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Last modified
11/23/2009 12:58:08 PM
Creation date
10/4/2006 10:20:44 PM
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Template:
Floodplain Documents
County
Statewide
Title
Using Multi-Objective Management to Reduce Flood Losses in Your Watershed
Date
1/1/1996
Prepared For
EPA
Prepared By
Association of State Floodplain Managers
Floodplain - Doc Type
Educational/Technical/Reference Information
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<br />. <br /> <br />Watersheds vary in size. They can be divided into smaller subwarersheds. <br />The important thing is that the stream and its watershed are connected. What <br />happens in the watershed-even in the higher elevations up near the <br />divide-affects things both in the low-lying lands near the stream or lake and also <br />farther downstream. <br /> <br />. The floodplain <br /> <br />The size of a river depends on how much water it collects from the watershed. <br />Most of the time, a river stays within its banks. However, after heavy storms or <br />snowmelt, the extra amount of water can be too much for the channel, causing it <br />to overflow onto normally dry land. <br />A floodplain can be defined according to the frequency of the flooding that <br />covers it; An area that is flooded every <br />year (a I-year floodplain) is smaller than <br />the floodplain that is inundated less <br />frequently (a IS-year, 7S-year, or even <br />larger floodplain). <br />TIle base flood-The national <br />standard for floodplain management is the <br />base or one percelll dlance floodplain. <br />This is the floodplain that has at least one <br />chance in lDO of being flooded in any <br />given year. It is also called the lOO-year <br />floodplain. This is the area shown on the <br />Flood Insurance Rate Map (FIRM) for <br />your town issued by the Federal <br />Emergency. Management Agency. <br />The'term "IDO-year flood" has <br />caused a lot of confusion. Some folks <br />think that it will only happen once in 100 <br />years, but that is definitely not true. <br />Instead, "I DO-year flood" is a statistical <br />term that refers to the odds of a flood of <br />that size happening in any given year (see <br />box). It is possible to have 100-year floods <br />two years in a row. <br />The flood way-Another term you <br />will see is jloodway. This is the river <br />channel and .the overbank area near the <br />channel that carry the deeper and faster- <br />moving flood waters. Some maps show a <br />regulatory jloodway, an area where construction regulations require special <br />provisions to account for this extra hazard. <br /> <br />. <br />WHAT ARE THE.ODDS OF <br />A lOO-YEARFLOOD7 <br /> <br />The term "100-year flood" has caused much <br />confusion for people not familiar with statistics. <br />Another way of looking at it is to think of the <br />odds that a 1OQ-vear flood will happen <br />sometime during the life of a 30-vear mortgage <br />(26% chance). <br /> <br />Chance of Floodino over a Period of Years <br /> <br />Time Flood Size <br />Penod 10-vear 25-vear 50-vear 100-vear <br /> <br />1 year <br />10 years <br />20 years <br />30 years <br />50 years <br /> <br />10% <br />65% <br />88%' <br />!l6%. <br />!l9% <br /> <br />1% <br />10% <br />18% <br />26% <br />3!l% <br /> <br />4% <br />34% <br />56% . <br />71% <br />87% <br /> <br />2% <br />18% <br />33% <br />45% <br />64% <br /> <br />Even these numbers do not convey the true <br />flood risk because they focus on the larger. <br />less frequent, floods. If a house is low <br />enough, it may be subject to the 10- or <br />25'vear flood. During the proverbial 30-vear <br />mortgage, it may have a 26% chance of being <br />hit bV the 100-vear flood, but the odds are <br />!l6% {nearlv guaranteedl that it will be hit bV <br />a 10-vear flood. Compare those odds to the <br />onlv 5% chance that the house will catch fire <br />during the same 30-vear mortgage. <br /> <br /> <br />. <br /> <br />Flood problems <br /> <br />As noted above, flooding is a natural occurrence but flood problems are the result <br />of human development. Floods may be acts of God, but flood damage results from . <br />human actions. Flood problems result from: <br /> <br />6 <br />
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