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increment from the curve. Averaging the incremental change between the original points is usually a satisfactory alternative, since the depth-duration plot normally approximates a straight line after the first several values. Once an accumulated depth for each interval has been determined, the depths are incremented to compute that portion of the depth that occurred in each period. 3.7. STORM ARRANGEMENT. The final step in the storm definition is arrangement of the storm rainfall into a specific pattern. The pattern used most often by the Corps of Engineers is a "triangular" arrangement, with the peak period in the center of the storm. For out example, for a 24-hour duration storm with a computation interval of 15 minutes,the peak 15-minute depth would be placed in (assigned to) the thirteenth hour or forty-ninth period of the twenty-four period storm sequence. The next-highest depth is placed just ahead of the peak (Period 48), the next highest depth just behind (Period 50), and so on until all 96 values are systematically arranged about the peak period. If a storm with a duration longer than 24 hours is to be arranged, all 24- hour periods outside of the peak 24 hours can be represented by an average value for each 24-hour period. The rainfall increments cannot be moved outside the 24-hour periOd from which the increment was developed, however. 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 100-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. 7-17