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Mr. Tenney February 1, 2012 Page 2 meets the SEO mapping termination guidelines. If the PMF-plus-breach simulation is modeled, we will use the recently updated PMF inflow that was approved by the SEO in July of 2011. The reservoir routing and dam-breach analysis will be simulated with the U.S. Army Corps of Engineers (USACE) HEC-1 computer program. Downstream cross-sections will be extracted from the 1/3 arc second (-10m) National Elevation Dataset (NED), using the HEC-GeoRAS ArcGIS extension developed by the USACE then imported into USACE HEC-RAS River Analysis System computer model. The channel routing of the breach hydrograph will be simulated using the unsteady flow option in the HEC-RAS model. The maximum water surface will then be imported into ArcGIS to develop the maximum inundation limits. We will use color aerial photos as our base map from the USGS/SCS unless the Colorado River District has better aerial photos that can be made available to us. Our inundation mapping will include data boxes at key downstream cross-section locations that will summarize key information, such as: distance downstream of dam, peak flood elevation, peak flood stage, estimated flood wave arrival time, and estimated time of maximum flood stage. Although the SEO guidelines suggest that velocities be included in the data boxes, we typically do not include flood velocities to simplify the mapping because flow velocities are generally irrelevant to most emergency planners. The inundation mapping will be produced on 11-inch x 17-inch sheets with an index sheet provided as a cover sheet. We will also provide a brief analysis memorandum that documents our key assumptions, analysis methods, and results of our dam-failure analyses. We generally neglect the impact of bridges in the downstream floodplain because dam-failure discharges are expected to significantly exceed the bridge hydraulic capacities.