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4.1.4 Previous Design Analyses <br />The previous design analysis for the South Diversion Ditch was documented in Exhibit D, <br />Section D.6.5.6 "Diversion Ditches" of the 112 Permit Application Amendment Permit <br />No. M -88 -112; September 1989 (BMRI, 1989). Appendix H Diversion Ditch Design <br />Calculations and Appendix I — PMF Storm Calculations both from the 112 Permit Application <br />provide additional analyses that illustrate part of the previous design basis. Information from <br />Section D.6.5.6 is summarized above in Sections 4. 1.1 and 4.1.2. <br />Calculations indicate the Diversion Ditch was designed for 224 cfs from where Area A enters to <br />where Area B enters, for 285 cfs from Area B to Area C and for 292 cfs below area C. The ditch <br />was designed as a trapezoidal section with a 10 ft bottom width, 2 horizontal to 1 vertical side <br />slopes, a Manning's `n' roughness coefficient assuming gravelly soils of 0.03 and a slope of <br />0.25 percent. The calculated flow depth ranged from 3.25 ft where Area A entered the ditch to <br />3.73 ft after Area C entered the ditch. Calculations were completed assuming normal depth and <br />steady flow conditions. <br />The purpose of the Drop Structure is to lower captured runoff diversion flow from the elevation <br />alongside the tailing impoundment to the channel elevation below the dam. The drop structure <br />includes a concrete approach Inlet Acceleration Chute, a section of 48 -inch CMP, a concrete <br />Energy Dissipation Structure and a grouted riprap -lined Plunge Basin. The channel is lowered <br />approximately 49 ft vertical by use of the Drop Structure facilities as shown on the record <br />Drawings. <br />The purpose of the Inlet Acceleration Chute is to accelerate the flow from the velocity in the <br />approaching diversion ditch, calculated by the designers as 4.50 fps, to a velocity of 29.5 fps at <br />the entrance to the culvert (assuming a depth of flow in the culvert of 2.96 ft). The design <br />established a convergence section where the width of the channel narrowed while the slope <br />increased. The design approach was based on a formula from Design of Small Dams (USBR, <br />1987) wherein the angle of convergence is related inversely to the Froude Number (Tan angle = <br />1/(3 * Fave). The angle of convergence was calculated to be 11.52 degrees (max) and an <br />associated length of convergence of 50 ft was determined. Calculations were not made of the <br />slope or head change required to accelerate the flow. <br />Calculations provided in the cited documentation for the Dissipation Structure are not for the <br />facility shown in the construction drawings. Accordingly, no presentation of the previous <br />calculations is included. The facility defined by the Construction Drawings, and which was <br />!_ observed during this inspection, is a flip bucket at the end of the 48 -inch CMP drop pipe. The <br />bucket is five ft wide by 16 ft long. The flip bucket has a radius of 9.5 ft, a bucket lip height <br />(depth of bucket) of 1.5 ft, and a trajectory angle of 33 degrees; the flip bucket discharges into a <br />plunge basin that has a bottom extending from 30 to 46 ft downstream and 10.5 ft below the lip <br />San Luis Project Miller Geotechnical Consultants <br />Tailing Dam Data Report 25 February 2014 <br />