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<br />10 <br /> <br />The Army Engineers have made wide use of the dry reservoir <br />idea to protect lind ted areas. Chart VII shows the location of these <br />dams, 50 of them, including those of the MusldngtUll District. There are <br />probab~ SClIIle omissions'. There are at least 60 more under construction <br />or authorbed. As far as is known, these projects have been successful, <br />aJ.though some of them probably do not afford complete protection to the <br />areas below. <br /> <br />The dry retarding basin is not a panacea for floods. It is <br />most effective for limited drainage areas. The storage capacity needed <br />for larger drainage areas is very much greater than for the areas w.here <br />the idea has been successful - the increase being all out of proportion <br />to the increase in drainage area. The effect of the retarding basins <br />on the crest of the floods decreases as the floods go down stream and <br />fin.,lly disappears entirely. Then, too, an intricate "tiJRing" problem <br />is presented which becomes more and more complicated as the nWllber of <br />retarding basins are increased and the drainage area to be handled <br />becomes larger. An oversimplified example will illustrate the diffi- <br />culty. EveIY river is fed by tributaries. During a heavy storm, these <br />side streams aJ.so flood. But, generaJ.ly, the crests of the floods on <br />the lower tributaries reach the main stream before the flood from the <br />headwaters gets that far and thus the water from the tributaries gets <br />out of the w~ ahead of the main crest. If one of the lower tributaries <br />has a retarding basin on it, the flood waters III8.Y be held back just <br />long enough so that the crest may arrive at the main stream at the same <br />time as the crest from upstream with the result that the flood level is <br />much higher than it would have been if no retarding basin bad existed. <br />Thus, in plenning a flood control project involving reservoirs or re- <br />tarding basins, great skill must be used in locating and determining <br />the size of the component parts so that they fit together proper~ and <br />perform their fmctions at the proper time and in the right order. A <br />drainage area can be so large and involved that retarding basinE are <br />not practicable. <br />