Hydrological and energy performance of multifunctional green roofs

Abstract

Green roofs are a classic example of multifunctional, sustainable urban drainage systems, which have the potential to retain stormwater on the roof's surface and lower a building's energy consumption. To maximise storm water management benefits, an increasing number of modular green roof systems have begun incorporating a drainage and water storage layer into their design.

The main aim of this PhD study is to quantitatively evaluate the hydrological and thermal benefits of modular green roofs by conducting experiments and developing a water and energy balance model. During the first phase, three full-scale experimental green roofs were installed on a student residence building in London, UK. The two extensive systems are composed of a shallow substrate covered by drought-tolerant plants. For the latter, a drainage layer with water storage capacity and a wicking mat was implemented. A third, low-maintenance, intensive system was designed with large water storage cells and a wicking mat to improve water availability. The results of 18 months of data demonstrated that the drainage layer with water storage capacity improved both the hydrological and energy performance of the extensive green roof. The intensive green roof outperformed both extensive green roofs in terms of retaining and delaying runoff and regulating roof surface temperature.

In the second phase, a one-dimensional water and energy balance green roof model was developed to predict runoff from green roofs, model soil moisture changes within the substrate layer and simulate temperatures of foliage and the substrate surface. In comparison to the experimental results, the proposed model exhibited good ability to represent the water and heat dynamics in the modular green roof systems. Future work that evaluates the developed green roof model over a broader range of rainfall events and documents comparisons of green roof performances with conventional roofs is needed.