2013-09-20_REVISION - C1992080 (2)

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sampling may yield a lower runoff volume and subsequently indicate a smaller required ditch capacity. 4. The HEC -RAS information provided for the upper segment of the Carbon Junction Channel indicates that the existing channel capacity is adequate to convey the design event. Predicted velocities (5.17 -8.08 fps), however, are in excess of the acceptable range (3 -5 fps, depending on soils) for a mixed grass vegetated waterway. Please revise the proposed channel designs to include lining as necessary (in areas where the channel will be disturbed) to ensure that the channels will be stable and not contribute suspended solids to streamflow and runoff outside the permit area (Rule 4.05.3(4)). Also, the design information submitted indicates that the channel is designed to convey the 100 yr — 24 hr event. The watershed contributing to the channel is less than one square mile, so the requirements of Rule 4.05.3 would apply to this section of Carbon Junction. Rule 4.05.3(3) requires the channel to be sized to adequately convey the runoff from a 10 yr — 24 hr event. The diversion portion of the channel, however, must be sized for the 100 yr — 24 hr event. Please see comment 7, below, for further clarification. 5. The sixth page of Appendix 5 -2 includes information on the Permanent Channel Realignment of Carbon Junction Channel to run through the reclaimed pit area. Since the Carbon Junction Diversion will be a permanent structure and Carbon Junction Channel will not be restored to its original configuration, this page should be eliminated or revised to indicate that the design will not be implemented. 6. The design information and maps provided with the technical revision application have not been certified by a registered professional engineer. Please provide the required certification for all of the maps, cross - sections, and plans submitted (Rules 2.10.3(2) and 4.05.4(2)). 7. The design event requirements for the Carbon Junction channel are as follows: • Unnamed tributary to Carbon Junction Channel — 10 yr -24 hr (Rule 4.05.3) • Carbon Junction Channel upstream of the Carbon Junction Diversion Channel 10 yr -24 hr (Rule 4.05.3) • Permanent Diversion of Carbon Junction around the fill — 100 yr— 24 hr (Rule 4.09) 8. It will likely be necessary to extend the rip rap upstream of the Permanent Carbon Junction Diversion Channel to ensure that flow from Carbon Junction is directed to the Permanent Carbon Junction Diversion Channel and not in the direction of the former East Diversion. Should you or the operator have any questions or need additional information, please let me know.