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In recent decades, green roofs are encouraged as effective tools of sustainable urban drainage system for stormwater management. They contribute to the reduction of runoff peak flows and volume discharges to sewer systems. Green roof reliability in stormwater control is mainly a function of storage capacity, given by the growing medium and the drainage layers. While the thickness of this last layer is usually defined by standards, the growing medium thickness strictly depends on vegetation type and rainfall regime. This paper presents an analytical probabilistic approach to evaluate this thickness as a function of the reliability of green roofs in term of runoff reduction. The possibility of pre-filling from previous events was also considered, by mean of chained rainfall events. The proposed model has the advantage to combine the simplicity of design methods with the accuracy of continuous simulation. The proposed equations were validated by an application to a case study in Milano, Italy. Monthly analyses were carried out to highlight monthly differences in roof operation due to rainfall distribution and evapotranspiration rate all along the year. Results showed a good agreement with those obtained by the continuous simulation.
green roof, probabilistic model, stormwater management, SuDS
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