A Comparison of Green Roof Systems with Conventional Roof for the Storm Water Runoff

Abstract

The overall goal of this chapter is to understand how extensive green roofs influence the storm water runoff reduction for typical Japanese rainfall patterns and to propose a new concept for controlling storm water runoff in densely populated cities. In this study, we calculated the effect on storm water reduction in an urban area and compared the delay time and runoff volume for three roof types: a traditional green roof system, a green roof system with a drainage layer, and a conventional roof. First, to examine how drainage layer thickness affects storm water reduction, runoff was examined for green roof systems with drainage layers of various thicknesses (25, 30, 40, 45, and 55). A typical Japanese rainfall pattern called “local torrential rainfall” with a rate of 150 mm/h was used in an experiment to demonstrate the effect of drainage layer thickness on storm water reduction. The mean storm water reduction was least at 30 mm (0.005 L/min), with a maximum of 45 mm (0.169 L/min), and the mean runoff coefficient was the lowest at 45 mm (C = 0.63) and highest at 30 mm (C = 0.97). Next, to estimate the effect that roof structure has on storm water reduction, runoff was calculated for the three different roof types in a case study involving 17 flood-prone districts in Chiyoda Ward, Tokyo, Japan. The storm water volume was calculated assuming that traditional green roofs or green roofs with a drainage layer were installed on buildings in a commercial district, on public facilities, and on buildings in an industrial district. Runoff volume was simulated for typical Japanese rainfall events: a local torrential rainfall (July 4, 2000) and a typhoon (August 27, 1993). The calculation results revealed the following: the traditional green roof was able to reduce the peak runoff to 71.7 %, and the extensive green roof with a drainage layer was able to reduce the peak runoff to 57.1 %. Finally, we discuss the significance of green roof systems on new storm water control strategies in urban areas of Japan.