2023-05-25_PERMIT FILE - M2022048 (9)

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ENVIRONMENT, INC. PAGE 5 L.G. EVERIST, INC. -WEST FARM RESERVOIR ADEQUACY RESPONSE 01 MAY 23,2023 wall. (A copy of the Corps approval letter dated 05/10/23 is enclosed, "RE: Regional General Permit Verification - Corps File # NWO-2020-01166-r)EN, West Farm, Weld County, Colorado".) Comments 13-16 are based on the guidance provided by the Urban Drainage and Flood Control District- Technical Review Guidelines for Gravel Mining and Water Storage Activities Within or Adjacent to the 100 Year Floodplain. 13) From section 2.6.1: "When the river is allowed to maintain an existing unstable alignment, and no bank protection is provided beyond the necessary revegetation, the minimum allowable top width of riverside berms will be 400 feet. This is the maximum setback requirement imposed on gravel mining operators." "When the river is left in an existing unstable alignment and no riverbank protection is provided, but pitside slope protection is provided in accordance with the requirements of Section 2.4- Pitside Bank Protection, the allowable minimum top width will be 300 feet."Please adjust the mining setbacks shown for Phase 2 to be a minimum of 300 feet from the river. Revisions have been made to the Exhibit C-1 Mining Plan Map, Figure E-1 and the Exhibit F - Reclamation Plan Map showing the required 300 foot setback from the top of the slope to the adjacent river bank. As we discussed, LGE will mine closer than 300 feet from the South Platte River in Phase 2 but no closer than 215 feet since the Slurry wall will be installed on a 200 ft. setback. This will be done in 500 ft. sections that stay open no more than 12 months. Backfilling will include filling in the 85 foot wide section from the slurry wall setback to the Top of Slope (T05) to its original elevation so there is 300 feet of setback from the river and construct the bank slopes into the reservoir. The newly filled area will be armored and seeded. Exhibits D and E have been revised to explain the mining and backfilling plans proposed. 14) For Phase 1, no pitside slope protection has been proposed, therefore, please adjust the mining setback from the river to be no less than 400'. If you wish to use a 300'setback for Phase 1, provide pitside slope protection in the mining and reclamation plans for that phase. We have added armoring in Phase 1 where the river is within 400 feet of the Top of Slope (TOS) along the east side of the Phase. This was added to the Mining and Reclamation Maps. It is approximately 1305 feet long and will require approximately 3,080 yards or armoring materials. Note that at its closest point, the river is still 347 feet away from the top of slope and east of the railroad embankment that was not overtopped in 2013 by flooding. 15) Due to the immediate and direct connection between the irrigation return flow ditch on the west side of Phases 1 and 2 and the South Platte River, the west sides of the Phase 1 and 2 reservoirs should also have pitside slope protection in