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• postmining conditions using the 700-year, 24-hour storm resulted from lower slopes and <br />slope lengths in the larger subwatersheds. <br />Impact Significance. Results from the modeling exercise indicate that runoff from <br />reclaimed areas proposed for the Seneca 11-W Mine will feature higher rates and volumes, <br />and will generate more sediment yield and higher concentrations in comparison to premini ng <br />conditions. However, the magnitude of the increased runoff and sediment loads predicted <br />from each drainage modeled assuming postmining conditions are not as significant as the <br />increases indicate. <br />For instance, the 100-year, 24-hour storm was predicted to yield 2.52 acre-feet of runoff <br />from the 005 Culch basin (down to the 005 pond). This translates to less than .2 inches <br />of water yield from this area due to more than 2 inches of rainfall. Similarly, the <br />postmining 006 Culch area was predicted to yield 11.7 acre-feet of runoff from the <br />100-year, 24-hour storm depth, which is also less than .2 inches of water yield from more <br />than 2 inches of rainfall. Although SEDIMOT II predicted significantly lower runoff <br />volumes from these areas using inputs reflective of premining conditions, water yields of <br />• less than .2 inches from a 2.6 inch storm depth do not indicate potential for erosion due <br />to increased runoff generated by precipitation. Both the 006 and 005 ponds are proposed <br />to be left intact after mining as permanent impoundments. Both impoundments will be <br />designed and constructed to adequately pass runoff and store sediment generated by the <br />100-year, 24-hour event (see Tab 13, Facilities). <br />The peak discharge rates predicted by SEDIMOT II using the 100-year, 24-hour storm have <br />been used to evaluate the stability of reclaimed drainage channels (see Tab 20, <br />Bac kfilling and Cra di ng). The reclaimed channels have been designed and will 6e <br />constructed to convey peak discharge rates at least as great as predicted by SEDIMOT II <br />(6.4 cfs, 005 Gulch, and 26.4 cfs, 006 Culch), without allowing the discharge to attain <br />excessive flow velocities that will cause degradation. The channels will be constructed <br />in order to maintain stability and minimize additional contributions of sediment. <br />Interestingly, none of the peak sediment concentrations predicted by SEDIMOT II exceeded <br />70 mg/l, which is the upper limit allowed by Peabody's NPDES permit for point-source <br />discharges. These predictions were made for runoff prior to entrance to a sediment pond <br />• or, proposed permanent impoundment. Both 005 and 006 ponds will provide sufficient <br />treatment (detention time) of any runoff with elevated sediment concentration prior to the <br />73 <br />