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<br />DRAINAGE CRITERIA MANUAL <br /> <br />RUNOFF <br /> <br />concentration is converted to the average rainfall intensity for the 4It <br />time of concentration. <br />3. The maximum runoff rate occurs when the entire area is contributing <br />flow. However, this assumption has been modified from time to time <br />when local rainfall/runoff data was used to improve calculated <br />resul ts. <br /> <br />3.3 Limitations <br />The Rational Method is an adequate method of approximating the peak rate <br />of runoff from a rainstorm in a given basi n. The greatest drawback to the <br />Rational Method is that it normally provides only one point on the runoff <br />hydrograph. When the basins become complex and where subbasins come together, <br />the Rational Method will tend to overestimate th~ actual flow, which results <br />in oversizing drainage facilities. The Rational Method provides no direct <br />data to route hydrographs through the drainage facil ities. One reason the <br />Rational Method is limited to small areas is that good design practice <br />requires the routing of hydrographs for larger basins to achieve an economic <br />design. . <br />Another disadvantage of the Rational Method is that with typical design <br />prccedures one normally assumes that all of the design flow is collected at <br />the design point and that there is no "carryover water" running overland to <br />the' next design point. However, this is not the fault of the Rational Method, <br />but of the design procedure. The Rational Method must be modified, or another <br />type of analysis must be used, when analyzing an existing system that is <br />underdesigned or when analyzing the effects of a major storm on a system <br />designed for the minor storm. <br /> <br />3.4 Time of Concentration <br />One of the basic assumptions underlying the Rational Method is that <br />runoff is a function of the average rainfall rate during the time required for <br />water to flow from the most remote part of the drainage area under <br />consideration to the point under consideration. However, in practice the time <br />of concentration can be an empirical value that results in reasonable and <br />acceptable peak flow calculations. The time of concentration relationships <br />recommended in this manual are based in part on the rainfall/runoff data 4It <br />collected in the Denver metropol itan area and are designed to work with the <br /> <br />5-1-84 <br />