Evaluation of Seasonal and Large Storm Runoff Volume Capture of an Infiltration Green Infrastructure System

Abstract

Green infrastructure, including stormwater control measures (SCMs), meant to reduce runoff volumes are typically designed for smaller, frequently occurring events (e.g., 2.5 cm or the 2-year, 24 h event), and their performance are not always considered for larger, more infrequent extreme event management. However, even during larger rainfall events, a SCM is still capturing at least the design rainfall and reducing runoff volume. Further, if a SCM relies on infiltration as its primary volume reduction strategy, infiltration continues throughout the rainfall event indicating that SCMs can be considered in flood mitigation strategies. This phenomenon of greater than designed runoff volume removal has been noted in several infiltrating SCMs, and the present work uses the Villanova green infrastructure site comprising an infiltration trench with pretreatment (i.e., linear vegetated SCMs) designed for the 2.5 cm rainfall event as a basis for discussion. The analysis focuses on the infiltration trench volume capture and includes the effect of storm volume, duration, intensity, and antecedent dry time. Seasonally, infiltration rates within the infiltration trench were shown to vary with temperature in the lower ponding depths. While there was some decrease in infiltration rates in the lowest depth increment (0–0.3 m) in the systems’ third winter and spring seasons, there was a general increase in infiltration rates over the course of the study period demonstrating the infiltrating SCMs’ ability to be a reliable stormwater management solution. With regards to large storm event performance, the results from 2 years of monitoring the infiltration SCM system have been quite impressive. The system has consistently met the volume reduction design goals and usually exceeded them by significantly contributing to storm runoff mitigation during large extreme events. The infiltration SCM system captured and removed at least 59% of the volume of every storm event analyzed from July 2012 through June 2014, with an average of 93% capture for events greater than the design volume of 2.5 cm. Furthermore, there were 159 storm events totaling 259 cm of rainfall during the study period. Given the drainage area loading, approximately 2,365 m3 of rainfall entered the linear vegetated component, with approximately 420 m3 (18%) reaching the infiltration trench and only 103 m3 (4%) of rainfall determined to overflow from the system. These performance results demonstrate that infiltration SCMs can reduce significant volumes of runoff during larger events and that new design strategies are needed to account for their performance.