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Craig Ranch Pit — General Stormwater Comments <br />Page 3 <br />June 23, 2011 <br />ii. If the sandstone is not rippable, consider keeping the proposed islands out <br />of the existing drainages such that the vegetative medium is out of the <br />100 -year flow path and less susceptible to erosion. <br />iii. The upgradient contact edge of the islands with the sandstone should be <br />parallel to elevation contours such that runoff tends to pond against the <br />island rather than flowing along the contact edge and eroding the island <br />material. <br />iv. Consider constructing collection/diversion ditches along the upgradient <br />edge of the islands. <br />4. Page E -4, Post - Reclamation Site Drainage — The text states "All onsite flows will end up <br />in a sediment pond either in Mining Area 1, 2 or 4." Map F does not show a sediment <br />pond in Mining Area 2. The Applicant shall provide: <br />a. A location of the sediment pond in Mining Area 2 on Map F and on Map C -2 (if <br />applicable). <br />b. Sediment pond sizing for the sediment pond in Mining Area 2 as discussed in <br />Comment No. 2. <br />c. Sufficient information (e.g., hydrologic analyses and hydraulic computations) for <br />the Division to evaluate the collector ditch sizing used to divert runoff to the <br />Mining Area 2 sediment pond. Please note because these ditches are to be part of <br />the post mine land use, the Division recommends they be sized to convey runoff <br />from the 100 -year, 24 -hour storm event, not the PMP. Also note Comment No. 5 <br />below. <br />5. Page E -5, first paragraph/precipitation — The Applicant states the 100 -year, 24 -hour <br />precipitation event is 2.2 inches. No reference is provided, but it appears the Applicant is <br />using Figure S -7 in "Procedures for Determining Peak Flows in Colorado ". The <br />isopluvial maps in that document are outdated. Current 24 -hour precipitation depths <br />should be obtained from NOAA Atlas II, Volume 3, available online at: <br />http://hdsc.nws.noaa.gov/hdsc/pfds/other/co_pfds.html, and specifically at <br />http : / /www.nws.noaa.gov /oh/hdsc /PF documents /Atlas2 Volume3.pdf. <br />The Division estimates the 100 -year, 24 -hour storm depth to be 2.6 inches for the area. <br />Please correct all calculations accordingly. <br />6. Page E -5, first paragraph/Curve Number — The Applicant states a conservative curve <br />number (CN) of 85 was used for calculations. Based on the areas provided in Table E -1 <br />(page E -1), the Division estimates a composite curve number between 87 and 88, <br />assuming 82.88 acres (or 41 percent) of the 200 contributing acres are impervious <br />sandstone. The Applicant shall use CN = 88 to be conservative, or provide rationale for <br />the CN = 85. <br />7. Page G -1, Hydrology and Sediment Control — The Post - Reclamation Site Drainage <br />section on page E -4 combined with Map F indicates there are at least four sediment <br />ponds (one large pond in Mining Area 6, two small ponds in Mining Area 1, and one <br />unknown pond in Mining Area 2). The calculation on page G -1 is for one pond. <br />m:\min \tc1\m- 1986 -002 craig ranch\ stormwatercomments _mem21jun11.docx <br />