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<br />4. STANDARD PROJECT STORM. The Standard Project Storm (SPS) is defined as that
<br />combination of severe meteorological events that gives the maximum precipnation
<br />reasonably characteristic of the geographic region of interest, excluding extremely rare
<br />events, Since the SPS is an infrequent event, no specnic frequency can be assigned to
<br />n. It may range from a return interval of a few hundred years to a few thousand years,
<br />The SPS has been used by the Corps of Engineers as a design storm in which only a
<br />small degree of risk of exceedance can be tolerated such as in the design of an urban
<br />f1oodwall, It is usually used for comparison wnh the recommended protection for a
<br />particular project, Because the Standard Project Storm (SPS) is used mainly wnhin the
<br />Corps of Engineers, only a limned number of publications describe its derivation and use,
<br />in contrast to materials available on hypothetical-frequency storms and the Probable
<br />Maximum Stonn, EM 1110-2-1411 describes the SPS derivation for the Unned States east
<br />of 1050 longitude, SPS development for the remainder of the Unned 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based on various published and unpublished Corps District reports and procedures,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 defined 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 melting when applicable)
<br />and is used as a design storm where virtually no risk of flooding can be tolerated, The
<br />PMS has historically been used in dam design to ensure the adequacy of spillways and
<br />top-of-dam elevations for high dams,
<br />
<br />As wnh the hypothetical frequency storms, one set of generalized criteria is applied
<br />to the majorny of the U.S" and a variety of regional crneria, accounting primarily for
<br />orographic effects, to the balance, Details for constructing a PMS for a particular region
<br />are given in the various Hydrometeorological Reports and Technical Papers listed in the
<br />references, The reports, HMR-51 and HMR-52, for the Unned States east of the 105th
<br />meridian, apply generally to the eastern plains portion of Colorado, The mechanics of
<br />stonn breakdown and arrangement presented in these reports are similar to methods for
<br />the western Unned Stated presented in other publications,
<br />
<br />5.1. PMS FOR EAST OF THE '103rd MERIDIAN. The steps for deriving a PMS,
<br />using Probable Maximum Precipitation data from HMR-51 and data for the detennination
<br />of shape, orientation, and distribution from HMR-52,
<br />
<br />5.1.1. Determine isohyetal Probable Maximum Precipnation (PMP) values for the
<br />study area for desired drainage area sizes (10,100,200, 1,000,5,000, 10,000,20,000
<br />square miles) and for corresponding storm durations (6-, 12-,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 72-holJrs) for PMP
<br />
<br />Colorado Flood
<br />Hydrology Manual
<br />
<br />DRAFT
<br />
<br />7,11
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