A simple method for sizing modular green–blue roof systems for design storm peak discharge reduction

Abstract

Typical green roof systems are used to improve downstream water quality and reduce long-term total runoff. However, they perform poorly at peak discharge reduction during large design storm events. This performance can be significantly improved by the addition of storage a layer (blue roof) underneath a green roof system. This paper presents a simple design methodology for designing modular green–blue roof systems to reduce this peak discharge. In particular, the methodology can be used for the preliminary design of the blue roof module’s outflow control sizing. It is then shown how the resulting design can be incorporated into a standard hydrologic modeling system for a more detailed analysis. Results of an example design show that the addition of just 3.8 cm (1.5 in) of storage can result in a 38.6% reduction in peak discharge for a rainfall depth of 17.2 cm (6.78 in) compared to a green roof without underlying storage. Increasing the storage depth to 7.6 cm (3 in) for the same storm resulted in a 78.2% reduction of peak discharge.