The Illusion of Rainfall Intensity
When cities experience record-breaking rain, the immediate assumption is that the volume of water is simply too much for the earth to handle. However, infrastructure experts argue that rainfall intensity is rarely the sole culprit. The difference between a functional underpass and one that turns into a basin depends on a city's specific topography, the direction of surface runoff, and the state of its drainage network.
The Role of High-Density Development
Modern urban expansion often works against flood mitigation. As cities become more densely populated, the amount of impervious surfaces—like asphalt and concrete—increases significantly. These surfaces prevent rainwater from being absorbed into the ground, forcing it to travel rapidly across the surface toward low-lying areas, such as underpasses.
- Increased impervious surface area limits natural soil absorption.
- Obsolete drainage networks often struggle to handle modern runoff rates.
- Encroachment on natural water channels removes the city's primary drainage relief valves.
- Hotspot zones develop where drainage nodes collectively overflow, overwhelming local systems.
The Infrastructure Gap
The problem is exacerbated by legacy systems. Many drainage networks were designed decades ago, based on climate models that no longer reflect the reality of current, more frequent extreme weather events. When these systems are combined with inadequate waste management—where debris clogs storm drains—even minor rain can lead to catastrophic waterlogging.
Since 2022, urban flooding in India has become more severe in cities like Mumbai, Chennai, Hyderabad, Bengaluru, and Kolkata, which are particularly vulnerable due to a combination of obsolete drainage and rapid, uncontrolled urban expansion.
— Urban Drainage Research Journal