![]() This means that a flood of that magnitude has a 1 in 100 chance of occuring in any year. When a major flood occurs, you might have heard that the stream stage reached the "100-year flood level". Floods, on the other hand, can be slow- or fast-rising, but generally develop over a period of hours or days.Įven though you may never have heard of "recurrence interval", it may be familiar to you. Most flash flooding is caused by slow-moving thunderstorms, thunderstorms repeatedly moving over the same area, or heavy rains from hurricanes and tropical storms. Topography, soil conditions, and ground cover also play important roles. ![]() Intensity is the rate of rainfall, and duration is how long the rain lasts. Two key elements are rainfall intensity and duration. ![]() Because of the buoyancy effects and power of moving water, even a foot of moving water can be enough to carry away some cars. During periods of urban flooding, streets can become swift moving rivers and can be dangerous for drivers. Urbanization increases the amount of impervious areas, causing runoff to be two to six times over what would occur on natural terrain. Severe thunderstorms can bring heavy rain in the spring and summer or tropical cyclones can bring intense rainfall to the coastal and inland states in the summer and fall.Īs land is converted from fields or woodlands to roads and parking lots, it loses its ability to absorb rainfall.
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