Design of Integrated Bioinfiltration-Detention Urban Retrofits with Continuous Simulation Methods

This paper presents the elements involved in the design of a bioretention planter/trench infiltration-detention system as an urban retrofit project. The system was designed to intercept all of the runoff from a synthetic 5.08mm 24-hr rainfall event. Diverted flows were conveyed into bioretention planter for treatment. The bioretention systems were fingerprinted into areas comprising 0.8% of the contributory drainage areas, with an associated stone trench comprising another 3.4%. The system was modeled using SWMM 5.0.014 continuous simulation (CS) software. Under existing conditions, over 80% of annual runoff exceeded the 3.50 L—s^–1—ha^–1 (0.05 cfs—ac^–1) threshold for initiation of combined sewer overflows (CSOs). Nearly all runoff was intercepted by the planter/trench infiltration system, and even with a soil infiltration rate of only 2.54 mm—h^–1, nearly 46% was infiltrated, and less than 8% was discharged at rates that could initiate CSOs. The number of CSO events was reduced from 74 to 4, a reduction of 95%. The volume of flows exceeding the threshold decreased by over 91%.