12000000053 Contract - Laserfiche WebLink

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