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I I I I I I I I I I I I I I I I I I I 3.0 LITERATURE REVIEW In 1977, Engineering Consultants, Inc. (ECI) completed a study that focused on the reaches of the Gunnison River and Tomichi Creek near the City of Gunnison (ECI, 1977). The reach of the Gunnison River examined in this study extended from Ohio Creek to Tomichi Creek. A Flood Insurance Study published by the Federal Emergency Management Agency in 1982 (FEMA, 1982) reports 1 percent chance exceedence flows, of 8,930 and 2,050 cfs, for the Gunnison River near Gunnison and Tomichi Creek at Gunnison, respectively. One additional study was identified on the upper reach of the Slate River. This study focused on the geomorphology, wetlands, and potential restoration options for the reach (HRS Water Consultants, 1995). 4.0 METHODS The purpose of this analysis is to develop existing conditions flow-frequency relationships based on the natural flows occurring in the basin. There are eight reaches that are studied with the ultimate goal of estimating the 10, 2, 1, and 0.2 percent chance exceedence events. Two reaches with adequate stream gage data (East River at Almont and Tomichi Creek from the Gunnison River upstream to the Gunnison Airport) are analyzed based on peak flows recorded at the gage locations within each reach. The remaining six reaches require additional analysis prior to performing the flood frequency analysis. Two of the six reaches (Gunnison River from A1mont to Ohio Creek and Gunnison River from Ohio Creek to Tomichi Creek) are affected by the Taylor Reservoir operations; thus, the flow records are transformed to represent natural conditions. The remaining four reaches require correlation with other reaches and gages to extend or supplement annual peaks and average daily flows to provide sufficient data for frequency analysis. Detailed discussions on the methods used and analyses performed are contained in the following sections. The flood frequency analysis was performed using REGFREO (Regional Frequency Computation Program) and the HEC-FFA (Hydrologic Engineering Center - Flood Frequency Analysis) computer programs. Both programs use the Log Pearson Type III distribution assumption to perform calculations and estimate percent chance exceedence values based on annual peak information. HEC-FFA includes the ability to test for high and low outliers in the data, while REGFREO has the ability to estimate the correlation between two gages. The reader is referred to "Guidelines for Determining Flood Flow Frequency" Bulletin No. 17B. (Water Resources Council, 1983); "HEC-FFA, Flood Frequency Analysis User's Manual" (USACE, 1992); and "Regional Frequency Computation Program User's Manual" (USACE, 1972) for further description of the methodologies and calculations used to support this study. For this analysis, the snowmelt runoff season is defined as March through June. The season was determined by visual inspection of the annual hydrographs and analysis of the streamgage records. 4.1 . REACHES WITH SUFFICIENT STREAMGAGE DATA For the two reaches with adequate streamp-age data (at least ten years of flood flows that are not appreciably altered by reservoir regulation) to support an analysis, the annual maxima series (largest instantaneous peak flow value for each year) from the natural flow data is identified. The two reaches with adequate data are Reach 5 - East River from Alkali Creek to Taylor River at Almont, and Reach 8- Tomichi Creek from the Gunnison River upstream to the Gunnison Airport (see Table 4.2.1). Prior to inputting the data to the REGFREO and HEC-FFA programs, the peak values were located on the annual hydrograph and compared with available precipitation data to determine whether the peak occurred during a snowmelt runoff or rainfall runoff event as described in Section 4.1.1. lWater Resources Council, Bulletin 17B, 1983. 7