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Mr. Elliott R. Russell <br /> January 4, 2018 <br /> Page 3 <br /> 18. The spillway structure, as depicted on the Final Reservoir Grading Plan (Sheet 10), <br /> does not contain a footer. The Division has approved similar structures which have <br /> contained a footer to help prevent the structure from settling over time and overturning <br /> during a flood event. Please provide information regarding how the spillway will remain <br /> stable overtime as well as during a flood event or revise the plans accordingly. <br /> As discussed above,the spillway design consisting of an earthen channel with a <br /> concrete crest wall has been changed. The new spillway design consists of a roller <br /> compacted concrete(RCC)channel with concrete cutoff walls at the top,midpoint, and <br /> toe of rundown slope. A footer for these cutoff walls will not be necessary as they will <br /> be cast directly against trench bottom and sidewalls. It is very common to construct <br /> these types of walls (knee wall,weir wall, cutoff wall)without footers, and this design <br /> exists on numerous jurisdictional dam spillways throughout Colorado. Settlement is <br /> not an issue since there is little to no additional load placed by the wall(soil at 110 to <br /> 130 pcf density replaced by concrete at 150 pcf density). For example, at the spillway <br /> crest cutoff wall, 6 feet of soil will be removed, or a load of 720 pounds(6 feet x 120 <br /> pcf)per lined foot of trench and replaced by 750 pounds(5 feet x 150 pcf). This is an <br /> additional 30 psf pressure and a typical sand and gravel deposit on which the wall will <br /> be founded has an allowable bearing of about 5,000 psf. <br /> Overtopping during a flood event does not appear to be a concern. The velocities do <br /> not exceed 7 fps, and are subcritical through the full range of spillway flows at the <br /> upstream cutoff, and this is considered a low scour velocity. The middle wall is <br /> attached to and covered by RCC. The downstream wall is embedded within the Pierre <br /> Shale bedrock. <br /> 19.Please discuss the relationship between the Phase I Parcel 4 spillway and the future <br /> Phase III reservoir. <br /> The Phase I,Parcel 4 spillway will flow southeastward towards the Arkansas River. It <br /> will not discharge into the Phase III reservoir. The area south of the spillway is being <br /> mined and backfilled, and a channel towards the river will be built. <br /> 20. Within the September 2017 Flood Analysis and Flood Control Plan (Appendix D), the <br /> Operator states riprap protection on the downstream portion of the spillway is not <br /> necessary due to the mild slope of the drainage swale tie-in. Please submit a cross- <br /> section which depicts the spillway and drainage swale tie-in. This cross section will be <br /> similar to the Spillway Section B on the Final Reservoir Grading Plan (Sheet 10), <br /> however, the cross section should depict from the pit floor to the natural grade of the <br /> swale. <br />