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I I I I I I I I I I I I I I I I I I I The REGFREO program was used to determine the recorded gage data frequency statistics (mean, standard deviation, and skew) for the annual peak, 1-,3-, 5-, 7-, 15-, 30- and 60-day family of curves. The family of curves and associated gage data (median plotting positions) were plotted on a log-probability graph. In order to provide a consistent family of curves for each reach, the standard deviation and skew values were manually smoothed. The smoothing prevents the crossing of any of the curves and provides a consistent set of curves. The resulting smooth statistics are provided in the Chapter 5. In addition to the recorded data frequency statistics, the REGFREO program also produces a set of adopted frequency statistics. The adopted frequency statistics are created using smoothing functions built into the program combined with the reconstituted frequency statistics to produce a smoothed set of frequency statistics for the family of curves. The REGFREO adopted frequency statistics were used during the smoothing process as a guide, but not used outright. When the adopted statistics were plotted along with the gage data, the result was not found to provide an adequate fit in all cases. Two additional guides used in the smoothing process were plotting standard deviation and skew values against the mean for the family of curves. These plots were used to aid in the creation of a consistent trend in the standard deviation and skew throughout the family of curves. This methodology is further described in Chapter 3 of "Hydrologic Frequency Analysis - EM-111 0-2-1415" (USACE, 1993). The smoothed statistics were estimated, plotted, and inspected for consistency and adjusted until a consistent set of curves was developed that described the raw data for each individual member curve of the family. The results are discussed separately for each reach in Sections 5.4 and 5.7. 4.1.1 Snowmelt, Rainfall, and Rain on Snow Events A review of the nature of flood events is presented in regards to snowmelt, rain on snow, and cloudburst event flows. The review indicates the hydrology of the streams in this system is driven by snowmelt runoff, the peak of which typically occurs between March and June (one peak occurring in early July). Rain on snow occurs infrequently. For the purposes of this study, a rain on snow event was defined as 0.5 inches of precipitation on any given day. According to Section 4-07 Runoff Characteristic in the Water Control Manual for Blue Mesa Dam and Reservoir, Gunnison River, Colorado (USACE, 1988), "Cloudburst thunderstorms frequently happen during the months of June through September but they are isolated to the lower elevation of the basin below the snowpack and limited to a relatively small area of the basin. They last from a few hours up to a day or two and contribute little to the total annual runoff." For the purposes of this study cloudburst events, as defined above, were combined into the category of rain on snow events. This was done since sufficient information was not available to differentiate between rain on snow and cloudburst events. In general, for the study area, the higher elevations of the study area and the March to June season investigated greatly reduce the likelihood of inclusion of cloudburst in the analysis. The Manual for Estimating Flood Characteristics (Colorado Water Conservation Board, 1976) adds "Many streams in Colorado originate in high mountain areas and flow out of the mountains to much flatter plain or plateau areas. Floods on these streams at higher elevations are caused by snowmelt runoff but at lower elevations, floods can result from snowmelt, rainfall, or a combination of rain on snow. " 8