Simulation of Runoff Reduction Capacity of Green Roofs and Their Impacts on Urban Flooding: A Case Study of Beichen District, Kunming City

Abstract China is constructing sponge cities, with focus on green roofs to counter increasing incidences of urban flooding. However, very few studies have investigated the performance of green roofs in real urban environments. Here, the Storm Water Management Model was used to simulate changes in surface runoff with green roofs while analyzing the impact of green roofs on urban flooding in Kunming, Beichen District, China. Comprehensive evaluations using the technique for order preference by similarity to an ideal solution (TOPSIS) were performed—from cost and benefit perspectives—to provide a research basis for green roof construction in Kunming. This study revealed the following: (1) higher rainfall intensity led to lesser runoff reduction. For rainfall recurrence intervals of 1, 2, 5, and 10 y, the runoff reduction rate for a green roof coverage of 10 % was 2.7, 2.1, 1.6, and 1.4 %, respectively. (2) The peak flow decreased with increasing coverage rate. However, under heavy rainfall, low coverage rates did not significantly reduce the peak flow. The delay time of the peak flow significantly decreased with increasing rainfall intensity. (3) The number of ponding points and the longest ponding duration exhibited a gradual downward trend with increasing green roof coverage. Green roofs with low coverage played an optimal role in mitigating flooding under heavy rainfall conditions. (4) Entropy weight-TOPSIS revealed that the optimal green roof coverage area in the study area was 60 %, which can be used as the priority construction goal for the popularization of green roofs in Kunming.