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reclaimed drainage channel. The peak flow modeling assumed the total watershed of the reclaimed <br />channel at the time of bond release. The modeled peak flow for 006 Gulch for the 100-year, 24-hour storm <br />event was 26.4 cubic feet per second. <br />In 2003, as the North Area final backfilling and grading were conducted and the final postmining <br />topography determined, the 005 and 006 Gulch postmine drainage channels were redesigned for the actual <br />postmine configuration. The redesigns were based on peak flows calculated using the SEDCAD4 computer <br />model (Civil Software Design, 1998). This program is an updated, more sophisticated version of the <br />SEDIMOT II model. All of the postmining channels in the 005 and 006 watersheds were redesigned for the <br />10-year, 24-hour storm event. The resulting channel lining (i.e., vegetated or rock riprap) is based on <br />channel slope and velocity. The peak flow for the reclaimed drainages were determined at the downstream <br />end of the channel, so most of the reclaimed channel reach will not have to handle that large a peak flow <br />rate. This was done to ensure that the reclaimed drainage channel capacity will be adequate and <br />conservatively designed. A limiting velocity of 5.0 feet per second (fps) was used to insure that the <br />postmining drainage channels would be nonerodible and stable. The limiting velocity was estimated from <br />the tables in bedded in the SEDCAD4 model for a vegetated channel composed of smooth brome. The <br />vegetated condition at Seneca II-W mimic that of smooth brome closer than the other vegetation types. <br />The shape of the reclaimed channels was assumed to be trapezoidal with 3H:1V side slopes as a worst <br />case condition. <br />Attachment 20-1, Postmine Drainage Channel Designs, presents the SEDCAD4 model input and output <br />information. Table 20-2, Postmining Drainage Channel Summary, summarizes general channel <br />geomorphology for pre- and postmining conditions. All reclaimed stream channels will be conservatively <br />constructed, with the dimensions presented representing minimum geometry. Some channel reaches will <br />require a riprap lining for channel protection. Please refer to the individual calculations for the proper riprap <br />sizes to be utilized. <br />After the watershed has been graded and the location of the drainage channel determined, the channel will <br />be constructed to the required shape. Initially the channel will be bare soil only (i.e., no vegetation) to <br />allow for initial stabilization. During the period when the watershed area is revegetating, the drainages will <br />be inspected for stability according to the Rill and Gully Plan presented in Tab 22. Maintenance or <br />monitoring that may be necessary as a result of the inspections will be submitted to CDMG for review and, <br />if necessary, approval. The methods used to stabilize the channels could include installation of approved <br />check dams and livestock ponds, erosion control matting, riprap and straw dikes. The reclaimed drainage <br />channels will then be topsoiled and revegetated according to Tabs 21 and 22, respectively after the <br />channel has stabilized. The side slopes of the meanders of the reclaimed drainage channels will be <br />protected from erosion by revegetation with mesic seed mix and shrub plantings according to Tab 22. <br />Maintenance of reclaimed drainage channels will be identified according to the Rill and Gully Plan in Tab <br />22. <br />TR-59 12 Revised 08/07 <br />