Laserfiche WebLink
APPENDIX 6.4.7-A2 <br /> 1. All work will be done in the overbanks and no work will be done in the river itself. Included <br /> with the proposed work for the mined areas is a section of raised grading to be constructed no <br /> less than 1 foot above the base flood elevation that a permanent hot asphalt mixing plant is to <br /> be built on. The analyses to determine the base flood elevation for the raised surface to place <br /> the mixing plant was completed separately from the analyses presented in the main report and <br /> is presented in Appendix C. <br /> In respect to Weld County code Section 23-11-360(0) the fill material used to build up the <br /> terrain for the asphalt mixing plant will be suitably resistant to erosive forces associated with the <br /> base flood. The results of the hydraulic analysis revealed that the maximum velocity seen in this <br /> area for the 100-year event was approximately 2.5 feet / second which is the maximum <br /> permissible velocity for a firm loam soil. Therefore, if the terrain is constructed from a firm loam <br /> soil and is vegetated or is constructed from another soil more resistant to erosive forces than a <br /> firm loam then there is no potential for erosion in this area from the base flood. <br /> Effective Flood Insurance Study <br /> The current Federal Insurance Study, FIS, for the project site was made effective as of 2016 <br /> and includes Weld County, Colorado and Incorporated Areas. The FIS reports the Big <br /> Thompson River at the Larimer-Weld County line as having a drainage area of 1,662 sq. miles <br /> and having a 100-year event of 10,000 cfs. The upstream edge of the project (Weld County <br /> Road 13) is the county line boundary between Larimer County and Weld County. <br /> The project site is spilt between the following FIRM panels 08123C1490E, 08123C1495E. The <br /> FIRM panels define the project site as being in Zone AE —the regulatory floodplain in which the <br /> base flood elevations have been determined. In this region of the Big Thompson River, there is <br /> no mapped floodway. The Big Thompson River has an average floodplain width of 1,750 feet <br /> and an average slope of 8 feet per mile. The valley is generally U-shaped, and the floodplain is <br /> of mainly agricultural use. The FIRM panels that cover the project area are included in <br /> Appendix B. <br /> One-Dimensional Analysis <br /> HEC-RAS version 5.0.4. was used for the modeling analysis. The model that was created for <br /> this project used a preliminary model created by AECOM Denver for the Colorado Hazard <br /> Mapping Program (CHAMP) in July 2016. The model is currently in review to be the effective <br /> model for FEMA, however as of the date of this report is not the effective model. Changes made <br /> to the original model for its use in this project are explained below in the following section. Both <br /> existing condition and proposed condition models were created for this project, the methods <br /> used to model and results are presented below. The focus of this analysis is the 100-year event,- <br /> however, 50-year and 10-year events were also examined for thoroughness. <br /> CHAMP Model Modifications for Existing Conditions <br /> For use in this project, the original CHAMP model was modified. Only a small portion of the <br /> CHAMP model was used which includes the lower Big Thompson reach cross sections 82986- <br /> 71810. The model also includes the WCR 13 Split reach that was originally put in the model to <br /> handle flow overtopping WCR-54. Additional changes to the model include: <br /> - Changed hydrologic conditions to 2016 effective FIS flows <br />