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<br />. <br /> <br /> <br />. <br /> <br />, <br />, <br />I <br /> <br />. <br /> <br />CHAPTER 5 <br />HYDROLOGY <br /> <br />SECTION 1.0 <br />RAINFALL <br /> <br />COLORADO STATEWIDE <br />DRAINAGE AND FLOODPLAIN MANAGEMENT CRITERIA MANUAL <br /> <br />1.4 <br /> <br />RAINFALL DISTRIBUTION FOR RATIONAL METHOD <br /> <br />Time-intensity-frequency curves and the Rational Method can be used to produce <br />rainfall-runoff data for drainage basins of less than 160 acres. The time-intensity- <br />frequency curves can be developed for a desired location utilizing the rainfall depths <br />of durations less than 1-hour for storm events between 2- and 100-year. <br /> <br />Utilizing the estimated rainfall depths of the 5-, 10-, 15-, 30-, and 60-minute durations <br />for a given recurrence frequency, rainfall intensities can be estimated by dividing the <br />rainfall depth by the duration of the storm. <br /> <br />Ix,v= Dx,v/Durationv <br /> <br />(Eq. CH5-108) <br /> <br />Where Ix.v=' "X"-year, Y-minute rainfall intensity (Inches/Hour) <br />Ox,v = "X"-year, Y-minute rainfall depth (Inches) <br />Durationv = Duration Y minute divided 60 (Hour) <br /> <br />A time-intensity curve for a given recurrence frequency can be developed by plotting <br />the intensity values versus their corresponding storm duration values. An example <br />showing the development of a time-intensity-frequency curve is given in Section <br />1.7.2 of this chapter. <br /> <br />1.5 RAINFALL DISTRIBUTION FOR CUHP METHOD <br /> <br />The CUHP (Colorado Urban Hydrograph Procedure) <br />computer model has been used widely within the Urban <br />Drainage and Flood Control District (UDFCD) <br />jurisdictional area to estimate urban sub-basin <br />hydrographs. The CUHP method was developed and <br />calibrated to simulate short duration convective storms <br />in the Denver Metro area and other similar urban <br />drainage environments. Convective storms are <br />commonly known to be responsible for high peak flows <br />and flooding problems for many small drainage basins. <br /> <br />The CHUP method <br />should only be used <br />for urban areas that <br />have similar <br />hydrologic <br />characteristics as <br />the Denver Metro <br />Area. <br /> <br />1.5.1 CUHP STORM DISTRIBUTION <br /> <br />The rainfall intensity and distribution analysis performed by UDFCD using 73- <br />years of rainfall record data at the Denver rain gage revealed that the <br />majority of the past intense rainstorms produced their largest rainfall within <br />the first hour of the storm. The analysis further discovered that out of the 73 <br />storm events analyzed, 68 events produced the most intense rainfall <br />beginning and ending within the first hour of the storm and 52 events <br />produced the most intense rainfall beginning and ending within the first half <br />hour of the storm. The UDFCD analysis concluded that these "leading <br />intensity" convective storms were the main cause of most of the flooding <br />problems in the Denver Metro Region (UDFCD, 2001). <br /> <br />The rainfall distributions recommended to be used with CUHP were <br />developed to reflect the "leading intensity" characteristics of the previously <br />recorded convection storms in the Denver Region, and they vary from 2- to 6- <br />hours depending on the size of the drainage basin. The rainfall distributions <br /> <br />AUGUST 2002 <br /> <br />RAINFALL <br /> <br />CH5-106 <br />