Vegetated roofs for managing stormwater quantity in cold climate

Vegetated roofs, i.e. green roofs, were continuously measured for runoff to quantify their capacity to manage stormwater in a field experiment in southern Finland, with distinct seasonality and varying weather conditions. Attention was paid to heavy storms to study the potential in mitigating urban flooding. In addition, the impact of biochar amendment (10% v/v) on rainfall retention was studied. Meadow vegetation was established on crushed brick based substrate either by introducing plants as seedlings and seeds or by using pre-grown readymade mats. Annual retention varied from 40% to 70%. The highest retention, ca. 80%, was found in summer even though it was the rainiest season. Wintertime retention was 30-40%. The coarse substrate enabled infiltration even when frozen and, thus, the roofs operated also in winter. Heavy storms occurred mainly in summer, the season with the highest hydrologic performance of the roofs, resulting in >80% peak attenuation and slow release of runoff. In individual rain events, runoff from meadow roofs was largely a function of rainfall depth. However, retention was weakly explained by the amount of rainfall or by other variables, such as rainfall intensity or antecedent dry period, indicating the difficulty in capturing the complex phenomena behind variable weather conditions. Biochar improved retention only slightly, at maximum by ca. 10%. The empirical evidence of this study highlights vegetated roofs as a feasible technology to be applied in urban runoff management even in cold climate.

Automated summary

The study in southern Finland showed that vegetated roofs with meadow vegetation can effectively retain rainwater, with the highest retention rate in summer and still functioning in winter. Biochar amendment had minimal impact on rainfall retention.