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The Drop Structure including the Inlet Acceleration Chute and 48 -inch CMP were modeled with <br />several different approaches to allow a thorough representation of its possible performance. One <br />method included use of a culvert and roadway overflow deck, believed to provide the best <br />estimate of probable overflow conditions, another included use of a separate culvert and lateral <br />structure approach to enable reentering of overflow below the Flip Bucket thereby maintaining it <br />unsubmerged and the third included a lateral structure with an internal culvert outlet which <br />allowed modeling of overflow down the embankment. <br />For the ditch system from station 27 +25 to 38 +43, the direction of flow within the ditch (whether <br />upstream or downstream from a point of entry) was found to depend on the rate of flow in the <br />system (sum of flows from Areas A, B and C), and the time on the hydrograph (defining the <br />relative location of flow entry). To function properly, a requirement of HEC -RAS is that it must <br />be setup with stationing decreasing in the direction of flow. Accordingly, for the area between <br />27 +25 and 38 +43, it was necessary to create several models depending on where the flow was <br />going. Operational conditions for these models are generally as follows: <br />1. Condition 1: All flows are downstream from any point of entry for flows less than <br />approximately 200 to 300 cfs depending on where flow enters the system (i.e., which side <br />of hydrograph is in effect may result in a greater percentage of the flow from Area A <br />versus Area B or Area Q. Model No 1 is used for this condition. <br />2. Condition 2: Flow entering from Area A will split, a portion going upstream (and over <br />the overflow at 38 +43) and the remainder going downstream; all other areas will drain <br />downstream. Applies for flows ranging from approximately 200 -300 cfs to <br />1600 -3900 cfs depending on where flow enters the system. Models No 1 and No 4 are <br />used for this condition. <br />3. Condition 3: Flow entering from Areas A and B will split, with a portion going upstream <br />and the remainder going downstream; Area C continues to drain downstream. In this <br />condition, the saddle overflow to the sedimentation impoundment at approximately <br />Station 30 +34 will be fed from both Areas A and B. Area A will also drain upward to the <br />overflow at Station 38 +43. Area B drains upward to the overflow at 30 +34 and <br />downward toward the drop structure. This flow condition is for flows above the range of <br />1600 -3900 cfs. Models No 2 through No 5 are used for this condition. <br />The multiple models were run at different flows and iterated to establish the flow split. The <br />hydraulic criteria established to determine the appropriate flow split was that the water level <br />calculated by a model at the location where flow entered the system (i.e., Area A and /or Area B) <br />had to be at the same elevation for the backwater from the two models for which each are <br />effectively flowing downstream from that point. <br />The hydraulics documentation includes the basis of channel roughness conditions (including <br />photographs), a tabulation of the key results from HEC -RAS analysis completed as part of this <br />San Luis Project Miller Geotechnical Consultants <br />Tailing Dam Data Report 31 February 2014 <br />