Methodology for holistic assessment of grey-green flood mitigation measures for climate change adaptation in urban basins

A novel methodological framework for urban drainage adaptation to climate change is introduced and tested in a small urban basin. The impact of climate change on Intensity-Duration-Frequency curves and on urban drainage is reported. The flood mitigation measures comprised both green and grey alternatives, i.e., permeable surfaces and sewer enlargement. The Storm Water Management Model (SWMM5) was calibrated and employed. Prior to calibration, sensitivity analysis of SWMM hydrologic-hydraulic and permeable surfaces input parameters was conducted using a global sensitivity analysis method. Two novel indices, the Index for Reliability of Urban Drainage Adaptation (IRUDA) and the Climate Variability Adaptation Index (CVAI), were also introduced. IRUDA assesses the total reliability of an urban drainage network in hydraulic, temporal and volumetric terms. Sizing of flood mitigation measures was undertaken employing construction cost along with IRUDA. CVAI was used for alternative selection among different flood mitigation scenarios. CVAI assesses the total adaptive capacity of the drainage network based on hydrological, hydraulic and economic aspects of the proposed adaptation plan. A forward uncertainty propagation scheme was employed on the selected alternative in order to examine input data and parametric uncertainty with respect to the peak discharge at the outlet of the basin. Finally, monetization of ecosystem services offered by green measures is presented. The simulation results for the study area illustrated that under the non-stationarity assumption future rainfall intensities and urban flooding risk will increase, and therefore, further flood mitigation measures will be required. Furthermore, the majority of the adaptation strategies examined were found to improve the capacity and reliability of the drainage network. Moreover, the results revealed that permeable pavements perform better for storm events with return period of up to five years. Overall, it can be concluded that design criteria need to be reviewed and also green measures, such as permeable surfaces, can be introduced in the design procedure.