Towards climate resilient green-blue roofs

Abstract

A green-blue roof consists of a water storage layer with on top a substrate layer covered with vegetation. Due to the presence of the water storage layer, a green-blue roof is better capable of retaining heavy rain events. A movable valve makes it possible to manage the amount of water on the roof and the timing of drainage from the roof to the sewer system, while in addition the stored amount of water is made available to the vegetation layer via a passive capillary irrigation system. This could potentially result in a higher evapotranspiration rate and therewith a reduction of the sensible heat flux compared to green roofs. Because of its qualities, green-blue roofs have been added to the list of measures that contribute to mitigation of the Urban Heat Island (UHI) effect and pluvial flooding. However, during dry summers a third climate related challenge arises namely drought. The question arises whether it is sustainable to increase the amount of vegetation in cities, as this increases the water demand during droughts. During long dry spells it can be challenging to store enough water for vegetation and cooling while keeping sufficient empty storage available at the same time. A conflict in water related functionalities of the roof is the result. It was the aim of this thesis to investigate how implementation of green-blue roofs can be made climate resilient by defining its strengths and weaknesses regarding temperature management and water consumption and come up with possible ways to improve the roof system. By conducting a measurement campaign in the summer of 2020, it was investigated if the presence of a water storage layer indeed enhances the cooling effect of a green-blue roof on the indoor and outdoor environment. Thermal fluxes at a green-blue roof and a conventional black roof were analysed, as well as two situations with either an empty of full water storage layer at the green-blue roof. Furthermore, a bucket model was designed to study the climate resilience of green-blue roofs for the climate scenarios of the KNMI for 2050. Based on the results, it is concluded that additional adaptation measures are required to make sure green-blue roofs can still contribute to a better and more resilient urban area towards the future. Several measures are available to improve the performance on water retention and drought resilience, like valve management, enlargement of the storage capacity on the roof or on ground level and irrigation. Closing the water cycle locally is important to make green-blue roofs self-sustainable in water consumption, which reduces the risk on conflicts on water use during droughts. Only regarding UHI mitigation, other measures like creating shade could be more efficient as the enhanced cooling of the urban area due to unlimited water availability is small, unless largescale application of green-blue roofs.