2006-11-13_REPORT - M1986015 - Laserfiche WebLink

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Additional Pages 2006 Annual Report RECEWEiD Pueblo East Pit NOy 13 M-1986-015 p~l~O"~R ~ Transit Mix of Pueblo Minin9and ~. Anniversary Date: November 12 Overview of Permit Year: The 2005-2006 permit year was eventful, to say the least. The operation was practically shut down fox the second half of the summer due to severe flooding on the Arkansas River. As a result of these problems, the amount of disturbed land was expanded a bit more rapidly than was initially expected, but is now returning to a more normal pattern. Work on preparing the amendment fox Phase 2 was nearly completed by the end of the permit year and the amendment is nearing the point of being submitted. Annual Report Maps: The annual report map prepared fox this year is based on the Phase 1 mining plan map and contains a bit more detail than has been provided in other annual reports. This is largely the result of the major impacts of the floods on the operation. Affected Lands: During the first half of the year, up until July, the operation proceeded as had occurred in the past. That is, a slow and steady expansion very closely following the permitted plan. Dewatering continued to be a growing problem with volumes of discharge reaching hlghex and higher levels. But everything remained quite manageable and work proceeded according to plan. During the summer of 2006, especially in July and August and into mid-September, the monsoonal moisture was quite intense with the Colorado Springs-Pueblo-Cation City triangle being hit very hard by intense rains, often day after day. This was a relief from the extremely dry winter and spring earlier in the year when precipitation was almost non-existent. Unfortunately, the amount of moisture received created a series of flood events that peaked in two major floods. The first was the largest flood since the floods at the end of April 1999. River flow in the first flood adjacent to the pit was estimated at between 17,000 and 20,000 cfs. Although the April flood which was about 12,000 to 13,000 cfs caused more damage due to its long duration, the floods of 2006 were more of a flash flood character -quick but very intense. The second flood was somewhat smaller, but still larger than the 1999 peak volumes. It too was a flash flood type. Although prolonged flood events often create more damage, flash floods create extreme damage where the circumstances are ideal for damage to occur. Although nobody witnessed the breaching of the pit wall, it is suspected, based on the resulting damage, that it occurred rather quickly in a series of two events. As the river rose it is suspected the initial water flowed in from wetlands just south of the south end of the pit. Across-channel in the meander to the south carried this water and normally it is discharged into the river on the east side of the meander. But the volume was so great it flowed down small rivulets, cut under the protective berm and poured into the pit at one location. It is not known if this was the fast event or the second in the sequence. The main event was caused by the river making its way around some in-channel sand bars that had developed over the past few years on the downstream side of the meander. Prior to the flood the river bank was stable and well vegetated, but during the flood the water was forced into the bank by a sand bar in the middle of the channel. This probably began to undercut the bank thinning the 100 foot setback. As the river tore higher and reached a peak flow the river appears to have overflowed the bank and poured into the pit. Headwaxd erosion quickly cut through the thinned and weakened wall until the river could pour into the pit and quickly fill the pit creating a lake that covered about 30 to 40 acres. During the breaching process the initial break rapidly widened to about 300 feet. Once the pit was full the breaching process stopped. Thus the size of the breach was probably a function of the time it took to fill the pit with water and stabilize the inflow. A smaller pit would have had a smaller break, but if the pit had been larger the break would have likely been even larger. Transit Mix of Pueblo 2006 Annual Report M-1986-015 Pueblo East Pit Page 1 of 3