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<br />o:n4'HI <br /> <br />21 <br /> <br />STORAGE REQUIREMENTS AT STREAMFLOW STATIONS <br /> <br />Prior discussion has been concerned with the maximum rate of natural <br />streamflow for various durations. It is now possible with knowledge of these <br />quantities to evaluate the storage required to reduce outflow to safe rates. <br />The outflow rates as set by the design engineer must be large enough to re- <br />lease the stored water quickly but also must be small enough to stay within <br />the natural stream channel. <br /> <br />Storage requirements to control either the maximum known flood volume <br />or the flood volume of a certain frequency are described in the following <br />parts of this section for constant gross outflow rates. Gross outflow includes <br />evaporation and seepage losses which are usually small for the time of stor- <br />age being considered. However users must decide whether evaporation and <br />seepAge losses are important at the specific reservoir site under consider- <br />ation. <br /> <br />Storage Requirements for Maximum Flood of Record <br /> <br />For many years mass curves of actual runoff have been used to de- <br />termine the storage requirements for regulating streamflow, (Johnstone and <br />Cross, 1949, p. 93-95). This method has been applied herein to the 14 long- <br />term stations of less than 10,000 square mile drainage area in Kansas to <br />show the storage required to control the maximum known flood experienced <br />since streamflow records were started. A comparison of the storage re- <br />quirements for the maximum observed flood volume with that expected on the <br />average of once in 50 years, for example, indicates the possible recurrence <br />interval of a storage requirement design based upon the maximum of record. <br /> <br />Figure 3 illustrates the mass-curve analyses for station 66, Delaware <br />River at Valley Falls during the period of the 1951 flood. The 1951 flood vol- <br />ume is the greatest recorded since daily records started in 1922 and is prob- <br />ably the greatest short-term volume since the flood of 1865 according to <br />historic information on peak stages. The ma ss curve in figure 3 is shown as <br />a heavy line, depicting the summation of daily observed streamflow, called <br />cfs-days, plotted against time. The ma ss curve rises slowly when the flow <br />is small and rapidly when the flow is large. Thus the slope of the mass curve <br />is a measure of the rate of flow of the stream or the inflow to a reservoir <br />located at that site. Similarly the slope of any straight line on the diagram <br />represents a constant rate of streamflow which may be considered as a se- <br />lected draft or rate of gross outflow from a reservoir. Several rates of out- <br />flow are shown by rays in the lower right portion of figure 3, and the rates <br />of 5 and 10 cubic feet per second (cfs) per square mile are superimposed on <br />the mass inflow curve to illustrate two uses. <br />