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For simplicity, HEC-RAS modeling assumed that corresponding flows from each basin, at a <br />given time, entered the ditch at the same time and did not attempt to compensate for lag times. <br />Since the total volume is similar for both evaluations and because the tailing impoundment can <br />hold the entire storm runoff, the storage capacity evaluation within the tailing impoundment <br />should not be significantly affected by the inflow hydrograph methodology utilized. <br />The results of hydrology analysis are presented in Appendix A and include a tabulation of the <br />key results from HydroCAD analysis together with output tables and graphs. <br />4.3.2 HEC -RAS Modeling of South Diversion Ditch and Drop Structure <br />Description of Model <br />Figure 16, which depicts the South Diversion Ditch System, shows topography along the <br />alignment of the Diversion Ditch from downstream of the Plunge Basin (identified as Station <br />0 +00), upstream through the Flip Bucket, the Drop Pipe, the Inlet Acceleration Chute, and then <br />along the South Diversion Ditch upstream to its end at approximately Station 38 +43. Flows <br />from contributing drainage basins enter the system at the following approximate station <br />locations: <br />o Area A at approximately Station 34 +00 <br />o Area B at approximately Station 27 +25 <br />o Area C at approximately Station 11 +55 <br />Cross Sections were developed from topographic mapping at various locations to define the ditch <br />geometry, including changes in alignment (horizontal or vertical), cross sectional geometry, <br />and/or top of bank elevation. Roughness coefficients (Manning's "n ") were estimated based on <br />available field observations and photographs provided from site visits. Locations of potential <br />overflow from the ditch system to either the tailing impoundment area or to the area downstream <br />from the embankment (at the Drop Structure) were determined through trial runs, and lateral <br />overflow structures were inserted in the model to enable modeling of the top of bank as a side <br />channel weir, thereby accounting for losses from the ditch system. A minor constriction of the <br />existing channel exists at around Station 22 +50. For purposes of analysis, modeling assumed <br />removal of that constriction. Key locations of potential overflow from the ditch system to the <br />tailing impoundment area were found to be located at around Station 30 +34, and at the far <br />upstream end of the ditch near Station 38 +43 both as shown on Figure 16. Other very minor <br />overflow locations to the tailing impoundment occur along the south embankment <br />(approximately Station 6 +00 to 23 +00). <br />San Luis Project Miller Geotechnical Consultants <br />Tailing Dam Data Report 29 February 2014 <br />