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<br />4. STANDARD PROJECT STORM. The Standard Project Storm (SPS) is defined as that <br />combination of severe meteorological events tllat gives tile maximum precipitation <br />reasonably characteristic of the geographic region of interest, excluding extremely rare <br />events. Since the SPS is an infrequent event, no specific frequency can be assigned to <br />it. It may range from a return interval of a few hundred years to a few tllousand years <br />The SPS has been used by the Corps of E:ngineers as a design storm in which only a <br />small degree of risk of exceedance Can be toleraled such as in the design of an urban <br />floodwall. It is usually used for comparison with the recommended protection for a <br />particular project. 13ecause the Standard f'roject Storm (SPS) is used mainly within the <br />Corps of Engineers, only a limited number of publications describe its clerivalion and use. <br />in contrast to materials available on hypothetical-frequency storms and tile Probable <br />Maximum Storm. EM 1110-2-1411 describes tile SPS derivation for the United States east <br />of 1050 longitude. SPS development for lhe remainder of the United States must be <br />based on various published and unpublished Corps District reports and procedures. <br /> <br />5. PROBABLE MAXIMUM STORM. <br /> <br />The Probable Maximum Storm (PMS) is ddined as the most severe combination <br />of meteorological occurrences considered reasonably possible in a particular region. It is <br />felt to be an upper limit of flood-producing rainfall (or snowpack meltin() when applicable) <br />and is used as a dOlsign storm wllere virtually no risk of 1I00ding can be to:erated. Tile <br />PMS has historically been used in da'11 design to ensure the adequacy of ,;pillways and <br />top-of-dam elevations for high dams <br /> <br />As with the hypothetical frequency storms, one set of generalized criteria is applied <br />to the majority of the U.S., and a variety of re"ional criteria, accounting primarily for <br />orographic effects, 10 the balance. Details for constructing a PMS for a particular region <br />are given in the various Hydrometeorological Reports ancl Technical Papers listed in the <br />references. The reports, HMR-51 and HMf'l-52. for the United States east of the 105th <br />meridian, apply generally to the eastern plains portion of Colorado. The mechanics of <br />storm breakdown and arrangement presented in tllese reports are similar to metllods for <br />the western United Stated presented in other publications. <br /> <br />5.1. PMS FOR EAST OF THE 103rd MERIDIAN. The steps for cleriving a PMS. <br />using Probable Maximum Precipitation data from HMR-51 and data for the determination <br />of shape, orientation, and distribution from HMR-52. <br /> <br />5.1.1. Determine isohyetal Probable Maximum Precipitation (PMP) values forthe <br />study area for desi,'ed drainage area sizes (10, 100, 200. 1,000.5.000.10.000.20,000 <br />square miles) and for corresponding storm durations (6-. 1 ~)-, 24-, 48-. and 72.hours) using <br />the appropriate plates from the report <br /> <br />5.1.2. Plot a family of duration curves (6-. 12 . 24-, 48-. and }2.hours) for PMP <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />DRAFT <br /> <br />7.11 <br />