Floods_I_notes

Floods_I_notes - 1 Floods are one of the most common...

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Floods are one of the most common hazards in the United States. They can occur in any state and at virtually any time of the year. They can last just a few minutes, or as long as a few months. Their impacts can be restricted to a single neighborhood, or an entire city. On average, floods cost the American economy approximately 2.4 billion dollars per year. Each year, approximately 200 people in the United States die from flooding, most of which occur during flash flooding events. 2
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Almost every U.S. county has experienced major flooding, i.e. a presidential disaster was declared. Not only are most counties susceptible to severe flooding but it is also a reoccurring hazard as indicated by the counties highlighted in red. 3
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Between 1960 and 2005, flooding caused $84 billion in direct losses. This does not include insured losses or indirect losses (e.g. lost revenue, etc.). If one would add storm surge losses (in above graphic classified as partially coastal), direct flood losses exceed $100 billion. As such flooding is the second most expensive hazard type after tropical storms and hurricanes. Data Source: SHELDUS 5.1 4
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In order to fully understand how and why floods occur, it is important that you learn some basic terms that are common to the discipline of hydrology. This slide and the next define 9 key terms that you will need to know for this lecture. 5
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All rivers gather their water from runoff. Runoff to a river will occur only within the confines of its drainage basin. A drainage basin, which is also sometimes called a watershed, is that area which surrounds a river within which runoff is drained to that river. The boundary of the drainage basin is usually the highest point of elevation between two different rivers. This boundary is referred to as the drainage divide, because it acts as a divide for drainage from one river to the next. Rivers often have smaller branches that merge into the main river channel. These branches of the river are called tributaries. A river many have many tributaries some of which are very large significant rivers in and of many have many tributaries, some of which are very large, significant rivers in and of themselves. For example, the Ohio and Missouri Rivers are both tributaries of the Mississippi River. Tributaries also have their own, smaller drainage basin, which are called sub-basins. In the upper reaches of the sub-basins, we find the first order streams. First order streams do not have tributaries and their headwaters are usually the highest point of elevation of flowing water. Since water flows downhill, the mouth of the river is usually the lowest point of elevation in the drainage basin. Rivers can empty into an ocean, lake, or another river as is the case for the Ohio River. When a river mouth occurs at a lake or ocean, the gradient of the river is typically quite low, and it therefore represent a reach of the river, where the stream velocity is lower than say half way upstream. As a result, rivers tend to
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This note was uploaded on 04/03/2010 for the course DSM 2000 taught by Professor Romolo during the Spring '08 term at LSU.

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Floods_I_notes - 1 Floods are one of the most common...

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