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<br />The Standard Project Storm is used widely within the Corps of Engineers <br />(USCE, 1965). Development of the SPS is described in detail in the Appendix, e <br />Bnd eXBmple calculations are given, However, because development Dr the SPS <br />for the Western states requires site-specific criteria, procedures for the <br />western states are not covered in this report. <br /> <br />The Probable Maxinun Storm (PMS) is considered the maximum storm that <br />can be reasonably expected to occur in a given region. Accepted <br />meteorological procedures are used to determine the upper limits of rainfall <br />amounts, which are then assembled into critical chronological sequences. PMS <br />data are usually developed by using depth-area-duration relationships for <br />precipitation from major storms that were measured in the region or could <br />have occured. Precipitation amounts for the storms are adjusted to <br />correspond to maximum moisture conditions and maximum rate of moisture inflow <br />to the storm location. Envelope curves based on the adjusted values for all <br />storms are then used to develop the PMS depth-area-duration curves. <br /> <br />Generalized charts for PMS rainfall are available for the united states <br />from National Weather Service publications. Two publications covering the e <br />U.S. in general are NWS (1956) for areas east of the 105th Meridian and NWS <br />(1960) for western areas. The Appendix gives an extensive list of <br />publications with PMS data for specific areas, <br /> <br />4,3 Applications for Hypothetical Storms <br /> <br />There are three general types of studies in which hypothetical storms <br />are used: <br /> <br />(1) Planning Investigations of Potential Flood Control Components -- <br />Hypothetical frequency storms are used in the development of <br />frequency curves for existing and modified conditions as input into <br />economic evaluations. Standard Project Storms can be used to <br />provide information on the performance of potential modificatiDns. <br />Probable Maximum storms are generally used for sizing spillway and <br />outlet works. <br /> <br />e <br /> <br />20 <br /> <br />..__,,~l <br />