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Stream Channel Parameters and Changes <br /> Exhibit 55B Due to Longwall Mining-Induced Subsidence Page 14 <br /> Subsidence along the South Prong channel(Basin 39)ranges from zero to approximately nine feet. <br /> As shown in Table 3,the maximum decrease in slope is 3.7%, and the maximum increase is 1.5%. <br /> Given the average pre-mining slope of 18.1%, slope changes due to subsidence are not expected <br /> to create any reaches with negative or flat slopes. Figures 27A and 27B show subsidence along <br /> the full extent of South Prong and zoomed in to the western edge of LWSS4, respectively. <br /> Subsidence along the Lion Gulch channel(Basin 41)ranges from zero to approximately nine feet. <br /> As shown in Table 3,the maximum decrease in slope is 3.0%, and the maximum increase is 1.8%. <br /> Given the average pre-mining slope of 24.2%, slope changes due to subsidence are not expected <br /> to create any reaches with negative or flat slopes. Figures 28A and 28B show subsidence along <br /> the full extent of Lion Gulch and zoomed in to the western edge of LWE14, respectively. <br /> As shown in Figures 27B and 28B, the existing slope at the downstream limit of the subsidence <br /> was projected to decrease due to the projected subsidence.Reduction of the stream slope will likely <br /> result in an increased width to depth ratio, a reduction in channel velocity, increased floodplain <br /> connectivity, and increased potential for sediment deposition. South Prong will likely maintain a <br /> step-pool morphology where larger pools are developed, and riparian plants establish upon the <br /> gentler channel side slopes. Morphological changes to Lion Gulch are predicted to be less <br /> prominent due to the ephemeral nature of the drainage and relatively low flow rates. <br /> The existing slope at the upstream limit of the subsidence is projected to increase. An increased <br /> stream slope generally results in a decreased width-to-depth ratio, an increased channel velocity, <br /> reduced floodplain connectivity, and increased potential for erosion. As the channel in the upper <br /> reaches currently maintains a cascade morphology, steepening of this reach will not fundamentally <br /> change the stream type. Furthermore, the observed outcroppings of bedrock in South Prong and <br /> larger material indicative of these steep streams will likely further mitigate against the potential <br /> for erosion where slopes have steepened beyond their existing condition. The Lion Gulch drainage <br /> will experience minimal change in its headwaters as the subsidence extends to the top of the <br /> watershed. <br /> 831-032.921 Wright Water Engineers, Inc. <br /> December 2020 <br />