Hydraulic and Hydrologic Invariance: Effectiveness of Green Roofs and Permeable Pavements

In this paper, the capability of selected sustainable drainage systems (SuDS) to accomplish hydraulic hydrological invariance (HHI) was analyzed. The HHI principle is achieved if both the peak flow and the volume runoff of the urbanized catchment remain equal after any increase of impervious surfaces. In particular, the hydraulic performance of green roofs and permeable pavements was evaluated by considering a standard case study frequently encountered in design practices. The numerical simulations were conducted using Storm Water Management Model (SWMM) software. The efficiency of SuDS was assessed by varying the effective impervious area (EIA) along with the main rainfall characteristics. The findings of this research show that the global performance of a SuDS-retrofitted catchment improves with the EIA reduction. Moreover, the rainfall characteristics, namely the rainfall duration and the return period, influence the hydrologic efficiency moderately. The experimental results confirm the positive impact of SuDS in mitigating the effect of the escalating urbanization on the existing urban drainage system. This effect was evaluated by quantifying the reduction of the choking occurrence in the sewer pipe and structures downstream the urban lot. However, the HHI principle is barely achievable with the standard implementation of SuDS, as also proved in a design graph to appraise how close SuDS are to HHI compliance.