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Urban areas often contain large amounts of concrete, asphalt, rooftops, and other surfaces that absorb little water. During heavy rain, much of the water runs directly into streets and drainage systems rather than soaking into the soil.
Drainage networks can manage ordinary rainfall but may struggle when water arrives at an exceptional rate. Blocked drains, construction debris, fallen leaves, and overwhelmed pumping systems can make the situation worse. Low-lying roads, tunnels, underground parking facilities, and underpasses are particularly vulnerable because water naturally collects in depressions.
The result can be sudden flooding with very little visible warning. A road that appears passable at the beginning of a journey may be covered by water minutes later. The depth can be difficult to judge, especially at night or when rain reduces visibility.
Floodwater is also more dangerous than it may appear. It can conceal open drains, damaged road surfaces, submerged objects, and strong currents. Water moving across a road may exert enough force to shift a vehicle or knock a person off their feet. A flooded route should never be treated as safe simply because another vehicle has passed through it.