12000000064 Ritschard Dam Inundation Report

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Ritschard Dam Emergency Action Plan Appendix A-2 - Inundation Map A-2-2 Appendix A-2 The incremental water surface elevation difference is negligible between the 100 year floodplain and the dam-failure inundation limits at this location. The flood wave arrival time at this location is also 59 hours which is more than 2 days for emergency warning and evacuation purposes. A constant lateral inflow was input to the HEC-RAS simulation for the larger rivers flowing into the Colorado River along the model reach. The lateral inflows were based on two-year floods based on a Log Pearson Type III analysis of selected stream gages. The lateral inflows used in the model are shown in Table A-2.1. TABLE A-2.1 - 2-yr Inflows to the Colorado River Lateral Inflow Location Lateral Inflow (cfs) Distance Downstream of Ritschard Dam (Miles) Colorado River 4,100 5.7 Eagle River 3,800 76.9 Roaring Fork River 7,750 96.4 Gunnison River 11,400 199.5 BREACH PARAMETERS The initial reservoir water surface was assumed to be at the service spillway crest prior to the reservoir routing of the IDF. The spillway flow discharge data did not consider the fuse plugs (emergency spillways) in the calculations because the maximum water surface associated with routing the IDF through the reservoir does not reach the elevation of the fuse plugs pilot channels. Other key dam breach parameters used in the dam-failure simulation of Ritschard Dam are shown in Table A-2.2. TABLE A-2.2 - SUMMARY OF BREACH PARAMETERS Ritschard Dam Input IDF-Plus- Dam-Failure Breach Bottom Width (feet) 210 Breach Development Time (hours) 0.5 Breach Side Slopes 0.5 Breach Bottom Elevation 7380 Breach Initiation Water Elevation 7496.5 Dam Crest Elevation 7498 Initial Reservoir Water Elevation 7489 The dam breach parameters were chosen based on Wheeler’s experience with other earth dam breach analyses and a review of several empirical breach relationships (FERC, 1991; MacDonald and Langridge-Monopolis, 1984; Froehlich, 1987; Fread, 1981). As part of the breach parameter selection process, a sensitivity analysis of a range of reasonable breach parameters was conducted and compared against erosion rate equations developed from historic dam-failures (Von Thun and Gillette, 1990). The breach bottom width was established by the restricted channel width immediately downstream of the dam. This analysis indicated that the parameters listed above are considered to be appropriate for this dam.