SWMM5 Unsaturated Drainage Models for Stormwater Biofiltration with an Internal Water Storage Zone

Abstract

Denitrifying stormwater biofiltration systems are designed to enhance nitrogen removal and operate under saturated and unsaturated conditions; however, unsaturated drainage expressions for these systems are unavailable for use in stormwater modeling programs. This study presents two strategies to simulate unsaturated drainage within the Stormwater Management Model version 5.1 (SWMM5): the low-impact development (LID) bioretention module, and the main processing platform. Drainage flux simulation data from HYDRUS-1D were used to calibrate the conductivity slope (HCO), an unsaturated drainage parameter, for different medium types (sand, loamy sand, and sandy loam) and nitrification layer depths (3.48–121.92 cm). The results indicate that HCO is a function of medium type and depth; however, current guidance manuals only provide HCO values as a function of medium type for conventional biofiltration systems. The calibrated HCO values can be directly applied within the LID bioretention module. The second strategy used the calibrated HCO values and Darcy’s law to develop a novel unified equation that can simulate both unsaturated and saturated drainage within the main SWMM5 processing platform. Three alternative configurations are presented for this strategy.