The Passive Rainwater Harvesting Toolkit
When people think about a rainwater harvesting system, they often picture a rain barrel, cistern or other storage device. That's natural as in active systems, these are often the most visible, prominent components. When considering your options for using rainwater that falls on your site, however, it is important to think about all the tools that are available to you. Passive systems don't utilize tanks, pumps and filters but they can hold on to water for you, and in doing so, help to support a healthy, self-sustaining ecosystem on your property while simultaneously mitigating problems associated with excessive runoff.
Whereas active rainwater harvesting systems generally seek to collect water from the cleanest surface available, normally the roof, passive systems can deal with water flowing from any impermeable surface. This can include driveways, patios, parking lots and even lawns and gardens where the soil has become too saturated to retain water.
A rain garden is a shallow depression that collects, absorbs and filters stormwater runoff that isn't able to soak into the earth elsewhere. They can be strategically sized and shaped to accommodate the anticipated volume of water and are designed to allow a pond to form temporarily following a rainfall. They are not, however, intended to be permanent ponds. Rain gardens are constructed using soil mixes that allow water to be rapidly absorbed and cleansed while supporting plant growth. They should be outfitted with an overflow drain to allow for a controlled outcome when the volume of water that the rain garden accepts is greater than it was designed for. Rain gardens can be planted with a variety of vegetation and can be integrated into most landscapes.
Rain gardens have many benefits. They are a beautiful way to provide habitat for beneficial insects and birds, can filter any contaminants that may exist in the runoff to protect groundwater and reduce flooding to neighbouring properties or to the storm sewers.
Permeable paving materials are porous, allowing some of the rain that falls on them to infiltrate into the soil beneath. They can be used in place of conventional paving for surfacing sidewalks, driveways or parking areas. There are various types of permeable paving including paving blocks, cobble stones, interlocking pavers and concrete, plastic or fibrous grid systems filled with sand, gravel or plants.
Swales are long, low ground depressions that collect and move stormwater runoff. They can be constructed to transport water but their main function is to moderate its flow and encourage infiltration into the ground. Swales are relatively narrow, distinguishing them from wide depressions that become basins or ponds. Swales can be grassed, planted in other vegetation and designed to be wet or dry. In many cases, swales are built on contour. This provides significant water storage and results in reduced overland flows.
Infiltration chambers are underground tanks or constructed chambers that have permeable bottoms, allowing large volumes of water to collect and then be released slowly to the groundwater over time.