Bioretention Drains Help Watershed

Town of Danville uses innovative ways to remove pollutants from storm water runoff

Although we relish every drop of rain that falls during drought years, rainwater can also be an unfortunate transporter of pollutants into our creeks.  Rainwater washes everything from candy wrappers to harmful chemicals into drainage inlets where underground pipes quickly convey the pollutants to downstream waterways.  Since storm drains in Danville ultimately discharge to San Ramon Creek, pollutants gets a free ride north to Suisan Bay and the Pacific Ocean every time it rains, with dire consequences for aquatic wildlife and fish.

Combatting this environmental issue has become a priority not only in Danville, but throughout California.  Local agencies are mandated to stop storm water pollution at its source.  This means not only educating community members about pollution reduction efforts, but also retrofitting storm drains with filters that can screen out pollutants for proper disposal.

Several filter devices have been tested in recent years, but the simplest system, called bioretention, has proven to be the most effective.  Simply running storm water through landscaped areas mimics the filtering effect that nature performs in undisturbed forests when it rains.  What sounds simple, though, can be costly and difficult when retrofitting roads and other public spaces that were built many years ago with traditional storm drains.  Developers of new neighborhoods or commercial spaces are also asked to set-aside valuable land to design landscaped areas with storm water filtering in mind.

The term “bioretention area” or “bio-filter” has been coined to describe areas where storm water is directed into landscaping.  The water is allowed to pool in the landscaping and then slowly migrate through the soil into a sub-surface collection system connected to traditional storm drain pipes.  Accumulated trash can then be easily picked-up during routine maintenance activities, and chemical pollutants can be absorbed by plant roots and metabolized.  The areas often appear as a large depression within a landscaped area, or as a basin bounded by concrete curbs with periodic openings to let the water enter.

Two current Danville Capital Improvement Projects have been designed as “Green Streets” to incorporate these bioretention/bio-filter elements:  Railroad Avenue Improvements andHartz Avenue Beautification.

For both roadway retrofit projects, landscaped islands will be constructed along the road edge to capture storm water for filtering.  The islands will resemble traditional landscaping areas except the soil surface will be depressed to allow the ponding and saturation necessary for water filtering.  These islands will replace the conventional storm drain inlets that residents are accustomed to seeing alongside the road.  The landscaping will not only be beautiful but functional – Good for Danville and good for Mother Nature.

For more information about the Town’s Clean Water Program or Environmental Stewardship efforts, contact Chris McCann at (925) 314-3342 or [email protected]