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<br />The a\('rag>, anllual flood of 75 cis was then <br />n1!lltiplil!d by these ratios and plotted as shown <br />in figlll'e .-, at the :2.~)-Yl~ar interval. These <br />\'uiues form the basis of a series of frequency <br />Clll";eS for combinations of sewered area and <br />lmperyious area. The shapes of the curves are <br />gUldc'cl b~' the principle that the illost infrequent <br />thuds l)("cur under conditiuns that are not ap- <br />pJ'(~'cjabl;/ aft\leted by irnperviousn('ss of the <br />basin. <br /> <br />The JYLo::..;t frequent fto\vs are t.herefore in- <br />creased b)' smaliEr ratios than would be the <br />average anliual flood. Also, the most frequent <br />flows are decreased in nUITlber because low <br />flows frUIn an urbanized area .-1re not sustained <br />by ground wa tel' a" in a natural basin. The <br /> <br />8 r-- <br /> <br />curves representing urbanized conditions there <br />fore converge at low flow values. <br /> <br />Obviously the frequency curves in figure 5 <br />are extrapolations based on minimal data and <br />require corroboration or revision as additional <br />field data become available. <br /> <br />The flood-frequency curve under original <br />(unurbanized) condition passes through a value <br />of 67 cis at a recurrence interval of 1.5 years. <br />At bankfull condition natural channels gener- <br />ally can carry the flow having that recurrence <br />interval. If one assumes that this flow approxi- <br />mates the capacity of the natural channels, the <br />intersection of the estimated curves for differ- <br />ent degrees of urbanization with the discharge <br />value of 67 cis can be used to estimate the in- <br /> <br />I <br /> <br />I <br /> <br />II <br /> <br />, <br /> <br />V) 6 . <br />:5: <br />0 <br />-' <br />u.. <br />"" <br />z <br /><I: <br /><Xl <br />e>:: 4 <br />..u <br />> <br />0 <br />u.. <br />0 <br />0 <br />>- <br /><I: <br />e>:: <br /> <br /> <br />Percentoge sewered 20 <br /> <br />40 <br /> <br />50 <br /> <br />80 <br /> <br />100 <br /> <br />Percenioge impervious 20 40 50 60 60 <br />I I <br />NONE MEASURE OF URBANIZATION COMPLETE <br /> <br />Figure 6.-Increase in number of flows per year equal to or exceeding original channel capacity (l-square- . <br />mile drainage area). as ratio to number of overbank flows before urbanization, for different degrees of <br />urbanization. (Derived from figure 5.) <br /> <br />10 <br />