When it rains on a large parking lot, water does not soak into the ground. It flows across the pavement, picks up oil, debris and other pollutants, and rushes into storm drains and local waterways. In heavy storms, that runoff can contribute to flooding and stress drainage systems. Designers have developed a range of approaches that help parking lots handle rain more like natural ground.
Why Pavement Causes Problems
Ordinary asphalt and concrete are impervious, meaning water cannot pass through them. A large lot can generate a lot of runoff very quickly. That water often carries pollutants, and because it moves so fast, it can cause erosion downstream and overwhelm storm drains. Paved surfaces also absorb and hold heat, which can raise local temperatures, a phenomenon related to what is called the urban heat island effect.
Permeable Pavement
Permeable pavement allows water to pass through the surface into a stone layer underneath, where it can soak into the soil or be slowly released. There are several types:
- Porous asphalt and pervious concrete, which have open spaces that let water through
- Permeable pavers, interlocking blocks with gaps filled with small stones
- Grid systems filled with gravel or grass, sometimes used for overflow parking
Permeable surfaces require maintenance, such as periodic vacuuming, to keep pores from clogging with sediment. They are often used in parking spaces while drive aisles use conventional pavement.
Bioswales, Rain Gardens and Trees
Many green parking lot designs include planted areas that collect and filter runoff. Bioswales are shallow, vegetated channels that slow water and let it soak in. Rain gardens are depressions planted with species that tolerate both wet and dry conditions. Trees in islands and along edges provide shade, which can help keep cars and pavement cooler. Curb cuts allow water from the pavement to flow into these planted areas rather than into drains.
Benefits and Tradeoffs
Green parking lot features can reduce runoff, filter pollutants, provide shade and make lots more pleasant. They can also help property owners meet local stormwater regulations. The tradeoffs include upfront design and construction costs, ongoing maintenance needs and space used for plantings. Soil conditions and climate affect which techniques work best in a given location. Requirements and incentives vary by locality, so property owners typically work with engineers and check with local agencies.
What Drivers Might Notice
If you park in a lot with green features, you may see pavers instead of asphalt, planted strips between rows or small openings in curbs. Respect planted areas by not driving or parking on them, and follow posted signs, which sometimes mark areas that should be kept clear for drainage. Shaded spaces under trees may also be the coolest place to leave your car on a warm day. These features are quiet reminders that parking lots are part of the larger environment, not separate from it.
How a Green Parking Lot Handles a Storm, Step by Step
It helps to follow a single rainstorm through a lot designed with green features, compared with a conventional lot.
On a conventional lot
- Rain hits the asphalt and immediately begins to flow toward the lowest point.
- As it moves, it collects oil drips, tire particles, litter and dust.
- Water gathers at drains, which carry it quickly into pipes and then to a stream, river or other waterway.
- In a heavy storm, drains may back up and water may pond in low areas.
On a lot with green features
- Rain falling on permeable spaces passes through the surface into a stone base below.
- Water running off drive aisles flows through curb cuts into bioswales or rain gardens.
- Plants and soil slow the water and help filter out some pollutants.
- Some water soaks into the ground, while the rest is released more gradually through overflow drains.
The difference is not that rain disappears, but that it moves more slowly and more of it is handled on site. That is the core goal of most green stormwater design.
Keeping Green Features Working: A Maintenance Checklist
For property owners and managers, green parking features perform well only when they are maintained. Common tasks include:
- Vacuum permeable surfaces on a schedule recommended by the designer or manufacturer to keep pores open
- Avoid sanding permeable areas in winter, since sand can clog the surface; ask the designer about appropriate winter treatments
- Inspect curb cuts and inlets for leaves, trash and sediment that block water from reaching planted areas
- Care for plants by weeding, replacing failed plantings and watering new ones until they are established
- Check after big storms for erosion, standing water that lingers too long or damaged edges
- Train snow removal crews so plows do not pile snow on rain gardens or damage pavers
- Keep records of inspections and maintenance, which can help if local stormwater rules require reporting
What This Means for Different Readers
Drivers and shoppers
Green features can make lots more comfortable, especially shaded spaces in summer. Keeping vehicles off planted areas and not dumping anything into drains or rain gardens helps them work as intended.
Small business owners and property managers
If you are resurfacing or expanding a lot, it is worth asking a civil engineer whether green features could help meet local stormwater requirements or reduce drainage problems. Local agencies may have guidance, and in some places incentives, though programs vary and change over time.
Neighbors
Large lots affect nearby streets and waterways. Public review of new developments is a chance to ask how stormwater from parking areas will be managed.
Stormwater Questions
Does permeable pavement work in cold climates?
It is used in many cold regions. Proper design, including the stone base, and maintenance practices suited to winter are important to its performance.
Can a whole lot be permeable?
It can, but many designs use permeable surfaces in parking spaces and conventional pavement in drive aisles, where traffic is heavier.
Why does water stand in a rain garden after a storm?
Rain gardens are designed to hold water briefly while it soaks in. Water that lingers much longer than intended can signal a maintenance issue.
Do green lots cost more?
Costs depend on the site and design. Owners typically compare upfront and maintenance costs with stormwater benefits and local requirements.
Can I wash my car in a lot with rain gardens?
Soapy water and other substances can harm plants and soil. Follow posted rules, and use a car wash instead.
