What's a Bioswale, and why should we care?
Short definition: landscape elements designed to remove silt and pollution from surface runoff water. They consist of a swaled drainage course with gently sloped sides (less than six percent) and filled with vegetation, compost and/or riprap. Why should we care? San Francisco has a combined sewage system - which means both storm water AND waste water from homes and businesses go into the same pipes. Anytime we get more than 1/2" of rain in one hour, it all gets dumped raw and untreated into the bay and ocean, since the system cannot handle the overwhelming volume. So, enter bioswales - a way to keep stormwater out of sewer pipes by capturing rainwater, collecting it in a location where the soil and plants can filter out the pollutants before allowing the water to naturally seep in the groundwater supply. Very clever.
In the photo below is a bioswale in action taken last week during our heavy rains. The spout in the background is draining water off an apartment building roof where the water falls on boulders, and then drains into the depression (really a just a shallow ditch - this is the "swale" referred to above). The swale is lined with plants that are particularly good at filtering pollutants (Rushes are commonly used). There are built in mechanisms to make sure in extreme rainfall events the system doesn't get overwhelmed and flood the building. #bioswales