2007-03-06_REVISION - C1994082 - Laserfiche WebLink

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widths were measured at each drainage channel to evaluate compliance with design criteria. The results of these measurements aze provided in the table below. Post-Mine Channel Bottom Width Grade PM-13 u er 11 feet 29% PM-13 mid 14 feet 26% PM-13 lower 21 feet 28% Avera e ri ra size ~ 6-9 inches PM-10 u er 13 feet 25% PM-10 mid 14 feet 22% PM-10 lower 15 feet 18% Avera e ri ra size ~ 12-15 inches PM-9 u er 13 feet 14% PM-9 mid 11 feet 22% PM-9 lower 13 feet 21 Avera e ri ra size ~ 7-9 inches Based on the field measurements taken and comparison with design criteria, it was determined that all channels met minimum design specifications except for two locations as described below. All of the channels were overbuilt with regard to size (bottom width and side slopes), and riprap specifications. Channel slopes measured with an inclinometer were all generally consistent with design slope information. The location and density of the reconstructed drainages are consistent with the pre- mining tributary drainage system. Post-mine drainages PM-9 and PM-10 were constructed at the approximate location ofpre-mining intermittent streams. Additional drainage density was added with PM-13. The entire tributary drainage system within the reclaimed North Pit has been reconstructed and is functional with exception of two locations that will require additional work or repair. These locations include upper to mid PM-13 and upper PM-10 and these portions of these channels will be excluded from the Phase I bond release as described below. At the head of PM-13 erosion is occurring in the form of rills above the start of the channel. The uppermost portion of PM-13 is void of riprap and erosionally unstable. There is a gully down the center of the channel and fabric is showing through in several azeas of the upper channel. At about the mid section of the channel significant bank erosion on the east side is occurring. Photographs of the upper and mid sections of PM-13 aze shown in Photograph Numbers 9 and 10. Erosion has been repaired above Pond 011 A by use of hummocks to slow down runoff. The runoff flows through the roughened area directly into PM-13 on the southeast side. A gully has eroded through the hummocky azea and will likely impact the long-term stability of Post-Mine Channel PM-13. A photograph of the unstable azea southeast and directly adjacent to PM-13 is shown in Photograph No. 11. These unstable azeas associated with the PM-13 channel will be excluded from the Phase I bond release. At the upper section of PM-10 the channel does not have a trapezoidal configuration with 3.0:1 sideslopes as indicated by the approved design. There is too much riprap stockpiled on the uphill side of the channel preventing water from flowing into the channel. This has resulted in gully formation 10