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Stream Channel Parameters • Exhibit 55A and Changes Due to Mining-Induced Subsidence Page 10 CHANGES IN SEDIMENT YIELD The mean annual sediment yield for each of the 12 basins will not change except for minor chammel cutting and channel filling extending over a 3- to 5-year period resulting from the geomorphological tendency for the existing channels to either be cut or filled. Overall, there will be a tendency for these changes in sediment production to balance out within the basins. Referring to Table 2, it is noted that the mean annual sediment yield for the subject basins ranges from approximately 6 to 220 cubic yazds per year. These sediment yields are not expected to change (i.e., increase or decrease) by more than about 5 percent due to any change in hydraulic chazacteristics resulting from the increase in slope as identified in Table 4. SUMMARY Stream channel chazacteristics will change as a result of subsidence. This analysis of stream • channels was undertaken to determine the magnitude of change resulting from the change in chammel slope. The changes to stream channel parameters were analyzed using standard procedures of the sedimentation and geomorphic engineering professions based on the effects of thalweg slope changes (either increase or decrease) due to mining-induced subsidence. The results aze summarized in Table 5. • The maximum estimated change in channel width is seven feet, and the maximum change in channel depth is 0.4 feet. Changes in new regime width and depths occur in a slow process that may take from three to five years or more. The likely change in sediment yield is not expected to be more than 5 percent from the values given in Table 2. The mining-induced subsidence impacts on existing stream channel pazameters and basin sediment yield aze not significant. Any changes in channel characteristics will occur gradually and reach the new regime over a period of several years. Maximum channel geometry changes will most likely be localized rather than reach wide. Monitoring of any channel morphology and geometry changes will allow for site-specific prescriptive mitigation as deemed necessary. 831-032.620 Wright Water Engineers, Inc.