Water: Sometimes too much of a good thing
San Francisco State, February 25, 2004. Photo by the SFSU Department of Geosciences
Did you notice during the last big rain? Certain storm drains around town were backing up and flooding the road. Just a taste of what might happen this winter if we get a big rainstorm, like the flash flood of 2004.
Most of San Francisco has a combined sewer system, meaning that both stormwater and sewage go in the same pipe to the treatment plant. Treated water goes into the Bay and Ocean. This works fine in good weather, but in a hard rainstorm, if the treatment plant is overrun, untreated sewage and stormwater is discharged into the Bay and Ocean. This happens more often than you might think.
A cleaner ocean means healthier fish, safer seafood.
Even under normal conditions, the rainwater that goes down the drain runs along dirty streets and sidewalks, picking up pollutants that may not all be removed during treatment. The key to water quality in the rainy season is to get as much water directly into the earth as possible.
Here’s where sidewalk gardens can really earn their keep. Besides looking good, the 20th Street garden has several design features enabling it to capture 100% of the rainwater that falls on its over 5,000 square feet.
- Trees and plants: It’s not enough just to have unpaved surfaces; the plants really make a difference. Foliage slows water down, so water has time to infiltrate the soil rather than run off. Mature tree canopies do the best job of this because they cover a larger area. Once water is in the ground, plant roots absorb some of it so the soil takes longer to become saturated.
- Permeable pavers: You can walk, drive and put tables and chairs on these pavers. They act just like pavement with an important difference: rainwater goes right through them. Permeable pavers come in a variety of styles and colors.
Ever wonder what that diagonal groove was for?
Slopes and channels: The sidewalk itself is sloped ever so slightly towards the garden. The soil is lower than the sidewalk, and slopes on either side slightly towards the middle of the garden. On the Florida Street side, grooves in the sidewalk channel water to the tree wells rather than the curb. All these nearly imperceptible details are vital to directing water flow into the soil. Without them, you have a garden. With them, you have stormwater catchment facility, or rain garden.
Look at the pavement in your area. Is all of it necessary? Do you see a potential rain garden? Find out more from these community organizations:
SF Baykeeper (includes information for outside of San Francisco)