2018-09-17_PERMIT FILE - C1981025 (4)

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nivnrcinnc �. Overland flow from undisturbed areas is directed away from disturbed areas by temporary diversions which are designed to safely pass the peak runoff from a 10 year, 24 hour precipitation event. The existing diversions, sediment ponds, and culverts are shown on Drawing D-4-7 and D-4-8. Design criteria for these structures are included in Appendix 4-B, "Final Engineering Calculations". Appendix 4-8, Section "Final Design - Ditches" shows calculations for the size of the area disturbed, slope adjustments, and design velocities for all the ditches in both the vegetated and unvegetated conditions. Temporary diversions systems are shown on Drawings D-4-7, D-4-8, and D-4-6. Drawing D-4-5 summarizes the designs of the ditches and shows the ditches profile. Temporary diversion ditches draining the refuse disposal area are shown on Drawing D-4-9. All temporary diversions will be removed as part of the post -mining reclamation plan. Ditches D-7, D-9, and D-10 which are associated with the refuse dispo- sal pile are the only permanent structures required for post -mining reclamation. These ditches are designed to pass the runoff from a 100 year -24 hour storm. The permanent diversion ditches are shown on Drawing D-4-11. The concrete conduit that contains North Thompson Creek is shown on maps D-3-4 and D-3-5. The conduit is large enough to pass the 100 year flood. See Appendix 3-N, North Thompson Creek Flows. Sediment Control Ponds Sediment control ponds P-3, P-9, and the Refuse Disposal Pond are designed to control the amount of sediment reaching North Thompson from the disturbed areas. The structures are designed to store runoff from a 10 year, 24 hour precipitation event while discharging the runoff from a 25 -year, 24-hour event. In addition to these three sediment control ponds there are seven structures P-1, P-2, P-4, P-5, P-6, and P-8 which are used as energy dissipators for the surface runoff drainage ditches. 4-17