Extreme Rainfall Intensification and Urban Flood Risk in Rapidly Growing Cities

Abstract

Rapidly growing cities in the Global South face a compounding hazard: warming-driven intensification of
short-duration rainfall extremes arriving on urban surfaces whose capacity to absorb and drain water has been
eroded by decades of unplanned impervious expansion. We combined a 42-year (1982-2023) sub-daily rain-gauge
and radar-adjusted precipitation archive with a time series of remotely sensed impervious surface fraction and
municipal drainage design records for a rapidly urbanizing delta metropolitan area (population approximately 5.2
million in 1990, rising to an estimated 15.8 million in 2023) to quantify how much of the observed growth in
flood-affected area is attributable to rainfall intensification versus land-surface and drainage change. Sub-hourly
and hourly annual maxima were analysed for trend using quantile regression and compared against the theoretical
Clausius-Clapeyron rate of roughly 7% per degree of warming using local dew-point temperature as the
thermodynamic covariate. Fifteen-minute and thirty-minute annual maximum intensities increased at a
super-Clausius-Clapeyron rate of 11.4% and 9.8% per degree Celsius of dew-point warming respectively, while
daily annual maxima scaled sub-linearly at 4.6% per degree, consistent with a shift toward more convective,
shorter-lived storm structure. Over the same period, impervious surface fraction across the built-up catalogue rose
from 38% to 71%, and the estimated composite runoff coefficient rose from 0.46 to 0.74. A reconstructed
rainfall-runoff model driven alternately by observed rainfall trends and observed land-surface trends attributed 58%
of the increase in modelled peak discharge for the 10-year return period storm to land-surface and drainage change
and 42% to rainfall intensification, with the land-surface contribution dominating in the historically low-lying,
densifying peripheral wards and the rainfall contribution proportionally larger in the already-saturated urban core
where further imperviousness gains have been small. Drainage infrastructure, designed predominantly to a 10-year
or shorter return period standard installed before 2000, was estimated to be undersized for the current 10-year
storm by a factor of roughly 1.4 in conveyance capacity. These results indicate that in fast-growing cities, rainfall
intensification and urbanization act as roughly co-equal but spatially distinct drivers of rising flood risk, and that
adaptation strategies focused solely on either climate-adjusted drainage design or land-use control alone will
address only part of the problem.