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1. INTRODUCTION <br />As of the mid -term review of 2006, a complete inventory of all surface water control structures <br />has been made and a complete new list of culverts, collection ditches and diversions has been <br />made for the permit area. Some culverts are new, others have been redesigned to account for <br />a more accurate drainage area, and others have been removed from past permit text because <br />they no longer exist on the site. <br />This mid -term review inventory of 2006 has the latest up to date topography for the site and <br />basically all designs east of 2700 Road show the post- mining topography design, while the area <br />west of 2700 Road uses the worst case drainage areas so that the structures do not have to be <br />redesigned after mining. <br />The topography of the land at New Horizon Mine slopes gently to the southwest at approximately <br />3 %. The northernmost part of the permit area generally drains to Tuttle Draw, the main natural <br />drainage immediately to the north of the mine disturbance, and most of the southern portion of the <br />permit area drains to the southwest to Calamity Draw, which is the immediate natural drainage to <br />the south of the mine disturbance. See Map 2.05.3(3) -1. This map shows the drainage basins for <br />each ditch and culvert, as well as pre -mine and post -mine topography. <br />2. PARAMETERS FOR DESIGNS <br />All temporary structures have been designed for the 10 year 24 hour event using a Type 11 storm. <br />All permanent structures have been designed using a 100 year 24 hour event, also using a Type <br />II storm. It is assumed that the antecent moisture condition for all designs is 2. <br />Curve numbers for original rangeland range from 80 to 87 depending upon the hydrologic soil type, <br />aspect, land use and condition. Most designs, however, are not based on this condition since it is <br />not the worst case. Curve numbers for disturbed land with no vegetation range from 85 to 90. <br />Curve numbers for reclaimed land range from 77 to 86 depending on the age of the reclamation, <br />% cover, type of cover and hydrologic soil type. For lands that will later be disturbed, the worst case <br />is used assuming the land is disturbed. <br />Revised June 2011 (TR -60) Attachment 2.05.3(3) -1 -4 <br />