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The runoff curve number which indicates the imperviousness of a given tract of land is based on <br />standard values from the NRCS for different land uses. A Land Use Table showing runoff curve <br />numbers for the different hydrologic groups can be referenced in the appendices. The land type <br />Rangeland (Pinyon- Juniper - Poor) was assigned to the drainage basins 1A and 2A. Rangeland <br />(Pinyon- Juniper— Fair) was assigned to drainage basin 3A. Per the Land Use Table the <br />qualifiers Poor, Fair and Good are used to identify the percent of ground cover for a particular <br />land type. Poor is less than 30 %, Fair is 30 -70% and Good is more than 70 %. The ground <br />cover was estimated from recent aerial photos for the site. In the TR -20 model, a runoff curve <br />number of 89 was applied for basins 1A and 2A. For basin 3A the runoff curve number used <br />was 80. <br />The Time of Concentration for all three drainage basins was calculated using the SCS Lag For- <br />mula which is applicable for larger undeveloped watersheds up to 2000 acres. The SCS Lag <br />Formula uses flow length (L), Slope (S) and the relative Curve Number (CN). These calcula- <br />tions are summarized in Table 2. <br />Analysis <br />In summary, the 10 year peak discharge from the three drainage basins totals 183 cfs, the <br />discharge for the 100 year event is 472 cfs (see Table 1). The one hour retention volume for <br />the 10 year storm is 15.1 AF. The existing pond capacity of 15.8 AF is adequate to retain the 10 <br />year storm volume from all three drainage basins, however routing runoff from drainage basin 3 <br />would require an extensive series of drainage channels which would be difficult to maintain. A <br />more cost effective solution is to construct a new drainage pond north of the existing ponds. <br />This pond should be constructed so the total volume is in excess of 4.2 AF, this would exceed <br />the storm volume total for this basin for the 10 year storm. The current drainage system of <br />ponds and swales has been in place for over 10 years and has proven to be an effective way to <br />handle the runoff from Tunnel Drive mine. In general all storm retention and detention basins <br />should be sized to handle the 10 year storm volume. All runoff conveyances including channels, <br />swales, ditches and pond overflows should be designed to handle the 100 year discharge (see <br />Table 1). During a July 2012 site visit it was noted that a few areas could use some <br />maintenance. These areas are outlined below: <br />1.) Add additional armoring to the berm where runoff from basin 1 flows off of the high wall <br />onto the pit floor. This berm currently is made only of dirt and during a large storm event <br />is subject to excessive erosion. <br />2.) Maintain and clear the existing drainage channel along the entire length of the high wall. <br />3.) Increase the rip rap size at the existing drainage pond outfall. Rip rap 12 -18" which is <br />readily available on -site would be more than adequate. <br />4.) Relocate the construction material stockpiled between the existing pond outfall and the <br />existing natural drainage channel. <br />5.) Swale the roadway between the existing pond outfall and the existing natural drainage <br />channel. <br />Currently there is little mining activity in drainage basin 3, however before any activity begins the <br />following drainage improvements should be completed. <br />1.) Remove fines stockpiled in the natural drainage and armor the natural drainage with rip <br />rap. <br />2.) Construct a minimum 4.2 AF pond north of the existing ponds in basin 3. <br />3 <br />