raingardens

The first rain gardens were built in the U.S. state of Maryland in the early 1990s as a way to counteract the erosion, water pollution and flooding caused by stormwater in urban areas. Rain gardens are now found throughout many cities and towns around the world.

In undeveloped areas like the bush or grassland a slow natural groundwater filtering process takes place. Rainwater slowed by vegetation and rough surfaces flows into low places, where plants soak up and release a small portion of the water back into the atmosphere. The rest slowly percolates through the soil leaving behind any contaminants, feeds into the ground water and eventually into the creeks and rivers.

Unfortunately, in urban environments, the water system no longer works this way. Instead after rain, large quantities of storm water run off, often at great speed from roofs, driveways, footpaths and roads directly into the storm water drains. The high velocity and large quantity of storm water causes erosion, water pollution and flooding. Directing stormwater straight into the drains also prevents the rain topping up the ground water levels. Installing rain gardens in urban areas allow us to mimic some of the natural filtering system lost through urban development and reduce the negative impacts of storm water.

Rain gardens come in many shapes and sizes but all follow the same basic pattern. Storm water is directed into an artificial pond planted with native species that are adapted to wet conditions. The plants take up some water, while the rest percolates down through several layers of soil, sand and gravel that removes many contaminants from the water before finally entering the groundwater or storm water system.

An additional benefit of rain gardens and other methods that increase ground water infiltration is a reduction of the heat island effect that is created in urban environments from large areas of concrete and bitumen. Trees and other vegetation not only cool by shade but water vapour released from pores in their leaves can reduce air temperatures by as much as one to five degrees.