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<br />calculated earlier. Both the Big Thompson Canyon and Frijole Creek storms <br />produce 6-hour PMP rainfall whicl1 exceed the values of the Masonville <br />storm. The design of water retention facilities should take into account the <br />rainfall potential of complex convective storms. <br /> <br />For time periods of 24 hours or more, the ~Ieneral PMP storm is dominated <br />by the steady accumulation of rainfall 'from the Big Elk Meadow storm <br />which exceeds even the total accumuliation of the Cherry Crelek and Gibson <br />Dam storms beyond 36 hours. Given tile split timing of the most intense PMP <br />rainfall, a combination of the heaviest rainfall at 1-hour and 6-hours from the <br />complex convective storms could be mer!Jed with the 24-hour and beyond values <br />of the Big Elk Meadow storm to produce the most rainfall. This suggestion is <br />supported by consideration that historically embedded convection in a general <br />storm is a common occurrence. This rationale can be compared to discussions <br />in HMR 55A (Section 10.1.1) which refer to "among storms" PMP values, Le., <br />different storms or storm types may control PMP at 5 hours and at 24 houl's. <br />Although the historic storms of record transposed to Mason Reservoir for the <br />present study do not exhibit this feature, HMS believes it is still a physical <br />possibility. This point is discussed further in Section 5.0 below. <br /> <br />5.0 HMS site-specific local and gene,ml storm PMP calculations <br /> <br />During Phase I HMS reviewed the site-spec:ific local storm and general storm <br />PMP values obtained by BV. These values were obtained by BV through the <br />application of standard HMR 55A methodology which incorporated PMP values <br />given in the series of HMR 55A Plates. HMS considered these values fOl' <br />possible application and modification based on the availability of enhanced data <br />sets and new technology. Phase I identified the potential utilization of the HMS <br />Convective Storm Methodology (CSM) operating on the Extreme Precipitation <br />Event Atmospheres to assist in the storm maximization and elevation adjustment <br />portions of the HMR Methodology. Each of these areas of PMP calculation <br />offers an opportunity for the consultant's subjective input based on experience <br />and science in local precipitation patterns.. <br /> <br />This section of the report presents a discussion of the adjustments <br />recommended by HMS to be used in addition to the standard HMR PMP <br />calculations of storm maximization, orographic effects and elevation adjustments <br />for both the site-specific local storm and general stOl'm PMP's. These <br />adjustments are based on the discussions and references presented in the <br />enhanced data set section. HMS compares the results of the HMR 55A and the <br />HMS site-specific local storm and general storm PMP's in this section. <br /> <br />34 <br />