## 5 Conclusions  
This study addressed the challenges related to high peak discharges and low-flow issues in the creek that enters the sewer system in the study area (i.e., Ljubljana, Slovenia). The analysis was conducted using HEC-HMS, evaluating different scenarios incorporating various nature-based solutions (NbS), as well as combinations of these solutions. The implementation of permeable parking (Scenario 2) was the most efficient individual solution in reducing both runoff volume and peak flow in most simulations compared to green roofs, permeable streets and retention ponds. This is because permeable parking is key to reducing runoff by restoring permeability. Event 1 exhibited the greatest reduction in peak discharge, particularly in Scenarios 4 and 6, with a decrease of more than 70 l/s. Scenario 6, which combined all proposed nature-based solutions (NbS), was shown to be the most effective overall, with the largest reduction in peak flow. The implementation of multiple NbS is important for maximising benefits. Ponds were effective at increasing the lag times in most cases, helping to increase water storage and reduce peak discharges. The events that showed the best response to NbS implementation were small or shorter precipitation events. The impact was smaller for more extreme precipitation events with higher durations. Extreme rainfall volumes appeared to overwhelm the proposed solutions, as they became saturated part way through the event, at which point they lost their effectiveness. Ponds can be effective in reducing peak flows under certain conditions, but their storage capacity is limited during extreme rainfall events, and they require improvements in flow control (e.g., outflow gates) to maximise their effectiveness, which were not considered in this study. It was found that changing the impervious % for each scenario had an important impact on runoff and discharge. This highlights the benefit of NbS in terms of breaking up sealed urban areas and reflects a key benefit of NbS over grey infrastructure. The importance of this parameter in hydrological models is also reinforced. HEC-HMS is a hydrological modelling software that, through adjustments to the CN and impervious areas, provides valuable insights into the impact of different infrastructures. Although HEC-HMS offers a useful approximation, exploring other hydrological models may improve the accuracy of the results and facilitate comprehensive performance evaluations of NbS. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app142411813/s1](https://www.mdpi.com/article/10.3390/app142411813/s1)]([https://www.mdpi.com/article/10.3390/app142411813/s1](https://www.mdpi.com/article/10.3390/app142411813/s1)), Figure S1: Calibration performance of the model for Event 2; Figure S2: Calibration performance of the model for Event 3; Figure S3: Calibration performance of the model for Event 5; Figure S4: Calibration performance of the model for Event 6; Figure S5: Calibration performance of the model for Event 7; Figure S6: Calibration performance of the model for Event 8; Figure S7: Calibration performance of the model for Event 9; Figure S8: Calibration performance of the model for Event 10; Figure S9: Elevation-area curve for southeast (SE) pond; Figure S10: Elevation-area curve for northwest (NW) pond; Figure S11: Elevation-area curve for existing pond; Table S1: Pond dimension and design parameters; Table S2: Evaluation matrix of nature-based solutions; Table S3: Final evaluation of nature-based solutions matrix with weighted percentages; Text T1: Selection process for nature-based solutions. **Author Contributions:** Conceptualisation, L.P.-C., S.E.H. and N.B. ; methodology, software, validation, formal analysis, investigation, resources, data curation, writing\(-\)original draft preparation, visualisation, L.P.-C. and S.E.H. ; writing\(-\)review and editing, L.P.-C., S.E.H., N.B. and M.B.A.; supervision, N.B. and M.B.A.; project administration, N.B. ; funding acquisition, N.B.