2025-05-05_PERMIT FILE - C1981010 (23)

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4.8.1.2 Surface Drainage During Mining The following section discusses diversions and culverts which will be used to control runoff during the current permit term. Since submittal of our original permit document in 1980, Trapper has submitted Technical Revision (TR -25) which altered design criteria for all future diversions and culverts. It has been agreed -to by the CMLRD that Trapper will not be required to alter structures which were constructed prior to the partial approval of TR -25 (Johnson and Pyeatt sediment control) September 5, 1986. Haul Road Culvert Sizing Haul road culvert sizes were determined using the 10 -year, 24-hour precipitation event predicting maxi- mum flow rates. These maximum flow rates and sizes were predicted through hydrologic modeling tech- niques. The results are presented in Table 4.8-2. Some haul road culverts are placed in undisturbed drainages. Based on the Soil Conservation Service Curve Number Method for quantifying excess rainfall or runoff volumes, the formula shows undisturbed drainages yield no runoff from the approved 10 -year, 24-hour precipitation event. In order to maintain proper drainage during high snowmelt or other exceptional events, the culverts in undisturbed areas were sized at eighteen (18) inches diameter. Table 4.8-2 presents the culvert sizing information for existing and future haul road culverts. Table 4.8-2a lists most non -haul road and auxiliary culverts to identify and quantify culverts within the active mining and reclamation area. Premine culverts installed outside of Trapper's active mining area are not shown in permit tables or on the M51 maps. The locations of all required culverts listed in both tables are shown on the Drainage and Sediment Con- trol Map (M-51). Actual future culvert locations may vary from those shown on Map M51. If so, the pro- cedures described in this section will be followed to ensure proper haul road culvert sizing and drainage continuity. Normal mining operations and weather conditions may require additional culvert installations within pit areas (includes associated spoils) to provide environmentally sound access corridors across drainage points or to divert runoff to existing drainageways. Culverts established within pit areas (often temporary installations for a short period of time) will not be shown on Tables 4.8-2, 4.8-2a, or M51 map and will not require Division approval prior to installation. Culverts established in these areas will be considered a Best Technology Currently Available (BTCA) practice with the intent of minimizing erosional impacts to Trapper's surface water hydrology. 4-173 Revision: PP ` 07 JAN 2 8 2014 approved: