|Summary:||Researchers in Maryland, North Carolina, and Pennsylvania have been
conducting parallel bioretention/bioinfiltration research since fall 2007. Various
designs have been tested including those that rely on underdrains, have internal water
storage (IWS) layers, or are underdrain-free. The cells provide a range of watershed
practice size ratios and employ a variety of land covers. Researchers are pooling
water quantity and quality data to help create new design standards. Initial results
from the shared data will be presented, with specific attention to bioretention design
parameters that control flow modification and water quality improvement.
Two bioretention cells of varying vegetative cover are being monitored in
Rocky Mount, NC. This site is located in the upper coastal plain with sandy in-situ
soils. These cells were designed with a 0.9 m media depth and a 0.6 m deep internal
water storage (IWS) layer. Another bioretention cell is being monitored in Silver
Spring, MD. It was constructed with a 0.9 m media depth and a 0.3 m pooling depth.
Finally, there are two bioinfiltration cells being monitored in Villanova, PA. The first
is the “Traffic Island” bioinfiltration cell, which has been monitored since 2003. The
bowl is only designed for 1.2 cm over the impervious surface, yet overflow rarely
occurs for events less than 5.1 cm. The site had groundwater wells installed in 2007
and is the subject of an ongoing study on the groundwater effects. The second site has
only been monitored for approximately six months.
Cumulatively, the four bioretention cells extensively examined have
dramatically reduced outflow volumes, completely assimilating all events less than
1.2 cm – and in some designs much greater events. By aggregating the data, runoff
reduction by bioretention can be profound.|