2009-11-06_REVISION - C1981019 (2)

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the West Pit Ditch to verify its ability to safely pass the appropriate sized storm event (10-year, 24-hour event). Curve number changes may be warranted based on the Technical Revision 73 changes and the requirements of question 17 below. This information for the West Pit Ditch is contained in Exhibit 7-14K of the permit. Please review the West Pit Ditch profile and current SEDCD modeling and revise if necessary. Please provide the Division with new SEDCAD hydrologic modeling for the West Pit Ditch if necessary or advise the Division if no updates are required. 15. Please update the general information for the West Pit Ditch contained on Page Exh. 7-14K-2 (in Exhibit 7-14K) if necessary. The revised information should contain any proposed changes to the West Pit Ditch associated with Technical Revision 83, including possible curve number changes. Please provide the Division with the updated Page Exh. 7-14K-2 for the West Pit Ditch. Rule 4.05.5 - Sediment Control Measures 16. Colowyo is currently in the beginning stages of reclamation of the West Pit and reclamation of the major portion of the East Taylor watershed. The changes proposed in Technical Revision 83 will influence the reclamation success in the East Taylor watershed. These proposed changes will also directly impact the East Taylor Pond both from a water treatment and stability standpoint. As in the Prospect watershed, Colowyo will have a large number of acres of young reclamation (recently backfilled, topsoiled and seeded and 1-2 year revegetation) in the East Taylor watershed, which will be reporting to the East Taylor Pond. Similar to the Prospect watershed, the success of the reclamation and the East Taylor Pond will be dependent on how much water reaches the pond and how fast it gets there. The use of upstream sediment control devices in the East. Taylor watershed will be essential for successful reclamation and the successful treatment and stability of the East Taylor Pond. The use of sediment control devices, such as those listed in Rule 4.05.5(2)(f), which help to reduce overland flow velocity, reduce runoff volume and/or trap sediment will be critical for the success of the reclamation in the West Pit in general and the East Taylor watershed specifically. Colowyo must include a specific and detailed plan for the use of alternate sediment control devices in the East Taylor watershed within the framework of Technical Revision 83. Rule 4.05.6 - Sedimentation Ponds 17. With the submittal of Technical Revision 83 Colowyo is proposing to change the locations, grades and configurations of the permanent post-mining drainage channels and Colowyo is proposing to change the post-mining topography in the lower portion of the East Taylor watershed. These proposed changes will affect the peak flow and velocity of the runoff into the East Taylor Pond. Based on the