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2025-05-05_PERMIT FILE - C1981010 (23)
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2025-05-05_PERMIT FILE - C1981010 (23)
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Last modified
6/12/2025 9:58:10 AM
Creation date
6/12/2025 9:52:05 AM
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Template:
DRMS Permit Index
Permit No
C1981010
IBM Index Class Name
Permit File
Doc Date
5/5/2025
Doc Name
pages 4-172 to 4-222z
Section_Exhibit Name
4.8 Hydrologic Impacts
Media Type
D
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No
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All drainage channels were assumed to be unvegetated as representative of newly reclaimed lands. All of <br />the channels were modeled as "erosive channels" with a width of 20 feet and with a 10 percent channel <br />slope and a surface of "gravel, non -colloidal". These dimensions were representative of the typical <br />condition at Trapper Mine. The limiting erosive velocity for this case is 4.0 feet per second, significantly <br />less than the 6.0 feet per second for a vegetated channel. This design methodology allows for the <br />establishment of a grass -lined channel in the future. <br />Results of the SEDCAD modeling presented in Appendix Q, Section 37 will be used to design and <br />construct all future postmining drainage channels. This information has been summarized in Figures 4.8- <br />1 c through 4.8-1 h. <br />Figure 4.8-1c provides a recommended minimum number of check dams based on the temporary storage <br />needed per the drainage area of the watershed and conditions of the reclamation with a curve number of <br />67. Similarly, Figures 4.8-1d, 4.8-1e and 4.8-1f provide the same information for curve numbers 72, 78 <br />and 83, respectively, at various channel slopes. The volume of storage capacity needed subsequently <br />dictates the number and spacing of check dams and/or dozer basins that would typically be required. <br />Notice the linear nature of all of these figures. Also notice how quickly the required storage volume <br />changes as the curve numbers decrease, Figure 4.8-1g. Although moving from a curve number of 83 to <br />72 as reclamation becomes mature could occur in as little as 2 years, there is a large decrease in the <br />storage volume needed. <br />Appendix Q, Section 12, lists representative curve numbers, hydrologic soil groups, and cover for native <br />and reclaimed land. Generally, the check dams are constructed about one growing season after the <br />upstream area has been seeded. A representative curve number of 86 is appropriate for the freshly <br />topsoiled condition, and a curve number of 80 will be used for reclaimed lands with 1 year's reclamation <br />growth. The curve number for reclamation with more than two year's growth is 67. Because <br />reconstructed drainage basins generally have various reclamation time periods and slope conditions, an <br />area weighted approach in applying these values is appropriate. <br />Dozer Basins. Trapper Mine uses dozer basins to contain sediment in areas near recently reclaimed <br />lands. These structures are always less than 2.0 acre feet in capacity and are excavated (subgrade). <br />Figure 4.8-7 in Section 4.8.1.7 gives a typical dozer basin design. The location and capacity of dozer <br />basins is largely dependent on the surrounding topography, with flatter areas being more conducive to <br />achieving adequate capacity. Generally, one or more dozer basins will probably be needed for each year's <br />reclamation activities depending on the drainage area. <br />Figure 4.8-1g provides a graph of Total Runoff Volume versus Drainage Area. As such, the calculation of <br />runoff volume is only dependent on drainage area and curve number and is independent of slope. Figure <br />4.8-1 h provides the Annual Sediment Yield from a 10 Year, 24 Hour storm. Notice how the quantity of <br />sediment generated drops off sharply prior to the second year of reclamation once vegetative cover <br />begins to establish. <br />Temporary storage is provided by a combination of check dams and/or the construction of dozer basins. <br />Dozer basins are complementary structures that could potentially offset the storage capacity of check <br />dams, as dictated by the drainage design. <br />4-183e <br />Revision: _.__ R _ G <br />t,PR 01 2015 Approved: <br />
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