Laserfiche WebLink
two to three percent. However, the overall average watershed slopes measured in both the • <br />005 and 006 basins will change less than one-half of one percent after mining and the <br />reestablishment of the postmining landscape. <br />The drainage system within each basin will experience minor changes as a result of mining. <br />Stream order and drainage pattern will remain unchanged, but the overall drainage density <br />and the inverse parameter, consta nce of channel maintenance, will change slightly (see <br />Table 17-21). Both ponds 005 and 006 will be designed and constructed as permanent <br />impoundments below the upper portions of both basins in which drainage density will be <br />altered. The ponds will be constructed in order to safely handle and treat runoff and <br />sediment loads generated from a 100-year, 24-hour storm (2.6 inches). Reconstructed <br />channels above both ponds within the reestablished postmining landscape will be designed <br />for stability and to prevent additional contributions of sediment. Changes in drainage <br />density will be limited to the areas above the impoundments, and the impacts of these <br />changes will be minimal, although long term. <br />Stream Channels. Table 17-22 includes pre- and postmining measurements of the longest <br />stream channel in both the 005 and 006 Gulch basins. Within 005 Gulch, no changes will • <br />occur in the length, elevation change, sinuosity, or gradient of the longest stream <br />channel. The length of the longest stream channel within the 006 basin will decrease <br />after mining by just over 300 feet, resulting in slightly lower sinuosity and a slight <br />increase in gradient (less than two percent). However, these changes are minor, as the <br />channel will remain a "straight" channel as indicated by sinuosi ties of less than 1.1, and <br />the gradient will change by less than two thousandths. <br />Tab 20, Bac kfill ing and Grading, discusses prominent channels within the permit boundary <br />that will be disrupted by mining and reconstructed upon reestablishment of the postmining <br />topography. Table 17-22 presents a summary of lengths, slopes, and changes in elevation <br />of these channels as measured prior to mining and proposed for the postmining landscape. <br />Exhibits 7-7-1 through 7-7-4 depict the locations of the prominent channels prior to <br />mining, and Exhibit 20-2 presents the channel locations proposed for the postmining <br />landscape. Overall, the postmining channels will be constructed to have comparable or <br />lower gradients than prior to mining. Most channels will be shorter in length, with <br />corresponding lower differences in elevation. The reconstruction of drainage channels <br />that wilt, for the most part, feature lower gradients, and be constructed for stability • <br />according to approved engineering criteria, will have a negligible although long-term <br />impact. <br />56 <br />