Runoff retention assessment for extensive green roofs and prioritization of structural factors at runoff plot scale using the Taguchi method

Abstract

In spite of the well-known green roof benefits, their widespread adoption in the management practices of urban green infrastructures requires the design of adequate structural factors/levels or its optimal combination. Using the Taguchi method, this study assessed the structural factors affecting runoff retention of extensive green roofs. The structural factors considered in this study consisted of substrate material, substrate depth, slope and vegetation type, each of which had three levels. Nine runoff plots of extensive green roofs were designed according to the Taguchi’s orthogonal array L9 (four factors at three levels). An analysis of the signal-to-noise (S/N) ratio was used to evaluate the runoff discharge of green roofs. The results showed that substrate material S2 has lower runoff discharge than substrate S1 and S3. The amount of runoff discharge increases as the substrate depth increases from 5 cm to 15 cm. There were slight differences in runoff discharge among vegetation types of green roofs. The runoff discharge of green roofs with 2% slope was higher than the runoff discharge of 7% and 12% slope gradients. Substrate material aspect made the greatest contribution of runoff retention (53.9%) and followed by substrate depth contribution (26.2%). The results from regression analysis of the predicted versus measured runoff discharge showed that the Taguchi method is able to predict the runoff discharge well (R2 = 0.96, p < 0.001), and it confirmed the high credibility of the Taguchi method to predict runoff retention in response to different combinations of factors and levels.