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• <br /> Channel Width Channel Depth <br />Slope Increase Larger Smaller <br />Due to Subsidence <br />Slope Decrease Smaller Larger <br />Due to Subsidence <br />The deformation of the ground surface due to subsidence results in a change in the existing <br />channel slope. The magnitudes of these slope changes were first computed based on the <br />subsidence model output, then the changes were applied to the existing channel slope to <br />determine the ensuing channel slope. For purposes of determining changes in channel <br />hydraulic characteristics, the channel slope near the confluence or mouth of each drainage was <br />used so that the contribution of the entire basin was included. <br />Utilizing channel regime relationships developed by the geomorphic engineering profession, it <br />was determined that over a 3- to 5-year period or more, the channel hydraulic characteristics <br />would change as shown in Table 5. The changes in depth and width are the maximum <br />computed values considering both a slope increase and slope decrease. However, it should be <br />recognized that the maximum changes in channel width and depth would not occur over the <br />same channel reach. <br />CHANGES IN SEDIMENT YIELD <br />The mean annual sediment yield for each basin will not change except for minor channel cutting <br />and channel filling extending over a 3- to 5-year period resulting from the geomorphological <br />tendency for the existing channels to either be cut or filled. Overall, there will be a tendency for <br />these changes in sediment production to balance out within the basins. <br />Referring to Table 2, it is noted that the mean annual sediment yield for the subject basins <br />ranges from approximately 6 to 220 cubic yards per year. These sediment yields are not <br />expected to change (i.e., increase or decrease) by more than about 5 percent due to any <br />change in hydraulic characteristics resulting from the increase in slope as identified in Table 4. <br />SUMMARY <br />0- <br />Stream channel characteristics will change as a result of subsidence. This analysis of stream <br />channels was undertaken to determine the magnitude of change resulting from the change in <br />channel slope. The changes to stream channel parameters were analyzed using standard <br />procedures of the sedimentation and geomorphic engineering professions based on the effects <br />of thalweg slope changes (either increase or decrease) due to mining-induced subsidence. The <br />results are summarized in Table 6. <br />The maximum estimated change in channel width is seven feet, and the maximum change in <br />channel depth is 0.4 feet. Changes in new regime width and depths occur in a slow process that <br />may take from three to five years or more. The likely change in sediment yield is not expected to <br />be more than 5 percent from the values given in Table 2. <br />The mining-induced subsidence impacts on existing stream channel parameters and basin <br />sediment yield are not significant. Any changes in channel characteristics will occur gradually <br />and reach the new regime over a period of several years. Maximum channel geometry changes <br />will most likely be localized rather than reach wide. Monitoring of any channel morphology and <br />geometry changes will allow for site-specific prescriptive mitigation as deemed necessary. <br />Tetra Tech - 0907161P