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• 4.8.1.2 Surface Drainage Durina Minina <br />The following section discusses diversions and culverts which will be used to control runoff during the <br />current permit tens. Since submittal of our original permit document in 1980, Trapper has submitted <br />Technical Revision (TR-25) which altered design criteria for all future diversions and culverts. It has been <br />agreed-to by the CMLRD that Trapper will not be required to after structures which were constructed prior <br />to the partial approval of TR-25 (Johnson and Pyeatt sediment control) September 5, 1986. <br />Haul Road Culvert Sizino <br />Haul road culvert sizes were determined using the 10-year, 24-hour precipitation event predicting maxi- <br />mum flow rates. These maximum flow rates and sizes were predicted through hydrologic modeling tech- <br />niques. The results are presented in Table 4.8-1. Some haul road culverts are placed in undisturbed <br />drainages. Based on the Soil Conservation Service Curve Number Method for quantifying excess rainfall <br />or runoff volumes, the formula shows undisturbed drainages yield no nahoH from the approved 10-year, <br />24-hour precipitation event. In order to maintain proper drainage during high snowmelt or other <br />exceptional events, the culverts in undisturbed areas were sized at eighteen (18) inches diameter. <br />Table 4.8-1 presents the culvert sizing information for existing and future haul road culverts. Table 4.8-1 a <br />lists most non-haul road and auxiliary culverts to identify and quantify culverts within the active mining <br />and reclamation area. Premine culverts installed outside of Trapper's active mining area are not shown in <br />permit tables or on the M51 maps. <br />• The locations of all required culverts listed in both tables are shown on the Drainage and Sediment Con- <br />trol Map (M-51). Actual future culvert locations may vary from those sham on Map M51. If so, the pro- <br />cedums described in this section will be followed to ensure proper haul road culvert sizing and drainage <br />continuity. <br />Normal mining operations and weather conditions may require additional culvert installations within pit <br />areas (includes associated spoils) to provide environmentally sound access corridors across drainage <br />points or to divert runoff to existing drainageways. Culverts established within pit areas (often temporary <br />installations for a short period of time) will not be shown on Tables 4.8-1, 4.8-1 a, or M51 map and will not <br />require Division approval prior to installation. Culverts established in these areas will be considered a <br />Best Technology Currently Available (BTCA) practice with the intent of minimizing erosional Impacts to <br />Trapper's surface water hydrology. <br />__pGA?l 1995.._.