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<br />5.1.6. Storm shape is given by criteria in HMR-52. with the PMS having a general <br />elliptical isohyetal pattern with a ratio between major and minor axes ranging from two to <br />five, Areas less than 300 square miles may use a circular shape. if desired. the adopted <br />ratio of the axes will be that which gives the most hydrologically-severe storm (one which <br />maximizes volume) within the study watershed. This usually requires significant trail and <br />error work. <br /> <br />5.1.7. With the maximized storm pattern established, the spatial variability of the <br />precipitation is determined. again by HMR-52. through figures and nomographs. Spatial <br />variability is greatest for the maximum six-hour period, diminishes for the second and third <br />greatest six-hour periods. and has no variability for the remaining six-hour periods. <br />Precipitation profiles are given in HM R-52 to develop the spatial variation in the peak 18 <br />hours of the PMS. <br /> <br />5.1.8. Final development of the PMS calls for a temporal arrangement to give the <br />most critical hydrologic response (maximum runoff). the six-hour values of the PMS are <br />arranged such that they decrease progressively to either side of the greatest six-hour <br />value. the four greatest six-hour increments are placed at any position in the sequence <br />except during the first 24 hours. <br /> <br />Only the general outline of PMS development has been given in this chapter. The <br />engineer should refer to HMR-51 and HMR-52 for detailed guidance in determination of <br />the PMS. <br /> <br />5.2 PMS FOR FRONT RANGE. Regionalized estimates used In the derivation of <br />the all-season PMS for Colorado between the l03rd meridian and the Continental DlvIde <br />are found In HMR 55A. HMR 55A provides Information needed for determination of the <br />general PMS for durations 'up to 72 hours.. for area sizes up to 20.000 square miles in <br />basically nonorographic regions. and areas up to 5000 square miles In the orographic <br />infuenced portions of the study area. Also. local PMS criteria is provided for durations up <br />to six hours and area sizes up to 500 square miles. <br /> <br />Because of the complexity of terrain, variety of storm typs, and lack of available <br />storm areal precipitation data. a modified approach to the PMS determination to that used <br />in the eastem nonorographic United States was required. Because of the variation in <br />terrain features and difficulties encountered in transposing observed areal precipitation in <br />such complex regions. the committee decided to evaluated the maximum storm potential <br />using a "storm separation" method. This method assumes that total precipitation from <br />individual storms occurring in an orographic setting can be divided into two separate <br />components for evaluation. Using storm the storm separation method. the total 24-hour <br />storm precipitation was evaluated based on individual storm relationships of observed <br />precipitation. isohyetal patterns. and meteorological data and analysis to develop an <br />estimate on anount of precipitation due to free atmospheric forcing and amount due to <br />orography. <br /> <br />, . <br /> <br />HMR 55A presents 24-hour PMP for sepecific locations assumed to represent <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />DRAFJ' <br /> <br />7.9 <br />