Cities built for a drier century

The city's storm-water network was largely engineered in phases stretching back to the 1960s, sized for a monsoon that delivered its rainfall gradually across the season. That assumption no longer holds: meteorological data over the past decade shows an increasing share of seasonal rainfall now arriving in short, high-intensity bursts of a few hours rather than steady daily accumulation.

When a drainage network built for gradual absorption meets several hours' worth of rain compressed into a single afternoon, the failure is immediate and visible: waterlogged arterial roads, backed-up stormwater drains, and low-lying residential pockets that flood even when the surrounding areas stay largely dry.

A ward-by-ward review of flooding incident reports over the past three monsoon seasons shows the failures cluster overwhelmingly in a identifiable set of low-lying wards with the oldest storm-water assets — many of them areas that were on the city's periphery when the drainage network was first laid and have since been absorbed into dense urban cores.

"We keep discussing capacity as if it were purely an engineering problem, but half of what we're seeing is inconsistent desilting and maintenance scheduling," said an urban infrastructure researcher who has advised municipal bodies on adaptation planning. "A well-maintained smaller drain can outperform a larger one that hasn't been cleared before the monsoon."

Municipal officials counter that maintenance budgets have in fact increased in recent years, and argue the more binding constraint is capacity itself — retrofitting drainage networks in dense, already-built neighbourhoods is far more expensive and disruptive than building fresh capacity in newer areas.

The dispute matters because adaptation budgets are finite, and the choice between funding maintenance versus funding capacity expansion will determine which neighbourhoods are protected first as rainfall patterns continue to shift.