Comparison of Urbanization, Climate Change, and Drainage Design Impacts on Urban Flashfloods in an Arid Region: Case Study, New Cairo, Egypt

Abstract

Urban flooding is considered one of the hazardous disasters in metropolitan areas, especially for those located in arid regions. Due to the associated risks of climate change in increasing the frequency of extreme rainfall events, climate-induced migration to urban areas leads to the intensification of urban settlements in arid regions as well as an increase in urban expansion towards arid land outskirts. This not only stresses the available infrastructure but also produces substantial social instability due to unplanned urban growth. Therefore, this study sheds light on the main factors that are increasing the flood risk, through examining the consequences of rapid urban growth and the performance of drainage networks on urban flood volumes and comparing it with the effects induced by climate change on the surface runoff. The effect of urbanization is assessed through land use maps showing the historical urbanization conditions for the past 30 years, while considering the role of urban planning and its effect on exacerbating surface runoff. Six climate projection scenarios adopted from three Global Climate Models under two Representative Concentration Pathways (4.5 and 8.5) during the period (2006–2020) were compared to ground observed rainfall data to identify which climate scenario we are likely following and then evaluate its effects on the current rainfall trends up to the year 2050. The significance of the drainage design in the mitigation or increase of surface runoff is evaluated through capacity-load balance during regular and extreme storms. It is found that using impervious surfaces coupled with poor planning causing the blockage of natural flood plains led to an increase in the total runoff of about 180%, which is three times more than the effect induced by climate change for the same analysis period. Climate change decreased the intensities of 2- and 5-year rainfall events by 6% while increasing the intensities of extreme events corresponds to 100-year by 17%. Finally, the urban drainage had a distinguished role in increasing surface runoff, as 70% of the network performed poorly during the smallest rainfall event of 2-year return period. The study emphasizes the urgency to re-evaluate the existing and future urban drainage design approach: although urban development and climate change have inevitable effects on the increase in urban flood volumes, it could be alleviated through improved infrastructures.