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5. PROBABLE MAXIMUM STORM. The ProbablEl Maximum Storm (PMS) is defined as the most severe combination of meteorological occurrences considered reasonably possible in a particular region. It is felt to be an upper limtt of flood-produCing rainfall (or snowpack melting when applicable) and is used as a design storm where virtually no risk of flooding can be tolerated. The PMS has historically been used in dam design to ensure the adequacy of spillways and top-of-dam elevations for high dams. As with the hl/pothetical frequency storms, one set of generalized criteria is applied to the majority of tlll~ U.S., and a variety of regional criteria, accounting primarily for orographic effects, to the balance. Details for constructing a PMS for a particular region are given in the variclus Hydrometeorological Reports and Technical Papers listed in the references. The repJrts, HMR-51 and HMR-52, for the United States east of the 105th meridian, apply gene1rally to the eastern plains portion of Colorado. The mechanics of storm breakdown and arrangement presented in these reports are similar to methods for the western United Stated presented in other publications. 5.1. PMS FOR EAST OF THE 103rd MERIDIAN. The steps for deriving a PMS. using Probable Maximum Precipttation data from HMR-51 and data for the determination of shape, orientation, and distribution from HMR-52. 5.1.1. Determine isohyetal Probable Maximum Precipitation (PMP) values for the study area for desired drainage area sizes (10. 100,200,1,000.5,000, 10,000,20,000 square miles) and for corresponding storm durations (6-, 12-, 24-, 48-, and 72-hours) using the appropriate plate:s from the report. 5.1.2. Plot a family of duration curves (6-, 12-,24-,48-, and 72-hours) for PMP intensities versus dra.inage area size on semi-logarithmic paper. 5.1.3. Interpolate for the desired storm area, determine PMP intensities for each duration and plot on ordinary graph paper. 5.1.4. Determine rainfall in each 6-hour time interval by interpolation, then increment the rainfall by successive subtractions. The PMP will be the maximum value for the selected storm area only; areas greater or less than the selected storm area will show smaller values of PMP, Considerable trail and error computations will normally be requited to determinEl the storm area which maximizes average precipitation in the study watershed. 5.1.5. Studies for HMR-52 have found that major storms have a dominant orientation, which mayor may not be similar to the general orientation of the watershed. The PMP will often be reduced, depending on the drainage area size and the angle between the storm and watershed orientation, No reduction is taken for orientation differences less than j:40.o, regardless of area, or for drainage areas less than 300 square miles, regardless of orientation. Maximum PMP reduction due to orientation is 15 percent. Cowrado Fwod Hydrology Manual DRAFT 7.8