Reducing Impervious Surfaces
Impervious surfaces are land surfaces such as roads, parking lots and building roofs that repel rainwater and do not permit it to soak into the ground. Instead, it flows quickly into nearby streams, causing unnaturally large and sudden flows that contribute to stream erosion. As rainwater flows across impervious surfaces, it picks up pollutants such as oil and other engine fluids, and carries them into streams and marine areas, where they can harm aquatic plants and animals. Furthermore, when rainwater is diverted into storm drains, it is "wasted" in the sense that it is not available to nourish vegetation or to replenish groundwater. Groundwater is important for sustaining minimum flows in streams, and in some cases it is a source of drinking water for people.
What can I do to reduce impervious surfaces?
The options for reducing impervious surfaces range from large-scale planning of new developments, to simple back yard modifications. The principle is to encourage water to soak into the ground so that pollutants are filtered by soil and vegetation, and the volume of water that flows into streams is reduced.
Large-scale planning and development options
- New developments can be "clustered" to reduce the area of impervious surfaces required. By doing so, it is often possible to create or preserve publicly-accessible green space, which may include a natural stream corridor or wetland. "Smart Growth" is an example of this type of planning (see More Information).
- Existing wetlands can be preserved for their ecological importance and their ability to filter and store water. Constructed wetlands, can be built to mimic natural wetlands, and used to treat the runoff from impervious surfaces.
- Multi-storey parking structures or underground parking can be built instead of sprawling one-level parking lots to reduce the area of land required for parking.
- In many places, swales are replacing the traditional concrete curbs and gutters for managing stormwater. Swales are gradually sloping depressions or trenches, often lined with gravel and/or planted with vegetation, that allow rainwater to infiltrate the ground.
- Narrower streets and green "islands" can reduce the paved surface area of streets and increase infiltration of rainwater. They also calm traffic and beautify the neighbourhood.
- "Green roofs" (e.g. rooftop gardens) can transform an impervious surface into a beautiful amenity. By allowing rainwater to be partially taken up by plants, they reduce the amount of water that runs off the property. A green roof can also be a relaxation space, a food source, and an energy saver, since the layer of soil helps to insulate the space below.
- Alternative materials can be used in parking lots, driveways, sidewalks and roads to increase infiltration of rainwater.
- When community developments and redevelopments are planned on a "watershed scale," many of these technologies can be combined and huge cost savings in flood damage, water supply and pollution control can be realized.
- See More Information for examples of these and other techniques.
Options for individuals and land owners
- Plant a garden of native plants; studies show that lawns, while preferable to pavement, do not allow as much rainwater infiltration as native grasses, shrubs and trees. A "rain garden" can be created in a shallow depression where it will intercept the rainwater runoff from your roof or property. Native plants suited to the amount of water that flows off your property help to take up and filter the runoff, and create wildlife habitat. (See also natural gardening techniques.)
- Rain barrels or underground cisterns can be placed under rain gutter downspouts to capture runoff from the roof; this water can then be used to water the lawn and garden.
- Replace asphalt driveways with interlocking pavers, gravel, permeable pavement or a "ribbon" style driveway (two strips of pavement with grass in between). Where possible, limit the length and width of the driveway.
- When designing a new walkway, consider using gravel, mulch, wood chips or stepping stones instead of asphalt. An "S" design can help to direct rainwater off to the sides of the path, instead of "funneling" it to one location.
More Information
- Water Balance Model for BC
- Stormwater Planning: A Guidebook for BC: Ministry of Water, Land and Air Protection
- Impervious Surfaces: Duluth Streams
- Reducing Runoff: Nonpoint Education for Municipal Officers
- Impervious Surfaces: Leelanau Government and Community Center
- Smart Growth BC
- Smart Growth Online (American)
- Green Roofs for Healthy Cities
© Image courtesy of Chris Jones