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A typical meandering channel design is provided using a design flow of 1.0 cfs and a • <br />Manning's n = 0.035 (Barfield et al., 1981, page 159). See Appendix B for channel designs <br />SIIPM-8a and SIIPM-Sb. <br />The channel designs show that the velocity of the flow will be less than 4.5 fps during the 10- <br />year, 24-hour storm. What cannot be modeled are the small pools embedded within the <br />channel that also help in dissipating energy. <br />Alternative Riprap Channel Design: It is doubtful that the estimated flow from the 10-yeaz, <br />24-hour event would have caused the erosion that occurred. However, it is possible that the <br />rapid snowmelt conditions may be a recurring event and the meandering channel may not be <br />stable during these springtime events. An alternative to the existing meandering channels <br />would be to constmct rocked channels, which would extend down the hillside to channel <br />SIIPM-8. <br />An alternative channel design for steep channel (18% - 31%) is also provided. The channel <br />wou]d be a minimum of 4 feet wide and would be lined with rock (DSO=9"). This can be <br />compazed to a similar channel in the area, namely SIIPM-14. This channel has a slope of <br />18%-25% and is lined with rock that is 6-12". <br />See Appendix B for alternative riptap channel designs for SIIPM-8a and SIIPM-8b..:. • <br />Discussion And Recommendation <br />The existing gully repair (meandering channel) is expected to control the flow from the 10- <br />year, 24-hour storm, since the calculated flow is less than 1.0 cfs, However, the gullying in <br />the spring of 2004, caused by a rapid snowmelt event, probably resulted in flows larger than <br />the design flow. The actual flow (Q) that occurred during the snowmelt is unknown. <br />Most of the area in question has been reclaimed for more than 3 years. Previous to the <br />spring of 2004, no rills or gullies of this magnitude were observed in the area. <br />SCC will leave the meandering channels intact and monitor during the Spring 2005 rill and <br />gully survey. If it is determined that the channels are not stabilizing the slope, if it is <br />determined that the spring snow melt will be a recurring problem that will destabilize the <br />area, or if new major gullies occur, then SCC will commit to constructing riprap lined <br />channels. <br />The existing temporary diversion channels that are currently diverting flow within the area • <br />will be removed and reseeded. <br />TR-36 7-12-8 Revised 11/04 <br />