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Why do roads, parking lots, and rooftops increase the risk of local flooding?

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urban sprawl Mosaic of images taken by Landsat 5 of the western portion of Las Vegas in 1984 (top), 1999 (middle), and 2009 (bottom).

Roads, parking lots, and rooftops increase the risk of local flooding because they function as impermeable surfaces that replace natural, water-absorbing vegetation and permeable soils.

In a natural landscape, soil and plants absorb precipitation, allowing it to infiltrate the ground. However, built-up infrastructure prevents this absorption. Consequently, rainwater and snowmelt cannot soak into the earth and instead run off these hard surfaces. This runoff accumulates quickly in low-lying areas, which significantly elevates the risk of local flooding. Furthermore, this process often carries pollutants from the pavement into the water supply, which degrades water quality and harms downstream ecosystems.1