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SUeam Channel Parameters <br />• Exhibit 55A and Changes Due to Mining-Induced Subsidence Page 2 <br />The first portion of this hydraulic and hydrologic evaluation establishes pre-mining, or baseline, <br />conditions. The second portion of this evaluation describes the potential and IikeIy impacts of <br />mining operations on the surface drainage system and channel chazacteristics. <br />EXISTING STREAM CHANNEL CHARACTERISTICS <br />The existing stream channel characteristics have been defined for various parameters, including <br />channel slopes, peak flow rates for a variety of frequencies of occurrence, flow velocities for <br />each of the frequencies, sediment transport regime, stage-dischazge relationships, channel <br />profiles, and channel and over-bank stability. <br />The existing channel shapes were related to the dominant dischazge (2-year frequency of <br />occurrence). Sediment transport has been defined for each stream using the annual sediment <br />load; however, sediment transport is a long-term value and represents the full range of flows <br />• including the 10-yeaz and 100-year frequency peak dischazges. <br />For each of the 11 existing stream channels, many hydrologic and stream channel parameters <br />were defined and evaluated, including: <br />1. Mean annual runoff <br />2. Peak dischazges for the 2-year, 10-yeaz, and 100-yeaz frequency floods <br />3. Mean annual sediment yields <br />4. Average thalweg2 slopes <br />5. Channel chazacteristics for the dominant discharge (consisting of width and depth <br />relationships) <br />6. The range of the channel slopes from neaz mouth to upper reach <br />Other factors that influence the long-term channel chazacteristics include the forested azea and <br />Z Line following lowest part of a valley, i.e., invert of channel. <br />• <br />831-032.620 Wright Water Engineers, tnc. <br />