12000000064 Ritschard Dam Inundation Report

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Ritschard Dam Emergency Action Plan Appendix A-2 - Inundation Map A-2-1 Appendix A-2 Appendix A-2: Inundation Map Documentation FLOOD INUNDATION AREA The dam-failure flood inundation mapping, provided in the Ritschard Dam (Wolford Mountain Reservoir) Emergency Action Plan (EAP), was prepared by W. W. Wheeler and Associates, Inc. (Wheeler) in May 2012. The purpose of this document is to summarize the methods and assumptions used to develop the inundation mapping. The flood inundation base mapping consists of aerial imagery available at http://services.arcgisonline.com as of May 2012. The flood inundation index map was prepared with the StreetMapUSA data layer (ESRI, 2005). The inundation mapping for the failure of Ritschard Dam was developed from a computer simulation of the dam-failure and a channel routing of the flood wave’s travel through the river channel downstream of the dam. The first step of the simulation was to prepare a reservoir routing model of the failure of Ritschard Dam in combination with the Inflow Design Flood (IDF) to the reservoir. The IDF for Ritschard Dam was developed by URS in February 2012 (URS, 2012). This reservoir routing model was simulated with the U.S. Army Corps of Engineers (USACE) Flood Hydrograph Package, HEC-1 (USACE, 1998). The IDF inflow to Wolford Mountain Reservoir results in a peak inflow of 85,228 cfs and a volume of 8,000 acre-feet. Ritschard Dam was modeled to fail at the maximum water surface resulting from the IDF, approximately 1.5 feet below the dam crest. The reservoir routing was initiated at the spillway crest elevation of 7489 feet which corresponds to a normal water storage of 65,985 acre-feet. The second step of the analysis was to simulate the channel routing downstream of Ritschard Dam of the breach hydrograph obtained from the HEC-1 model. The flood wave flows south from the reservoir through Muddy Creek, to its confluence with the Colorado River to the Colorado/Utah State Line, approximately 240 miles downstream. The channel routing of the breach hydrograph was simulated using the unsteady flow option in the USACE HEC-RAS River Analysis System computer model (USACE, 2010). The cross-sections used in Wheeler’s HEC-RAS simulation model and flood inundation analyses were extracted from the 1/3 arc second (~10m) National Elevation Dataset (NED), (USGS, 2011). All of the cross-sections were extracted from the surface using the HEC- GeoRAS ArcGIS extension developed by the USACE (USACE, 2011). A Manning’s roughness value of 0.04 was used for the main channel and a value of 0.1 was used for the left and right overbank areas in the HEC-RAS model. Bridge modeling was not included in the HEC-RAS model. The bridges in the study reach were assumed to fail from the dam-failure flows. The more conservative case of the IDF-plus-breach scenario was modeled to prepare the inundation map because this case maximizes downstream flood stages and minimizes flood wave arrival time for emergency planning. The peak breach discharge from Ritschard Reservoir as a result of the IDF-plus-breach is 903,230 cfs. The IDF-plus-breach inundation limits represent the assumed worst-case scenario for the limits of downstream flooding. The inundation mapping was terminated at the Colorado/Utah State line, which is located about 240 miles downstream of Ritschard Dam. The inundation mapping was terminated at this location because the dam-failure flow was equivalent to the 100 year floodplain. Wheeler performed a Log Pearson Type III analysis of stream gage data at the Colorado River State Line gage (USGS No. 9163500). The 100-year discharge was estimated to be 68,760 cfs, which was considered to be equivalent to the dam-failure discharge of about 70,000 cfs at this location.