Use of Fragility Curves to Evaluate the Performance of Green Roofs

Abstract

Green infrastructure offers an alternative multibenefit method of runoff reduction in urban areas, using natural processes to infiltrate, store, and treat runoff near its source. However, green infrastructure performance is highly variable, and few consistent standards exist to help designers and planners decide whether or not a green infrastructure is performing as expected. This paper provides a reliability analysis–based methodology that can be used to evaluate green infrastructure performance, demonstrated for green roof peak runoff reduction. The authors characterize green roof failure graphically with a visual aid typically used in earthquake engineering: fragility curves. These fragility curves plot the probability of failure with changing storm intensity to show the runoff reduction provided by a green roof compared with a conventional roof under different storm scenarios. The use of reliability analysis as a part of green roof design code can help to test for weaknesses and areas for improvement, quantitatively characterizing the hydrologic performance of green infrastructure.