2009-10-05_PERMIT FILE - C1982057A (2)

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Channel Geometry Existing channels (constructed prior to 2007 were evaluated assuming a triangular, or V- shaped configuration, with a 3H:1V side slope. In certain situations, SCC designed a trapezoidal shaped channel with a bottom width of varying width and 3H:1V side slopes. Channel designed in 2007 as part of TR-?? were trapezoidal with a bottom width of either three (3) or 10 feet and 4H:1V side slopes. • The design parameters for peak flow, channel slope and channel geometry are summarized on Figure 20-1.2. Retardance Class The reclamation seed mixture SCC uses to revegetate reclaimed areas results in a mixed grass community. Consequently, the vegetation type selected as the model input is mixed grass. The flow resistance varies by grass type and stem length. Retardance classes have been divided into five classes designated by A, B, C, D and E. SCC (1979) developed guidance for selection of appropriate vegetation retardance classes and they are outlined in Table 20-3, Guide to Retardance Class Selection. TABLE 20-3: , - Guide to Retardance Class Selection Stand Length of Vegetation inches Retardance Class Good >30 A 11 - 24 B 6-10 C 2-6 D <2 E Fair >30 B 11 - 24 C 6-10 D 2-6 D <2 E Under short grass conditions, the channel is more prone to being eroded than during tall grass conditions. Consequently, stability is evaluated under short grass conditions. Retardance classes of D and E are typically used for stability analysis of vegetated channels. Tall grass conditions are used to determine channel capacity. The tall grass affords a great deal of resistance to channel flow as a function of stem Basel area and by laying over during high flows. The higher the resistance, the higher elevation of flowing water in the channel. Retardance classes of A through C (especially A and B) are used for the capacity analysis. TR-59 15 Revised 05/07