12000000053 Contract - Laserfiche WebLink

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Ms. Jill A. Baty December 16,2011 Page 2 for the Ish Reservoir study will be made available to assist in preparation of inundation mapping for the Highland#2 North Dam. Failure of the South Dam at Highland#2 Reservoir would result in overland flow from the reservoir through the east side of Longmont and into the St. Vrain River. The inundation mapping is expected to extend downstream to I-25. The City of Longmont has proposed a regional detention pond to the north of Highway 66 that will be represented in the modeling and the inundation mapping of the failure of the South Dam. Failure of the dam at Highland#1 Reservoir would flow to the east across I-25 and into New Thomas Reservoir. The surcharge storage volume in New Thomas Reservoir is expected to contain the majority of the flow resulting from a clear day failure of Highland#1. The inundation mapping is expected to extend from Highland#1 down to New Thomas Reservoir. A blue line will be shown on the mapping illustrating the flow path from the New Thomas Reservoir spillway down to the South Platte River. Failure of the dam at Highland#3 Reservoir would generally flow south toward the St. Vrain River located approximately 2 miles away. The inundation mapping is expected to extend downstream to I- 25. 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 I-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-