Performance Modeling of Right-of-Way Stormwater LID Practices-Exfiltration System

In order to assess the hydrologic performance of the exfiltration system (ES), the monitored ES inflow and overflow time series data were analyzed. Both continuous and event-based monitoring data analysis depicts that the ES has a significant impact on the water balance, reducing the surface runoff by 84% and achieving a substantial reduction of peak flow, thereby maintaining the contemporary stormwater management goals (post-development infiltration volume to predevelopment level). This typical ES can store up to 123 m3 of runoff volume and exfiltrates to the surrounding soils. The average ES exfiltration rate ranged from 0.76 to 1.05 mm/hr over four observed complete drainage periods (full capacity to empty), indicates that the system is draining at a much slower rate than that was assumed when designing the exfiltration system (7.7mm/hr). As a consequence, it causes the system to require a much longer period to achieve complete drainage (drawdown) than the estimated 78 hours.

PCSWMM model was developed to analyze the annual water balance cycle and determine suitable alternative measures to control the ES overflow. Eight meaningful events were selected to calibrate and validate the model, most sensitive parameters such as hydraulic conductivity and imperviousness of subwatershed were calibrated to fit simulated ES inflow with observed inflow whereas orifice discharge coefficient and hydraulic conductivity of storage trench were calibrated for ES-overflow. Rainfall events larger than 24mm exceeded the storage capacity of the granular trench and caused overflow from the exfiltration system (ES2). Model results indicate that ES2 overflow could be easily avoided by diverting some amount of ES2 inflow to the underutilized upstream exfiltration system (ES1) by constructing diversion storm sewer to drain flow accumulated at manhole 5 (MH5) to manhole 4 (MH4).