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<br />5 <br /> <br />. <br /> <br />From the sketch which is shown on Chart I and actual quanti- <br />ties such as those just 'quoted, the difference between the total volume <br />of water coming down a river during a flood and the relatively small <br />amount of this water which arrives at a given point at any one time is <br />very striking. The chief reason for the difference is the existence of <br />natural storage in the river channels and flood plains both on the main <br />stream and an the tributaries. Another term often used by engineers <br />for this factor is valley storage. Reference to Chart I shows that all <br />or the greater part of the great "slug" of water that constitutes a <br />flood, at any OI1e time while it is in progress, must be contained in the <br />valley; there is no other place for it to be. Thus the amount of natural <br />storage is the volume of the water that is moving along in the river and <br />its tributaries while the flood is 011. It varies in extent with the <br />size of the flood, as it must equal the total volume of water in the <br />flood, except where part of the flood is discharged from the mouth of <br />the drainage area before the flood has ended. <br /> <br />The extent of channel storage in specific sections of rivers <br />is very great indeed. The Army Engineers estimated that before the TVA <br />dams were built, the channel storage in the main stem of the river from <br />Inoxville to Aurora Landing, 42. miles above its mouth, was over <br />14,000,000 acre feet during a maximum flood. <br /> <br />Chart II has been prepared to illustrate the effect of channel <br />storage on flood crests at an actual location. (See "Water Power Engineer- <br />ing" by H J[ Barrows). The graph is based on an actual flood that came <br />from. about 60,000 square miles of drainage area. The total amount of <br />water involved was about 9,000,000 acre feet, which is by no means a <br />large flood for this location. The chart shows, by meanll of hydrographs, <br />the contrast between the actual flood at the particular point and what <br />the same flood would have been if no natural channel storage had existed. <br />For larger drainage areas, or for a larger flood an the same drainage <br />area, the difference between the two would be much greater. <br />