The first thing I wanted to point out was this pavement type is not meant for roadways, it’s meant parking areas, driveways, walkways, etc. Some people have voiced a concern about frost heaves with regard to roads which really isn’t what I was writing about.

However, even for parking lots and other non-road applications the creation of frost heaves will eventually deteriorate the pavement. However, the use of proper sub-bases will greatly reduce the occurrence and severity of frost heaves.

As you can see in the photo (Cahill Associates) the pavement surface is not the only component of a successful porous pavement application. The sub-base creates a layer for water storage and recharge. That layer contains stone aggregate with a large area of voids. Without writing a whole thesis on frost heaves, the large void areas will help drastically reduce the number and severity of frost heaves. Frost heaves are most prevalent in poorly drained silty type soils where the water can’t escape easily and ice lenses form.

It’s also worth pointing out that porous pavements have been used successfully in Norway and other parts of North America with very cold climates.

Advantages of Pervious Pavements

• Water treatment by pollutant removal.

• Less need for curbing and storm sewers.

• Improved road safety because of better skid resistance.

• Recharge to local aquifers.

There are two types of porous pavement: porous asphalt and pervious concrete. Porous asphalt pavement consists of an open-graded coarse aggregate, bonded together by asphalt cement, with sufficient interconnected voids to make it highly permeable to water. Pervious concrete consists of specially formulated mixtures of Portland cement, uniform, open-graded coarse aggregate, and water. Pervious concrete has enough void space to allow rapid percolation of liquids through the pavement. The porous pavement surface is typically placed over a highly permeable layer of open-graded gravel and crushed stone. The void spaces in the aggregate layers act as a storage reservoir for runoff. A filter fabric is placed beneath the gravel and stone layers to screen out fine soil particles. Another variation to the concrete pavement is the use of open celled concrete pavers which provide a durable driving surface and material to plant vegetation which further helps eliminate pollutants.

One of the many benefits to installing a green roof on a building is the ability to reduce stormwater runoff. Stormwater runoff from impervious surfaces such as building roofs, parking lots, roads and walkways create drainage and pollution problems. By using a green roof system a substantial amount of water that would normally run off the structure can be captured to sustain the plant life that’s growing on the roof.

The building in the photo has a daycare center at the visible layer with mercantile space below that and finally an underground parking structure. As you can see a good majority of the roof structure is green and covered with grass and trees. It appears that 60% to 70% of the structure is covered with a green roof. Therefore, this structure has probably reduced stormwater runoff by at least 50% or more.

Other Benefits of Green Roofs:

• They can provide a more pleasant and usable outdoor space compared to a concrete patio or stone roof.
• They can help reduce the urban heat island effect (typically a factor in urban areas).
• Increased wildlife habitat (typically birds and insects).
• Reduced heating and cooling loads on buildings from the additional mass and thermal resistance.
• Heavy metals and contaminants can be filtered out of rain water.

The biggest disadvantage to these types of roofs is the additional loading placed on the structure. The soil, water and plants add a significant load to the roof. However, the benefits of green roofs certainly outweigh the small costs associated with the stronger roof.

Green roofs are just one way of helping to reduce stormwater runoff. As stormwater management rules become restrictive green roofs will become more popular and useful. I guess you could say green roofs are as green as they come!

Why is Stormwater Runoff Bad?
Stormwater can pick up debris, chemicals, dirt, and other pollutants and flow into a storm sewer system or directly to a lake, stream, river, wetland, or coastal water. Anything that enters a storm sewer system is discharged untreated into the waterbodies we use for swimming, fishing and providing drinking water. Left untreated, stormwater pollution can damage natural habitats for wildlife, fish, plants and water supplies.

Typically storm water runoff is collected in detention ponds, catch basins and other drainage features. The water is stored in order to remove sedimentation and treat the water using biological and chemical means. Most stormwater detention structures are large and expensive to build. These structures usually require large pieces of land to be cleared and modified to build the structures.

Low Impact Development Features
There are quite a few low impact development features that can be used to reduce the stormwater runoff from a project. The most commonly used features are:

• Bioretention Cells (also known as rain gardens) – Bioretention is the process of biological removal of contaminants or nutrients as fluid passes through media or a biological system. Typically a rain garden is a depressed area that has a well draining soil and plant life (see photo above). These features are inexpensive to create and add aesthetically pleasing landscaping to the area.
• Cisterns and Rain Barrels – collecting rain water from gutters on buildings is a great way to reduce the amount of stormwater runoff. The cisterns can be used to store water for fire protection and irrigation. Smaller rain barrels are a great way to collect water for gardening and irrigation.
• Green Roofs – are roof surfaces covered with some type of vegetation and soil. The idea here is to capture rain water and hold it so it doesn’t run off the structure and create stormwater runoff from the impervious layer.
• Permeable Pavements – are porous pavements or pervious pavements that allow the movement of water through them to the soil below. Again the idea here is to reduce the stormwater runoff that you’d get from a traditional impervious pavement.
• Grassed Swales – or bioswales are designed to slow the flow of water and remove silt and pollutants. The swales have very gentle slopes with vegetation planted within it.

Benefits of Low Impact Development
The benefits to low impact development are fairly obvious. The biggest benefit is improved water quality and the protection of habitat for animals, fish and insects. LID also provides for much more aesthetically pleasing landscaping, reduced construction costs and a reduction in impervious surfaces. The other benefit to this approach is an improved rate of ground water recharge due to infiltration.

I hope you’ve found this brief discussion of Low Impact Development interesting and useful. If you have questions or comments I’d like to hear them. If this topic is of interest to my readers I can certainly provide future posts in more detail.