Investigating Major Causes of Frequent Flooding in Highly Urbanized
Metropolitans Using a Quali-Quantitative Approach
Abstract
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