Urbanization involves the expansion of cities and towns, transforming natural landscapes into developed areas. This converts permeable land, like forests and fields, into residential, commercial, and industrial zones. Such land conversion alters the natural water cycle, increasing urban environments’ susceptibility to flooding and exacerbating flood risk.
Impervious Surfaces and Increased Runoff
Urban development creates widespread impervious surfaces like roads, buildings, and parking lots, preventing rainwater infiltration. In natural settings, a significant portion of rainfall is absorbed by soil and vegetation, replenishing groundwater and slowing down surface flow.
When rain falls on concrete or asphalt, it cannot penetrate the surface and instead flows across it as surface runoff. This dramatically increases the volume and velocity of water flowing over the land compared to undeveloped areas. For instance, a natural meadow might generate 10% runoff, while a dense urban area with 75-100% impervious cover can generate 55% to 100%. This rapid accumulation of water overwhelms natural and engineered drainage systems, directly contributing to more frequent and severe flooding. Increased runoff speed and volume also lead to greater erosion and pollutant transport.
Altered Drainage Systems and Waterways
Urban development often modifies natural drainage patterns and water bodies for infrastructure. Engineered systems like storm drains, culverts, and channelized rivers quickly convey water away from developed areas. While protecting urban infrastructure by rapidly moving stormwater, these systems can become overwhelmed during intense or prolonged rainfall.
When these engineered systems are exceeded, excess water floods streets, basements, and low-lying areas. Channelization, which straightens and deepens natural rivers, increases water velocity. This accelerated flow reduces natural flood storage and transfers risks downstream, potentially causing more severe flooding further along the modified watercourse. Such modifications disrupt rivers’ natural meandering and flood-buffering functions.
Loss of Natural Water Retention
Urbanization often removes or degrades natural features that absorb, slow, or store water. Green spaces, forests, wetlands, and natural floodplains are converted into urban areas for buildings and infrastructure. These elements serve as sponges or reservoirs, mitigating flood impacts.
Forests and green spaces intercept rainfall and promote infiltration, slowing water movement. Wetlands and floodplains naturally retain large water volumes, temporarily storing excess runoff during heavy rains and releasing it gradually. This natural buffering reduces peak flood levels and allows for controlled water release. The loss of these features means less capacity to absorb and manage heavy rainfall, directly increasing urban flood risk.