Rain characteristics include how heavy the rain is and how long the rain lasts. These two characteristics are critical in determining the flood threat from rain events. The optimum condition for flooding is heavy rain that lasts a long time. In extremely heavy rain it may only take minutes though to produce flooding. Very heavy rain can occur from convective thunderstorms. This heavy rain can be more likely to produce flooding when the convective storms are slow moving or storms keep developing over the same area (called training). Surface conditions also influence the flooding threat. Frozen soil, desert soil, urbanized area, and saturated soil all help increase the flooding threat since less moisture is able to soak into the ground. Heavy rain on top of a ground surface that can not soak in more moisture can quickly lead to flooding. Tropical systems such as tropical storms and hurricanes often produce flooding since heavy rain occurs over an extended period of time. When these systems are slow moving they can produce extreme amounts of precipitation. FLOOD DEFINITIONS: RIVER FLOOD- Occurs when heavy precipitation (sometimes accompanied by snow melt) fills river basins with water beyond the holding capacity of the river banks FLASH FLOOD- Produced by very heavy rain in a short amount of time that creates a rapid flow of water over areas not prepared to handle the runoff. Most flooding deaths are the result of flash floods COASTAL FLOOD- Caused by winds and waves generated by tropical storms, hurricanes, or strong mid-latitude cyclones that drives ocean water well inland. Coastal floods can produce beach erosion and damage to beach property URBAN FLOOD- Urbanization increases runoff since concrete does not allow percolate of rain water. This type of flooding produces excess runoff that floods roads, basements and parking lots FLASH FLOOD DANGERS: 1. Get out of areas that are subject to flooding 2. Do not drive through water of an unknown depth 3. Do not drive through moving water 4. If your vehicle stalls, seek higher ground 5. Be especially cautious at night since most flash flood deaths occur in automobiles at night |