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<br />DRAINAGE CRITERIA MANUAL <br /> <br />RAINFALL <br /> <br />6. BASIS FOR DESIGN STORM DISTRIBUTION <br /> <br />. <br /> <br />Ra infa 11 in the Denver area is infl uenced by the orographi c effects of <br />the Rocky Mountains, the topography of the high plains and the semi-arid <br />cl imatology of the region. Rainstorms can often have an "upslope" character <br />where easterly flow of moisture settles against the mountains. These types of <br />rainstorms have durations that can exceed 6-hours and, al though they may <br />produce large amounts of total precipitation, they are rarely intense. <br />Although upslope storms can cause 1 oca 1 drainage problems, they are not the <br />cause of 2- through lOO-year type of urban flooding in the Denver area. <br />Very intense rainfall in the Denver area results from convective storms <br />or frontal stimulated convective storms. These types of storms are often less <br />than 1 or 2 hours in duration and can produce brief periods of high rainfall <br />intensities. It is these short duration intense rainstorms that appear to <br />cause most of the urban flooding problems. <br />Analysis of a 73-year record of rainfall at the Denver raingage reveals <br />that an overwhelming majority of the intense rainstorms produced their <br />greatest intensities in the first hour of the storm. In fact, of the 73 most . <br />intense storms analyzed, 68 had the most intense period begin and end within <br />the first hour of the storm and 52 had the most intense period begin and end <br />within the first half hour of the storm. The data clearly shows that the <br />leading intensity storms predominate among the "non-upslope" type storms in <br />the Denver Region. <br />The recommended design storm distribution takes into account the observed <br />"leading intensity" nature of the convective storms. In addition, the <br />temporal distributions were designed to be used with the 1982 version of the <br />CUHP, the publ ished NOAA I-hour precipitation values (1), and the Horton's <br />infiltration loss equation. They were developed to approximate the recurrence <br />frequency of peak flows and volumes (i .e. 2- through 100-years) that were <br />estimated for the watersheds for which rainfall/runoff data was collected. <br />The procedure for the development of these design storm distributions and the <br />preliminary results were reported earlier (2) (3). The recommendations <br />contained in this manual are the result of refinements to the work originally <br />reported in 1979. <br /> <br />. <br />