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<br />4 <br /> <br />still further down stream. By the fourth period the natural storage in <br />t~e river has commenced to flatten out the crest. ay the end of the <br />firth period the flood is discharging from the mout!! of the river, the <br />crest has been flattened still more, and has moved much further down <br />stream, and the danger to this particular watershed is almost over. <br /> <br />Now the sketch, drawn of necessity on a greatly distorted <br />scale and in an oversL~plified manner, sho~s one possible set of cir- <br />cumstances. Each flood is different; the shape of its flood wave <br />varies and its details are never the same. But there is one funda- <br />mental fact about a flood that never changes. It is a great, moving, <br />"slug" of water, generally several hundred miles long, a mile or two <br />wide, and thirty to fifty feet deep, with a crest, like a lump, usually <br />near its forward end. Once the water runs off the land into the creeks <br />and gullies, no'ching can be done to reduce the volume of the water and <br />therefore flood control is just what its name implies - the construction <br />of physical facilities by which this great volume of water can be handled <br />so as to prevent damage and loss of life. <br /> <br />Few persons realize the great volume of water which comes down <br />a big river during a large flood. For example, the Ohio River during <br />the 1937 flood discharged a total of over 135,000,000 acre feet of water <br />at Cairo, although but 153,000 acre feet of water passed that point dur- <br />ing the hour when the flood was at its crest. For those readers who do <br />not know what an acre foot of water is, it is 43,560 cubic feet, which <br />is the volume of water that will Cover one acre of land, one foot in <br />depth. During the same flood, 240,000,000 acre feet of water passed <br />Arltansas City, Arkansas, on the Mississippi, only 180,000 acre feet of <br />which went by during the hour in which the river was at its maximum <br />crest. The volume of water that went by Arkansas City was enough to <br />cover the entire state of Georgia to a depth of 6-1/2 feet. The very <br />size of these figures epitomizes the scope and magnitude of the problem <br />which confronts the engineer when an attempt is made to control floods <br />from large drainage areas by means of reservoirs. <br />