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<br />',. <br /> <br />precipitation in the study watershed. <br /> <br />5.1.5. The "Hop Brook adjustment" has often been applied to PMP <br />values in Corps studies for the Eastern and Central United States. It attempts to <br />adjust for the reduced probability of a Probable Maximum Storm occurring directly <br />over a small watershed. The smaller the watershed, the less is the likelihood of such <br />a large storm occurring directly over it. Factors for reduction of PMP intensities for <br />various drainage areas are shown on Table 1. <br /> <br />TABLE 1 <br />HOP BROOK ADJUSTMENT FACTORS* <br />DRAINAGE AREA ADJUSTMENT <br />(SO Mil FACTOR <br />1000 .9 <br />500 .9 <br />200 , .89 <br />100 .87 <br />50 .85 <br />10 .80 <br />1 .80 <br /> <br />*NOTE: Not all Corps of Engineer Districts apply this adjustment factor. <br /> <br />5.1.6. Studies for HMR-52 have found that major storms have a <br />dominant orientation, which mayor may not be similar to the general orientation of <br />the watershed. The PMP will often be reduced, depending on the drainage area size <br />and the angle between the storm and watershed orientation. No reduction is taken <br />for orientation differences less than .i:. 40.0, regardless of area, or for drainage areas <br />less than 300 square miles, regardless of orientation. Maximum PMP reduction due <br />to orientation is 15 percent. <br /> <br />5.1.7. Storm shape is given by criteria in HMR-52, with the PMS having <br />a general elliptical isohyetal pattern with a ratio between major and minor axes ranging <br />from two to five. Areas less than 300 square miles may use a circular shape, if <br />desired. the adopted ratio of the axes will be that which gives the most <br />hydrologically-severe storm (one which maximizes volume) within the study <br />watershed. This usually requires significant trail and error work. <br /> <br />7-20 <br />