Gestion à la parcelle des eaux pluviales: étude d'une toiture végétalisée, bilan et retour d'expérience Stormwater management at the local scale: a green roof study, review and lessons learned

The Geneva Canton (Switzerland) promotes a stormwater management policy at the plot scale aiming at better controlling/mastering the floods in densely urbanised areas. With the aim of better documenting the green roof approach, we studied the case of the Seymaz High school (Thonex commune), a building with a green roof surface of nearly one hectare and a substrate depth of 20 cm. The study is based on the continuous monitoring of local precipitations and outflow from the roof over a five-year period starting in 2007 when the construction was finished. The analysis allowed the evaluation of the actual effectiveness of a concrete example of green roof as a tool for water management as well as a quantification of its impact on the receiving waters. The results demonstrate system's capacity to absorb high-intensity, high-volume rainfalls having a return period of over two years without a sizeable drainage at the outlet. On the contrary, when substrate's storage capacity is exhausted a threshold effect is taking place with precipitation-to-discharge conversion rates approaching unity. The attenuation of peak-flows remains nevertheless better than that of a conventional roof. As the history of climate forcing on a longer period (days to tens of days) is determining system's response, we propose as an assessment and evaluation tool a simple but robust hydrological model. This model is able to provide statistics of drainage volumes and discharges by simulation on long series of input data. Moreover, a preliminary water quality study didn't show any contamination by herbicides and the quality of the drainage water is considered as sufficient for being returned to the natural environment.