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INTEROFFICE MEMORANDUM TO: Dan Mathews FROM: Mike Boulay DATE: August 1, 2001 SUBJECT: Roadside Mine (Permit No. C-81-041) Technical Revision No. 37 Adequacy Review As requested, I have reviewed the modified design for the 2 West Portal Relocated Drainage Channel. This technical revision incorporates a SEDCAD run for the channel and a revised Table 13-4 into the PAP. The primary change noted in the revised design is that the slope of the channel has been reduced from 8% to 6%. No problems were noted with the SEDCAD analyses provided by J.E. Stover & Associates. The SEDCAD run utilized the Simons/OSM method for steep slope design and recommends that D50: 6inch riprap be installed in the channel. However, J. E. Stover states that the design flow is based on the 100-year event and the channel will be monitored to determine if riprap is required. Please note that the design previously approved in the permit was fora 15 fr. wide channel with an 8% slope, and 4 inch riprap would be used (if necessary). I performed a separate analyses for the 10-year, 24-hour storm event to determine if riprap was required for a smaller event. Utilizing the same design parameter inputs, the Simons/OSM method recommends D50: 3 inch riprap for the 10/24 event. Without riprap protection, a 15 fr. wide channel with a 6% slope may be susceptible to excessive erosion, and downcutting at some point in the channel would likely occur. Unless otherwise agreed upon by you and the operator, I suggest that the recommendations of the Simons/OSM method be followed and riprap be placed in the channel. Site conditions or other constraints may justify installing the channel without the use of riprap. If riprap is not used, the operator should be required to monitor [he channel and ensure compliance is maintained relative to potential erosion. Please let me know if you have any questions. c: Sandy Brown