Modelling green roof stormwater response for different soil depths

Green roofs have been proposed as a way to mitigate stormwater run-off in urban areas due to the possibility of retrofit to existing buildings. The amount of run-off is influenced by the, humidity, evapotranspiration, as well as soil type and depth. A modelling approach was undertaken to evaluate the response of different soil depths to cumulative rainfall and the efficiency in stormwater flow rate attenuation. The soil hydraulics were modelled using HYDRUS-1D software developed for modelling water flow in variably saturated porous media. Model runs were carried out for three quarterly scenarios to determine run-off peak flow rates and the overall retention, based on evapotranspiration rates of succulent plants and rainfall registers from Auckland, New Zealand. The soil depths modelled ranged from 5 to 160 cm. The efficiencies in peak flow attenuation by the shallowest soil considered were reduced under extreme and longer rainfall events by 3%. Therefore shallow soil or extensive green roofs may, on a wide scale, overcome the performance of deep soils due to their lighter weight which adds limited loads to existing roof structures thereby making them suited to retrofit greater numbers of buildings.