Modelling of rainwater reduction and hydrological performance of selected green infrastructure (GI) facilities in urban catchments

Currently, effective rainwater management in urban areas requires a modern and comprehensive approach, with mathematical modelling methods at its foundation. The aim of this study was to estimate the impact of the use of dispersed green infrastructure (GI) – specifically, drainage-equipped rain gardens (bio-retention cells, or BC) – and their operational hydrological performance on the scale of an urban catchment, particularly in reducing the amount of rainwater discharged directly to the receiver. An additional objective was to determine the relationship between the total rainwater inflow per unit area and the capacity percentage reduction in individual BC facilities.

In this research, a hydrodynamic single-event precipitation model was built and calibrated for an existing housing estate in Cracow (Poland) using the Storm Water Management Model (SWMM). The functioning of the GI is represented by Low Impact Development (LID), which was simulated for a number of precipitation scenarios (S₀–S₇) with durations of 4.2, 15, 30, 45, 60, 90, 120 and 180 minutes, and a probability of occurrence of p = 10% (indicating a 10% chance of occurrence in any given year).

The results demonstrated significant potential for reducing the peak flow of rainwater by an average of 94%, and reducing the total volume of rainwater by an average of 86%, for all simulated precipitation at the outfall of the entire system. Analysis of individual BC facilities revealed reductions in rainwater ranging from 67% to 95% of capacity for GI facilities, depending on the duration of the rainfall. Notably, more significant reductions in GI facilities were observed in cases of short, intense rainfall. The analysis of the individual BC facilities with the highest efficiency showed that the maximum values of total inflow per 1 m² of their surface area, reduced to 100% for rainwater inflow, occurred during heavy rainfall of 15 min (S₁) and amounted to 29.72 L/m² (average 5.79 L/m²).