Stormwater management has become an ever more important issue in the urban setting.  As our urban areas become more Rain Gardens
developed and more impervious surfaces like concrete and asphalt are installed the issue compounds itself.  Most of us don’t realize what happens to our stormwater after it touches the ground.  Stormwater experts sometimes talk about the “first flush” which is the initial blast of stormwater that enters into the storm sewer system.  This is usually the dirtiest and most polluted water because it picks up dirt, oils, heavy metals and pollutants from our roads and parking surfaces.

Every urban area is different but all have to manage stormwater in some fashion.  Where I live up until 20 years ago had a combined stormwater and sanitary sewer.  All of our stormwater went to the wastewater treatment plant before it made its way into the river.  This caused several issues, one of which was that when we had a heavy rain event the wastewater treatment plant could not handle that much water.  When the plant was at capacity stormwater (and raw sewage) were dumped directly into our river which would cause a health advisory for several days.  Our city has slowly been separating the sewer systems but the dirty little secret is that as time goes on the water in our river is getting dirtier and more polluted.  Why?  The reason is that we are no longer sending the stormwater through the wastewater treatment plant.  The onus for good stormwater management and keeping our rivers cleans comes back to us as Architects and Real Estate Developers to properly handle the issue at the site.

A rain garden is a landscaping feature that can be installed to collect, filter, and infiltrate stormwater back to the water table naturally without having to send water into the storm sewer.  Typically rain gardens are dish shaped gardens located at a natural low point on the site that collects the stormwater.  They not meant to be retention ponds but may temporarily hold water during large rain events.   The plants in the rain garden vary but all must be carefully selected by a professional.  Certain plants are better than others at filtering heavy metals and pollutants through a natural uptake process called phytoremediation.  The plants at the bottom of the dish must be able to filter pollutants, have deep enough roots to encourage infiltration, and be able to stand wet conditions for short periods of time.  Plants that are on the higher edges of the rain gardens typically do not need to withstand the wet conditions.

Rain gardens work particularly well in conjunction with vegetative green roofs as stormwater management tools.  On one project that I worked on several years ago we were able to handle 100% of the stormwater that fell on the site with the vegetative green roof and the site rain garden.  It was the first zero-stormwater discharge site in our city.