What Is an Impervious Surface?
One method of reducing stormwater runoff is to minimize the amount of impervious surfaces such as concrete sidewalks and asphalt driveways. Impervious surfaces do not allow runoff to seep into the ground. Use pervious surfaces instead. A paving surface that allows water to soak in may seem impossible, but there are many materials that provide the durability of concrete while allowing rainwater to filter down into the ground. If you are planning a new patio, walkway or driveway, there are several attractive alternatives to concrete.
Some examples of these needs are:
- Wood or recycled material decks, usually installed for their functional good looks, can serve as a form of porous pavement. Redwood, cedar and treated pine are as durable as most nonporous surfaces such as asphalt or concrete. Decking allows rainwater to soak into the ground beneath it, and the space between the planks provides ample room for precipitation to drain directly onto the soil surface. As long as minimal air space is maintained between the soil surface and the decking, wood rot can be minimized.
- If you are installing a new patio or rebuilding a crumbling sidewalk, you do not need to use the typical slab concrete. Using bricks, interlocking pavers or flat stones (flagstone, bluestone or granite), you can construct an attractive, durable walkway. If placed on well-drained soil or on a sand or gravel bed, these modular pavers allow rainwater infiltration. Avoid using chemicals to control weeds growing in the joints between the pavers; Corsican mint or moss can crowd out weeds and add beauty to the paved area.
Pre-cast concrete lattice pavers also rest on a bed of sand and gravel and allow rain to soak slowly into the ground
Dutch drains, which are containers of gravel with holes used to infiltrate water from rooftops directly into the ground, carry water from rain spouts into the soil, where it gradually filters into the ground.
These types of materials can be used wherever natural soil drainage is good and there are no problems with either bedrock near the surface or seasonal high water table.
Significant strides have also been made in the last few decades in developing porous asphalt pavement. This material is similar to conventional asphalt in durability, but it contains a much smaller percentage of very fine particles. As a result, the asphalt allows water to soak through into the soil below.