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<br />TP 40 and TP 49 for the 35 states east of the Rocky Mountain area which are essentially <br />free of significant orographic effects. The particular pUblications to be used depends on <br />the storm duration under examination, The 13 mountain states containing the Rocky <br />Mountains and those areas to the west are covered by NOM Atlas 2 and other site- <br />specific publications. The procedures described in all the NWS publications are based on <br />statistical evaluations of long-term rainfall-gage records in a region. These evaluations <br />include estimates of the frequency of accumulated rainfall-depth versus storm duration at <br />each rain gage. Rainfall maps were made from these depth-duration values, and isopluvial <br />lines were drawn on these maps to define constant rainfall-depth relationships through a <br />region for a specific storm duration. Each of the NWS publications gives a detailed <br />discussion of the derivation of the rainfalJ-trequency depth-duration relationships. It is <br />strongly recommend,~ that users of NWS data thoroughly read the pertinent sections and <br />be familiar with the clpplications and limitations of the NWS material. <br /> <br />A hypothetical storm developed from NWS data is also referred to as a "balanced <br />storm," because a consistent depth-frequency relation is used for each peak duration <br />interval of the storm. That is, for a hypothetical l00-year retum period 48-hour-duration <br />storm, the rainfall de1pths for the peak 3D-minute, l-hour, 6-hour, 24-hour or other peak <br />period would each be equal to the l00-year depth for that duration. This consistent <br />frequency-depth-duration relationship throughout a storm would not occur in nature, <br />because of the randomness of the rainfall events. The balanced storm concept, however, <br />does allow for logical construction and arrangement of a storm event for a particular return <br />period. Balanced storms are discussed in more detail later in this section. The Standard <br />Project Storm (SPS) is used primarily by the Corps of Engineers as a standard design <br />criteria to design and evaluate performance of flood control projects. Development of the <br />SPS for states west of the Rocky MOuntains requires site-specific criteria, which are not <br />discussed here. <br /> <br />~ <br /> <br />The Probable Maximum Precipitation Storm (PMP) is developed primarily from <br />NOAA and NWS crit,ilTia. The area east of the 105th meridian is covered by HMR 51 and <br />HMR 52. For areas between the l03rd meridian and the continental divide, HMR 55A is <br />used. HMR 49 cover$ the Colorado River basin and the Great Basin areas. Special <br />studies for site-specific areas are prepared by the NWS upon request. For example HMR <br />33 was prepared for the design of Chatfield Reservoir and covers the upper South Platte <br />River Basin. <br /> <br />3. HYPOTHETICAIL-FREQUENCY-STORM DERIVATION. <br /> <br />Development of a storm from NWS data is straightforward and systematic. The <br />individual performing the study must: 1) establish the appropriate storm duration and the <br />time interval for subdividing the storm rainfall, 2) extract the rainfall values from NWS <br />publications for his area of interest, 3) make adjustments to the rainfall depth for size of <br />drainage area if needed, 4) adjust for partial to annual series (if required), 5) compute <br />incremental rainfall amounts, and 6) arrange the storm rainfall increments in time. Each <br />of these steps is described in the following paragraphs. <br /> <br />3.1. STORM DURATION. Before constructing any hypothetical event (including <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />DRAFT <br /> <br />7,2 <br />