INSPEC03556 - Laserfiche WebLink

Home Browse Search

i c MINE ID # OR PROSPECTING ID#: M-1990-098 PAGE: 2 INSPECTION DATE: 10-9-01 INSPECTOR'S INITIALS: ACS The Division of Minerals and Geology has been finalizing the review of the amendment AM-Ol application to the Perry Pit mined land reclamation permit. The final technical issue to be addressed prior to approval of the application is the proposal to eliminate construction of the side channel spillway that is required under the approval of the original (1990) application. In the Applicant's adequacy response letter dated September 21, 2001, several issues are discussed supporting the conclusion that a side channel spillway or other flood control measures will not be necessary to protect the Perry Pit and prevent redirection of river flow energy through the excavations. The Division's position on the points raised in the adequacy response will be enumerated below. The purpose of this inspection was to evaluate the Applicant's determination that the sinuosity of the river channel is confined by a historic levee in the reach where the channel bends toward and within approximately 220-feet of the south end of the pit. As the river flows north from this closest point of approach to the pit, it also moves away from the pit to the east. At a point approximately 800 feet south of Weld County Road 6, the riverbank is more than 400 feet from the mining limits and is aggrading. As a result, the potential for damage to the north half of the proposed pit during a 100-year flood is greatly diminished. This inspection was focussed on the area east of the south half of the pit where an outside bend of the river is approximately 220 feet from the mining limits (see the diagram included as page 5 of this report). At this location an eddy has developed at the approximate midpoint of the outside river bend. The western edge of the eddy pool is the closest approach of the river water to the pit. Just south of the eddy, the left riverbank is undercut and ezoding. The historic levee is located just to the west of the riverbank, and is in relatively good condition above the eddy and for a few tens of feet to the north of the eddy. The crest height of the levee drops as the structure proceeds north, and it was determined that if the levee were to be overtopped during a flood, the overtopping would occur initially at a location to the north of the eddy, at the approximate location illustrated on the included diagram. This assumes that the levee will not have already failed by some other mechanism prior to overtopping flows. The low point of the crest of the levee has been reinforced by past landowners through the installation of embedded tires. The reinforced low point in the levee will serve to control the location at which a flood will low onto the Perry Pit property, as long as the levee holds until overtopping occurs. Based on the geometry of the levee, the likely spill point at the crest of the levee, and the proposed mining limits of the Perry Pit, it was determined during this inspection that damage to the pit slopes during a 100-year overtopping flood event would be minimal. This is due to several factors, including: