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<br />DRAINAGE CRITERIA MANUAL <br /> <br />RUNOFF <br /> <br />. <br /> <br />1. <br /> <br />RUNOFF ANALYSIS <br /> <br />The analysis of storm runoff, that is, the hydrologic aspects of urban <br />drainage including determination of the peak rate of runoff, the volume, and <br />the time distribution of flow, is that part of the drainage engineer's work <br />which has the greatest effect on the success of failure of his effort. The <br />storm runoff peak, volume, and timing provide the basis for all planning, <br />design, and construction of drainage facilities. To be in error on the <br />hydrology means that the works are either unders i zed, oversized, or out of <br />hydraulic balance. On the other hand, it must be kept in mind that the result <br />of the runoff analysis is an approximation. Although much has been learned <br />since the original publication of this Manual in 1969 too little still is <br />known of the factors that influence urban rainfall-runoff. The intent of this <br />part of the Manual is to provide a reasonably dependable and consistent method <br />of approximating the characteristics of urban rainfall-runoff in areas having <br />similar meteriology and hydrology to what is found within the Urban Drainage <br />and Flood Control District in the Denver Region. <br /> <br />. <br /> <br />1.1 Analytical Methods <br />The review of current practice coupled with the recent analysis of <br />rainfall-runoff data show that two basic approaches provide reasonable results <br />in the Denver Region. They are the Rational Method and the unit hydrograph <br />design rainfall method called the Colorado Urban Hydrograph Procedure (CUHP). <br /> <br />. <br /> <br />1.2 Applicability of Mehtods <br />The Rational Method, in widespread use in the Denver Region, will <br />continue to be utilized for the sizing of storm sewers. In addition, it will <br />continue to be used for determining runoff magnitude from unsewered areas. <br />The new 1 imit of appl ication of the Rational Method is approximately 160 <br />acres. It has been concluded that for tributary basins in excess of 160 <br />acres, the cost of the drainage works justifies significantly more study, <br />thought, and judgement on the part of the engineer than is permitted by the <br />Rational Method. <br />When the urban drainage basin exceeds 160 acres, the CUHP method <br />represents better practice and should be used. It provides the engineer with <br />a higher degree of dependability in the approximations of runoff magnitude and <br /> <br />5-1-84 <br />