Runoff Quantity Dynamic from an Extensive Green Roof during Individual Rainstorms

This study evaluates the influence of an extensive green roof on runoff quantity dynamic during individual rainstorms in Central China. Controlled experiments on a simulated extensive green-roof plot were carried out firstly to produce a unit hydrograph (UH) by the methods of S-curve linearization treatment and curve fitting. Then in situ tests were conducted to compare runoff quantity per minute from the green roof plot and a conventional roof plot to verify the obtained UH and compare their runoff quantity dynamic during two rain storms in the year of 2009.The obtained UH is able to predict accurately peak flows and runoff volumes for any rain input. When the initial water content of the substrate was dry, the 7 cm thick green roof can delay the runoff initiating for nearly 8 minutes, lower runoff peak and volume by about 50% compared with conventional roof during a 10.1mm rainstorm on 7 June; and even when the substrate was wet, the extensive green roof can detain runoff effectively by 129 min longer than conventional roof and thus lower the peak flow of runoff by 43% during a 6.3 mm rainstorm on 12 March.