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<br />H <br />t' <br /> <br />Table 2. CUHP Shaping Factors <br /> <br />VARIABLE UNITS USE <br />coefficient (Ct) for time to number optional <br />peak. <br />coefficient (Cp) for peak number optional <br />rate of runoff <br />width at 50% (W50) minutes optional <br />width at 75% (W75) minutes optional <br />fraction of W50 before T p number optional <br />(default = 0.35) <br />fraction of W75 before T p number optional <br />(default = 0.45) <br />water quality capture inches optional <br />volume 0NQCV) <br />drain time for WQCV hours optional <br /> <br />HISTORICAL DATA <br /> <br />When preparing designs for storm water management, rainfall-runoff data <br />is often incomplete or non-existent for a specific catchment. As a result, when <br />developing the guidelines for controlling the anticipated urban drainage profiles, <br />engineers rely on historical data for a generalized area to determine what <br />controls are necessary. Historical precipitation data tells us what frequency can <br />be expected for major storm events. Based upon these frequency analyses, <br />decisions are made as to how construction should proceed. For example, . <br />Colorado requires that a culvert designed to convey storm water below a major <br />roadway must meet the needs of the 5-year storm. <br />Since it is impossible to predict what the profile of a specific future storm <br />will be, assumptions must be made to generate a synthetic hyetograph for the <br />event. The UDFCD has outlined the proportional distribution of rainfall events for <br />the 2-, 5-, 10-, 50-, and 1 OO-year storms in the Denver metropolitan area and <br />these ratios were incorporated into CUHP for the 1-hour rainfall option. <br />Although rainfall data may not be available for anyone given catchment, <br />Colorado, in conjunction with the USGS, has accumulated an abundance of <br />rainfall data for many areas in the State. The UDFCD has compiled this data, <br />performed frequency analysis, and published rainfall event statistics. The State <br />of Colorado and the Denver Regional Council of Governments have endorsed <br />the use of these statistical values. The 1-hour rainfall depths for the Denver <br />metropolitan area as determined by the UDFCD are presented in table 3 (Urban <br />Drainage Criteria Manual, 1984). Since these are the values used by industry, <br />they were used in this investigation for the 1-hour rainfall options. <br /> <br />Application and Evaluation of CUHP <br /> <br />Page 5 of 52 <br />