A long-term hydrological modelling of an extensive green roof by means of SWMM

Green roofs provide multiple environmental and social benefits, among which the opportunity to control storm water runoff as they limit the rate of runoff after urbanization to the rate that would have occurred before urban development. The hydrological behaviour of a green roof is site specific, thus the local environmental parameters, the characteristics of the vegetation and the physical properties of its layers have to be considered in the evaluation of its performance. Furthermore, the hydrological performance of a green roof is influenced by the size of the plot (full-scale vs small scale), by the definition of “event”, and by the number of events included in the research. From this broader context this paper first provides a review of the scientific literature, with a focus on the hydrological behaviour of experimental full-scale installations and on hydrological modelling of green roof performance. Second, the study presents the results of a monitoring activity of a full-scale extensive green roof in Bologna (Italy). Continuous weather data and runoff were collected between January and December 2014, resulting in 69 storm events suitable for the study. Experimental data show that single event rainfall attenuation ranged from 6.4% to 100% with an annual average value of 51.9% which is consistent with other author's findings. Last, the study uses the field data to calibrate and validate a numerical model realized with the commercial software SWMM 5.1. The model was used to simulate the long-term hydrologic response, over one year, of the same full-scale extensive green roof and to compare it to an adjacent impervious roof of the same size. Modelling results confirm the role of green roofs in restoring the natural water regime by reducing the annual runoff volume. The comparison of the results between the experimental green roof monitoring and the SWMM simulation proved that the suggested model has good capabilities in simulating the hydrograph of stormwater runoff from green roofs along the year, as demonstrated by the quite high values of NSE and the low value of RSR in both the calibration and validation phase. Furthermore, the low difference (< 9%) in total retention between the 69 measured and simulated events confirms the suitability of the model for long term simulations. The proposed modelling approach demonstrates that SWMM can be used for assessing the performance of LID systems (Low Impact Development), and consequently for supporting local authorities or designers in the evaluation of the hydrological efficiency of green roofs.