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<br />0011u3 <br /> <br />avoid public misconception. Houston, <br />Texas in fact experienced one percent <br />floods three times during 1979! Flood <br />frequency detennination is based on <br />conditions as they exist at the time an <br />engineer preforms an analysis. Increasing <br />urbanization, channel modifications and <br />similar changes will change flood fre- <br />quencies and magnitudes. Thus, last <br />year's "two percent" flood can be this <br />year's "one percent" flood. <br /> <br />With riverine floods, another criterion <br />is used as well: they are also classified <br />according to the rapidity of their onset. <br />known as "flashiness." Larger rivers <br />flowing through valleys of gradual slope <br />experience slow flooding. Residents of <br />the lower Mississippi River Valley for in. <br />stance. can receive warning several days <br />in advance of an approaching flood crest <br />and thereby can take necessary precau. <br />tions. By contrast, narrow mountain <br />canyons or valleys--,)( the Rockies or the <br />Appalachians, for example-experience <br />"flash floods" which crest within hours <br />after a heavy rainfall (Fig. 6 ). These re- <br />sult from precipitation falling on rocky <br />areas producing a high proportion of sur- <br />face runoff, and also from the steep <br /> <br />gradient of mountain streams. Flash <br />tloods also happen in arid regions where <br />normally dry gullies become raging tor- <br />rents within minutes due to thunder- <br />stonns, often a considerable distance <br />away. <br />B. Uses of Floodplains <br /> <br />Floodplains are attractive for a variety <br />of human uses-some compatible with <br />flooding, others not. Historically I the <br />most important human use of flood- <br />plains has been agriculture. Floodplains <br />are inherently fertile due to the deposi. <br />tion of silt and nutrients by floodwaters. <br />This fertility allowed the Cloodplains of <br />the Nile and the Tigris and Euphrates <br />Rivers to serve as "cradles of civiliza. <br />tion." Similar roles have been played by <br />the Yangtze in China, the Volga in the <br />Soviet Union, and the Mississippi in the <br />United States_ Flooding in such areas <br />was essential for agriculture because it <br />renewed soil fertility. AJthough crops <br />were sometimes destroyed by floods, the <br />choice of grain and the timing of plant- <br />ing were adjusted to minimize this haz- <br />ard. Traditional agriculture was in close <br />harmony with the natural nooding proc- <br />ess. <br /> <br /> <br />Figure 7 Wesl Springfield. Massachusetts Ounng Connecticut Rivet Aood of March. 1936. <br />(Photo' U. S Amry. Corps of Englneersl. <br /> <br />9 <br />