A physics-based routing model for modular green roof systems

Abstract

The ability of green roof systems to impact stormwater runoff from buildings has been covered in many research studies. However, a lot of these studies looked at long-term retention, with much less work focused on how to model a green roof's response to larger design storms. Work that has examined flow routing for individual rainfall events has focused on empirical routing models that are tuned to the specific roof being modelled. This paper presents a new physics-based model for flow routing based on the green roof module geometry and soil properties, which requires only a single discharge coefficient to be measured. The results of this model were compared with the results of a series of experiments to quantify a modular green roof system's hydraulic response to drawdown and steady rainfall.

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

“…Theoretical routing models have been developed and applied to green roof systems to predict rainfall-runoff behavior [9], to calculate the system's initial abstraction [10], to quantify the detention performance [11], and to quantify their ability to reduce the peak discharge from major rainfall events [12]. However, in general it is seen that extensive green roof systems have limited storage capacity relative to large design storm events and, therefore, have minimal impact on peak discharge during these more significant rainfall events.…”

“…However, in general it is seen that extensive green roof systems have limited storage capacity relative to large design storm events and, therefore, have minimal impact on peak discharge during these more significant rainfall events. In an experimental and modeling study of modular extensive green roof systems specifically looking at major rainfall events, Martin et al [12] showed that the drawdown time for a typical modular green roof system is of the order of a few minutes. These results match what has been seen for non-modular green roofs as well [13].…”

“…Even with the potential for the modular green roof systems to be designed to enhance runoff detention, Martin et al [12] found that the drawdown of the modules was very fast. One reason for the rapid drawdown time is the need for multiple drain holes in the base of green roof modules to avoid long-term pooling of water around the plant roots.…”

“…One reason for the rapid drawdown time is the need for multiple drain holes in the base of green roof modules to avoid long-term pooling of water around the plant roots. A second constraint on green roof outflow controls highlighted by Martin et al [12] is that the presence of soil in the module can lead to clogging of the drain holes if they are not sufficiently large. In their study, Martin et al [12] sealed all but one drain hole in each submodule tested to increase the drawdown time though the effect was to increase it from the order of 10-20 s to a few minutes.…”

“…A second constraint on green roof outflow controls highlighted by Martin et al [12] is that the presence of soil in the module can lead to clogging of the drain holes if they are not sufficiently large. In their study, Martin et al [12] sealed all but one drain hole in each submodule tested to increase the drawdown time though the effect was to increase it from the order of 10-20 s to a few minutes. To overcome the storage and discharge problems, socalled green-blue roofs have been developed and investigated [17,18] that combine the water quality benefits of a green roof system and the detention benefits of a blue roof.…”