Saurav KC

and 1 more

This study investigates the key reasons for repeated pluvial flooding in highly urbanized Kathmandu Metropolitan City (KMC), using an expert’ s-based questionnaire and quantitative validation through observed data and literature. The results obtained from 70 experts (onsite and offsite) showed that the majority experienced high rainfall intensities of shorter duration though the annual rainfall has declined. This result has been validated through the non-parametric Mann-Kendall test and Sen’s slope estimation for rainfall trends and through the RClimDex package for the rainfall extremes using the observed rainfall at KMC (1989-2018). Furthermore, the results indicate that the three major causes of frequent pluvial flooding in KMC are (i) increased impervious cover, (ii) unplanned settlement and (iii) clogging of sewers followed by other contributors such as the undersized design of sewers, lack of coordination, climate variability etc. The land-use change analysis validates that the imperviousness of the city increased to 75% from 25% in 1990, and the census data proves the unprecedented urban settlement surged threefold in three decades, pressurizing the combined sewerage system. Additionally, the study also ranked several possible adaptation and management options and their effectiveness based on expert’s perception to minimize frequent pluvial flooding in the study area. Research studies in KMC have shown that the combination of small-scale rainwater harvesting and overflow storage is likely to reduce flood volume by 20-35%. The study’s approach and the findings highlight the prominence of evidence and experience-based approach in investigating causes of pluvial floods in extremely urbanized metropolitans. Besides, it also provides a common ground to policymakers, managers, planners, and technical officers in rating the multiple causes based on multi-practitioners observation and experience, understand its trend and severity quantitatively, identify the possible solutions, develop the inter and intra institutional coordination mechanism and take a call-to-action accordingly. Keywords: Urban flooding, Mixed methodology, Combined sewers, Climate Extremes, Kathmandu Metropolitan City