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<br />5.2 PMS FOR FRONT RANGE. RE!gionalized estimates used in 1Ile derivation of <br />the all-season PMS for Colorado between ':Ile 103rd meridian and the Continental Divide <br />are found in HMR "5A. HMR 5SA provides information needed for determination of the <br />general PMS for durations up to 72 rlours. 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 />influenced 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 types, and lack of available <br />storm areal precipitation data, a modified approach to the F'MS determination to that used <br />in the eastern nonorographic United States was mquirecl. BeCause of the variation in <br />terrain features and difficulties encountered in transposing observed areal precipitation in <br />such complex regiols, 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 olJserved <br />precipitation, isohyetal patterns, and meteorological data and analysis to develop an <br />estimate on amoun': of precipitation due to Iree atmospheric forcing and amount due to <br />orography. <br /> <br />HMR 55A presents 24-hour PMP for specific locations assumed to represent areas <br />of 10 square miles. The calculations were determined for a dense grid covering the study <br />region, and a smooth analysis of tile data waS produced. After meteorological and <br />topographical inconsistencies were addressed, a tinal map of a 24-hour, 1 a-square mile <br />PMP WaS obtained. Using smoothed regional analyses of 1- to 6-llour, 6- to 24-llOur, and <br />72-hourto 24-hour ~recipitation ratios derived from observed storms of record. a 10-square <br />mile PMP WaS derived for 1-, 6-, and /'2-hour cfuralions. Using relationships from severe <br />storms of record and data supplied form acljacent PMP studies, depth-area relationships <br />were assembled for various subre,lions based on topographical and meteorological <br />variations in such parameters. Nomo(;lraphs were then constructed that allowed the user <br />to determine values of PMP for various size drainages within the HMR 55A study region <br />and within the areal and durationa.llimitations of the report. (Bureau 01 Reclamation. 1989) <br /> <br />6. SUPPLEMENTA,l. INFORMATION. <br /> <br />Additional material to consider in deriving these storm data which are also <br />applicable to hypotrletical storm development include: <br /> <br />6.1. EXTRAPOLATION OF FREQUENC1I DATA.. The tecllnical Papers of the <br />NWS indicate that a limited extrapolation (to 200-year return periods) is appropriate based <br />on the available generalized data. However. extrapolation to a 500-year return period, as <br />is often required for the hydrology nec:essa~i in flood-insurance studies, is of questionable <br />validity. Rainfall for tile extrapolated 50 a-year event may be only 20 to 30% greater than <br /> <br />Colorado Flood <br />Hydrology Manual <br /> <br />DRAFT <br /> <br />7.13 <br />