Evaluating the Influence of Imperviousness on Urban Runoff and Drainage System Efficiency

Urbanization leads to the conversion of pervious surfaces into impervious surfaces, significantly altering the hydrological conditions of urban catchments. These changes adversely affect the conveying capacity of urban drainage systems, resulting in increased overflow and eventually increase urban flooding risk. This study aims to assess the impact of imperviousness on rain induced urban flooding and to evaluate the effectiveness of (NbS) in improving runoff management using advanced modeling techniques for a neighborhood of the Trento Municipality (Italy). To achieve these objectives, 1D/2D hydrodynamic modeling approaches was applied to simulate the runoff generation, its routing within the designed drainage system and the flooding propagation on the study area. Within the 19 sub-catchments experiencing flooding under existing conditions and extreme rainfall events, NbS, specifically green roof and bio-retention cell were applied to manage runoff.Results demonstrate the capability of the proposed NbS to reduce flooded areas, runoff volume by mimicking natural processes (i.e., delaying runoff time and promoting infiltration). Reducing the imperviousness of the study area by 6% provides a reduction of the flooded area and runoff volume by 40% and 50%, respectively.Overall, the findings confirm that increasing NbS coverage significantly enhances urban drainage efficiency and mitigates urban flooding, highlighting the importance of sustainable urban planning and green infrastructure strategies for effective flood management.