Why cities flood more than they used to: Conclusion

Introduction

A city that flooded once a decade now floods every other year, and the rainfall alone does not explain it. Something has changed about what happens to water after it lands, and most of that change was built deliberately, for reasons that made sense at the time.

First argument

The first cause is surface. Soil absorbs rain and releases it slowly; concrete and asphalt absorb none and release all of it at once. As a city grows, the share of its area that can take water in falls, and the same storm that once soaked into the ground now arrives at the drains in a few minutes.

Second argument

The second cause is what was built over. Cities grow into the flat, low ground beside rivers because it is easy to build on, and that ground is flat and low precisely because the river used to flood across it. The water has not forgotten the route. It is simply arriving to find houses on it.

Counterargument

It is fair to point out that storms themselves have grown heavier, and that drainage built for the rainfall of 1960 would struggle now regardless of what was paved. That is true, and it matters. But it makes the surface problem worse rather than replacing it, because both effects arrive together.