A tour guest said it best, "Thanks so much for coordinating this field trip. I learned a lot. I've driven by that site and seen all the plants growing there but had no idea it was a biofiltration site, I guess that's the whole idea!”   

 

After a large-scale Lady Bird Lake cleanup on June 11th, 2011 volunteers were treated to a tour of a biofiltration project by Watershed Protection Department Environmental Engineer Lee Sherman. Click here to see a News 8 Austin interview about the event.

 

Volunteers met at the Open Room Austin art installation before strolling along the pathway while learning more about WPD's Stormwater Treatment Program.

   

 

 

 

The biofiltration pond located north of Cesar Chavez between Lamar and Sandra Muraida Way is complete with educational signage near a pathway so walkers and cyclist can learn more about this pilot project.

 

 

 

Guests listened carefully as they learned that the pond was jointly funded by the city and The Gables development to be pro-active in addressing increased runoff due to impervious cover in our urban environment. The pond slows the flow of stormwater full of sediment, oil, trash, and increased nutrients and gradually filters these materials out before releasing treated water into Lady Bird Lake.

 

Guests had lots of intelligent questions.

 

 

A University of Texas and Watershed Protection Department study showed that biofiltration outperformed sand filtration. Researchers created test columns and with sand, a sand/soil mixture, and a sand/soil mixture with plants on top and then ran synthetic storm water through each column. The biofiltration columns were the most effective in filtering storm water.  Test column diagram and photo courtesy of Watershed Protection Department.

 

When the storm water rushes into the pond it passes through "energy dissipators" aka bunch grasses to slow the flow.

 

 

 

 

 

 

 

 

 

Bunch grasses like switchgrass use the excess nutrients washed in with the stormwater to grow big and strong. Since the storm water includes excess nutrients the planting media does not include compost. A variety of plants are included in the basin so scientists can learn what plants will perform best in these conditions. Riparian plant species were chosen since they are hardy enough to meet the demand of a system with varying water levels. Plants that are thriving in this pond include: switchgrass, inland sea oats, wax myrtle, and obedient plant.

 

 

 

KAB Green Teens pose by the switchgrass to show just how big these grasses are. In a natural environment switchgrass roots may extend twice as far down in the ground as the leaves do above ground, but in the biofiltration pond there is geotextile 18" down so the roots grow horizontally forming a thick mat.  The Watershed Protection Department anticipates that plant roots will create pores in the collected sediment which will allow water to continue filtering through the medium. Naturally maintaining “hydraulic conductivity” aka water flow will reduce the frequency of maintenance.      

 

Pollutants are removed by physical and biological processes including straining (or sorption), denitrification, and plant uptake.  In addition to the removal of suspended sediment and dissolved pollutants, trash/litter is also captured in the biofilter and removed before entering Lady Bird Lake.

 

 

So now that you know how biofiltration works for large-scale urban developments, what can you do at home? Visit www.ci.austin.tx.us/growgreen/downloads/raingarden_factsheet.pdf to follow six steps to create your very own rain garden.