2009-02-04_REVISION - C1981019 (4)

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fine particles. Hydrologic soil cover complex (CN) values used for the three ditch sizing modeling runs are appropriate for land in a significant disturbed condition. Ditch grades for the three ditches that will divert runoff to the East Pit are I%, except over short runs where the grade may be steeper. The operator has committed to use channel lining/protection, if necessary, where steeper grades exist to limit downcutting and erosion. More importantly, from a performance perspective, is that the ditches are maintained to have adequate capacity to carry the peak flows, especially the uppermost ditch, Ditch A, that may carry up to 56 cfs during peak flows. Breaching of Ditch A and the potential for overtopping of Ditches B and C downslope could be catastrophic to the Prospect pond. The operator has included two feet of freeboard on these ditches to account for additional flows in excess of what the modeling predicts. The fourth and final SEDCAD modeling run is for storm runoff and snowmelt that will be captured by the sump in addition to runoff from the remainder of the areas that drain directly to Prospect pond, i.e. not diverted into the East Pit via ditches A, B, or C. If approved, this model will replace the currently approved Prospect Pond model for the 10 year, 24-hour storm event. The Division does not agree with the use of hydrologic soil cover complex (CN) values of 56 for 3 year and older reclamation within the Prospect watershed. In the Division's opinion and in the absence of significant, site-specific supporting evidence or documentation, a curve number this low does not accurately represent the hydrologic soil cover conditions for these more mature reclaimed areas. The Division waiting for responses from Colowyo on a pending technical revision (Technical Revision 73), that proposes to change the curve number values from specific values to a range of values. Please use a CN of 75 or higher, or respond to pending TR-73 to allow for proposed curve number ranges. Revise the Prospect Pond 10 year, 24-hour model as appropriate. The Division would prefer to promptly resolve the curve number issue pending in TR- 73. 2. To comply with Rule 4.05.9(2)( c)(ii), please submit SEDCAD modeling that demonstrates that the Prospect Pond spillway system will safely pass the 25 year, 24-hour storm event. Unless ditches A, B, and C are designed to divert the 25 year, 24 hour storm event, 25 year storm runoff will also need to be included in the Prospect Pond spillway design run. 3. Based on soils information from the approved permit, the majority of the soils in the Prospect watershed are comprised of one of the following soils shown in the table below. Selection of CN values should reflect these fine textured soils having a slow infiltration rate. With the potential for compaction upon reapplication of soils, considering all soils as category C is preferred. Please update the SEDCAD runs to include this change.