Optimal design of stepped bioretention cells for slopes

•A numerical model was developed based on laboratory results to simulate stepped bioretention cells.•Underdrains increased peak runoff and volume reduction but decreased exfiltration and water distribution uniformity under all environmental conditions.•Intermediate pipes enhanced exfiltration when underdrains were present.•Reducing the number of cells only improved peak runoff reduction under less-intense rainfalls. Stepped bioretention cells (SBCs) refer to multiple bioretention cells (BCs) built in series along slopes. They are green infrastructures that can possibly be implemented on slopes. However, their optimal design has not been thoroughly investigated. The use of multiple cells makes the design of SBCs distinct from that of traditional BCs. In general, the design of a single BC focuses on water distribution among different horizontal layers (i.e., surface, soil, and storage layers), while that of SBCs additionally focuses on water distribution among cells. To examine the optimal design of SBCs, this study constructed a physical model of SBCs and conducted eight representative experiments. A numerical model was built, calibrated, and validated based on the experimental results and then used to evaluate the effects of design measures, namely changes in the number of cells and installation of pipes (i.e., underdrains and intermediate pipes). Intermediate pipes connect the storage layers of two adjacent cells. Four metrics were used to compare the effectiveness of design measures under various environmental conditions: peak runoff reduction, runoff volume reduction, exfiltration, and water distribution uniformity (WDU). The first three metrics reflect the overall hydrologic performance of SBCs, while the last metric reflects the distribution of infiltrated water among the cells. The findings showed that reducing the number of cells increased peak runoff reduction under less-intense rainfall events and increased WDU. Underdrains significantly improved peak runoff reduction and runoff volume reduction but decreased exfiltration and WDU under all environmental conditions. Elevated underdrains encouraged more exfiltration than bottom underdrains. Intermediate pipes enhanced exfiltration but had a limited effect on peak runoff reduction. However, intermediate pipes decreased runoff volume reduction and even led to a poor WDU when underdrains were absent. Overall, the findings suggest that SBCs are an effective form of green infrastructure for slopes as the