An Improved Approach to Modelling Green Roofs with Below-Substrate Storage A Case Study on the Orlyplein Roof Park, Amsterdam

One of the challenges for green roof design and construction is the need to trap large volumes of water on roof structures with limited load bearing capacity. If this is attempted by using increased volumes of mineral substrate the ratio of water volume stored to total load can be insufficient to achieve the water attenuation aims of the system within the limited load constraints. One approach to this is to provide a reservoir directly below the substrate and supply the water by capillary action such as installed at Orlyplein , Amsterdam. By storing water immediately beneath the green roof substrate and making it available for the plants by capillary action, more water can be stored on the roof without adding more soil or substrate to the system; effectively shifting the weight balance on the roof from ‘storing soil, to storing rainwater. One of the advantages of of this system is that when available storage is limited because the reservoir is full and the substrate is at field capacity it can be rapidly increased to accomodate predicted rain events by draining water from the reservoir layer. Increasing the flood control capabilities of the green roof. Whilst providing a good start to the understanding of the performance of this type of green roof the a previously reported model to describe water availability was limited since the model assumed that potential evapotranspiration (PET) (using data supplied by the Dutch weather service) would always be achieved. The model reported here corrects for that assumption.