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<br />July 12, 1995 <br /> <br />Site-Specific PMP for The Cherry Creek Drainage in Colorado <br /> <br />1. Background <br /> <br />A site-specific study of the probable maximum precipitation <br />(PMP) over the Cherry Creek Drainage in Colorado was requested by <br />the District Office of the Corps of Engineers in Omaha. The <br />comprehensive study applicable for this drainage is Hydro- <br />meteorological Report No. 55A (HMR 55A), "Probable Maximum <br />Precipitation Estimates - united States Between the Continental <br />Divide and the 10Jrd Meridian" (Hansen et al., 1988). HMR 55A <br />provides the framework for the relations developed specifically <br />for the Cherry Creek Drainage. The PMP depths as related to area <br />size and duration found in HMR 55A were developed from an <br />ensemble of individual storm values and, as such, are designated <br />as "among-storm" relations. This term will be expanded upon just <br />below. <br /> <br />c. <br /> <br />It is recognized, however, that for a drainage such as <br />Cherry Creek, the probable maximum storm (PMS) will produce PMP <br />for only one storm area size. This concept was advanced in HMR <br />No. 52, "Application of Probable Maximum Precipitation Estimates- <br />United States East of the 105th Meridian (Hansen et al., ~982). <br />The relations between depth, area, and duration in HMR 52 are <br />those which pertain only within an individual storm and as such, <br />are designated as "within-storm" relations. This term also will <br />be elaborated upon just below. This study will use the concepts <br />developed inHMR 52 to define the PMS for the Cherry Creek <br />Drainage. <br /> <br />In section 1.8 of HMR 55A, it is stated that the western <br />limit of the applicability of HMR 52 techniques was the <br />orographic separation line (OSL) (Hansen, et aI, 1988). <br />Virtually all of the Cherry Creek Drainage lies just west of this <br />line as shown on Plate Vb of HMR 55A, except for a very small <br />portion in the northeast corner of the drafnage near Piney Creek. <br />It was decided to overlook this limitation in this study for the <br />following reasons: <br /> <br />A. AMONG-STORM, NON-OROGRAPHIC PMP DEPTHS CAN BE <br />CREATED FROM INFORMATION IN HMR 55A FOR STORM AREA <br />SIZES AND DURATIONS TO BE USED BY HMR 52 TECHNIQUES. <br /> <br />The terms among-storm precipitation values and <br />within-storm precipitation values are used throughout <br />this study. Among-storm precipitation values are those <br />which may come from more than one, individual storm for <br />a given range of storm area sizes and durations. A <br />