Low Impact Development Techniques:
Pervious Pavement

Another LID technique is to substitute impervious materials with pervious or porous surfaces that can help reduce surface flow and increase infiltration. There are several types of surface covers that work well for this purpose. Porous cement concrete, porous asphalt concrete, gravel pavers, grass pavers, variations on different types of grids and blocks, and loose aggregate. These pervious surfaces can be used in a variety of areas including driveways, parking stalls, walkways, emergency vehicle access ways, alleys, highway shoulders and other non-high vehicle traffic areas. However, under the right circumstances these surface cover materials can be used, with caution, in roadways and other moderate traffic flow areas. Well-designed parking and roadways can include a mixture of various porosity densities, with the more dense material being located in high traffic areas, and less dense or pervious material located in low traffic areas, or areas where wheel turning is at a minimum (e.g., parking stalls). Pervious surfaces should not be used when the surface grade exceeds 5%.

In cold climates pervious pavement can be used, but requires additional consideration and cautions. One method to avoid potential for freeze/thaw or shrink/swell (clay soils) problems with pervious surface materials is to install a perforated drainage pipe above the aggregate storage area and below the pervious surface. Overflow from the undersurface drainage can be directed to a grass swale to faciliate holding and infiltration during high flow periods. One of the most important considerations in cold climate or on poorly drained soils is the aggregate base should be sufficiently deep to ensure standing water does not accumulate within the top-most pervious grade. The exact depth will depend on base soil permeability, but should be at least 12 inches of one- to three-inch washed aggregate with 40% void space. The aggregate base should have sufficient storage to hold at least the two-year storm event and suitably drained within 72 hours of the end of the rain event. The two-year storage can be sufficient to meet peak-flow runoff from the 25-100 year storm event. Pervious pavers are readily available from a number of manufacturers to suit most every situation. With proper installation and maintenance, pervious pavement can infiltrate up to 80% of the annual runoff volume, and can remove more than 60% of undissolved nutrients, and more than 90% of the sediment runoff. The most common types of pervious pavement that would be appropriate to use include the following:

Porous grass or turf is a green open space that is in effect a pavement that can support pedestrian and vehicular traffic. Turf's permeability is positive as long as the soil is not compacted by excess traffic. Various manufacturers produce plastic geocell latticework that helps maintain the permeability of the turf. There is a maintenance requirement for porous turf areas including mowing, fertilization and irrigation. The use of this material is especially effective for overflow parking and emergency vehicle access ways. These systems work with a plastic geocell material filled with a soil media and seeded with grass. The geocell material adds support and rigidity to the turf. The most commercially available plastic geocell material and block pavers have been approved by many emergency management agencies and meet the requirements of California Fire Code 902.2.2.2, however this requirement should be verified with the manufacturer.

Gravel pavers are similar to grass pavers in the use of geocell support structure. It also helps to keep the gravel in place and does provide additional structural support. Most plastic geocell material is flexible so it can adapt well to shrink/swell and freeze/thaw conditions. Most commercially available geocell material is made from recycled material, an added environmental plus.

Open joined and open cell paving blocksare solid units of concrete, brick or stone laid side by side. They can bear traffic loads and are shaped to produce openings that are filled with porous aggregate or turf that allows for infiltration of storm water. They have a long useable life, are relatively easy to install and provide good infiltration. However, they are sensitive to deformation in the base or sub-grade and do require a thich base to prevent "heaving." In cold climates where areas need to be plowed for ice or snow removal, blocks may catch and cause damage to the blocks and/or plow.

Porous cement concrete requires special skills and training both in the mixing and laying of the product. Only trained and experienced concrete specialists should install porous concrete systems to ensure desired results. Improper installation of pervious concrete will result in low infiltration rates and structural problems. The void space for properly installed porous cement concrete is between 15-22%. Proper maintenance includes periodic, but routine, vacuuming of the surface to prevent clogging with sediment or organic material. With proper maintenance porous concrete can last a minimum of 20 years.

Porous asphalt concrete The use of porous asphalt can be a good option to promote infiltration in parking stalls, walkways and along highway shoulders. Clogging has less to do with surface debris and more to do with the "drain down" of the asphalt binder during hot summer months. The surface areas do need to be cleaned 2 to 4 times a year to avoid build-up of debris. Some research has found that even with 99% clogging the infiltration rate (IR) can be up to 10 inches per hour—which is faster than most soils. Similar to porous cement,porous asphalt should have between 18-22% void space. The aggregate underlayment should be 18 to 36 inches deep and will depend on storage requirements, frost depth in cold climates and site grading. Protection of edges is very important with all porous surfaces, but is especially important with porous asphalt. Porous asphalt does not require special training and can easily be supplied by conventional asphalt batch plants.