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Stream Channel Parameters and Changes <br /> Exhibit 55B Due to Longwall Mining-Induced Subsidence Page 19 <br /> SUMMARY <br /> Stream channel characteristics will change as a result of longwall panel subsidence. This analysis <br /> of stream channels was undertaken to determine the magnitude of subsidence along channel <br /> profiles and changes to channel slope due to longwall mining. The changes to stream channel <br /> parameters were analyzed using standard procedures of the sedimentation and geomorphic <br /> engineering professions based on the effects of thalweg slope changes(either increase or decrease) <br /> due to mining-induced subsidence. The results are summarized in Table 4. <br /> Of those streams and longwall panels analyzed, the maximum estimated change in channel width <br /> is 9 feet, and the maximum change in channel depth is 0.8 feet (Table 5). Changes in stream <br /> channel width and depths generally are expected to occur over a period of three to five years. <br /> However, there is potential for more rapid localized change due to factors such as soil saturation, <br /> forest cover, and channel stability. The likely change in sediment yield is not expected to deviate <br /> by more than 5 percent from the values given in Table 1. <br /> Subsidence in the studied basins can reach magnitudes of up to nine feet and extend along hundreds <br /> of feet of channel. Although the changes in slope may impact the geomorphic characteristics of <br /> the channel, the potential for adverse impacts to stream function remain low. The dominant <br /> discharge will naturally shape a channel dimension that will be in dynamic equilibrium with the <br /> existing up- and downstream reaches that are not impacted by the subsidence. <br /> In general, channel geometry changes and channel profile lowering are expected to occur over a <br /> period of three to five years. However, localized factors may lead to more rapid change in which <br /> certain channel segments lower more quickly than others. <br /> 831-032.921 Wright Water Engineers, Inc. <br /> December 2020 <br />