2022-09-29_PERMIT FILE - C1980007A (2)

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Stream Channel Parameters and Changes Exhibit 55B Due to Longwall Mining-Induced Subsidence Page 14 831-032.923 Wright Water Engineers, Inc. December 2021 Subsidence along the South Prong channel (Basin 39) ranges from zero to approximately nine feet. As shown in Table 3, the maximum decrease in slope is 3.7%, and the maximum increase is 1.5%. Given the average pre-mining slope of 18.1%, slope changes due to subsidence are not expected to create any reaches with negative or flat slopes. Figures 27A and 27B show subsidence along the full extent of South Prong and zoomed in to the western edge of LWSS4, respectively. Subsidence along the Lion Gulch channel (Basin 41) ranges from zero to approximately nine feet. As shown in Table 3, the maximum decrease in slope is 3.0%, and the maximum increase is 1.8%. Given the average pre-mining slope of 24.2%, slope changes due to subsidence are not expected to create any reaches with negative or flat slopes. Figures 28A and 28B show subsidence along the full extent of Lion Gulch and zoomed in to the western edge of LWE14, respectively. As shown in Figures 27B and 28B, the existing slope at the downstream limit of the subsidence was projected to decrease due to the projected subsidence. Reduction of the stream slope will likely result in an increased width to depth ratio, a reduction in channel velocity, increased floodplain connectivity, and increased potential for sediment deposition. South Prong will likely maintain a step-pool morphology where larger pools are developed, and riparian plants establish upon the gentler channel side slopes. Morphological changes to Lion Gulch are predicted to be less prominent due to the ephemeral nature of the drainage and relatively low flow rates. The existing slope at the upstream limit of the subsidence is projected to increase. An increased stream slope generally results in a decreased width-to-depth ratio, an increased channel velocity, reduced floodplain connectivity, and increased potential for erosion. As the channel in the upper reaches currently maintains a cascade morphology, steepening of this reach will not fundamentally change the stream type. Furthermore, the observed outcroppings of bedrock in South Prong and larger material indicative of these steep streams will likely further mitigate against the potential for erosion where slopes have steepened beyond their existing condition. The Lion Gulch drainage will experience minimal change in its headwaters as the subsidence extends to the top of the watershed.