often face stringent environmental requirements regarding water runoff. There's also a frequent desire to optimize land use in a way that improves your return on investment (ROI). One of the best ways to deal with stormwater runoff is to reduce its volume through the use of pervious pavement.

concrete parking lot
concrete parking lot

Stormwater Runoff Solutions

According to the Pennsylvania Department of Environmental Protection (DEP) manual, there are both structural and non-structural methods for controlling stormwater. Structural methods include the use of permeable pavements that allow stormwater to seep down into the subgrade.

Pervious concrete, pervious asphalt, permeable pavers and turf pavers are all examples. The durability of pervious concrete combined with high permeability make it an ideal choice for many applications, like light-duty parking lots.

Pervious Concrete Pavement Systems

As defined by the NRMCA, pervious concrete is an engineered pavement featuring interconnected voids that allow water to infiltrate and percolate right through it. Quality pervious concrete requires carefully controlling water and cementitious materials (with little or no sand) to create a paste that coats the aggregates.

It is possible to achieve 15 to 25 percent voids that promote water flow rates up to or even more than five gallons per square foot per minute. Although strength reduction occurs, sufficient strength remains for parking lot applications.

Pervious pavement systems typically feature three distinct layers:

  • A pervious surface pavement layer
  • An underlying layer of crushed stone or gravel with sufficient spaces to collect rainwater
  • Soil into which the water can eventually infiltrate

Design Considerations

When planning a parking lot, consideration must be given to the type of application, anticipated traffic (pedestrians, cars, trucks and/or buses) and the permeability of the underlying soil.

A subgrade percolation test is needed to determine the design of the aggregates detention base. Underdrain pipes address inadequate percolation rates. Pervious concrete is the one permeable pavement option that does not always require a reservoir layer.

Also, an accurate assessment of anticipated water volume is a must. The system must be adequate to avoid flooding, erosion and other problems. Extreme rainfall is more common in the state than you might imagine. The heaviest 3-hour U.S. rainfall, 28.5 inches, occurred in Smethport.

To minimize debris that limits permeability, avoid roof drains positioned directly above the pavement, and don't use mulch adjacent to the pervious pavement.

Pervious Pavement Benefits

There are a variety of important environmental and safety advantages that pervious pavement offers.

Replenish groundwater

Pervious pavements not only limit stormwater runoff into local lakes and waterways, they can also facilitate the recharging of local groundwater supplies.

Reduce retention pond size

When water infiltrates the pavement, the need for costly retention ponds and stormwater drainage systems is reduced. This promotes more productive land use.

Green construction benefits

Although pervious pavements have been around for decades, surging interest in green construction now drives growing demand. Its use supports initiatives like Cool Communities and Low Impact Development. Pervious pavement may also aid your quest for LEED points.

Address liability issues

Pervious concrete can address certain liability concerns by promoting better traction and enhanced snowmelt. Melting snow typically infiltrates the pavement rather than re-freezing on the surface. Also, the air in the voids better insulates the sub-base, and this limits freezing.

Without a doubt, pervious pavements are a best management practice (BMP) for stormwater runoff.

Placement of Pervious Concrete

These best practices promote successful placement of pervious concrete:

  • Position the concrete truck close to the site, and avoid delays
  • Wet the underlying aggregate before the pervious concrete is placed
  • Promote compaction by striking off about ⅜ to ½ -in with a vibratory screed
  • Use a steel pipe roller for compaction, but avoid over-compaction.
  • Cut ¼ in joints in the pavement
  • Cover the pavement with plastic less than 20 min after the strike-off to promote proper curing
  • Keep the plastic in place for a minimum of seven days, and do not permit traffic
  • Once the plastic is removed, establish baseline permeability per ASTM C1701

A baseline is important to future assessments of the permeability of the pavement system. Always log test results for future reference.

Finally, it is recommended that concrete installers complete the NRMCA's Pervious Concrete Contractor Certification Program.

Challenges of Pervious Pavement

It is important to maintain the permeability of the pavement/pavers. To minimize the flow of sediment onto the surface, maintain grassy upland areas and divert excess water carrying debris onto the pavement.

Periodic vacuuming reduces sediment accumulations that reduce permeability. Routine dry vacuuming greatly reduces sediment that has accumulated on the surface. Wet vacuuming is a deep cleaning method combining pressure washing and vacuuming to dislodge accumulated sediment and debris. It is important to maintain a service log that lists the date, responsible party, type of maintenance, weight and type of debris removed and other data.

Although pervious pavement still functions in cold and snowy weather, it is necessary to maintain its hydrologic capability. Only select deicing agents should be used, like coarse sand, small crushed aggregate and calcium chloride impregnated sand (after one year). Use a snow blower or a plow fitted with a rubber surface boot set at one inch. Avoid the use of skid loaders and front-end loaders.

About PACA

The Pennsylvania Aggregates and Concrete Association (PACA) educates, promotes and markets on behalf of those in the industry. For additional assistance with your pervious pavement project, please contact us.