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<br />Data from table 1 have been transposed into <br />the graph shown in figure 2. The ratios of peak <br />dischar"e of urbanized to rural areas are pre- <br />sented for different percentages of sewerage <br />and impervious area; lines of equal values of <br />the ratio are drawn through the data. Briefly, <br />these data show that for unsewered areas the <br />differences between 0 and 100 percent impervi- <br />ous will increase peak discharge on the average <br />2.5 times. For areas that are 100 percent <br />sewered, peak discharge for 0 percent im- <br />penious will be about 1.7 times the mean <br />annual flood and the ratio increases to about <br />eight for 100 percent impervious areas. Figure <br />2, then, reduces the basic data to the same units <br />applicable to a 1-sq-mi drainage basin and to <br />the mean annual flood. <br /> <br />A basin produces big flows from large and <br />intense storms and smaller flows from less in- <br />tense but more frequent storms. The great or <br /> <br />10,000 <br /> <br /> <br />0 <br />z <br />0 <br />u <br />u.J <br />v> <br />0:: <br />u.J <br />a... <br />I- <br />u.J <br />u.J <br />u... <br />u 5000 <br />CD <br />::J <br />u <br />Z <br />u.J <br />C) <br />0:: <br /><{ <br />::I: <br />U <br />v> <br />0 <br /> <br />catastrophic event is rare, and the storm Of. <br />ordinary magnitude is frequent. These events <br />can be arranged in order of magnitude and <br />counted. For example, all discharge events ex- <br />ceeding 400 cfs (cubic feet per second) can be <br />tabulated from the record at a stream-gaging <br />station and arranged in order of magnitude; <br />the val'Jes in the array can be plotted as a <br />discharge-frequency curve. This has been done <br />for the gaging station on West Branch Brandy- <br />wine Creek at Coatesville, Pa., for 9 years of <br />record (fig. 3). The theory and practice of <br />constructing such flow-frequency curves is well <br />known. The plotting position or frequency <br />often used is defined as <br /> <br />R=n+l <br />m <br />where R is the recurrence interval in years, n <br />is number of years of record, and m is the rank <br />of the individual event in the array. <br /> <br /> <br />f <br />I <br />I <br />I <br />I <br />/ <br />/ <br />/ <br />/ <br />/ <br /> <br />o <br /> <br /> <br />0.5 1 2 3 5 <br /> <br />RECURRENCE INTERVAL, IN YEARS <br /> <br />Figure 3.~Flood-frequency curve for partial-duration series, West Branch Brandywine Creek at Coatesville, pa.,. <br />based on data for 1942, 1944-51. <br /> <br />o <br />0.1 <br /> <br />0.2 <br /> <br />0.3 <br /> <br />10 <br /> <br />20 <br /> <br />6 <br />