STORMWATER MITIGATION BY EXTENSIVE GREENROOFS IN A TEMPERATE CLIMATE

摘要

Greenroofs are increasingly used as a stormwater management tool to mitigate the effects of altered urban hydrology. Literature cites retention of -50-68% of annual precipitation and 50-90% peak flow reduction for individual rainfall events for full-scale greenroofs. The performance of green roofs for stormwater control is being quantified in New Zealand at the University of Auckland (6 plots totaling 234 m~2) and Waitakere City (170 m~2 monitored plot within a 510m~2 greenroof). At the University, three substrate blends (including the Waitakere substrate) are installed in hydraulically isolated plots at depths of 50 mm and 70 mm. Runoff from the c.100 mm deep Waitakere greenroof is compared with that from a 77-m2 conventional roof. Hydrology is analysed on an individual rain event basis. Complementary glasshouse trials quantify the contribution of evapotranspiration to the availability of stormwater attenuation volume.rnThe University greenroof plots retained ~ 80% of the 191 mm of total rainfall from 17 spring/ summer events ranging from 0.8 to 59 mm. Peak flow reduction per storm was ~ 92% of peak rainfall intensity for 50 and 70 mm substrate depths. Data indicate that green roofs are most effective at mitigating low-intensity rainfall events, small events, and under dry antecedent weather. In contrast, attenuation of -10-20 mm winter events was 35-50% at the Waitakere greenroof. Glasshouse trials show the attenuation volume is optimized when plants are actively transpiring - unstressed, drought-tolerant plants contribute ~ 47% of evapotranspiration benefits and increase the speed at which attenuation volume becomes available compared to bare substrate. When plants are stressed soil moisture depletion is similar to bare substrate. Attenuation of stormwater volume runoff by greenroofs is therefore likely greater in temperate