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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />II <br />I <br />I <br />I <br /> <br />basins. The Rational Formula is: <br /> <br />Q = CfCIA <br /> <br />where Q = discharge in cfs, <br /> Cf = frequency factor (1.25 for 100-year storm), <br /> C = runoff coefficient, <br /> I = rainfall intensity in inches per hour, <br /> A = drainage basin area in acres. <br /> <br />Composite runoff coefficients (C) used in computations in this study <br />varied between 0.2 and 0.5 for various combinations of open fields, <br />residential and commercial land use. <br /> <br />The C values used are for existing land use conditions. Most of the <br />undeveloped land to the west and northwest is flat with many sump <br />areas that retain stormwater runoff. As the area develops, the run- <br />off rates can change for different reasons. Additional impervious <br />surfaces will tend to increase runoff; however, flat terrain will <br />tend to not significantly increase the runoff coefficient. Depend- <br />ing on how the drainage is directed to the receiving stream, accumu- <br />lated runoff mayor may not increase. If a new development is built <br />in an area increasing the impervious area, minimal regrading to di- <br />rect the runoff into a flat open channel, lengthening the time of <br />concentration will keep the additional runoff to a minimum. As a <br />result, assumptions for future conditions could affect calculations <br />in order that new developments do not increase the amount of runoff. <br /> <br />In addition, the designs for drainage facilities may be selected on <br />the basis of reasonable capacity, rather than a preselected storm <br />frequency for developed future conditions. <br /> <br />Rainfall data was taken from the Precipitation-Frequency Atlas of the <br />Western United States, NOAA Atlas 2, Volume III - Colorado. From this <br /> <br />-12- <br />