AbstractPermeable pavements equipped with an underdrain are one of the most widely used and efficient types of green infrastructure. They can greatly reduce, delay, and retain surface runoff, given their high surface infiltration rate and storage volume; however, their performance in shallow groundwater environments is poorly understood. Based on the monitoring data of three underdrained permeable pavements in Hong Kong collected from April to November 2017, this study demonstrates and quantifies the impact of shallow groundwater on the hydrologic performance of permeable pavements. All of the permeable pavements achieved 70%–100% and 90%–100% in peak and volume reductions of surface runoff, respectively, for 90% of the rainfall events, even after 1 year of service without maintenance. However, 4,000–10,000 mm of extraneous water—equivalent to three to six times the rainfall depth during the monitoring period—entered the reservoirs of two pavements and was discharged through their underdrains. The drawdown times of these two pavements, both of which were equipped with underdrains, were >24 and >72  h for 35% and 20% of the rainfall events, respectively. Underdrains did not reduce drawdown times; instead, they discharged the extraneous water from the subsurface into the sewer system. These findings demonstrate the deficiency of underdrains and the need for careful underdrain design for permeable pavements in shallow groundwater environments. In areas of shallow groundwater, detailed site investigations are recommended. Underdrains, when needed, should be elevated and installed with flow restrictors to restrict their maximum outflow and to strike a balance between drawdown time and underdrain outflow volume. The technical design of the underdrain is demonstrated to be a key factor for green infrastructure in shallow groundwater environments; it should be more highlighted and detailed in the design guidance of green infrastructure.

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