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Failure of the dam at McIntosh Reservoir would generally flow south toward the St. Vrain River located 1.5 <br />miles away. The inundation mapping is expected to extend downstream to I-25. <br />The inundation limits shall be prepared in UTM coordinates (NAD 83, Zone 13 – meters) which shall be <br />overlaid on base mapping that includes color aerial imagery (2009 or newer). The inundation map shall be <br />printed in color on a minimum sheet size of 11”x17”. The inundation extents shall be clearly illustrated along <br />with peak discharge estimates, peak flood wave arrival times and peak flood wave depths at critical <br />locations. Roads, highways and critical infrastructure (schools, hospitals, fire stations, etc.) shall be <br />annotated and any roads expected to be overtopped by the flood wave shall be identified. The map shall <br />include a legend noting the dam name and DAMID, a north arrow and graphic scale. <br />PROPOSED METHODOLOGY <br />D&A will perform a clear day dam failure analysis of each of the six dams and route the resulting outflow <br />hydrographs downstream using HEC-HMS. Breach parameters shall be estimated in accordance with <br />methodologies recommended by the Office of the State Engineer, Dam Safety Branch. Hydraulic parameters <br />including width of flow, depth of flow, and velocity of flow at critical locations shall be estimated using the <br />Corps of Engineers’ HEC-RAS computer model. A steady-state water surface profile resulting from the peak <br />flow rates at various locations shall be generated. <br />Topographic information for the floodplains shall be based on USGS 10-meter DEMs from the National <br />Elevation Dataset (NED) elevation data. NED data was initially derived from the USGS 7.5¬minute <br />quadrangle elevation data and was then processed to filter out production artifacts from the existing data. <br />The NED is updated every two months to include new and improved elevation source data and is considered <br />the best available raster elevation dataset. D&A has the ability to contour the DEM at any desired interval <br />using 3D Analyst in ArcGIS. The quality of the output elevation contours shall be controlled by comparing the <br />index contours to the USGS 7.5-minute topographic images. <br />Requests shall be made to Weld County Public Work, Boulder County Transportation, and the City of <br />Longmont to obtain information regarding the opening dimensions and roadway elevations at potentially <br />critical bridges in the study reaches. Prior to conducting the field survey of critical bridges, a meeting shall be <br />held with John Batka of the Colorado Division of Water Resources to present preliminary water surface <br />profile modeling and inundation mapping and to discuss the critical bridges that have been identified as <br />requiring field measurements. <br />Inundation maps of the clear day failures of the three dams shall be overlaid on 1-meter resolution aerial <br />photography. D&A currently has a digital National Agricultural Imagery Program (NAIP) mosaic of all of <br />Larimer and Weld Counties. The NAIP mosaic consists of aerial photos of quarter quadrangles that were <br />acquired with a precision aerial mapping camera in 2009. <br />DELIVERABLES <br />The deliverables for this project shall include a letter report for the inundation mapping analysis (stamped <br />by a Colorado P.E.), digital files of the inundation limits, Inundation Maps, and supporting spreadsheets and <br />hydraulic/hydrologic models.