Evaluating the Long-Term Performance of Bioretention Cell: A Five-Year Study from the Prague City Lab

Nature-based solutions (NbS) are gaining recognition as increasingly effective strategies to confront the escalating challenges posed by climate change and urbanization. By utilizing natural processes and ecosystems, NbS offers sustainable approaches to enhancing urban resilience, mitigating climate extremes, and improving environmental quality. These solutions, which are a form of blue-green infrastructure include green roofs and bioretention cells. They address critical issues such as flooding, heatwaves, and biodiversity loss while providing additional benefits like improved air quality and recreational spaces.

Within the Horizon Europe project NBSINFRA, the potential of NbS is investigated through City Labs experimental hubs established to implement, monitor, and refine NbS in diverse urban settings. Prague is one of the five City Labs, showcasing innovative NbS projects in collaboration with local stakeholders and institutions. A key site within the Prague City Lab is the University Centre for Energy Efficient Buildings (UCEEB), where advanced NbS technologies, such as green roofs and bioretention cells, are implemented and observed to assess their effectiveness in enhancing urban resilience. The presented study primarily focuses on the hydrological behavior and long-term performance of a small experimental bioretention cell at UCEEB, which serves as a critical component of the Prague City Lab.

This study outlines a five-year experiment conducted on the bioretention cell at UCEEB, designed as a multilayered system with a biofilter composed of 50% sand, 30% compost, and 20% topsoil, sand layer and a drainage layer, planted by perennial vegetation. Bioretention cell is isolated from the surrounding soil by a waterproof membrane and is instrumented by a system of sensors. Four time-domain reflectometry probes monitor soil water contents of biofilter and five tensiometers record the water potential in a biofilter. The amount of a discharge from bioretention cell is recorded by a tipping bucket flowmeter and inflow is measured by rain gauge. Over the course of five years, the study focused on parameters such as water balance, retention capacity, soil water potential, and plant growth to evaluate the cell's hydrological performance and its evolving efficiency.

Results of experimental study and modeling using HYDRUS 2D revealed significant temporal changes in the performance of the bioretention cell. The runoff coefficient decreased over time due to increased evapotranspiration. Peak flow reductions ranged from 30% to 100% for individual rainfall epizodes. Median runoff delays were approximately 50 minutes, and peak flow delays varied from 0 to 100 minutes, indicating increasing variability over time. Inverse modeling in Hydrus 2D demonstrated a fivefold increase in the saturated hydraulic conductivity of the biofilter, alongside with a decrease in the saturated hydraulic conductivity of the sand layer. These findings offer valuable insights into the long-term performance of bioretention cells and their contributory role in advancing sustainable urban stormwater management through NbS.