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Ms. Cynda Lower <br /> December 27, 2011 <br /> Page 2 <br /> PROPOSED METHODOLOGY <br /> D&A will perform a clear day dam failure analysis for the dam at Hyatt Lake and route the resulting <br /> outflow hydrographs downstream using HEC-HMS. Breach parameters shall be estimated in <br /> accordance with methodologies recommended by the Office of the State Engineer,Dam Safety <br /> Branch. Hydraulic parameters including width of flow, depth of flow, and velocity of flow at critical <br /> locations shall be estimated using the Corps of Engineers' HEC-RAS computer model. A steady-state <br /> water surface profile resulting from the peak flow rates at various locations shall be generated. <br /> Topographic information for the floodplains shall be based on USGS 10-meter DEMs from the <br /> National Elevation Dataset (NED) elevation data. NED data was initially derived from the USGS 7.5- <br /> minute quadrangle elevation data and was then processed to filter out production artifacts from the <br /> existing data. The NED is updated every two months to include new and improved elevation source <br /> data and is considered the best available raster elevation dataset. D&A has the ability to contour the <br /> DEM at any desired interval using 3D Analyst in ArcGIS. The quality of the output elevation contours <br /> shall be controlled by comparing the index contours to the USGS 7.5-minute topographic images. <br /> Requests shall be made to the Urban Drainage and Flood Control District(UDFCD) to obtain the <br /> effective floodplain models for the impacted reaches of Van Bibber Creek and Ralston Creek. If the <br /> magnitude of the breach outflow greatly exceeds the flow rate for which these floodplain models are <br /> valid, it may be necessary to obtain information regarding the opening dimensions and roadway <br /> elevations at potentially critical bridges in the study reaches. Prior to conducting the field survey of <br /> critical bridges, a meeting shall be held with Greg Hammer of the Colorado Division of Water <br /> Resources to present preliminary water surface profile modeling and inundation mapping and to <br /> discuss the critical bridges that have been identified as requiring field measurements. <br /> Inundation maps of the clear day failures of the dam shall be overlaid on 1-meter resolution aerial <br /> photography. <br /> DELIVERABLES <br /> The deliverables for this project shall include a letter report for the inundation mapping analysis <br /> (stamped by a Colorado P.E.), 10 hard copies of the inundation maps, and 2 CDs containing: digital <br /> files of the inundation limits that are compatible with ESRI ARCView software, PDF file of the <br /> inundation maps, and supporting spreadsheets and hydraulic/hydrologic models. <br /> SCHEDULE <br /> Work on this project will begin in January 2012 with delivery of the final product no later than May <br /> 31, 2012. <br />