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Mint Watn Convo! Plan <br />Sediment storage was estimated through SEDCAD+ by using the Revised Universal Soil <br />Loss Equation (RUBLE). This option was chosen to handle the steep slopes at the mine site. <br />Sediment volumes were based on a three-year period using a rainfall and runoff (R) factor <br />of 30. The mine recognizes that this is only an estimate. Sediment storage capacity in each <br />pond is monitored either through a survey or through use of a level indicator in the pond. <br />When sediment has exceeded the recommended storage capacity, the mine will drain the <br />pond and excavate the sediment, allowing for additional sediment storage. <br />Precipitation events were based on the data available through NOAA's atlas for Colorado <br />(TP40). The precipitation event fora 10-year, 24-hour event is 1.9 inches over a 24-hour <br />period. Precipitation for a 25-year 24-hour event equaled 2.3 inches of precipitation over <br />any twenty four hour period. The 100-year 24-hour event is estimated to be 2.8 inches of <br />precipitation over a 24-hour period. The precipitation was distributed over the watershed <br />using a Type II Distribution. No provisions were made for snowmelt or a rain on snow <br />event. <br />Hydraulic Design Considerations <br />The site was broadly classified into two distinct groups, Disturbed and Non-disturbed. Most <br />• of the area was considered as non-disturbed since the mining is underground. Only areas <br />such as open stockpiles, ]aydown areas and maintenance yards were truly classified as 100% <br />disturbed. All other areas were considered as non-disturbed or combined non-disturbed and <br />disturbed. These designations are important for the input parameters such as curve numbers <br />(CN), Land Use Types and Canopy covers. <br />Watersheds that have been highly impacted due to some form of disturbance have been <br />assigned a CN of 82 as compared to a watershed with little to no disturbance with a CN of <br />72. This correlates to a higher amount of runoff from the disturbed area as to the non- <br />disturbed areas (greater infiltration capabilities in anon-disturbed area). Graveled benches <br />have been assigned a CN of 87. Roller compacted concrete has been assigned a CN of 98. <br />Land use types were used to aid in lag time and velocity of runoff in the watershed. The <br />higher the ground cover and obstructions in the coarse of the runoff the slower the water <br />would travel. Water that is moving through the watershed at a slower rate will pick up less <br />sediment, therefore less erosion will occur. <br />Lag times were utilized in this analysis to model treatment of specific storm events. Time <br />of concentration and Muskingum routing parameters utilized the SCS upland curve method <br />• <br />V <br />