2014-07-08_PERMIT FILE - C1981025 (8)

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NORTH THOMPSON CREEK MINES POND 9 EMERGENCY SPILLWAY Calculation Rational and Results Summary Rationale: A new emergency spillway design is prepared for Pond 9 as a result of NOV- 2014 -001. Pond 9 is designed with a single open - channel spillway, in lieu of a combination of principal and emergency spillways. Pond 9 is a water treatment pond so it may at times be full. The design takes into consideration the very good vegetation at the mine site. The vegetation at the site has been granted Phase III bond release. The design assumes the pond is full when a 25 -year 24- hour storm event occurs. Therefore, all of the run -off from the storm event flows out through the emergency spillway. Since the drainage area for Pond 9 has such good vegetation, there is very little run -off that results from the design storm event. The ditch on the east side of pond 9 that conveyed water to the pond is mostly filled with sediment. Therefore, the drainage area for Pond 9 has been reduced. Soils: According to Map D -3 -14, the No. 1 Mine Site was constructed in the Dollard Shale outcrop soil type. Dollard soils are a soil type C. Vegetation The drainage area for Pond 9 is 11.0 acres. The majority of the drainage area for Pond 9 is considered Sagebrush with a grass understory. The results of the 2000 vegetation inventory at the mine are: • Total Plant Cover 50.28% • Rock 3.32% • Litter 37.8% • Bare ground 8.6% There is also a two track road that contributes drainage to Pond 9. The road is approximately 2000' long. The disturbed width is assumed to be 6.5 feet which yields a disturbed drainage area of 0.3 acres. The vegetation on the two track road is Herbaceous with poor cover. Curve Numbers Based on Table 2 -2d, the curve number for the sagebrush with grass understory with good cover and soil type C is 47. Based on Table 2 -2d, the curve number for the two track road with poor herbaceous cover and soil type C is 87. TR -28 P -9 -12 05/14