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Introduction <br />The Tunnel Drive mine, located at the southwestem edge of city limits in Canon City, Colorado, <br />has been the site of active surface mining activity since the early 1900's. Tezak Heavy <br />Equipment Company, Inc., purchased the property in the mid- 1990's and amended the limited <br />impact operation to a type 112 mining permit in 1997 and again in 2000 to reflect the current <br />231 permitted acres. The current amendment to mining permit M- 1977 -193 proposes <br />continuation of mining to the west of the current permit boundary onto USBLM property totaling <br />an additional 79 acres. The mine is located in Section 31, T18S, R70W 6th PM in Fremont <br />County, Colorado. The address of the site is 201 Tunnel Drive, Canon City, Colorado 81212. <br />(See Figure 1). <br />Existing Conditions <br />The contributing watershed for the project area is also shown in Figure 1. It consists of three <br />drainage basins totaling 299 acres + / -. The limits of the three drainage basins were determined <br />from the Royal Gorge USGS 7.5 minute Quadrangle map for this area. Based on Natural <br />Resource Conservation Service (NRCS) soils report for the area, there is one major soil type for <br />the contributing watershed, the Ustic Torriorthents, bouldery-Rock outcrop complex. This soil is <br />a hydrologic group D soil. The vegetation is mainly Rocky Mountain Juniper and Two needle <br />Pinyon with an understory of shrubs and grasses. See the Appendices for the soil survey or go <br />to websoilsurvey.nres.usda.gov. <br />Per the FEMA flood insurance rate maps, a portion of the drainage basin 1 is within the limits of <br />a Zone A 100 year flood plain. <br />On -site runoff from these three drainage basins flows through the Tunnel Drive mine. Runoff <br />from basins 1 and 2 is routed at the base of the high walls across the pit floor to the existing <br />drainage ponds in an earthen swale. The drainage ponds consist of 3 detention ponds totaling <br />about 15.8 AF which in over 10 years have never overflowed. Runoff from drainage basin 3 still <br />more or less follows its natural drainage course. <br />Hydrologic Modeling <br />To determine the runoff and peak discharge from the contributing drainage basin, The WinTR -20 <br />model, created by the Natural Resources Conservation Service (NRCS) was used. This model <br />is a storm event surface water hydrologic model applied at a watershed scale. It is based upon <br />unit hydrograph theory. Model input includes the lot area, the weighted runoff curve number and <br />the time of concentration to calculate the direct runoff from rainfall occurring over specified <br />areas. It is used to analyze current watershed conditions as well as assess the impact of <br />proposed changes (alternates) made within the watershed. <br />Method <br />To meet the drainage requirements for Fremont County, calculations for the 100 year - 24 hour <br />storm, and the 10 year - 6 hour storm are included. The associated rainfalls for each storm are <br />below. <br />Depth of Rainfall <br />Storm (inches) <br />100 Year — 24 Hour 4.0 <br />10 Year —6 Hour 2.2 <br />2 <br />