About Bioretention System (Bio II)
General Information Performance Data Water Quality Treatment Process Design


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Design

Bio II is comprised of a sedimentation fore-bay and a bioretention filtration basin. The basin is filled with a bioretention soil mix (BSM) 30 inches in thickness, and consisting of 60 percent sand, 20 percent woodchips, ten percent compost, and 10 percent native soil. The filtration basin is well vegetated. Researchers selected vegetation for flood and drought tolerance, the capacity for maximum ground cover, and aesthetics.

The forebay holds 25 percent of the water quality volume (WQ_V), and drains through a standpipe into the bioretention basin, which holds 75 percent of the WQ_V. The basin allows eight inches of ponding, and the BSM permits an infiltration rate of eight feet per day. Overflow contingencies exist for channel protection volume (CP_V) and conveyance protection volume (Q10) events. Typically Q2 events are conveyed over 24 to 48 hours, and Q10 events bypass to the surface.

Bio II is the second bioretention system installed at UNHSC, and reflects the 2005 best design standard. The first, Bio I, had a BSM that was higher in clay and silt—as much as 7.5 percent of the total mix—and it used non-woven geotextile between the BSM and the crushed stone bedding. Both features were consistent with 2001 BMP manual specifications; however, Bio I experienced hydraulic failure in 2005. Analysis indicated the primary reason was clogging and infiltration loss. UNHSC then solicited design input—primarily from the Maryland Low Impact Development Center. The result is Bio II, which maintains a higher infiltration rate than Bio I, virtually eliminates silts and clays from the BSM, and eliminates the geotextile under the BSM. The BSM rests on a graded subbase, the bottom of which consists of crushed stone overlain with pea gravel, which graduates to coarse sand just below the BSM.