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Under short grass conditions, the channel is more prone to being eroded than during tall grass <br />• conditions. Consequently, stability is evaluated under short grass conditions. Retardance <br />classes of D and E are typically used for stability analysis of vegetated channels. <br />Tall grass conditions are used to determine channel capacity. The tall grass affords a great <br />deal of resistance to channel flow as a function of stem base) area and by laying over during <br />high flows. The higher the resistance, the higher elevation of flowing water in the channel. <br />Retardance classes of A through C (especially A and B) are used for capacity analysis. <br />SCC used rtardance values of Class D for stability and Class B for capacity. <br />Freeboard <br />In accordance with Rule 4.05.3, SCC designed all postmine channels with a minimum <br />freeboard of 0.3 feet. <br />The minimum channel designs for the vegetated channels are summarized on the Channel <br />Design Schedule table at the front of Appendix B. SEDCAD4 demonstrations are presented in <br />Appendix B. <br />2.2.2 Rock Riprap Designs <br />Prior to 2007, all unstable channel reaches (e.g., flow velocities greater than 5 fps) were <br />armored with rock riprap. Riprap size was determined using the SEDCAD+ riprap lined <br />channel sub-routine, utilizing either the Simons/OSM (Simons, et al., 1982) or PARER <br />(Pennsylvania Department of Environmental Resources, Stover, 1990) method. <br />The minimum channel designs for the rock riprap lined channels are summarized on the <br />Channel Design Schedule table at the front of Appendix B. Given the uncertainty in <br />determining riprap Dso for steep slopes with the PARER Method, SCC adds an additional 3- <br />inches to the model recommended Dso size. SEDCAD4 demonstrations are presented in <br />Appendix B. <br />2.2.3 Turf Reinforcement Mat Design <br />Turf Reinforcement Mats (TRMs) allow vegetation to be used in areas where flow conditions <br />exceed the limits of natural vegetation (e.g., limiting velocities greater than 5 fps). They can be <br />used for steeper slopes (1.SH:1 V, or flatter) and under medium to high flow conditions. <br />In select situations SCC may use TRMs as an alternative channel lining. Typical matting <br />construction consists of athree-dimensional corrugated matrix (with either coconut/straw or <br />polypropylene fiber) that will anchor and reinforce roots and stems for long-term stability. In <br />addition, the TRM deflects flowing water away from [be soil surface and increases the surface <br />roughness. <br />TR-4 ] 7-12-7 Revised 04/07 <br />