2018-01-31_PERMIT FILE - C1981019A (2)

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EXHIBIT 7, ITEM 20, PART E PERMANENT CHANNEL DESIGN There are two permanent primary channels and eight permanent secondary channels associated with the South Taylor Pit. The primary and secondary channels are permanent channels that convey runoff from the areas near the watershed divide near the southernmost extension of the mining activity. The primary channels are associated with drainage from the face of the West Taylor permanent fill. Their purpose is to convey water to the north from the head of the Taylor Creek watershed towards the West Taylor Sedimentation Pond as shown on Map 41A. The secondary channels drain reclaimed pit areas near the watershed divide in Sections 16, 20 and 21 to the southeast toward Section 16 Sedimentation Pond and Section 28 Sedimentation Pond. The secondary ditches rely on several temporary ditches to convey the runoff to the respective sedimentation ponds until reclamation is complete and the ponds are no longer needed. These temporary ditches are described in other exhibits. Two of these temporary ditches, described in Exhibit 7, Item 20, Part D are the Section 29 Collector Ditch and the Section 28 Collector Ditch. Another temporary ditch, the Section 16 Pond Ditch also receives flow from some of the secondary channels. It is described in Volume 2C, Exh 7-14B. The remaining temporary ditch that collects flow from the secondary channels described in this Part E is the Section Sixteen Pond Ditch Extension. This temporary ditch is described in Volume 2C, Exh. 7-140. All of these ditches are shown on Map 41A. Channel profiles for these channels are presented on Map 33B. Watershed details are also shown in Exhibit 7-20C. Primary Channels The two primary channels are the following: • West Taylor, East Tributary Ditch • West Taylor, West Tributary Ditch By inspection, the area contributory to each ditch is less than one square mile, making the appropriate design storm the 10 year, 24 hour event as specified in Rule 4.05.3. However, portions of these ditches pass over permanent fills, so the design event required by Rule 4.09.2 becomes the 100 year, 24 hour event. For simplicity of design, the entire channel for each ditch is designed to this larger event. Exhibit 7, Item 14 describes the hydrologic design methodology used in all models. For channels protected by a riprap liner, selection of minimum rip rap size is done using the Simons/OSM method in SEDCAD. For channels to be protected by a vegetative liner, the permissible velocities are also determined using SEDCAD's routines. The upper segments of the permanent primary channels are flat enough that a vegetative liner will provide sufficient erosion protection, even for the 100 year flow. However, both ditches below the stock ponds require riprap lining to ensure long-term stability. Each. 7-20E-1 Revision Date: 11/15/17 Revision No.: TR-117