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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 1988-1992 permit term. Since submittal of our original permit document in 1980, Trapper has submitted Technical Revision (TR-25) which aRered design criteria for all future diversions and cul- verts. 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, 24hour precipitation event predicting maximum flow rates. These maximum flow rates were predicted through hydrologic modeling tech- niques. The hydrologic modeling resuRs are preserned in Appendix Q, Drainage and Sediment Control Plan. Some haul road culverts are placed in undisturbed drainages. Based on the Soil Con- servation 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 snowmeft or other exceptional events, the culverts in undisturbed areas were sized at twenty-four (24) inches diameter. Table 4.8-1 presents the culvert sizing information for existing and future haul road culverts. The selection of the culvert sizes was obtained from Table 4.8-2, Culvert Capacities, which was derived from the U.S. Bureau of Public Roads (USDI, 1974, p. 565). The ratio of head-water to the diameter of the culvert is measured from the bottom of the culvert, and a ratio of one (1) is a no-head flow condition. Table 4.8-1 a lists all non-haul road and auxiliary culverts to identify and quantify all 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 M-9 •Roads• maps. The locations of all culverts listed in both tables are shown on the Roads map [M9 (4 sheets)]. Actual future culvert locations may vary from those shown on Map M9. If so, the procedures 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 environmernally 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 4.8-1 or 4.8-1 a and will not require Division approval prior to installation. Culverts established in these areas will be considered a Best Technology Currernly Available (BTCA) practice with the intent of minimizing ero- sional impacts to Trapper's surface water hydrology. REVISION a-179 APPROVED ~pT z ~S a