Risk-Based Methodology for Evaluation of Bioinfiltration Design and Performance

Abstract

Green infrastructure practices are being implemented as a sustainable way to manage stormwater. Many communities are implementing green infrastructure practices on their own and in conjunction with gray infrastructure upgrades as an adaptation tool to mitigate risks and increase resilience in the face of climate change. Using a risk-based approach for design and planning has been common practice in flood control, but is not common in green infrastructure design. A risk-based approach for analyzing the performance of bioinfiltration practices was developed to assist in design and evaluation of performance over time. Bioinfiltration practices are vegetated basins that capture and retain runoff through infiltration and evapotranspiration without the use of an underdrain. Bioinfiltration facilities with varied ponding depths, media depths, and native soil saturated hydraulic conductivity based on regional design guidelines, and a field-calibrated site, were modeled over a 30-year period and compared to a predevelopment condition as the target design performance, which defined the acceptable risk. In this analysis, risk was determined for each bioinfiltration design based on the product of the probability of a rainfall event occurring, the probability of an overflow event occurring, and the average volume of overflow; the terms used to define risk can be modified to reflect local priorities. Cumulative annual risk for bioinfiltration designs was compared to acceptable risk to determine appropriate design. Results indicated that designs with increased ponding depth decreased cumulative risk. Designs with higher native soil hydraulic conductivity had reduced risk when compared to a predevelopment scenario and observed in the field-calibrated bioinfiltration site. The risk-based approach provides a helpful tool for municipal separate storm sewer system communities and combined sewer communities for design and evaluation of bioinfiltration practices.