Why Urban Areas Flood Faster
Urban areas have many hard surfaces like roads, sidewalks, and building roofs that water cannot pass through. In nature, soil and plants absorb rainfall, allowing it to soak into the ground slowly. In cities, when rain falls on pavement, it has nowhere to go except to flow downhill as runoff. This means more water moves faster through streets and neighborhoods compared to rural areas with natural ground.
Overwhelmed Drainage Systems
Cities rely on storm drains, sewers, and culverts to move water away from streets during heavy rain. However, these systems are designed to handle only a certain amount of water. When extremely heavy rainfall occurs, the drainage system fills up and cannot process water fast enough. The water then backs up and floods streets, basements, and other low areas. Older cities with older drainage systems are especially vulnerable to this problem.
Heavy Rainfall Events
Flash floods typically happen during intense rainstorms where several inches of rain fall in a short period of time, often within an hour or two. These heavy rainfall events can result from thunderstorms, tropical storms, or hurricanes. The faster the rain falls, the more likely it will exceed the drainage system's capacity and cause flooding.
Low-Lying Areas at Higher Risk
Certain parts of cities are more prone to flash flooding than others. Areas that are naturally lower in elevation, located near streams or rivers, or built in valleys are at greater risk. Streets that form natural pathways for water to flow downhill, such as those at the bottom of hills, also experience more frequent flooding.
Human Development Factors
As cities grow and expand, more natural areas are replaced with buildings and roads. Wetlands and areas that naturally absorb water are often removed for development. Additionally, when soil is compacted during construction, it becomes less able to absorb rainfall. These changes make modern urban areas even more susceptible to flash flooding than they were in the past. 
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