2009-11-06_REVISION - C1981019 (2)

Home Browse Search

5. The Colowyo Permit contains Map 12 - Hydrology-South Area, which shows both the proposed locations of the permanent post-mining drainage channels and the current construction status of these channels. Map 12 was last updated on 25 March 2008 in conjunction with Technical Revision 71. An updated Map 12 was not submitted as a part of Technical Revision 83. Since Colowyo has proposed changes to the locations of the permanent post-mining drainage channels in the West Pit and since some of the channels on the map have been at least partially constructed (Taylor Tributary Ditch, Prospect Ditch, Final East Pit Ditch and North Tributary, East Pit Ditch), Colowyo will need to update this map as necessary (including the proposed changes to the ditches in the East Taylor watershed and updating the construction status of all the ditches). Please submit three copies of Map 12 that have been signed by a registered professional engineer to the Division for review. Rule 4.05.4 - Stream.Channel Reconstruction 6. As noted in the general comments section above, Colowyo is proposing changes to the East Taylor Ditch that include reducing its length, changing its configuration, changing its slope and changing the point at which it intercepts the Taylor Tributary Ditch. Colowyo did not include any revised SEDCAD hydrologic modeling for the East Taylor Ditch to verify its ability to safely pass the appropriate sized storm event (100-year, 24-hour event). This information is contained in Exhibit 7-14I of the permit. Please provide the Division with new SEDCAD hydrologic modeling for the East Taylor Ditch. 7. In addition to the SEDCAD modeling requested for the East Taylor Ditch in question 6 above, Colowyo will also need to update the general ditch information on Page Exh. 7-14I-2 (in Exhibit 7-14I). This information must reflect the proposed changes to the East Taylor Ditch design as shown in Technical Revision 83. Please update Page Exh. 7-141-2 and submit it to the Division for review. 8. Colowyo is proposing to change the path of the lower portion of the East Taylor Ditch from a location that is more or less along the natural drainage to a location that requires the creation of a more artificial channel for the drainage. In doing this, the runoff from the area immediately south of the proposed East Taylor Ditch, including runoff from the north side of the West Pit Fill, will be allowed to flow outside of a channel until it reaches the lower 1,000 feet of the Taylor Tributary Ditch. This area of uncontained runoff is approximately 200 acres and will include flows that travel over 4,000 feet overland, outside of any constructed ditch. Additionally, this area includes the very steep lower portion of the East Taylor watershed, with slopes in excess of 20%. Colowyo must explain the reasoning for this change in configuration of the East Taylor Ditch and the reason why an additional ditch is not proposed for the south part of the East Taylor watershed where there is a very large, natural drainage being created.