July 7, 2012
Barbara J. Euser
Before man began constructing buildings and paving roads and parking lots, when rain fell on the earth it percolated down through layers of sand and soil and eventually entered the water table below the surface. When rain falls on solid buildings and paved surfaces, it runs around them often causing serious erosion. In modern cities, this runoff water is collected in storm drains which route the runoff to the nearest body of water.
In the Bay Area, storm drains typically flow into streams, eventually flow into the bay. Storm drains, which are expensive to build, collect water often containing petrochemicals, pesticides and herbicides. The runoff is often warmer in temperature than the water in the rivers, streams, lakes or ocean into which the runoff flows. Adding polluted or warmer water to natural water bodies causes changes in their ecology, sometimes killing native species or causing destructive plant or algae growth.
In 1990, while designing a subdivision in Prince George's County in Maryland, David Brinker developed a plan to collect rainwater in gardens, rather than allowing it to run off into storm drains or be collected in a storage pond. One immediate benefit of these "rain gardens" was economic: the cost to build storm drains for the subdivision would have been $400,000. The complete cost of the rain gardens was $100,000.
The gardens were aesthetically pleasing, and the water was retained in the local soil. The rain gardens reduced storm water runoff from 75 to 80 percent, according to flow monitoring done in later years. These prototype rain gardens spawned a whole landscaping movement now known as Low Impact Development.
The concept of rain gardens is simple:
• Identify where water will run off a building or parking lot.
• Plant gardens that will contain the water long enough for it to sink into the ground. Homeowners may check the drain spouts from the gutters that run along the edge of their roof. A pipe may be used to carry the water away from the foundation of the house to a garden area.
Make sure the plants in this area can tolerate damp roots during the rainy season of the year. Robust native plants that grow at the edge of wetlands, such as sedges, rushes, ferns, select shrubs and small trees, will do well in these conditions. If a low-lying area will contain water much of the time, it is called a wetlands rather than a rain garden. Rain gardens allow water to infiltrate — sink into — the ground in about 48 hours.
Commercial buildings and parking lots are more complex, but the principle is the same. Raised berms or depressed swales may be used to contain and collect water. In order to obtain a commercial building permit, the applicant must demonstrate how runoff will be dealt with.
In one case I am familiar with, the original plan called for the use of permeable paving stones in the parking lot rather than impermeable asphalt. A thoughtful landscaping plan using a series of planted berms to keep the water inside the property and out of the storm drains provided a much more economic solution.
For more ideas on rain capture and plant selection, visit the Sonoma County Master Gardener website or read the UC Cooperative Extension and SeaGrant joint publication.
One doesn't have to be a subdivision developer or a commercial contractor, however, to take a step toward keeping rainfall from running into storm drains. The first step is conscious awareness: each householder should examine his or her own property to determine where runoff goes. The second step is to find a way to create a rain garden to capture the runoff and, using the appropriate plants, keep the water in the soil.
If we, as individuals, take responsibility for minimizing runoff from our own homes and properties, we can help minimize the negative effects of runoff on our streams, lakes and incomparable San Francisco Bay.