A city where rain has nowhere to go
Rain that falls on forests and fields largely soaks into the ground or is held by plants. In a city covered with asphalt, concrete and roofs, there is little room for it to soak in, so most of it runs straight off the surface.
Surfaces that water cannot pass through are called impervious surfaces. The more of them there are, the more runoff there is, and the faster water gathers.
Three things that amplify urban flooding
More runoff
Less water infiltrates, so the same rain produces more surface flow.
Earlier peaks
Water collects quickly along smooth surfaces and pipes, so the peak arrives sooner after rain begins.
Fixed drainage capacity
Sewers and pumps handle rain up to their design standard. When more falls in a short time, water backs up or pools in low places.
So time is short
Urban flooding does not build over days like a river flood. Underpasses and low roads can flood within tens of minutes of intense rain starting.
Deciding within that window means watching minute-by-minute change rather than hourly totals, and the rain at the place at risk rather than the city average. That matters even more as climate change brings more short, intense bursts of rain.
What cities can do
- Green infrastructure that lets rain soak in and stay (permeable paving, rain gardens, detention)
- Revisit drainage design standards and risk maps with recent rainfall data
- Observe rainfall minute by minute near where water collects, and alert as soon as thresholds are reached
Sources
- Urban Flood Management (Integrated Flood Management Tools Series) — WMO / GWP Associated Programme on Flood Management
- Green Infrastructure — U.S. Environmental Protection Agency
- Climate Change 2021: The Physical Science Basis — IPCC Working Group I (AR6), 2021
