Even if you feel you live in a community with a low risk of flooding, remember that anywhere it rains, it can flood. Just because you haven't experienced a flood in the past, doesn't mean you won't in the future. Flood risk isn't just based on history, it's also based on a number of factors including rainfall , topography, flood-control measures, river-flow and tidal-surge data, and changes due to new construction and development.
Flooding in urban areas can be caused by heavy rainfall, flash floods, coastal flooding/storm surge, overflowing rivers and streams. Urban flooding is specific in the fact that the cause is a lack of drainage. As there is little open soil that can be used for water storage nearly all the precipitation needs to be transport to surface water or the sewage system. High-intensity rainfall can cause flooding when the city sewage system and draining canals do not have the necessary capacity to drain away the amounts of rain that are falling. Water may even enter the sewage system in one place and then get deposited somewhere else in the city on the streets. Sometimes you see overflowing drains and covers.
Urban floods are a great disturbance of daily life in the city. Roads can be blocked, people can’t go to work or to schools. The economic damages are high but the number of casualties is usually very limited, because of the nature of the flood. The water slowly rises on the city streets. When the city is on flat terrain the flow speed is low and you can still see people driving through it. The water rises relatively slow and the water level usually does not reach life endangering heights.
Flash flooding can be much more dangerous. These sudden local floods are usually due to periods of heavy rain and can inundate streams, streets, and low-lying areas in seconds, making them extremely da
ngerous. As such, the National Weather Service has devised a public warning system related to flash floods. These include:
A flash flood watch is issued by the National Weather Service when conditions are favorable for flash flooding in flood-prone areas - usually when grounds are already saturated from recent rains or when upcoming rains will have the potential to cause a flash flood. These watches are also occasionally issued when a dam may break in the near future.
A flash flood warning is issued when a flash flood is imminent or occurring in the warned area. A flash flood is a sudden, violent flood after a heavy rain, or occasionally after a dam break. Rainfall intensity and duration, topography, soil conditions, and ground cover contribute to flash flooding.
What is the difference between a Flash Flood Warning and a Flash Flood Watch?
A Warning is issued when flooding is imminent or occurring. A Watch is issued when conditions are favorable for a specific hazardous weather event to occur. For example: A Flood Watch is issued when conditions are favorable for flooding. It does not mean flooding will occur, but it is possible.
Ways to prepare for the event of a flood include developing a communication plan with your loved ones and participating in a CERT training. For more information regarding this and other types of hazards, visit http://www.ready.gov/.