Practical Steps for Urban Flood Risk Mitigation Using Nature-Based Solutions—A Case Study in New Cairo , Egypt

This study investigated the effectiveness of nature-based solutions (NBSs) as a resilient strategy for mitigating urban flood risks in a developing hot arid country. The research method included the following steps: (a) performing a flood hazard risk assessment for the Fifth Settlement district in New Cairo , Egypt, (b) selecting best-fit NBSs, and (c) performance assessment. The process started with flood hazard analysis using hydrological data, topographical maps, urban planning, and land use maps, in addition to the history of storm events. This step defined the urban areas located in flood depth zones and categorized their flood hazard level. Exposure assessment considered the number and characteristics of population and buildings exposed to flood hazards. Vulnerability assessment determined the vulnerable characteristics of exposed populations and buildings to flood risk. The result of this assessment step indicated that there were 2000 buildings distributed in almost twenty neighborhood areas facing high flood risk. One of these urban areas with 72 building units, including residential, public, and services buildings, was selected to test the potential of integrating NBSs for flood-resilient land use planning and disaster preparedness. The selection of best-fit NBSs was based on a weighted-average sum matrix considering their climatic and contextual suitability and applicability. As a final step, numerical simulation models helped assess the efficiency of the selected NBSs for stormwater runoff reduction and the percentage of the volume capture goal. Five simulation models tested the efficiency of each NBS individually. Rain gardens achieved the highest stormwater capture percentage, while green roofs performed the least effectively, with capture rates of 43.6% and 9.9%, respectively. Two more simulation models were developed to evaluate the efficiency of NBSs when implemented in combination compared to the base case of using no NBSs. Permeable paving demonstrated the highest effectiveness in volume capture. The result indicated that applying combined measures of NBSs over 54.1% of the total site area was able to capture 8% more than the required volume capture goal. Consequently, this study underscores the necessity of adopting tailored solutions and integrated approaches using NBSs for flood risk mitigation. This necessitates testing their performance under site-specific conditions and future climate projections.