Field experiment-based investigation on hydrological benefits of typical sponge facilities in the Yangtze Delta region

In order to enhance the ability of urban waterlogging prevention and improve urban adaptability of urban areas to changing environments, it is significantly important to investigate the hydrological benefits of different sponge facilities under the climate pattern of the Yangtze Delta region. Four typical sponge facilities in the Zhenjiang Sponge Base, the first national sponge pilot city, were selected as the study cases. Several indicators, such as runoff coefficient, runoff reduction rate, peak-cutting rate, and flood peak, were selected to evaluate the rainfall-runoff relationship, analyze the hydrological performance under different rainfall intensities and patterns, and explore the impact of operation time on the hydrological benefits of the sponge facilities. The results showed that the permeable pavement sponge facilities demonstrated a single rainfall-runoff relationship, and in contrast the green plant facilities showed a segmented rainfall—runoff relationship. For the green plant, the rainfall—runoff curve underwent a transition when the event rainfall amount exceeded a critical storm rainfall amount. As a result, the mean rainfall coefficient increased by 8.4 to 38.5 times, and the average peak-cutting rate and runoff reduction rate decreased by 50.4% and 44.6%, respectively. Under storm conditions, all sponge facilities exhibited the highest runoff generation capability and flood peak but displayed the lowest total runoff reduction and peak-cutting abilities. This effect was more remarkable in the conditions of storm to heavy rain than in the conditions of heavy rain to moderate rain. The hydrological performance of the sponge facilities was jointly affected by the rainfall pattern, the average rainfall intensity, and the maximum unit rainfall intensity. Except for the rain garden, the runoff coefficient of all sponge facilities was most sensitive to the changes of the aforementioned factors, followed by the flood peak, and the runoff reduction rate was the lowest. The permeable pavement showed a significantly weakened runoff control ability after 1-year operation, and its runoff generation capability and flood peak were significantly augmented by 1.7 to 2.1 times and 1.9 to 2.5 times, respectively.