2020-12-31_REVISION - C1981019 (3)

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d. In the structure network, it is unclear why Structure 917 flows directly into the channel below (Streeter Ditch) rather than into a null at the confluence. Within this model and other models for the Colowyo Mine, null structures are commonly used at the confluence of two channels. Please explain. e. Please confirm that the stage/storage information in SEDCAD for the Streeter Pond is accurate. Note that the information on the as -built drawing for this pond may not be correct, as the topography does not appear to reflect the current conditions: the large amount of sediment below the fill is not shown and the peninsula near the gate is not shown. If the stage/storage relationship for the pond has changed, this should be reflected in the SEDCAD model. VOLUME 2E 5. Please explain how the CN values were chosen for the post -mining conditions to model in SEDCAD. In Figure Exh. 7-14ET-2 the large majority of the drainage area has a CN of 62 (925 acres out of 1049 acres). Please explain why that is the worst -case hydrologic condition. 6. Please explain apparent discrepancies between the East Taylor Pond As -Built (Figure Exh. 7-14ET-1) and the SEDCAD model (page 18 of the 25-year model). On the topographical drawing the principal spillway is below elevation 6940, which does not agree with the model. The spillway perforations are at an elevation of 6944 on the Stage Storage Curve, but there is no discharge in the Detailed Discharge Table until the water reaches 6952. The principal spillway is at 6952 on the Stage Storage Curve but a foot lower in SEDCAD. Also, there are errors on the section for the principal spillway, such as the elevation for the top of the pipe (6872 is lower than pond bottom). 7. In the East Taylor Pond SEDCAD model, the discharge from structure 917 is 0.012 cfs (seepage 10 of the 10- year model). Per the introductory text for the East Taylor Pond, however, the flow should be 0.56 cfs. Please explain this apparent discrepancy. 8. When reviewing the Section 16 Pond text (Appendix Exh. 7-14S): a. In the text that discusses the temporary ditches, the listed depths of 0.5 foot and 1 foot may be too small when compared to depths in the SEDCAD model. For example, the depth for Structure 93 in the 10-year model is 1.17 feet. Please revise as appropriate and also explain if freeboard is not necessary. b. In the text that discusses the temporary ditches, it appears that the words "West" and "East" are mixed up in three or more locations. Please revise as appropriate. 9. Figure Exh. 7-14S-2 (Section 16 watersheds) appears to have an error with the stationing for East Section 16 Ditch. The label "13+15" should be revised to "23+15." 10. The scale on Figure Exh. 7-14S-2 is incorrect. Please edit this error. 11. Please explain an apparent discrepancy between the Section 16 Pond As -Built (Figure Exh. 7-14S-1) and the associated SEDCAD model. The spillway perforations are at an elevation of 7746 on the Stage Storage Curve of the drawing, but there is no discharge in the Detailed Discharge Table in the model until the water reaches 7753. VOLUME 7 12. On Maps 12 and 19 (and other maps), please check the disturbance boundary symbol. This symbol is not clearly shown in some areas such as the Section 16 Fill Ditch and Prospect Pond. 13. On Map 12, the ditch that crosses the West Pit Fill (the East Taylor Pond Ditch) is not shown. However, this ditch is still included in Appendix Exh 7-14F. Please address this apparent discrepancy.