Geography
Why Cities Are Built on Floodplains Despite Risks
Quick fact
Over 40% of the world's largest cities are located on floodplains, including Tokyo, Shanghai, and New Orleans.
Why this is interesting
Why do we build our greatest cities on land that is destined to flood?
Read the full explanation
Understanding Why Cities Are Built on Floodplains Despite Risks
Floodplains are low-lying areas next to rivers, formed by layers of sediment deposited during floods. This sediment makes the soil incredibly fertile, ideal for farming. The river itself provides a natural highway for trade and transportation—far cheaper and faster than land routes before railways. Flat land also simplifies construction. So early settlements clustered here for food, water, and commerce. As cities grew, they built levees, dams, and canals to control water, reducing the immediate danger. Over time, the economic value of the city—its buildings, roads, and businesses—became so high that moving became unthinkable. Even after devastating floods, people rebuild because the location's advantages still outweigh the risk in their calculations.
A deeper explanation
The persistence of cities on floodplains stems from a combination of economic inertia and the levee effect. Economic inertia means that once a city is established, the cost of relocating infrastructure (factories, ports, power grids) is enormous; the investment is 'locked in.' The levee effect occurs when flood defenses (dikes, walls) reduce the frequency of small floods but increase the potential for catastrophic failure because people build more valuable property behind them. When a levee inevitably fails, the damage is immense. Additionally, floodplains are naturally dynamic—rivers meander and deposit sediment, raising the land over time. This natural process actually makes floodplains safer over centuries, but humans disrupt it by confining rivers, which worsens flooding elsewhere. Policy often prioritizes short-term economic growth over long-term hazard mitigation, and insurance systems can subsidize risky development. Thus, the decision to build on floodplains is a rational trade-off at the city scale, but it accumulates regional and global risk.