From: Andy Soos, ENN
Published May 13, 2010 04:41 PM

Floods

Rivers throughout middle Tennessee crested at record high levels in the week of May 3. They exceeded previous highs by as much as 14 feet, according to preliminary estimates released May 13 by the U.S. Geological Survey (USGS). The highest flood levels were recorded on May 2 and 3, from Nashville west toward Jackson, extending about 40-miles north and south of Interstate 40, and affecting major tributaries to the Cumberland and Tennessee Rivers. Floods have always happened and can be devastating when people build in the wrong places.

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A flood occurs when water exceeds the capacity of the river or lake and covers land that is normally not covered by water. A high tide is also a sort of flood.

In the late summer of 2005, the remarkable flooding brought by Hurricane Katrina, which caused more than $200 billion in losses, constituted the costliest natural disaster in U.S. history.  However, even in typical years, flooding causes billions of dollars in damage and threatens lives and property in every State.

Floods can occur in rivers, when flow exceeds the capacity of the river channel, particularly at bends or meanders. Floods often cause damage to homes and businesses if they are placed in the natural flood plains of rivers. While flood damage can be virtually eliminated by moving away from rivers and other bodies of water, since time out of mind, people have lived and worked by the water to seek sustenance and capitalize on the gains of cheap and easy travel and commerce by being near water. That humans continue to inhabit areas threatened by flood damage is evidence that the perceived value of living near the water exceeds the cost of repeated periodic flooding.

The flood peak on the Cumberland River in downtown Nashville ranks as only the tenth highest in more than 200 years of record at that site. This peak was, however, the highest observed during the past 73 years in which much of the basin upstream of Nashville was regulated by several large flood control reservoirs that were implemented to control the area flooding.

At least four major tributaries to the lower Cumberland River met or exceeded warning levels established by the National Weather Service (NWS) for major flooding conditions last week. Flood peaks on the Harpeth River at Bellevue exceeded the expected level for major flooding by more than 5 feet, and exceeded the USGS record at that site (measured in 1948) by more than 9 feet. The Harpeth River near Kingston Springs exceeded the NWS major flood stage by almost 16 feet and was 14 feet higher than the previous recorded peak from 1946.

“Most tributaries to the lower Cumberland River had flows with only a 1 in 500 chance in any given year, causing the lower Cumberland to flood with a severity that was almost entirely unexpected,” according to Rodney Knight, surface-water specialist with the USGS Tennessee Water Science Center. “That a regulated river like the Cumberland could have such high flooding is unusual and is a testament to the severity of this event.

The world's four greatest recorded river floods are all in China with the latest in 1975 where a dam failed with help from a typhoon. Hundreds of thousands of people died in these floods. In comparison in the US the great Johnstown flood of 1889 is discussed where only 2,000 people died.

For further information: http://www.usgs.gov/newsroom/article.asp?ID=2461&from=rss&utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+UsgsNewsroom+%28USGS+Newsroom%29&utm_content=Google+Reader

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