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These types of storms are often less than one or two hours in duration and can produce brief periods of high rainfall intensities. The Standard Project Storm distribution can be used to develop an alternate arrangement of rainfall in time. However, although the SPS arrangement is sometimes applied to hypothetical-frequency storms, it was specifically derived for events much rarer than even the lOO-year return-period event. Since its application will give a more severe arrangement than may be reasonable for a hypothetical-frequency event, estimates of peak discharge may be excessively high when the SPS arrangement is used. 4. STANDARD PROJECT STORM. The Standard Project Storm (SPS) is defined as that combination of severe meteorological events that gives the maximum precipitation reasonably characteristic of the geographic region of interest, excluding extremely rare events. Since the SPS is an infrequent event, no specific frequency can be assigned to it. It may range from a return interval of a few hundred years to a few thousand years. The SPS is often used as a design storm in which only a small degree of risk of exceedance can be tolerated such as in the design of an urban floodwall. It is usually used for comparison with the recommended protection for a particular project. Because the Standard Project Storm (SPS) is used mainly within the Corps of Engineers, only a limited number of publications describe its derivation and use, in contrast to materials available on hypothetical-frequency storms and the Probable Maximum Storm. EM 1110- 2-1411 describes the SPS derivation for the United States east of 1050 longitude. SPS development for the remainder of the United States must be based on various published and unpublished Corps District reports and procedures. 4.1. SPS FOR EASTERN AND CENTRAL UNITED STATES. Deriving the SPS for drainage basins greater than 1000 square miles requires special studies by the National Weather Service. The general criteria in EM 1110-2-1411 are applicable to basins of less than 1000 square miles. The sequence of SPS derivation descnbed in the reference includes: selection of an 'index' rainfall, determination of the 24-, 48-, 72- and 96-hour SPS rainfall based on the index and the drainage area under study, adjustment of the rainfall for a basin shape factor, division into incremental rainfall amounts, and arrangement of the incremental rainfall values into the storm sequence. Colorado Flood Hydrology Manual DRAFI' 7.7