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

Abstract

This article presents the elements involved in the design of a bioretention planter/trench infiltration-detention system as part of a very large-scale urban retrofit project. The prototype system was designed to intercept all of the runoff from a synthetic 5.08-mm 24-h 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%. As layered systems, an approach that is capable of modeling vertical flows in addition to dynamic routing of outflows is used. The system was first modeled using HydroCAD, a design storm event modeling software. A four-compartment node system is used to model the dynamics of flow through the layers. The system was then modeled using SWMM 5.0.014 continuous simulation software. The resulting response to a design storm was computed by both of these models to compare the results of each method. The resulting SWMM model was then run on the 2005 design year rainfall distribution. Under existing conditions, over 60% of annual runoff volume exceeded the 3.50 L糭1稡-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稭1, 47% was infiltrated, and less than 6% was discharged at rates that could initiate CSOs. The number of CSO exceedance pulses was reduced from 233 to 6, a reduction of 97%. The volume of flows exceeding the CSO threshold decreased by 90% in the planter/trench system.