As land is converted from fields or woodlands to roads and parking lots, it loses its ability to absorb rainfall. Urbanization increases runoff two to six times over what would occur on natural terrain. During periods of urban flooding, streets can become swift moving rivers, while basements and viaducts can become death traps as they fill with water. Several factors contribute to flooding. Two key elements are rainfall intensity and duration.
Intensity is the rate of rainfall, and duration is how long the rain lasts.
Topography, soil conditions, and ground cover also play important roles. Most flash flooding is caused by slow-moving thunderstorms, thunderstorms repeatedly moving over the same area, or heavy rains from hurricanes and tropical storms. Water may become contaminated or service may be interrupted. Bring outdoor belongings, such as patio furniture, indoors. Unsecured items may be swept away and damaged by flood waters.