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Colowyo, C1981-019, PR3 adequacy 13 February 25, 2011 <br />Rule 4.05.6 Sedimentation Ponds <br />1. Detailed design information was provided for both the in-pit sump and the sediment pond <br />below the Little Collom X pit. Should worst-case sediment modeling be re-run due to mine <br />plan changes, please ensure that the designs are modified, if necessary, for these two structures. <br />2. In accordance with Rule 4.05.6, sediment ponds are required to be as close as practicable to <br />disturbed areas, and this is also stated in PR-03. However, there is a very long distance from <br />the Collom Lite pit to the upper sediment pond (aka the sump in the Little Collom X pit). <br />Additionally, the temporary spoil pile appears to be an impediment to surface runoff finding its <br />way to the sump/pond system. Colowyo may want to consider additional sediment ponds <br />above the temporary spoil pile. <br />3. Regarding the SEDCAD model documentation for the worst-case scenario (Exhibit 7, Item 23), <br />the following deficiencies were noted: <br />• Curve numbers in the hydrologic model are old values rather than the new values agreed <br />upon in TR-73. However, in the worst-case scenario, the old curve number values provide <br />for a more conservative estimate of runoff and sediment production. Should the worst-case <br />scenario be revisited, Colowyo may want to use currently approved curve number values <br />for the various land conditions. <br />• Locations of some ditches on figures are apparently not accurate. For example, ditch D7 <br />(shown on Exhibit 7, Item 23, Figure E1) does not follow the contours; furthermore it drops <br />over 100 feet and rises back up approximately 50 feet per the contours on the figure. <br />• Figure Al and Figure E1 do not show all of the channels in the model, namely those <br />channels with descriptions beginning with "NC" and "RD." For example, Structures 2 <br />through 6 and Structures 11 through 16 are not shown. <br />• Elevations are not shown for contours on many of the maps (e.g., Map 1013, Map 12B, Map <br />22B). <br />• The SEDCAD model documentation (including structure networking tables, maps, and <br />SEDCAD output) does not clearly delineate which sub-watersheds drain to which channels. <br />Some of these network linkages are obvious, but others are not. Using the existing <br />information, it is nearly impossible to compare the SEDCAD model to the watershed maps. <br />• Some explanation of the selection of SEDCAD parameters for sub-watershed <br />sedimentology is necessary for the Division to perform a thorough review. For example, <br />the procedure for determining SEDCAD lengths is not described <br />• The locations of some ditches and areas on the maps look like they could be errors (or if <br />correct, then the purpose of the structures appears uncertain):